INVESTIGATION OF NEW PRODUCTION RATE ESTIMATES FOR IN SITU
COSMOGENIC 14C FROM THE HUANCANÉ CALIBRATION SITE, QUELCCAYA
ICE CAP, SOUTHEASTERN PERU
by
Adam M. Hudson
A Prepublication Manuscript Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements
for the Degree of
MASTER OF SCIENCE
In the Graduate College
THE UNIVERSITY OF ARIZONA
2011
Investigation of new production rate estimates for in situ cosmogenic 14C from the
Huancané calibration site, Quelccaya Ice Cap, southeastern Peru
Adam M Hudson1*, Nathaniel A. Lifton2, A.J. Timothy Jull1
1)
2)
Geosciences Department and Arizona AMS Facility, University of Arizona, Tucson, AZ
85721, USA
Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN
47907, USA
*Corresponding author: amhudson@email.arizona.edu
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Abstract
Terrestrial cosmogenic nuclides (TCN’s), produced directly in rocks and sediment near Earth’s
surface by cosmic radiation, have become increasingly useful as geochemical tools to investigate
timing of a variety of Earth surface processes. While these nuclides can be very accurately
measured in terrestrial samples, incomplete understanding of the systematics of nuclide surface
production cause significant uncertainty in the geologic interpretation of TCN measurements.
Because of this, researchers for the CRONUS-Earth project seek to provide a consistent
methodology for treatment and interpretation of TCN measurements and to reduce the
uncertainty in the global scaling of TCN surface production rates. This is in part accomplished
with a network of geological ‘calibration’ sites with independently well-constrained exposure
histories to which production rate scaling models can be calibrated. The Huancané calibration
site in southeastern Peru provides preliminary estimates of sea level high geomagnetic latitude
(SLHL) TCN production rates for in situ 14C in quartz that are, depending on scaling model, 1830% lower than the best-fitting estimate based on other CRONUS sites.
estimates for
10
Be and
26
Production rate
Al are also 5-17% lower than estimates from other CRONUS sites.
Based on geologic evidence and surface production modeling, there is no feasible complex
exposure history that can explain the disagreement between Huancané and the other global sites,
suggesting Huancané is extremely important for production rate calibration at low latitude and
high altitude. The in situ
14
C data from that site have significant uncertainty, however, and
replicate measurements are needed to confirm this conclusion.
Keywords: terrestrial cosmogenic nuclides, production rates, in situ
CRONUS-Earth
14
C, Quelccaya Ice Cap,
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#!
1. Introduction
Radionuclides produced by cosmic radiation (most commonly
3
10
Be,
26
Al,
14
C,
36
Cl, and
He) in rocks and sediments (terrestrial cosmogenic nuclides, or TCN’s) have become
increasingly useful tools for investigating Quaternary geologic questions, including timing of
glacier advance and retreat, surface erosion/exhumation rates, recent fault slip rates, and timing
of volcanic eruptions. The most common application of TCN’s is surface exposure dating,
because it is often viable in situations where lack of dateable material or old age precludes the
use of traditional radiocarbon or other dating methods. This method provides an estimate of the
length of time that a sample has been exposed to cosmic radiation at the surface, often by
measuring only one nuclide.
In addition to simple exposure histories, multiple nuclides
measured in the same samples can be used to unravel complex exposure histories, in which
alternating periods of burial and exposure can be constrained by modeling the interaction of the
different rates of production and radioactive decay of each nuclide. Gosse and Phillips (2001)
provide a comprehensive review of the development history and various applications of
terrestrial cosmogenic nuclides.
The most commonly measured TCN is 10Be in quartz because of its long half-life (1.36
Myr: Nishiizumi et al., 2007, 1.387 Myr: Chmeleff et al., 2010, Korschinek et al., 2010), the
wide availability of quartz-bearing substrates on Earth, and the increasing accuracy of
measurement by accelerator mass spectrometry (AMS).
26
Al (half-life 0.705 Myr: Nishiizumi,
2004) is also routinely measured in quartz and is often coupled with 10Be to investigate complex
burial and exposure histories. In situ cosmogenic 14C (in situ 14C) is a third nuclide that can be
measured in quartz. It is unique among TCN’s due to its short half-life (5.73 kyrs). Its potential
for use in surficial process and Quaternary geologic studies has long been recognized, but while
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in situ
14
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C has been measured in meteorites and lunar samples routinely for many years (e.g.
Fireman et al., 1976, Goel and Kohman, 1962, Suess and Wänke, 1962), development of
terrestrial in situ 14C has lagged that of other TCN’s because concentrations in terrestrial surfaces
are approximately two orders of magnitude lower than in extraterrestrial samples. This required
development of specialized extraction and purification techniques to reliably isolate terrestrial in
situ 14C in quartz from the ubiquitous atmospheric and organic
14
C in the environment. Some
early work suggested this was feasible (Jull et al., 1989, 1994), but these techniques were first
systematically described by Lifton et al. (2001) and have been subsequently refined and
expanded (Dugan, 2008; Hippe et al., 2009; Pigati et al., 2010; this work).
Calculation of exposure ages using TCN’s relies upon accurate knowledge of the
production and decay rates of the nuclides.
One of the biggest uncertainties remaining for
geologic applications of TCN’s is the spatial and temporal variation in TCN production rates.
TCN production rates are directly related to the flux of secondary cosmic radiation incident on
the Earth’s surface, which varies systematically because of the strength and character of the
Earth’s geomagnetic field, the intensity of the solar cycle, and variations in atmospheric pressure
(Gosse and Phillips, 2001). These spatial variations in cosmic radiation are also known to vary
through time. A variety of empirical models have been published (e.g. Lal, 1991/Stone, 2000;
Dunai, 2000, 2001; Desilets and Zreda, 2003, Desilets et al., 2006, Lifton et al., 2005, 2008) to
enable temporal and spatial scaling of TCN production rates based on modern physical
experiments and calibration to sites with well-constrained exposure history.
Each of the
published scaling models uses different neutron monitor datasets or incorporates the factors
affecting TCN production at the surface in different ways.
Therefore, discrepancies exist
between the latitudinal, altitudinal and temporal scaling factors in each model needed to generate
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site-specific production rates used to calculate exposure ages. The discrepancy in production
rate scaling between scaling models imparts a similar range of variability in calculated exposure
ages, which can have large effects on the geologic interpretation of TCN-based chronologies.
This uncertainty illustrates the need for a commonly accepted framework for TCN measurements
and production rate scaling among the TCN scientific community.
1.1 CRONUS-Earth
The CRONUS-Earth project (Cosmic-Ray Produced Nuclide Systematics on Earth)
(https://www.ees.nmt.edu/cronus/) was conceived in response to the increasing demand for
applications of TCN’s to geological and geochronological research questions. As the accuracy
and precision of analytical measurements has improved, the uncertainty in our understanding of
the physical behavior of TCN’s in the environment has become one of the main factors limiting
the scope of geologic questions that can be addressed with cosmogenic nuclides. The increasing
demand for applications of TCN’s in the geosciences highlights the need for collaboration
between researchers, laboratories, and educational institutions to collectively advance
understanding of the systematics of production of TCN’s at the Earth’s surface and to develop
standardized methods for collection, processing and measurement of TCN’s.
CRONUS-Earth project’s goals are to 1) facilitate the development of a framework for
comparison and standardization of TCN measurements between many research groups, 2) to
improve the uncertainties in TCN production rate scaling and exposure dating, and then 3)
provide a user-friendly, consistent, and accurate means of calculating exposure ages or erosion
rates for users in the scientific community.
The first goal has been approached with a ‘laboratory intercomparison’ project in which
CRONUS-Earth has provided reference materials, such as homogenous quartz materials
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CRONUS-A, and CRONUS-N, to laboratories across the US and internationally. Individual
laboratories were tasked with measuring the TCN concentrations in these materials and reporting
results to assess and mitigate systematic and random uncertainty in inter-laboratory
measurements. Multiple measurements of in situ 14C in the CRONUS-A standard are presented
in this study (Table 4) and the results of the intercomparison are forthcoming (Jull et al., in prep).
The third goal has already been partly achieved with the CRONUS-Earth Online
Calculator (Balco et al., 2008), an online application that provides the software foundation for
users to easily and quickly calculate 10Be and 26Al exposure ages and/or erosion rates by entering
their TCN data in a standardized format. The calculator employs a single calibration dataset to
derive the reference sea level high latitude (SLHL) production rates used in each of the included
scaling models, ensuring valid comparison between model-specific exposure age estimates. The
standardized format and calculation methods also facilitate easy comparison with other TCN
datasets and can be used to assess sensitivity of individual datasets to the exposure age results
calculated using different scaling models.
The second goal, specifically that of reducing the uncertainties in the global production
rate scaling, is the backbone of current approaches to improving TCN applications. The Online
Calculator of Balco et al. (2008) includes a calibration dataset for
10
Be with a relatively wide
range of latitudes and altitudes represented, but for other TCN’s (26Al,
14
C,
36
Cl, 3He), good
calibration datasets are fewer, and have greater uncertainty. CRONUS-Earth investigators have
therefore sought to create a larger network of calibration sites of known exposure age,
incorporating new measurements of multiple TCN’s. TCN concentrations are measured in
samples from new sites of independently well-constrained exposure histories by multiple
laboratories participating in the CRONUS-Earth project. These data will form the basis for
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assessment of published scaling models at each site and a new empirical calibration dataset of
production rates for the globe. Under the CRONUS-Earth project, we present new in situ 14C
results from the high altitude, low latitude Huancané calibration site located in the Andes of Peru
and compare them to 26Al and 10Be results obtained by other CRONUS-Earth laboratories.
1.2 Huancané Calibration Site
The Huancané calibration site, located in the Cordillera Vilcanota of southeastern Peru, is
a critical location for production rate calibration because it is the sole low latitude and highest
altitude CRONUS site. Previous calibration efforts (Balco et al., 2008 and references therein)
have focused mostly on sites at mid- and high geomagnetic latitude. The Huancané site provides
essential new information about the performance of scaling models at low geomagnetic latitude
where the effects of temporal variation in the geomagnetic field are strongest (Dunai, 2000,
2001; Lifton et al., 2008), and modern atmospheric pressure conditions are shown to differ
significantly from the ‘standard atmosphere’ approximation (Farber et al., 2005; Balco et al.,
2008).
The glacial valleys draining the Quelccaya Ice Cap (described below) have been
continuously sculpted by periods of glacier advance and retreat, depositing moraines that create
dammed peat bogs in the valley floors. These bogs are continuously growing during periods of
retreat and being overrun by subsequent advances, preserving dateable organic material both
stratigraphically above and below the moraines. This section will describe the site, focusing in
detail on the best-dated moraines of the Huancané valley (Mercer and Palacios, 1977, see Section
2), including the organic 14C sampling sites used for independent age determination, and the 10
boulders sampled for CRONUS-Earth for TCN measurement. We will present and discuss the
independent age determination for the moraines and the in situ
will compare sea level high latitude (SLHL) in situ
14
14
C measurements. Lastly, we
C production rate estimates from other
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CRONUS calibration sites to those calculated for Huancané, and then investigate the observed
14
C concentrations by comparing them to
10
Be and
26
Al measurements performed on the same
samples using a simple surface production model.
2. Study Site
2.1 Regional Setting and Quelccaya Ice Cap
The Quelccaya Ice Cap (QIC) is an unusually large glacial ice body located in
southeastern Peru, where ice cover elsewhere in the tropical Andean ranges is restricted to steep
mountain glaciers. The ice cap is located in the southeastern arm of the Cordillera Vilcanota,
directly upslope from the Amazon Basin. It lies on a broad plateau of Miocene-age ignimbrite
(Mercer and Palacios, 1977) and extends above 5,600 m asl at the summit. It has a broad
hypsometry that covered ~55 km2 in the mid-1980’s (Thompson et al., 1985), but since has been
undergoing rapid retreat well documented using photogrammetric monitoring of the Qori Kalis
outlet glacier (Brecher and Thompson, 1993; Mark et al., 2002; Thompson, 2000, Thompson et
al., 2006). The modern western ice margin of the QIC terminates around 5,200 m asl above the
Huancané Valley (Fig. 1). The Qori Kalis outlet glacier extended down ~4,950 m asl in 1963
through 1991 (Brecher and Thompson, 1993).
Remote-sensed Tropical Rainfall Measurement Mission (TRMM) precipitation data for
the grid square (0.25°x0.25°) containing the western margin of the QIC gives an estimate of
~1090 mm/yr of precipitation averaged over 12 years of data (1998-2010) (Kummerow et al.,
1998). This is similar to the estimate cited by Mark et al. (2002) of 800-1000 mm/yr. The QIC
region has a pronounced rainy season from November to March, corresponding with the austral
summer position of the Inter-tropical Convergence Zone (ITCZ). Eighty percent of precipitation
on average falls between December and February (Garreaud et al., 2003). Easterly circulation
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brings primary moisture up from the tropical and sub-tropical Atlantic and recycled moisture
from the Amazon Basin. Almost no precipitation comes from the west due to the large distance
over the Andes to the Pacific and cold upwelling off the western coast decreasing evaporation.
Snow accumulation data from the QIC summit suggests that precipitation is also affected by El
Niño-Southern Oscillation (ENSO) variation, with significantly reduced precipitation during El
Niño (Thompson et al., 1984). This is the regional pattern across the Altiplano, which Garreaud
et al. (2003) suggest is linked to increased frequency of dry westerly winds during the rainy
season and reduced frequency of the easterly winds responsible for bringing Amazon-derived
moisture.
2.2 Huancané Valley and South Fork Valley
The Huancané and South Fork Valleys (Fig. 1) are located adjacent to each other on the
southwestern margin of the QIC.
These valleys were selected for the TCN production
calibration site because they contain well-preserved glacial moraines with large boulders ideal
for TCN sampling. Mercer and Palacios (1977) first described and radiocarbon dated 3 distinct
sets of glacial moraines in Huancané Valley region. They were designated H1, H2 and H3,
increasing down valley from the ice margin (Fig. 1).
The moraines (specifically the H2
moraines) are bounded stratigraphically, above and below, by bog sediments containing aquatic
plant material used for conventional radiocarbon dating, which provides an excellent
independent age estimate for calibration.
The Huancané Valley curves northwest from the modern ice margin and is separated
from the South Fork Valley by a steep bedrock ridge reaching up to 5,000 m asl. This bedrock
ridge forms the headwall of the South Fork Valley cirque and encloses a small lake. Core site
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TL08-14 (Fig. 4,Table 1) is located at the center of the lake. The two valleys converge about 5
km down valley from the head of the Huancané valley.
2.2.1 H1 moraines
The H1 moraines (Fig. 2a) are located ~1 km down valley from the modern ice margin at
5100 m asl. These moraines are extensive features in the upper Huancané Valley. They stand 25 m high with sharp crests and are composed of poorly sorted sediment grading up to ignimbrite
boulders up to 1.5 m in diameter with well-preserved glacial polish. Upslope from the H1
moraines in the Huancané Valley is a pro-glacial lake named Boulder Lake by Buffen et al.
(2009). It is located at the modern ice margin dammed behind a lip of glacial-polished bedrock.
At some point between field seasons in July 2007 and June 2008, the lake level is thought to
have risen above the bedrock lip and flooded into the Huancané Valley (M. Kelly, pers. comm.).
It scoured a channel up to 20 m deep through the H1 moraines and deposited a large amount of
fine sediment into the Huancané Valley floor. This new channel provides an accessible view of
the H1 moraines in cross-section (Fig. 2a). Sampling site TL08-19 (Fig. 4, Table 1) is located in
this channel, which exposes alluvial fan sediments beneath the H1 moraines.
2.2.2 H2 moraines
The H2 moraines are located 4 km down valley from the modern ice margin. In the
Huancané Valley they are sharp-crested, standing up to 20 m high (Fig. 2b). The moraine
complex extends ~0.5 km parallel to the valley axis between two bedrock ridges in more than
five distinct moraine ridges. The areas between the moraine ridges are filled with shallow ponds
and bogs that support numerous aquatic plants. On the northern side of the H2 moraine complex
a small stream cuts through the moraine ridges. Stratigraphic section TL06-02 (Fig. 4, Table 1)
is located in the streamcut through the most valley-ward H2 moraine. Numerous boulders
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**!
ranging in size from 0.4 to >3.0 meters in height are located on the crests and slopes of these
moraines. Five CRONUS calibration samples discussed here were collected from the two most
valley-ward moraine ridges in the Huancané Valley (Fig. 1, inset, Fig. 3, Table 2).
In the South Fork Valley the H2 moraines are wider, more subdued and discontinuous
(Fig. 2c). They are located about 1.5 km down valley from the lake. They consist of only two
main moraine ridges on the valley floor standing 3-5 m high that connect to a strand of large
boulders extending uphill on the south side of the valley. The more subdued expression of the
moraines in the South Fork valley is most likely due to the greater width of that valley, which
allowed the ice of the QIC to spread more thinly over the valley floor, in contrast to the high
valley walls of the Huancané valley that confined the ice into an elongate valley glacier. The
boulders located on or near the crests of the two moraine ridges in the South Fork valley are
similar in size to those found in the Huancané valley. Stratigraphic sections TL08-01A and
TL08-01B (Fig. 4, Table 1) are located in a streamcut through the outermost moraine ridge in the
center of valley. Section TL08-02 (Fig. 4, Table 1) is located about 100 m up valley from the
other two sections in the same stream channel, but located stratigraphically above the H2
moraines.
The remaining five CRONUS calibration samples were collected from the H2
moraine ridge furthest down valley in the South Fork Valley and from the boulder moraine
extending up the southern hillslope (Fig. 1, inset; Fig. 3; Table 2).
2.2.3 H3 moraines
The moraines identified as H3 are a discontinuous set of boulder lines and 1-5 meter
rounded ridges about 8 km down valley from the ice margin. No stratigraphic sections were
measured for this moraine set and no calibration samples were collected.
3. Methods
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The sampling strategy of CRONUS-Earth researchers for the Huancané calibration site
focused on two types of samples: those that provide independent organic 14C age control for the
H2 moraines, and the calibration TCN samples taken from the boulders on the moraines.
Although the H2 moraines in the Huancané valley had been previously dated by Mercer and
Palacios (1977) using organic
14
C, those authors provided only maximum limiting ages on the
H2 advance limited to one stratigraphic section and measured only in the Huancané valley.
CRONUS-Earth researchers expanded their search into the South Fork valley and collected
samples to provide both maximum and minimum limiting ages for the H2 moraines to tightly
constrain the age of the H2 ice advance that deposited them. This ensures that the true exposure
age of the sampled boulders deposited on the moraines is well constrained for production rate
calibration.
3.1 Organic 14C
3.1.1 Field collection:
All samples used for independent age determination in this study were collected in situ,
either from stratigraphic sections or from lacustrine sediment cores.
The independent age
determination of the H2 moraines is based on 14C dating of these samples. Sediment cores were
removed from the corer at the sample site and were then packaged in rigid plastic tubing lined
with plastic wrap and sealed for shipping. Once shipped to the US, the cores were halved,
measured, described and sampled by Thomas Lowell at University of Cincinnati. Samples from
stratigraphic sections were collected in the field from measured depths and placed in sealed
plastic bags for shipping.
All sample locations, stratigraphic sections, and cores were
photographed in the field. Samples of organic material were collected from the cores, packaged
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*#!
in sealed plastic bags and shipped to the University of Arizona for analysis, along with the
previously packaged samples from the stratigraphic sections.
3.1.2 Laboratory pretreatment:
All samples were composed of well-preserved, fibrous plant material. At University of
Arizona, samples were initially dried overnight in a 70°C drying oven. Samples were then
placed in 50 mL centrifuge vials and subjected to a standard acid-base-acid (ABA) treatment
with HCl and NaOH, rinsed thoroughly with Type 1 water and again dried overnight.
3.1.3 Sample combustion, CO2 extraction and graphitization:
Approximately 5 mg of pretreated sample was placed in a quartz tube with 100 mg of
CuO powder and a small sliver of Ag foil and evacuated to <2x10-5 torr pressure. The tubes
were sealed with a torch and samples were combusted at 900°C for 4 hours. Sample CO2 gas
was then extracted under vacuum and cryogenically purified, passed through a 600°C Cu/Ag
furnace to further remove contaminant gases, frozen in liquid nitrogen (LN) and sealed in glass
tubes. Purified CO2 samples were then graphitized using 100 mg of zinc powder and Fe powder
in a 2:1 proportion to the mass of carbon in the sample following the procedure outlined in Slota
et al. (1987).
3.2 In Situ 14C
3.2.1 Field collection:
CRONUS calibration samples were collected from boulders > 1 m in height on or near
the crest of the outermost H2 moraine ridges in the Huancané and South Fork valleys (Fig. 1,
inset; Fig. 2b,c, Table 2). Boulders were selected to minimize the amount of erosion on their top
surfaces. Only two boulders, HU08-04 and HU08-10, had glacial polish preserved on their
surfaces, but this polish was located on the side of the boulder near the ground in both cases and
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was not sampled directly. Erosion from each sampled surface was estimated by measuring the
height difference between the tallest pedestal and deepest pit on the sampled surface. 3-5 kg of
rock was collected from most un-eroded top surface of each boulder. Sampling was conducted
by drilling a !” diameter hole into the surface using a battery-powered hammer drill, inserting a
small explosive charge and detonating it, breaking off a chunk of rock, following the method of
Kelly (2003). Samples were then placed in canvas sample bags to be shipped to the US.
Topographic shielding horizon measurements were made using a compass and eye level in
azimuth intervals of 40°. Photos were taken of each sampled boulder with views from each of
the four cardinal directions, and the sample location on the boulder before and after sampling.
3.2.2 Laboratory pretreatment:
Samples were first crushed and homogenized at PRIME Lab (Purdue Rare Isotope
Measurement Laboratory) before distribution to participating CRONUS-Earth laboratories for
extraction and measurement of
10
Be,
26
Al,
36
Cl and in situ 14C. Pretreatment of samples for in
situ 14C measurement was performed at the University of Arizona. Crushed samples were sieved
and the 0.25-0.5 mm fraction was retained. Samples were then treated overnight in 6N HCl with
~0.25% H2O2 at 60-70°C, rinsed thoroughly with Type 1 water and dried under low vacuum at
70°C overnight. Samples were then magnetically separated using a Frantz magnetic separator
and the non-magnetic fraction was retained. The non-magnetic fractions were etched in 1%
HF/1% HNO3 in lidded 2.2 L Teflon beakers in an ultrasonic water bath at 95°C for 4 hours and
then cooled overnight. Samples were thoroughly rinsed with Type 1 water between etching
steps, and the HF/HNO3 etching process above was repeated until only pure quartz remained
(typically 4-5 cycles). The samples were then thoroughly rinsed and then sonicated in Type 1
water with no acid for the final rinse for 4 hours at 95°C to ensure complete removal of HF
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residues prior to extraction. Samples were then dried overnight under low vacuum at 70°C and
sealed in air in clean glass jars for storage.
3.2.3 Sample extraction and graphitization:
Quartz samples were etched a final time in 10% HNO3 ultrasonically for 30 minutes at
room temperature, then left for an additional 90 minutes, thoroughly rinsed with Type 1 water
and dried overnight as above. This was done in order to remove any contamination on the
sample incurred during storage and handling prior to extraction. Samples were then loaded and
extracted following the methodology of Lifton et al. (2001) modified by Pigati et al. (2010) but
using a newly automated extraction system at University of Arizona. The system uses a stainless
steel bellows recirculating pump in the extraction stage, but with the exception of being
automated, both extraction and purification stages were operated essentially identically to the
methods described in those references.
To summarize the procedure, each quartz sample was dissolved using a degassed lithium
metaborate flux (LiBO2) at high temperature in ~50 torr of Research Grade (RG) O2 to liberate in
situ 14CO2, then the gas was purified cryogenically and using a Cu/Ag furnace, measured, diluted
with
14
C free CO2 gas and sealed in an evacuated glass tube. The extraction and purification
process is spread over three days. On the first day ~20 g of LiBO2 is degassed and purified by
combustion in RG O2 at 1200°C for one 1 hour. The LiBO2 was then allowed to cool to room
temperature overnight. During the second day, ~5.0 g of sample quartz was added to the LiBO2,
combusted for 1 hour in RG O2 at 500°C to remove remaining organic or atmospheric C, then
combusted for 3 hours at 1100°C to release in situ 14C as 14CO and 14CO2. The gas was further
passed through a 1000°C furnace to oxidize all the gas to 14CO2, then frozen in LN and sealed in
an evacuated glass tube. On the third day the gas sample was subjected to a rigorous purification
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procedure to remove all gases other than CO2 (see Pigati et al., 2010 for full description) and
diluted before graphitization.
The purified and diluted CO2 samples were graphitized on a separate system reserved for
in situ 14C samples following a modification of the methodology of Pigati et al. (2010). In the
modified methodology, prior to admitting the sample gas for graphitization, the Fe and Zn
powders used for the catalytic reduction were first reduced in ~0.5 atm of >99.9999% pure H2
gas at 400°C for 1 hour and the gas was then pumped slowly away until the graphitization
chambers reached <5x10-6 torr pressure. The graphitization chambers were then “scrubbed” two
times by admitting ~0.5 atm of RG He gas for 5 minutes, and then slowly pumped away to
<5x10-6 torr each time, with the Zn at 425°C and the Fe at 600°C. After graphitization, the
resulting graphite samples were placed in glass vials with PTFE septum caps. The vials were
alternately evacuated and filled with >1 atm RG Ar gas two times by piercing the septum in the
vial lid with a syringe needle (attached to a low vacuum pump and the Ar gas supply cylinder) to
fully remove the ambient atmosphere. These vials were then placed into an outer vial which was
also subsequently evacuated and filled with Ar gas, as above, to prevent any atmospheric
contamination on the graphite between graphitization and AMS measurement. Extraction blanks
were extracted following the extraction and graphitization procedures above, but with no sample
material added on the second day of extraction.
3.2.4 AMS measurement:
Graphite targets for all in situ and organic radiocarbon samples were pressed into
aluminum sample holders and measured by the Arizona AMS Facility. Approximately 20% of
the diluted CO2 gas for each sample was split form the main aliquot, and was measured for "13C
correction by IRMS (Isotope-Ratio Mass Spectrometry) at the Arizona AMS Facility.
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3.2.5 Calculation of in situ 14C concentrations:
The concentration of 14C atoms per gram of sample (C14) is given by
!!" !
(1)
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where Fm is the fraction modern carbon (the
!
14
C/13C ratio of the sample vs. that of a standard,
both corrected to "13C = -25 VPDB and to AD 1950) (Burr and Jull, 2009), A14 is the fractional
abundance of 14C in “modern” (1950) carbon (1.177x10-12 14C/12C), VS is the volume of CO2 in
cm3 STP collected from the sample, VA is the volume of one mole of CO2 in cm3 STP, NA is
Avogadro’s number, M is the sample mass in g (set to one for blanks), and B is the number of
14
C atoms associated with the corresponding extraction blank (the small amount of residual
atmospheric
14
C not removed during the extraction and cleanup) (Lifton et al., 2001, Eqn. 5).
The Fm value used is corrected for the mass-dependent graphitization blank following Lifton et
al. (2001).
Following the methods of Lifton et al. (2001), measurement uncertainties from all
extraction and analytical procedures have been fully propagated through each calculation using
the law of combination of errors, neglecting covariance terms (Bevington and Robinson, 1992).
When averaging measured values, we use relative-error-weighted means to minimize biases from
correlated measurements and uncertainties, unless otherwise noted (Bevington and Robinson,
1992, Eqn. 4.17). We then calculate the uncertainty in the weighted mean (Bevington and
Robinson, 1992, Eqn. 4.19) and the weighted average variance of the data (Bevington and
Robinson, 1992, Eqn, 4.22). We accept the larger of these uncertainties as the uncertainty in the
weighted mean. All uncertainties are presented as 1# unless otherwise noted.
4. Results
!"#$%&'()'*+,!
*(!
4.1 H2 moraine radiocarbon dating
CRONUS-Earth researchers focused on the H2 moraines in both valleys over the H1 or
H3 moraines for calibration because their age of deposition can be tightly constrained using both
maximum and minimum limiting ages.
The most probable age for the H2 moraines was
computed using twenty six radiocarbon ages (Table 1) from the TL06-02, TL08-02, TL08-01A, 01B, and TL08-19 stratigraphic sections and the TL08-14 sediment core from the South Fork
valley lake (Fig. 1, 4).
Stratigraphic interpretation was a joint effort between CRONUS
researchers; all the authors included in Kelly et al. (in prep) provided input. 15 total ages from
TL06-02 and TL08-01A and -01B are from locations stratigraphically below the moraines and
are maximum limits on the moraine age. 11 total ages from TL08-02, TL08-14 and TL08-19 are
located within the H2 glacial limit and are minimum limits on moraine age. The following
descriptions provide evidence supporting the stratigraphic interpretation of each maximum and
minimum limiting sampling location. All ages reported here have been calibrated from 14C kyrs
BP (ka: BP = before present, present is AD 1950) to calendar kyrs BP (cal ka) using the CALIB
6.0 software, which incorporates the IntCal09 calibration curve (Reimer et al., 2009).
4.1.1 H2 moraine maximum limiting age localities: TL06-02 and TL08-01A,B
Well preserved fibrous organic material from TL06-02, located 2 m below the outermost
H2 moraine crest in the Huancané valley is interbedded with sandy till material.
Eight
radiocarbon ages from six discreet stratigraphic locations (Fig. 4) range from 12.52-12.80 cal ka
(Table 1). This organic matter must predate the ice advance that deposited the H2 moraine
material above it and so these are maximum ages for the H2 landforms.
Sections TL08-01A and -01B are located directly adjacent to each other in the South Fork
valley (Fig. 4). The organic horizons are located 6.5 m below the crest of the outermost H2
!"#$%&'()'*+,!
*)!
moraine ridge. Fibrous organic matter horizons sampled from these sections are contorted where
they were thrust into the moraine by ice along with sandy and silty till. This indicates they were
closely contemporaneous with, but deposited prior to moraine deposition. Seven radiocarbon
dates from these two sections range from 12.53-13.34 cal ka (Table 1). Similar to TL06-02,
these dates are maximum ages for the H2 moraines.
4.1.2 H2 moraine minimum limiting age localities: TL08-02, TL08-14 and TL08-19
Section TL08-02 is located 100 m up valley from the outermost moraine inside the H2
moraines in the South Fork valley (Fig. 1). Five radiocarbon dates from two horizons range from
12.25-12.40 cal ka (Fig. 4, Table 1). Organic matter in this section is interbedded with sand and
the basal organic matter directly overlies a glacially striated boulder, making this a close
minimum estimate for age of the H2 moraines.
Section TL08-19 is located 4 km up valley from the H2 moraines, but provides the sole
minimum age constraint for ice retreat in the Huancané valley. The layered organic matter is
interbedded with sand in fan deposits exposed in the walls of the outburst flood channel about
200 m down-valley from the outermost H1 moraines (Fig. 1). Four radiocarbon ages from two
horizons range between 11.25-11.65 cal ka (Fig. 4, Table 1).
The final two radiocarbon dates used for age determination come from the basal organic
matter in lake sediment core TL08-14. The organic material in the core is located directly above
a hard, massive, gray silt horizon with no organic material, interpreted to represent deposition
when the lake was immediately proximal to the ice margin. These dates are indistinguishable
from each other at 11.24 cal ka, and provide an additional minimum limiting age in the South
Fork Valley (Table 1).
4.2 Analyses of CRONUS intercomparison material CRONUS-A, and extraction blanks
!"#$%&'()'*+,!
Prior to extracting in situ
"+!
14
C from the calibration samples, a reference material
developed as part of the CRONUS-Earth project (CRONUS-A) was extracted and measured to
assess possible changes in the performance of the extraction system due to the automation. This
ultimately confirms complete extraction of in situ
14
C.
During the period in which the
CRONUS-A and the calibration samples were extracted, extraction blanks were measured to
provide the blank correction for the calibration samples (see Eqn. 1) and also to serve as a second
monitor of the performance of the new automated system.
4.2.1 Analyses of CRONUS-A
CRONUS-A is a homogeneous quartz intercomparison material distributed among
laboratories participating in the CRONUS-Earth project to facilitate laboratory intercomparison
and provide a standard material for measurement in the scientific community. In situ
14
C
concentrations were measured for CRONUS-A before and after the automation of the extraction
and purification lines to assess the effects of automation on experimental results (Table 3).
Analysis RN-869 was performed prior to automation. Three analyses, ISC-0006, -0008 and 0009 were performed after automation, but prior to analysis of the Huancané calibration samples.
The pre- and post-automation concentrations are comparable, with a weighted mean value
indistinguishable within error. This demonstrates that both manual and automated procedures
achieve complete extraction of in situ 14C from the CRONUS-A quartz.
4.2.2 Extraction blank analyses
Four extraction blanks were also analyzed, interspersed between the standards and
calibration samples discussed above (Table 3). ISC-0005 and -0007 were analyzed prior to the
calibration samples, ISC-0016 during the middle of the sequence and ISC-0026 directly after the
last calibration sample. The concentrations show a significant increase over time, especially
!"#$%&'()'*+,!
"*!
over the period during which the calibration samples were run, with the last two blanks being
several times greater than the mean pre-automation blank value of 141±13x103 14C atoms (Pigati
et al., 2010). This unexplained apparent blank instability following the automation creates
uncertainty in the blank correction applied to the calibration analyses, which significantly
complicates their interpretation.
4.3 H2 moraine boulder TCN concentrations
The measured TCN concentrations for
samples are given in Table 4.
The
10
14
C,
Be and
10
Be and
26
26
Al in the H2 moraine calibration
Al measurements performed by several
participating CRONUS laboratories for each individual sample are listed here for comparison to
14
C concentrations to aid in the TCN surface production modeling and interpretation of the 14C
dataset.
4.3.1 In situ 14C concentrations
14
C concentrations are listed for two scenarios to try to constrain the effects of the
observed blank instability (Table 4). In the first scenario, the long-term mean blank correction
for the pre-automation extraction system (Pigati et al., 2010) is applied (14CAvg). The second
scenario assumes the blank value associated with a given sample can be linearly interpolated
(based on respective extraction dates) between the two measured blank values immediately
bracketing the sample extraction date (14CInt). We suggest that these two scenarios represent the
end member blank corrections for the sample concentrations and that the reality is likely
somewhere in between. Values for both scenarios have been corrected to surface conditions and
no topographic shielding for direct comparison in Table 5. An additional 5.44% (one standard
deviation of all University of Arizona and Lamont-Doherty Earth Observatory CRONUS-A 14C
measurements) uncertainty has been added to the analytical uncertainty for each
14
C
!"#$%&'()'*+,!
""!
measurement to better express intra- and inter-laboratory reproducibility. The
14
CInt scenario
results in less scatter in the measurements than the 14CAvg scenario (Table 5, Fig. 5). The 14CAvg
scenario has a significantly higher mean value, however, due to the lower blank correction. The
relative-error-weighted means of the corrected
14
C data for interpolated and long-term average
blank scenarios are 7.58±0.19x105 atoms g-1 and 8.26±0.24x105 atoms g-1 respectively (Table 5).
The 14C mean calculations exclude sample HU08-06, which is an obvious outlier in the dataset.
We suspect this analysis reflects incomplete extraction of
14
C arising from a thermocouple
malfunction (Table 4,5, Fig. 5). During extraction of HU08-06, the thermocouple that measures
the extraction furnace temperature malfunctioned, reading temperature that was incorrectly too
high, and the software routine that monitors and controls furnace temperature in the automation
software was unable to maintain the temperature at the optimal 1100°C, and instead kept the
furnace much cooler, dropping as low as 800°C. HU08-06 is therefore not included in any
further analysis or discussion of in situ 14C.
4.3.2 10Be and 26Al concentrations
10
Be concentrations considered here for comparison reflect twenty-seven total
measurements made by three
10
Be laboratories. All ten Huancané calibration samples were
processed for 10Be by John Stone’s laboratory at University of Washington. Second, all samples,
except HU08-03, were processed for
10
Be by Joerg Schaefer’s laboratory at Lamont-Doherty
Earth Observatory. Third, samples HU08-01 through HU08-04, HU08-06, and HU08-10 were
processed for
10
Be by Meredith Kelly’s laboratory at Dartmouth College. All
10
Be and
26
Al
AMS measurements were performed at the Center for Accelerator Mass Spectrometry (CAMS)
facility at Lawrence Livermore National Laboratory. Analytical details for these samples are
forthcoming in a publication by CRONUS-Earth investigators on the Huancané calibration site
!"#$%&'()'*+,!
"#!
(Phillips et al., in prep). The actual individual measurements for each sample for all nuclides are
given in Table 4. For analysis,
10
Be concentrations for each sample are represented as the
unweighted average of all (up to four) measurements made for that sample so that all
measurements are weighted equally (Table 5).
26
Al concentrations were only from John Stone’s
laboratory (Table 4, 5).
In contrast to the in situ 14C data, the surface- and shielding-corrected average estimates
of each
10
Be concentration, and individual
26
Al concentrations show good internal consistency
with relative-error-weighted means of 5.28±0.06x105 and 3.58±0.69x106 atoms g-1 respectively
(Table 5, Fig. 5). Compared to the
10
Be and
26
Al datasets, for which the individual 1# errors
overlap their respective means, the nine included 14C data show much greater scatter, varying up
to 13% from the mean for the
14
CInt set and 15% for the
14
CAvg set, well outside the individual
analytical errors (Fig. 5).
Weighted means were also performed separately for each valley (Table 5). For 14C, the
South Fork mean is 9% higher than the Huancané mean for the 14CInt scenario and 6% higher for
14
CAvg. In both scenarios, the South Fork mean has better internal consistency than the Huancané
mean as well. The 10Be and 26Al means for the South Fork and Huancané valleys overlap within
error.
5. Discussion
Discussion of the results can be broken down into three sections: 1) discussion of the
calculations used for determining the most likely independent age for the H2 moraines; 2)
estimation of the site and sea-level-high-latitude-normalized (SLHL) 14C production rates based
on the Huancané calibration site data in comparison to other CRONUS calibration site estimates;
and 3) investigation of the cosmogenic nuclide results based upon the independent age and
!"#$%&'()'*+,!
"$!
nuclide production modeling to test possible explanations for the observed discrepancy between
Huancané and other estimates of the global reference production rate of in situ 14C.
5.1 H2 moraine independent age determination
Kelly et al. (in prep) determined the most probable age for the H2 moraines using the
calibrated probability density functions (pdf, generated by CALIB 6.0, Reimer et al., 2009), and
based on the stratigraphic constraints that 1) the true age of the moraine must be older than the
true ages of all samples stratigraphically above the H2 moraines and 2) the moraine must be
younger than the true ages of samples below the moraines (Fig. 6). The pdf of the moraine’s age
is calculated by Kelly et al. (in prep) as follows
(2)
! ! !
!
!!! ! !!
! !"
!
!!! !! !!
! !"
where G(t) represents the probability that any individual age t represents the true age of the H2
moraines.
P(t) is the calibrated pdf of any individual radiocarbon age.
Set A includes
radiocarbon ages i = 1,2,…m that come from samples stratigraphically below the moraine. Set B
includes radiocarbon ages j = 1,2,…n that come from samples that are stratigraphically above the
moraines.
The implicit assumption associated with assigning an independent age to the CRONUS
calibration sample is that the moraines in both valleys are the same age. The statistical analysis
of Kelly et al. (in prep) is also based upon this assumption and suggests that the ice advance that
deposited the H2 moraines culminated at 12.4±0.1 cal ka. If the two valleys are examined
separately, without the minimum limiting age constraint of the TL08-02 section, the age
determination in the Huancané valley has greater uncertainty. However, there are several lines
of evidence suggesting the assumption of equivalent age is valid. First, organic material several
!"#$%&'()'*+,!
"%!
km up the Huancané valley (TL08-19) suggests that the ice margin had retreated well up valley
by 11.6 cal ka, implying at most the Huancané H2 moraines could be ~6% younger than those in
the South Fork. However, there are also 3 additional moraine sets identified by Kelly et al. (in
prep) between the H2 moraines and the location of those dates, suggesting retreat was not
uniform, but rather time was required for intermittent readvance or pause of the glacier up-valley
from the H2 moraines. Also, the youngest maximum limiting ages from both valleys are
indistinguishable at 12.53±0.17 cal ka and 12.52±0.12 cal ka, suggesting close maximum age of
ice advance in both valleys. We therefore have no reason to reject the assumption that the
moraines are the same age, and adopt 12.4±0.1 cal ka as the landform age for production rate
calibration.
5.2 Site-specific and SLHL-normalized 14C production rate estimates
The utility of a calibration site depends on the ability to scale the estimated site
production rate to other study sites of unknown exposure age. This requires a scaling model that
accurately accounts for the variability of production rates due to altitude, and position and
strength of the geomagnetic field during the exposure period. Typically this is accomplished by
scaling calibration site production rates to sea level and high (>60°) geomagnetic latitude
(SLHL) where the cosmic ray flux is unaffected by changes in the geomagnetic field strength
and is only a function of altitude (Lal, 1991). Using the corrected 14C concentrations (Table 5)
and the independent age of the H2 moraines we can estimate the total local production rate for in
situ 14C for the Huancané site. We can then scale to SLHL using a variety of scaling models to
compare the production rates estimated at Huancané to other CRONUS calibration sites for in
situ 14C.
5.2.1 Huancané calibration site production rate estimates
!"#$%&'()'*+,!
"&!
The weighted mean site production rate estimate, corrected to the ground surface with no
topographic shielding and incorporating the estimated erosion rate for each sample (see Table 2),
is 122.6±3.0 atoms g-1yr-1 (n=9) for the 14CInt scenario and 133.1±4.0 atoms g-1yr-1 (n=9) for the
14
CAvg scenario (Table 6). This is the time-integrated mean production rate for the exposure
period (12.4±0.1 cal ka to present) only, since production rates vary temporally. This estimate
reflects the sum of production by spallation, fast muon interactions and slow muon capture. The
proportion of production due to each of these mechanisms for in situ 14C have not been measured
in natural samples, but were derived from irradiation experiments by Heisinger et al. (2002a,b)
and we adopt those SLHL muogenic production rates here. Estimated production by individual
pathways can be calculated using those values.
5.2.2 Sea level high latitude production rate estimates
To compare the calibrated 14C production rate estimates from the Huancané site to other
CRONUS calibration sites, we must scale them to SLHL and compare them to the SLHL
production rate estimates of those sites. We will compare to six other CRONUS in situ
14
C
calibration sites: Promontory Point, in the western United States (Lifton et al., 2001, Miller et al.,
2006, Dugan, 2008) Corrie Nan Arr and Maol Cheann Dearg, both in northwestern Scotland
(Dugan, 2008), and three unpublished altitude transects from the White Mountains, California,
northern Chile and west Antarctica (Lifton et al., in prep) (Table 7).
The SLHL production rate estimate for a TCN depends upon the scaling model used.
Because the sample datasets and methods differ between scaling models, each scaling model
calculates a unique SLHL reference production rate for a calibration site. These values are not
interchangeable; when using a particular scaling model to calculate production rates for exposure
dating applications at a study site, one must only use the reference production rate derived
!"#$%&'()'*+,!
"'!
specifically for that scaling model. We use six scaling models here to calculate and compare
SLHL reference production rate estimates for in situ
14
C. These models, Lal/Stone (St; Lal,
1991, Stone, 2000), Lal-modified (Lm; Lal, 1991, Stone, 2000), Dunai (Du; Dunai, 2000, 2001),
Desilets (De; Desilets and Zreda, 2003, Desilets et al., 2006), Lifton (Li; Lifton et al., 2005,
2008), and Lifton-Sato (LS; Lifton and Sato, in prep) are reviewed briefly below.
The scaling model of Lal (1991) reflects the legacy of TCN research over the last 50
years, and consists of scaling functions for geographic latitude and altitude empirically derived
from photographic emulsion and neutron detector data from the 1950’s. This scaling model is
applied with the assumption that on time-scales greater than thousands of years, the geomagnetic
field averages to a geocentric axial dipole (GAD) rendering geomagnetic and geographic
latitudes equivalent. Stone (2000) subsequently reparameterized this model in terms of
atmospheric pressure, rather than the original altitude parameterization, to account for the effects
of long term regional heterogeneities in atmospheric pressure. This model is the simplest and
does not incorporate time-dependent scaling of production rates.
The modified Lal (1991)/Stone (2000) (Lm) model uses the same scaling functions as the
Lal/Stone model, but incorporates a simple dipolar geomagnetic correction scheme following
Nishiizumi et al. (1989) to allow for temporal production rate variation deriving form
geomagnetic field variability.
Global scaling has also more recently been addressed in the models of Dunai (2001),
Desilets and Zreda (2003, extended by Desilets et al., 2006), and Lifton et al. (2005) using
neutron monitor latitude and altitude surveys. In addition to neutron monitor surveys, the Dunai
(2001) model also incorporates cloud chamber data. Each of these models is parameterized in
terms of atmospheric depth and some form of effective vertical cutoff rigidity (RC), the former
!"#$%&'()'*+,!
"(!
addressing the effects of atmospheric pressure variations, and the latter accounting for dipolar
and non-dipolar magnetic field variations.
Cutoff rigidity is defined as the minimum
momentum-to-charge ratio that primary cosmic rays must have to penetrate the geomagnetic
field at a given point. Vertical cutoff ridigity, RC, further accounts for the effects of the zone of
alternating allowed and forbidden cosmic-ray trajectories near the Earth known as the penumbral
region (Cooke et al., 1991; Shea et al., 1965). Numerous studies have shown that RC is the most
effective means of ordering cosmic ray data, accounting for both dipole and non-dipole field
heterogeneities (e.g., Shea et al., 1965; Carmichael et al., 1965; Dorman et al., 2000; Nagashima
et al., 1989). The Dunai (2001) model approximates temporal and spatial variation in global RC
using a dipole approximation and incorporating records of geomagnetic field paleointensity and
geomagnetic inclination to account for non-dipole variations. The Desilets and Zreda (2003) and
Lifton et al. (2005) models are both based on RC values derived from trajectory tracing (e.g. Shea
et al., 1965), where an anti-proton is traced backwards from the surface through a high-order
spherical harmonic approximation of the geomagnetic field. Desilets and Zreda (2003) derived
trajectory-traced RC values for a GAD field of varying intensity as a function of geomagnetic
latitude. Lifton et al. (2005) addressed potentially persistent non-dipole field effects on RC by
using a quasi-dipolar model fit to a global grid of trajectory-traced values derived for the 1955.0
geomagnetic epoch (Shea et al., 1968). Lifton et al. (2005) also used the largest neutron monitor
dataset and incorporated the effects of solar variability. Lifton et al. (2008) extended this model,
incorporating a continuous geomagnetic model covering the last 7 cal ka.
Versions of the previous five models were all included in the CRONUS-Earth Online
Calculator of Balco et al. (2008). The final model, Lifton-Sato (LS), is based on the Monte
!"#$%&'()'*+,!
")!
Carlo modeling of atmospheric cosmic-ray fluxes by Sato et al. (2006, 2008), and was developed
specifically for the CRONUS-Earth project (Lifton and Sato, in prep.).
The reference spallation production rates calculated for all scaling models (Table 7) were
calculated using the preferred CRONUS-Earth geomagnetic framework (Lifton and Sato, in
prep), rather than those originally included in the scaling models. Muogenic production of 14C is
scaled using one SLHL value for all scaling models and all sites, 3.78±0.37 atoms g-1yr-1,
following Balco et al. (2008) and Heisinger et al. (2002a) (Table 7).
production rate estimates from our in situ
14
Total mean SLHL
C results from the Huancané site range between
11.20±0.40 atoms g-1yr-1 for the De model and 12.93±0.41 atoms g-1yr-1 for the LS model. To
provide a ‘grouped’ estimate from the other six calibration sites, we calculated the unweighted
average of the individual sample SLHL estimates from each calibration site. Samples with more
than one measurement were combined into a weighted sample mean before averaging. The
grouped average production rates range from 15.70±1.01 atoms g-1yr-1 for the Lm model to
16.80±1.09 atoms g-1yr-1 for the Li model. The estimates for Huancané from all scaling schemes
are 19-31% lower than grouped average from other sites (Table 7). This suggests either a serious
bias in all of the CRONUS-Earth production rate scaling models at low latitude and/or high
altitude, or undiagnosed site-specific effects on the production history of TCN’s at the Huancané
site.
5.3 Comparison to 10Be and 26Al data
We will compare the 14C data to the 10Be SLHL spallation production rate estimates from
Huancané and other sites to further investigate possible site-specific exposure effects on nuclide
production at Huancané. Unpublished CRONUS
10
Be calibration sites include Promontory
Point, Corrie Nan Arr, and Maol Cheann Dearg (to be included in Stone et al., in prep), and
!"#$%&'()'*+,!
#+!
additional data from New Zealand (Putnam et al., 2010). The Huancané 10Be SLHL estimates,
ranging between 3.14±0.06 and 3.88±0.08 atoms g-1yr-1, are ~4-25% lower than the grouped
average SLHL production rate estimates from the other CRONUS sites (Table 7). These rates
appear to be somewhat less than the grouped estimates (4.03±0.39 to 4.50±0.44 atoms g-1yr-1,
Table 7), depending on scaling model, although not significantly so. If it is significant, the
mismatch for the
10
Be estimates is less than for
Huancané are consistent with the estimated
26
14
C. Also, the measured
26
Al/10Be ratios from
Al/10Be production ratio of ~6.75 (Balco and
Shuster, 2009) (Table 5), which because of the long half-lives of those nuclides, does not change
appreciably over only 12.4 kyrs of exposure.
26
Al also appears to be in agreement with the lower
Huancané production rates overall, but similar to 10Be, and less than 14C. Lastly, the individual
measurements of both
26
Al and
10
Be have little scatter, while the
14
C dataset scatters up to 9%
around the mean. Thus, discounting variable inaccuracy in global scaling and calibration at the
site, any feasible site specific effects must reduce
26
14
C concentrations more relative to
10
Be and
Al to explain the experimental dataset. The comparison to other sites suggests that all scaling
models may be predicting production rates that are too high for all nuclides and this site is
essential for good global calibration. Conversely, however, this may also be symptomatic of a
problem with the site. Based on geologic evidence, there is no reason to suggest that the
sampled boulders have undergone any complex exposure.
In fact the site was selected
specifically to minimize this possibility. However, because complicating factors such as surface
burial would be expected to affect 14C more than either 10Be or 26Al, due to the large difference
in half-lives, we need to investigate whether geologically reasonable scenarios can proposed to
explain the lower-than-predicted 14C concentrations.
5.4 Site Production Modeling
!"#$%&'()'*+,!
#*!
We use a cumulative nuclide production model similar to that used in the CRONUS
Online Calculator (Balco et al., 2008) to investigate the nature and feasibility of complex
exposure scenarios on TCN concentrations at Huancané. Modeling the concentrations and
concentration ratios of more than one nuclide (i.e. 26Al/10Be or 14C/10Be) in terms of production,
radioactive decay and erosion, and comparing modeled results to the measured datasets can
provide important clues to the exposure history of a site. Understanding exposure history is
especially important for calibration sites because any uncertainty in the history of a calibration
site can lead to inaccurate production rate estimates and reduces its utility for production rate
calibration.
The majority of variables affecting nuclide concentrations under ‘complex’ conditions
(differing from simple exposure) affect the production of all TCN’s equally. These include high
surface erosion rates, or exposure of the sample after moraine deposition. We instead have to
focus on the differences in production and decay between
14
C,
10
Be and
26
Al to explain the
observations. For example, the proportion of total production by muons for
estimated at ~22-24% at the surface, while both
10
Be and
26
14
C at SLHL is
Al it is <2% (Heisinger et al.,
2002a,b). At the Huancane site, because of the difference in scaling of muons and spallogenic
nucleons these proportions are less: 10-15% for 14C, depending on scaling model, and ~1.5% for
26
Al and
10
Be. Under extremely high erosion conditions, on the order of 1-10 cm/kyr, the
proportion of production in a sample due to muons increases because it attenuates less rapidly
with depth (Gosse and Phillips. 2001). Therefore we would expect greater production of
14
C
relative to 10Be and 26Al under those conditions.
The production of all nuclides is simultaneously accompanied by loss from nuclear
decay, and
14
C decays much more quickly than
10
Be and
26
Al. This is the strongest factor
!"#$%&'()'*+,!
determining
14
#"!
C concentration in the long run. Over timescales of thousands of years, nuclear
decay causes the net production of
14
C to slow relative to
10
Be and
26
Al.
14
C reaches secular
equilibrium (where production is balanced by decay and the concentration cannot increase
further) under surface exposure conditions after ~25-30 kyrs and the 14C/10Be concentration ratio
evolves continuously from the initial production ratio towards zero over long exposure periods.
Therefore, exhumation of the boulders after 12.4 cal ka also cannot explain the observed
14
C/10Be ratios because this would instead result in ratios apparently higher than expected
because of the shorter period of decay. This essentially leaves two scenarios, both involving the
rapid decay of 14C relative to 10Be and 26Al, that could result in lower 14C concentrations relative
to 10Be and 26Al.
The first scenario involves the possibility of snow cover at the site. If, over the period of
nuclide production, the boulder surface was covered with an increasing amount of snow due to
long-term climate change, as suggested by the H1 ice advance of the QIC culminating during the
Little Ice Age (LIA), then the
relative to
10
Be and
26
14
C could decay preferentially with lower production at depth
Al, resulting in lower-than-expected
14
C/10Be ratios, but with
26
Al/10Be
ratios still consistent with simple exposure. This surface coverage would also result in lower
nuclide concentrations for all nuclides and could also explain why the SLHL production rate
estimates for all nuclides from Huancané are less than the grouped site means.
The second scenario requires a violation of the assumption that all nuclide production in
the Huancané samples post-dates H2 moraine deposition. Once again, because of the difference
in half-lives, it is possible that the Huancané samples could have an inherited concentration of
10
Be and
26
Al from an earlier exposure period, perhaps if the QIC glaciers receded during the
Bølling/Allerod warm period or some earlier period, and then ice cover resumed long enough to
!"#$%&'()'*+,!
##!
decay away some of the
than-expected
14
14
C produced during this prior exposure. This would result in lower-
C/10Be ratios, but does not explain why all TCN concentrations are lower than
expected. However, there is no recognized geological evidence at the site to refute this scenario
and it is modeled for completeness.
5.4.1 Model Structure
Table 5 shows the surface- and topographic shielding-corrected TCN concentrations and
TCN ratios we are comparing to with the model. We consider these datasets as the ‘target’ for
the modeling scenarios. There is evidence to suggest that most TCN concentrations measured on
moraine boulders yield age estimates that are younger than the moraine age, and that the ‘oldest’
boulders, with the highest TCN concentrations, are a better estimate of the true landform age
(Heyman et al., 2011). Because of the small scatter in the Huancané
10
Be and
26
Al data, the
highest concentrations are not very different from the mean, and the mean concentrations are
more representative of the actual variability in the dataset. Considering this, and the fact that all
sample concentrations will be used for production rate calibration, examining all samples and
sample means makes the most sense here.
The equation governing the production model is modified from Balco et al. (2008), and
incorporates the currently accepted decay constants and production pathway-specific attenuation
lengths for each TCN. These values and corresponding references are shown in Table 8. The
half-life of
10
Be was measured more recently by Chmeleff et al. (2010) and Korschinek et al.
(2010), but the measured
10
Be concentrations discussed in this study are normalized to the
07KNSTD standard using the half-life cited by Nishiizumi et al. (2007) so we use a weighted
mean of the three measurements in the model. The new half-life determinations differ by less
than 2% from that of Nishiizumi et al. (2007) and, over such a short exposure history, do not
!"#$%&'()'*+,!
#$!
change the results of the modeling appreciably. The site-specific spallation and muogenic
production rates used in the model are derived using values for mean latitude, longitude and
altitude at the site, and scaling codes based on those in the CRONUS Online Calculator (Balco et
al., 2008). Reference production rates were calculated using the preferred CRONUS-Earth
geomagnetic framework and data from other CRONUS geological calibration sites discussed
above (Lifton et al., in prep, Stone et al., in prep). This provides time-dependent spallogenic
production rates averaged over 100 year time steps for each of the 6 scaling schemes (St model
production rates are equivalent in step, and therefore time-invariant) and constant production
rates for fast and slow muon interactions (Table 8). The Lifton-Sato (LS) scaling scheme reflects
the most current CRONUS scaling framework and is the model used for comparison to the
measured data in Figure 7 and for discussion. However, the results for the five other scaling
schemes are also included in Table 9 for reference.
The model calculates TCN concentrations produced during each 100 year interval
starting at 12.4 cal ka, based on the production rates, erosion rate, and amount of surface cover
designated for that step and sums them, corrected for nuclear decay, using equation (3) modified
from Balco et al. (2008, Eqn. 1):
(3)
! !! ! !
!
!
! !
! !
!!!"!
!
!!"#
!!
!!"#
!! !
!
!!" ! ! !!!"! ! !!"
!! ! !!!"! !
!!"#
!!
!!"!
!! ! !!"
! !!!"!
!
! !!"
!
!! !
where N(z,t) is the final total nuclide concentration (atoms g-1), t (yr) is the number of years
before present, Ps(t) (atoms g-1yr-1) is the time-dependent production rate due to spallation, Pµf
and Pµs (atoms g-1yr-1) are the constant production rates due to fast and stop muon interactions
respectively, ! (yr-1) is the decay constant for the TCN, " (cm yr-1) is the surface erosion rate
during the exposure period, # (g cm-3) is the density of the sample or surface coverage material,
!"#$%&'()'*+,!
#%!
and $s, $µf, and $µs (g cm-2) are the absorption mean free paths for spallation, fast and stop
muons (Table 8). For production of
10
Be and
26
Al, an additional term is added to the equation
where Ni (atoms g-1) is some inherited nuclide concentration corrected for nuclear decay and
erosion over the whole exposure period, T (yr).
The erosion correction for the inherited
concentration is calculated for spallation only for simplicity. Because of the low production
rates by muons for 10Be and 26Al, specific terms for inherited concentrations due to muons can be
neglected.
5.4.2 Model Scenarios
We will compare the modeled results of three scenarios to the measured data, and then
discuss the feasibility of those scenarios based on evidence from the site. First, the case of
‘simple exposure’ 12.4 cal ka to present with no surface coverage is compared with measured
data as a ‘control’ simulation to further investigate the disagreement of the measured data with
predictions derived from the global mean production rates. In the second scenario, ‘snow cover’,
we add variable amounts of surface coverage near the end of the exposure period (2.0 cal ka, or
younger, to present) to simulate increased snow cover during the LIA, allowing
relative to
10
Be and
26
Al to reproduce the measured concentrations and
14
14
C to decay
C/10Be ratios. In the
third ‘inheritance’ scenario, we introduce an inherited concentration of 10Be and 26Al at 12.4 cal
ka to attempt to reproduce the measured 14C /10Be ratios and maintain the 26Al/10Be ratios under
simple exposure conditions. We use the maximum estimated surface erosion rate (4x10-4cm yr-1)
for all model scenarios because there is obvious geologic evidence of erosion on all sampled
boulder surfaces. However, the model is very insensitive to these low erosion rates, and varying
this rate by ±100% does not significantly change the results.
!"#$%&'()'*+,!
#&!
Modeling results for the LS model are summarized in Figure 7. TCN concentrations for
the measured data are normalized to the LS model-estimated modern production rate for their
respective locations. Modeled results are normalized to the LS modern mean site production
rate. This removes the minor effect of differing elevation between samples (~60 m total spread)
for comparison to the LS model in Figure 7. This results in units of years for normalized 10Be
concentration on the x-axis (10Be atoms g-1/10Be P atoms g-1yr-1).
dimensionless.
14
C/10Be on the y-axis remains
The un-normalized modeled concentrations for all six scaling models are
compared to the un-normalized means of the experimental data in Table 9 under the assumption
that the measured means are representative of production at the mean elevation of the site to
which the modeled production rates are scaled.
5.4.3 Model Results: Simple Exposure
The simple exposure scenario is the simplest case and is the ideal scenario for a
calibration site because the TCN concentrations are overwhelmingly a function of the sitespecific production rates, and age. The evolution of the modeled 14C/10Be ratio (14Cm/10Bem) in
the simple exposure scenario (Fig. 7, red line) is very similar to that of the simple exposure/no
erosion line (Fig. 7, gray dashed line). The erosion rate used in our calculation reduces the final
concentrations of all TCN’s slightly and results in a 14Cm/10Bem value ~1% higher than that for
12.4 kyrs exposure with no erosion. Using the maximum erosion rate results in 10Bem and 26Alm
concentrations similar to the mean of the measured data (Table 9). However, modeled
41% higher than the mean of the
14
CInt data and the
14
14
Cm is
CInt/10Be calibration data (Fig. 7, purple
dots) plot well below the modeled 14Cm/10Bem.
This exposure scenario is best supported by geologic and paleoclimatic evidence for the
Huancané site. The radiocarbon ages from sections TL08-02, TL08-19 and core TL08-14 (Fig.
!"#$%&'()'*+,!
#'!
4) suggest that the glacier retreated quite rapidly up valley from the H2 moraines following the
YD, suggesting all samples were exposed at a similar time, and not covered after that time. The
sampled boulders all stand >1 m above the moraine surface, making post-deposition exhumation
from the moraine surface unlikely. While erosion is evident on the boulder surfaces, several
boulders sampled by M. Kelly (pers. comm.) preserved glacial polish on the raised pedestals of
the boulder surfaces. Paired
10
Be measurements from these pedestals and directly adjacent
weathered surfaces suggest an erosion estimate of up to 8 mm/kyr. The modeled results agree
with the measured 10Be and 26Al data, suggesting this erosion estimate is sound. Although snow
cover at the site during the year is somewhat unconstrained, we observed one minor storm during
June 2008 that left 10-20 cm of snow on the ground, which subsequently melted away in less
than 8 hours. During the rainy summer season (Dec. to Feb.) snow accumulation is most likely
greater, but presumably also melts quickly or blows off the boulders in the almost constant winds
blowing to and from the ice cap. Under modern conditions, at most the boulders would have
snow cover <1m thick for 1-2 months per year. During the exposure period, paleoclimatic
evidence suggests less precipitation and/or warmer temperatures relative to modern since YD
time with the exception of the LIA period. Radiocarbon dating of plant material preserved near
the modern ice margin at the head of the Huancané valley suggests that the extent of the QIC
today is the smallest since ~5 cal ka and was likely within that extent or smaller between ~7 and
5 cal ka (Buffen et al., 2009). This is consistent with the tropical ice core records from
Huascarán and Nevado Sajama (Thompson et al., 2000). After ice retreat from the H2 moraines
at 12.4 cal ka, the QIC region along with the rest of the tropical Andes was warmer/drier than
today up until ca. 1.0 cal ka, during the LIA. In all, snow cover was probably not a major factor
!"#$%&'()'*+,!
#(!
during most of the exposure history of the H2 calibration samples. Although it is geologically
most likely, the measured 14CInt data are inconsistent with the modeled results of this scenario.
5.4.4 Model Results: Snow Cover
The ‘snow cover’ scenario explores the possibility that increased snow cover during the
LIA period can reproduce the measured dataset by attenuating production under deep snow. The
H1 (LIA) advance of the QIC had to result from increased accumulation and/or lower
temperatures, which could increase the thickness and residence time of snow in Huancané valley
as well. To reproduce even the highest measured
14
CInt/10Be ratios within error, we introduced
year round snow cover for two time periods starting at both 2.0 cal ka (Fig. 7, light blue line) and
1.0 cal ka (dark blue line). It is unrealistic that snow cover would increase so abruptly, or persist
year round, but is sufficient to demonstrate the feasibility of this scenario with the model. Paths
shown in Figure 7 approximate the evolution of 14C/10Be ratios under these conditions.
Mercer and Palacios (1977) collected organic material from beneath the H1 moraines
(Fig. 1) that constrains the maximum moraine age to 300-900 cal yrs BP (originally reported as
14
C yrs BP, calibrated for comparison here; Reimer et al., 2009). This latest glacial advance is
also broadly synchronous with the trend towards higher "18O values and increased accumulation
in the ice core records during the LIA, though the record of increased QIC accumulation is
limited to only 600-200 cal yrs BP (Thompson et al., 1986). Isotope values increase in both the
Huascarán and Sajama ice cores around 1.0 cal ka (Thompson et al., 2000). This suggests the
1.0 cal ka scenario is most consistent with the timing of the LIA. However, it requires 10 m of
water equivalent (1 g cm-3 density) to reproduce even the highest measured normalized
14
CInt/10Be ratio, which is roughly 10 times the modern annual precipitation total (Table 9).
Assessment of accumulation in the QIC ice core during the LIA by Thompson et al. (1986)
!"#$%&'()'*+,!
#)!
suggests, at most, a 30% increase in total accumulation. If, however, snow cover begins at 2.0
cal ka, it only requires 2.5 m of water equivalent.
14
Both scenarios still yield normalized
Cm/10Bem ratios ~16% higher than the normalized 14CInt/10Be mean. Both scenarios, 1.0 cal ka
and 2.0 cal ka, also underestimate the measured normalized 10Be and 26Al concentrations by ~3%
and ~7% (Fig. 7). If snow cover were to be increased earlier in the exposure history, this
underestimate would increase further. Moreover, the assumption of year-round snow cover is
also highly unlikely. Ultimately, the magnitude and timing of snow cover required to even
partially explain the experimental data is unrealistic and does not simultaneously account for the
14
C and 10Be inventory in the Huancané samples.
5.4.5 Model Results: Inheritance
The ‘inheritance’ scenario assumes the same conditions as the ‘simple exposure’
scenario, except that a concentration of 7.50x104 atoms g-1 of
26
10
Be and 5.06x105 atoms g-1 of
Al is added to the initial time step to simulate inherited concentrations of TCN’s. This
corresponds to roughly 1.6 kyrs of prior exposure using the mean modeled production rates.
These values were chosen because they reproduce the mean normalized
14
CInt/10Be ratio.
However, they overestimate the normalized measured concentrations of 10Be and 26Al by ~20%
(Fig. 7).
The SLHL production rate estimates for
10
Be and
26
Al (Table 7) suggest that, under
simple exposure conditions, the Huancané site is slightly lower, but not significantly different
than the global average. Introducing an inherited concentration of those TCN’s therefore implies
instead that the modeled production rates are too low, similar to 14C. Our confidence in the 10Be
and 26Al measurements is ultimately higher than the 14CInt data, so this is an unlikely explanation
given the small mismatch. Also, the samples come from moraines in two separate valleys at the
!"#$%&'()'*+,!
$+!
study site deposited by different glaciers. Thus, they are likely derived from a variety of original
locations with differing exposure histories. It is therefore unlikely that every sample would have
an essentially identical inherited concentration, as suggested by the lack of scatter in the
measured 10Be and 26Al data. Although this model scenario can reproduce the measured ratios,
we argue it is an unrealistic explanation given the geologic evidence.
5.5 Implications for production rate calibration
There is no satisfactory explanation that correctly incorporates all aspects of the
experimental data for the Huancané calibration site. The LS scaling scheme is favored for
production scaling for this site because it is physically-based and most closely fits the
experimental
10
Be and
simple exposure).
26
Al under the exposure conditions suggested by geologic evidence (i.e.
Although the
14
C data indicate complex exposure may complicate that
interpretation, these data must be reproduced before any concrete conclusions can be made.
6. Conclusions
The H2 moraines in the Huancané and South Fork valleys have a well-constrained age of
12.4±0.1 cal ka based on upper and lower bounding radiocarbon dates. The maximum bounding
ages in both valleys are in agreement, while the minimum age is most tightly constrained in the
South Fork valley, and less certain in the Huancané valley. Because of the close agreement in
radiocarbon ages and measured TCN concentrations, we assume the true moraine landform and
boulder ages in both valleys to be equal and 12.4 cal ka is used as the independent age for
production rate comparison and calibration.
TCN concentrations of
10
Be and
26
Al in ten quartz samples collected from boulders
deposited on or near the crests of the H2 moraines in both valleys are consistent with simple
exposure with limited erosion, based on the independent age determination, but suggest SLHL
!"#$%&'()'*+,!
$*!
production rates between 4.1% and 25% lower than those from other CRONUS calibration sites,
depending on scaling model. The measured 26Al/10Be ratios for the samples are consistent with
simple exposure since 12.4 cal ka, but this ratio is not diagnostic because of the long half-lives of
the TCN’s.
In situ 14C concentrations measured in the same samples are also consistently lower than
those predicted using scaled production rates, but to a greater magnitude - 19.0% to 30% lower.
Correspondingly, the measured
14
C/10Be ratios are 7.5% to 17% lower than predicted, which
suggests either complex exposure histories, or a fundamental difference from the current
estimated production rate ratio between
14
C and
10
Be. Based on the results of the site surface
production model, which incorporates the most current scaling and physical parameters for TCN
production, there is no feasible complex exposure scenario that satisfactorily explains all the
measured TCN concentrations and ratios.
We cannot be fully confident that the in situ 14C data presented here do not suffer from
some systematic loss of
14
C due to the recent automation of the extraction system without
duplicate analyses of the Huancané calibration samples. This could help to explain the apparent
low measured
14
C/10Be ratios relative to the model. However, procedural blank and reference
material analyses performed before and after automation provide no evidence for this at present.
Until these discrepancies are resolved, we suggest that this site should not currently be included
as a primary geologic calibration site for
14
C production in the CRONUS-Earth exposure age
calculator.
Acknowledgements
This research was funded by NSF Grant EAR-0345150 (CRONUS-Earth Project) to Lifton and
Jull, and supported by the University of Arizona Accelerator Mass Spectrometry Laboratory.
!"#$%&'()'*+,!
$"!
Thanks to Meredith Kelly and Thomas Lowell for introducing us to the Huancané site and access
to unpublished data. Thanks to all the CRONUS researchers for access to unpublished data and
for helpful discussion on the data analysis and production modeling included in this manuscript.
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Figure Captions
Figure 1. Huancané and South Fork valleys on the western margin of the Quelccaya Ice Cap.
Moraine crests of H1, H2 and H3 moraines (Mercer and Palacios, 1977) are shown in blue,
red, and gold respectively. Mapped moraine crests are modified from (Kelly et al., in prep).
Independent moraine age sampling sites are shown by triangles (Table 1). Contour interval
is 50 m with the 5,000 m contour highlighted by the thick black line. Red triangles denote
maximum limiting ages for H2 moraine deposition, blue triangles denote minimum limiting
ages. Inset: Close up of H2 moraines with specific boulder calibration sample locations
shown by blue dots.
Figure 2. Huancané calibration site overview photos: A) H1 moraines and channel scoured
through moraines and underlying fan sediments by the outburst flood at the head of the
Huancané valley from Boulder Lake in 2007. B) H2 moraines of the Huancané valley and
visible calibration sample boulders. C) H2 moraines in the South Fork valley and visible
calibration sample locations. Photos courtesy of Fred M. Phillips.
!"#$%&'()'*+,!
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Figure 3. Huancané site calibration sample boulders. See Figure 1 for boulder locations. Tape
measure shown for scale is 2 m high divided into alternating red and white intervals of 10
cm. Boulder surface pitting is clearly visible on all boulders. Boulder HU08-04 has glacial
polish visible on the side near the ground (black circle). Photos courtesy F. M. Phillips.
Figure 4. Stratigraphic sections and radiocarbon dates from moraine age sampling sites. All
ages are listed uncalibrated in
14
C yrs BP. Calibrated age ranges are listed in Table 1.
Sections TL08-01A, -01B and TL06-02
14
C ages are maximum limiting ages for moraine
deposition. Sections TL08-02, TL08-19 and core TL08-14
14
C ages are minimum ages for
moraine deposition (see text). No suitable photos are available for TL06-02 or TL08-14 so
they are shown as schematic diagrams, courtesy of Tom Lowell, modified from Kelly et al.
(in prep). Section photos were provided by F.M. Phillips.
Figure 5. Measured
14
C,
10
calibration samples. A)
Arizona (UA). B)
14
Be, and
14
26
Al surface-corrected concentrations for ten Huancané
CInt data for samples processed and measured at University of
CAvg data; processed and measured at UA. C)
10
Be data; processed at
University of Washington (UW), Lamont-Doherty Earth Observatory (LDEO) or Dartmouth
College and measured at Lawrence Livermore National Laboratory (LLNL). D)
26
Al data;
processed at UW, measured at LLNL. Solid horizontal lines indicate unweighted average of
all samples. Thick dotted lines indicate one standard deviation from the average. Thin grey
dotted lines are 5% intervals around the average value. Individual sample error bars denote
1# total lab and analytical error. Samples are in the order they are listed in Table 3 from left
to right. Sample numbers are listed below panels C and D. Averages of
datasets do not include HU08-06 (see text).
14
CInt and
14
CAvg
!"#$%&'()'*+,!
%+!
Figure 6. Summed probability density function (PDF) curves of twenty six, unaveraged,
minimum-limiting (red) and maximum-limiting (blue) radiocarbon ages for the H2 moraines
in the Huancané and South Fork valleys (see text for methods). Based on this comparison,
the best estimate of the H2 moraine age is 95% likely to lie in the range from 12.37 to 12.16
ka, shown in green. Figure modified from Kelly et al. (in prep).
Figure 7. LS Model modeled evolution from 12.4 cal ka to present of normalized
10
14
Cm and
Bem boulder surface concentrations for model scenarios. Model values are normalized to the
modern calibrated production rate estimates for the LS scaling model (Lifton and Sato, in prep)
at the mean location and elevation of the Huancané site. Modeled continuous exposure with no
erosion (gray dashed line) and end point for 12.4 kyrs of exposure (gray cross) shown for
reference. The simple exposure scenario with 8.1x10-4 cm/yr (Kelly et al., in prep) surface
erosion (red line) terminates near this end point. The snow cover scenarios, 2.0 (light blue line)
and 1.0 cal ka (dark blue line) to present, are approximated by deviation from the simple
exposure line to their end points (see text). The inheritance scenario (green line) begins with an
inherited
10
Be concentration and
14
Cm/10Bem increases as
14
C accumulates, then decreases by
decay of 14C. Sample concentrations (purple dots) are normalized to the modern production rate
estimates for their respective locations and elevations using the LS model.
!"#$%&'()'*+,!
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Hudson et al.
FIGURE 1
!"#$%&'()'*+,!
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Hudson et al.
FIGURE 2
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Hudson et al.
FIGURE 3
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Hudson et al.
FIGURE 4
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1
14C
14C
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UA
atoms/g (x105)
atoms/g (x105)
UA
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B) 14CAvg
1
1
5%
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10Be
5%
atoms/g (x106)
atoms/g (x105)
UW
C) 10Be
D) 26Al
01 02 03 04 06 10 11 14 15 16
Hudson et al.
FIGURE 5
01 02 03 04 06 10 11 14 15 16
!"#$%&'()'*+,!
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Hudson et al.
FIGURE 6
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Hudson et al.
FIGURE 7
Tables
Hudson et al.
58
B!(.,*H8*3IJ*'!#$%&!'(%)*#!/,-*:%'*%'+!)$&*=!/,'$!.*&%)-/'!$)$)+*/1,*!+,*%:*/1,*K?*=%'!$),-*$)*/1,*K0!)&!)L*!)#*J%0/1*M%'N*C!..,E-8
14
Sample Site
Sample ID
Lab Number
C age ±uncert. 95% Calibrated age
range (cal yr BP)a
(14C yr BP)
Relative
areab
Median age
(yr)
Stratigraphic
Age Constraint
South Fork valley
TL-08-02
TL-08-14
TL-08-01A
TL-08-01B
08-02-A
AA82738-1
10,490 ±100
12080-12610
1
12394
Minimum H2
08-02-A
AA82738-2
10,400 ±140
11766-12605
1
12246
Minimum H2
08-02-A
AA82738-3
10,470 ±160
11809-12671
1
12310
Minimum H2
08-02-A
AA82738-4
10,400 ±130
11814-12600
1
12253
Minimum H2
08-02-D
AA85307
10,450 ±100
12041-12595
1
12330
Minimum H2
08-14b-2_19cm
AA82731
9,820 ±90
11071-11613
0.96
11243
Minimum H2
08-14b-2_25cm
AA82733
9,820 ±90
11071-11513
0.96
11243
Minimum H2
08-01A samp A
AA82730
11,780 ±120
13367-13871
1
13623
Maximum H2
08-01A-samp F
AA82740
11,470 ±110
13120-13582
1
13332
Maximum H2
08-01A-samp F
AA85308
11,380 ±100
13084-13454
1
13253
Maximum H2
08-01A-samp G
AA82741
11,060 ±110
12670-13165
1
12943
Maximum H2
08-01B-samp A
AA82737
11,480 ±110
13126-13593
1
13341
Maximum H2
08-01B-samp I
AA82736
10,610 ±100
12371-12715
0.9
12530
Maximum H2
08-01B-samp I
AA85305
10,840 ±200
12375-13191
0.98
12756
Maximum H2
Huancané valley
TL-08-19
TL-06-02
!
08-19A
AA82725-1
10,080 ±100
11270-12006
1
11649
Minimum H2
08-19A
AA82725-2
9,980 ±140
11067-11833
0.92
11373
Minimum H2
08-19B1
AA85305
9,850 ±150
11190-11359
0.99
11252
Minimum H2
08-19B2
AA85306
9,930 ±120
11165-11832
0.97
11444
Minimum H2
06-02A
AA74022
10,661 ±52
12532-12709
0.99
12602
Maximum H2
06-02A
AA79186
10,195 ±54
11699-12087
0.98
11894
Maximum H2
06-02B
AA74023
10,614 ±51
12516-12667
0.83
12571
Maximum H2
06-02B
AA79187
10,568 ±60
12388-12652
1
12518
Maximum H2
06-02C
AA79188
10,564 ±55
12395-12637
1
12518
Maximum H2
06-02D
AA79189
10,718 ±60
12548-12748
1
12631
Maximum H2
06-02E
AA74024
10,828 ±52
12591-12874
1
12702
Maximum H2
06-02G
AA74025
10,931 ±53
12629-12968
1
12797
Maximum H2
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