Ice-rafted debris in the Southern Ocean:
Potential uses and limitations of 230Th-normalized fluxes
by
Jessica E. Fujimori
Submitted to the Department of Earth, Atmospheric and Planetary Sciences
in Partial Fulfillment of the Requirements for the Degree of
Bachelor of Science in Earth, Atmospheric and Planetary Sciences
MASSACHUSETT~IS INS
at the Massachusetts Institute of Technology
OF TECHNoLOGY
May 12, 2014
Qa Lr 20 f
Copyright 2010 Jessica E. Fujimori. All rights reserved.
JUN 10 2014
LIBRARIES
The author hereby grants to MIT permission to reproduce
and to distribute publicly paper and electronic copies of this
thesis document in whole or in part in any medium now
known or hereafter created.
Signature redacted
Author
Depiftment of Earth, Atmospheric and Planetary Sciences
May 12, 2014
Signature redactedCertified by_
David McGee
Thesis Supervisor
Accepted by
Signature redacted
Richard P. Binzel
Chair, Committee on Undergraduate Program
E
2
Abstract
We measured IRD fluxes from 22-5 ka BP in two sediment cores from the Scotia Sea using two
different methods. The first, commonly used method, uses the linear sedimentation rate (LSR),
dry bulk density, and weight percent of IRD in the sample. The second uses
2 30
Th normalization,
which has been proposed as an improved way to determine sediment fluxes in sites with
significant lateral redistribution. We found that IRD fluxes calculated using the LSR produced a
chronology in closer agreement with prior studies than those calculated using
2 30
Th
normalization. Based on the differences in records between the two cores, we conclude that IRD
flux records more likely provide information about local ice sheet dynamics than about ice sheet
behavior as a whole. IRD flux records may be influenced by differences in local sediment
focusing, currents, and distance from the ice sheet.
3
Acknowledgements
I would like to thank David McGee and Elizabeth Pierce for their support and guidance
throughout this project. I would also like to thank Irit Tal, Christopher Kinsley, and Elena
Steponaitis for their mentoring and help within the lab.
For her valuable coaching on communications skills throughout my time as an EAPS
undergraduate, and for her compassionate provision of bagels at early-morning classes, I thank
Jane Connor.
For the freshman-year field trip that inspired me to pursue an EAPS degree, and for his
support over the past four years as my professor and academic advisor, I thank Sam Bowring.
Finally, I want to thank my parents, brothers and sister, friends, and Paul for their patience,
love, and support that keep me grounded and happy.
4
Contents
Abstract ...........................................................................................................................................2
Acknowledgem ents ........................................................................................................................ 3
Contents ..........................................................................................................................................4
1. Introduction ................................................................................................................................ 5
2. Background ................................................................................................................................ 7
Z1 Ice-rafteddebris ........................................................................................................................ 7
Z2 Estimating deep-sea sedimentfluxes ........................................................................................8
3. M aterials and M ethods ............................................................................................................ 10
3.1 Sedim ent cores ........................................................................................................................ 10
3.223OTh measurement................................................................................................................. 11
3.3 Calculations............................................................................................................................. 12
4. Results .......................................................................................................................................13
4.1 Comparison of m ethods .......................................................................................................... 13
4.2 Comparisonof cores ............................................................................................................... 14
4.3 Focusingfactors...................................................................................................................... 15
5. Discussion .................................................................................................................................15
5. 1 Comparisonto priorice retreatreconstructions.................................................................... 15
5.2 Comparisonbetween cores ..................................................................................................... 16
5.3 Role of lateralredistribution.................................................................................................. 17
5.4 Limitation of '0 Th for IRDfl ux ............................................................................................17
6. Conclusions ...............................................................................................................................19
7. References .................................................................................................................................20
5
1. Introduction
Despite their direct implications in climate and sea level changes, the dynamics of massive
ice sheets such as Antarctica are not yet fully understood, nor have their historical changes been
fully characterized. In the Arctic, massive releases of iceberg armadas, called "Heinrich events,"
have been observed in the geological record (Heinrich, 1988). Williams et al. (2010) postulated
the occurrence of Heinrich event-like discharges in the Southern Ocean during the late Miocene
and early Pliocene. It is not yet known, however, if such sudden and catastrophic collapses are
typical for ice sheets, or how local behaviors along the ice sheet edges differ.
The Antarctic ice sheet is the largest in the world: the East Antarctic ice sheet contains a
volume of ice equivalent to 52m of sea level rise, while the West Antarctic ice sheet contains
~5m. In dry, cold periods, that volume was larger; in warm, wet periods, it was smaller. A
crucial question that remains to be answered concerns the transition periods: do ice sheets grow
and shrink steadily, or in sudden, dramatic events? This question is of interest both for its
implications in paleoceanography as well as recent warming and sea level rise.
Reconstructions of ice sheet extent have been attempted using subglacial bedforms and
glacial erosional unconformities; these generally allow crude and relative age estimations, and
only as far back as the bedforms are preserved. Marine sediment cores could provide
chronological information about past changes to the ice sheet using radiocarbon ("C) dating of
calcareous microfossils (Smith, 2010); however, these microfossils are rare in the Southern
Ocean (Hillenbrand, 2013). Furthermore, radiocarbon dating can only be used to date strata from
around 60 ka BP or younger.
Ice-rafted debris (IRD) records from deep-sea sediment cores can be used to track changes in
ice sheets over time, in a potentially more continuous and longer history than glacial bedforms or
calcareous microfossils. IRD are grains of continental origin carried by icebergs that calve off of
ice sheets; these sediments drop into the water column as the icebergs drift and melt. IRD grains
can be analyzed in provenance studies (e.g. Hemming et aL, 1998; Roy et al., 2007; Pierce et al.,
2011), indicating where the sediments originated and thus providing information about iceberg
trajectories and local ice sheet behaviors. Reconstructing IRD fluxes can also provide
information about ice sheet dynamics over time; higher IRD flux would indicate a greater
amount of ice breaking off of the ice sheet and a more erosive and dynamic ice sheet (Alley et
6
al., 1997). IRD has been studied in both of these contexts (provenance and flux) in the North
Atlantic (e.g. Bailey et al., 2013; Stickley et al., 2009) and the Southern Ocean (e.g. Manoj et al.,
2013; Passchier, 2011).
The traditional method first calculates total sediment flux using linear sedimentation rate
(LSR) and bulk density, then calculates IRD flux based on the estimated weight percent of IRD
in the sample. Francois et al. (2004) reviews potential flaws in this method in locations with high
lateral sediment redistribution and suggests the use of
2 30
Th isotope measurements to better
estimate IRD fluxes. In this study, we test these two different methods of calculating IRD fluxes
for two core sites in the Scotia Sea. We present two comparisons: 1) the fluxes from the LSRbased method versus from
23 0
Th normalization within each core, and 2) the reproducibility of
IRD records between sites. We examine IRD fluxes throughout the late Pleistocene and early
Holocene and compare the implied ice sheet changes with prior work on Antarctic ice sheet
reconstructions.
7
2. Background
2.1 Ice-rafted debris
As ice forms over Antarctica and flows outward towards the terminus of the ice sheet, the
underside of the ice scrapes against the surface of the continent. Sediment of various sizes
becomes embedded in the bottom layer of the ice. Eventually, the ice may reach the edge of the
ice sheet and calve off of it, becoming a free-floating iceberg in the currents of the Southern
Ocean. As the iceberg drifts, it gradually melts, releasing the embedded continental grains into
the surface waters of its drift path. Most of these grains fall through the underlying water column
to the deep-sea floor below. These sediments, carried by icebergs to locations potentially far
from their continental origin, are called "ice-rafted debris" (IRD).
IRD in deep-sea cores can be useful records of ice-sheet behavior over time. Within a
sediment sample, IRD is distinguished as detrital grains in the coarse fraction, often defined as
above 150 pim. IRD fluxes are a proxy for iceberg quantity; during periods where Antarctica was
shedding many icebergs, we would expect to see more ice-rafted sediment. Large and sudden
fluxes of IRD could be interpreted as sudden and catastrophic ice sheet collapses, analogous to
the Heinrich events that have been documented in the Arctic (e.g. Heinrich, 1988).
Grains from IRD samples can also provide insight into historical iceberg tracks. Because
different geographical areas of the Antarctic continent have distinct geochemical signatures,
particularly in Argon (Ar) isotope ratios, geochemical analysis of IRD from deep-sea cores
indicates the origin of the sediment, and thus the margin of the ice sheet that produced the
iceberg.
Attempts to estimate IRD fluxes over time are challenging. Relying solely on the IRD weight
percent over segments of the core is not a dependable option, since the concentration of IRD is
affected by the flux of other types of sediment over time. One method uses the linear
sedimentation rate (LSR) to calculate the mass accumulation rate of all sediment types, then
multiplies by the concentration of IRD. This method, though common, is limited by uncertainty
between age tie points and inability to distinguish between vertical and lateral fluxes.
The use of
23 0
Th as a normalization method for deep-sea sediment fluxes was first proposed
by Bacon (1984) in a study of carbonate and clay sedimentation. Francois et al. (2004) reviewed
8
the method, urging greater use of 2 30Th in estimating late-Quaternary deep-sea sediment fluxes
and noting advantages of
23 0
Th normalization over more widely used methods that depend on
dated sediment horizons and dry bulk density. Here, we explore the potential for
2 30
Th
normalization for reconstructing IRD fluxes.
2.2 Estimating deep-sea sediment fluxes
We will briefly review the LSR-based method and the
230Th-normalization
method for
calculating sediment fluxes, describing some of their advantages and their limitations.
LSR-basedflux
If an age model is available for a sediment core, the estimated linear sedimentation rate
(LSR) between two age model tie points can simply be multiplied by the dry bulk density (pdry)
of the sediment to calculate the overall mass accumulation rate (MAR):
MAR = pdry * LSR
(1)
LSR =
(2)
(Z2-Z1)
(t 2 -tl)
The MAR of any sedimentary constituent (MARi) can be easily calculated as the product of
its concentration in the sample and the total MAR:
MARi = MAR * [i]
(3)
An unavoidable limitation of this method arises from uncertainty in the age model on which
it depends. If age tie points are taken close together, the uncertainty in the LSR becomes large; if
they are taken far apart, the temporal resolution of the LSR becomes low. Furthermore, this age
model-based method does not differentiate between vertically and laterally supplied sediment
fluxes. In a depositional environment where lateral accumulation is a relatively high proportion
of sediment flux, a MAR calculated by (1) will be much higher than just the vertical settling rate.
9
To avoid these sources of error,
230Th
normalization has been proposed as an alternative method
of determining sediment flux.
23 0
Th normalization
The
Th normalization method depends on the known, constant production rate of 2 30Th in
2 30
230Th,
the ocean:
produced from decaying
waters into the deep ocean. These
scavenges onto particles falling from the surface
234
23 0
Th-bearing sediments accumulate, lithify, and become part
of the oceanic sedimentary record; the concentration of
2 30
Th in these sediments becomes a
proxy for sediment flux. Since the production rate of 2 30Th is constant, greater sediment flux will
"dilute" the concentration of
230
Th in a given volume of sediment; this is known as "detrital
dilution." Therefore, if we observe a decreased concentration of
23 0
Th in a particular sediment
horizon, we would infer increased sediment flux during the time period represented by that
segment.
Using the known decay rate of
234U
to
(3 = 0.0267dpm/m 3 /yr, where dpm stands
230Th
for disintegrations per minute), the depth of the water column (z) at the core site, the weight
fraction of IRD, and the concentration of
23 0
Th in the core sample, IRD flux can be calculated as
follows:
MARIRD= [IRD]
*2 3 0
[
(4)
Th]
Unlike the LSR-based method, which can only provide MARs between dated horizons, this
method provides a MAR for every sample point within the core. It depends solely on the IRD
and
2 30
Th concentrations at a particular point.
2 30
Th normalization thus avoids the trade-off
between resolution and precision inherent to the LSR-based method.
The core assumptions behind the validity of
rate of
2 30
2 30
Th normalization are 1) that the production
Th in the ocean is constant, 2) that all produced
2 30
Th scavenges onto falling particles
and is promptly removed from the water column, 3) that any lateral sediment fluxes will contain
the same concentrations of
vertical fluxes.
230
Th and any sediment constituent of interest (in this case, IRD) as
10
3. Materials and methods
3.1 Sediment cores
Two sediment cores from the Scotia Sea were analyzed: TPC288 and TPC290/078. PC
means piston core; TC means trigger core; TPC means the TC+PC composite. TPC288 and
TPC290 were collected on cruise JR48 aboard RRS James Clark Ross in March 2000. TPC290 is
a repeat core of TPC078, which was collected in 1993. TPC290/078 was located at
55.55 S/45.02W, at a water depth of 3826 m. TPC288's location was 581.5 km southeast of
TPC290/078 at 59.14'S/37.96"W and a water depth of 2864 m.
( TI
WAUV41&IU4
Figure 1. Core location and context. Top left: Location of cores (Pugh et aL, 2009). Top right: Modern iceberg
tracks (1999-2010) at core locations (Madsen et al., 2013). Bottom: Location of cores with surrounding
bathymetry (Google Earth).
11
Because we are studying sediment that was carried by drifting icebergs and then dropped into
the currents of the Scotia Sea, it is important to understand the ocean circulation patterns in this
region. Circulation in the Scotia Sea is dominated by the Antarctic Circumpolar Current (ACC),
the largest ocean current in the world. The ACC is driven by surface stress from westerly winds
between 45'S and 550S. Its flow speed is highest at the surface and decreases with depth;
however, it extends to the ocean floor in most places. Flow at the seafloor is strongly influenced
by local bottom topography. These local influences on flow could affect sediment redistribution
and therefore IRD flux estimates.
Both of the cores had previously been dated (Pugh et al., 2009) using correlation of magnetic
susceptibility in the cores to the EPICA Dome C ice core dust record and further confirmed
through
14 C
dating and radiolarian abundance stratigraphy of Cycladophoradavisiana. Samples
from each core were sieved into size fractions; IRD weight percent was estimated using the >150
ptm fraction.
3.2
23OTh
measurement
Sediment from each horizon in both cores was crushed using a mortar and pestle. Th and U
spikes were added to 40 mg of each sample. The sediment was then digested in HF, heated, and
ultrasonicated. Fe oxyhydroxides were precipitated, scavenging U and Th, and the samples were
then centrifuged and the supernatant decanted to waste. The remaining sample was then digested
and put through chromatography columns to remove Fe and separate the Th and U.
The separated Th and U isotopes were measured on a Thermo Scientific Neptune Plus multicollector Inductively Coupled Plasma Mass Spectrometer (ICP-MS). We measured Th with 229Th
and
232
Th on Faraday cups and
measured U with
235U
236U,
and
2 30
Th on the central secondary electron multiplier (SEM). We
2 38
U on Faraday cups and
2 34
U on the central SEM. A 50ng/g
natural U standard (CRM 112a) was used to monitor mass bias and the Faraday/SEM relative
yield (gain). Uncertainties for all measurements on the instrument are <1%. Measurements from
the ICP-MS were corrected for detrital and authigenic Th and U using a U/Th activity ratio of 0.6
in the detrital fraction, consistent with the average U/Th ratio of the upper continental crust.
12
3.3 Calculations
To determine IRD fluxes using the LSR, we used Equations (1)-(3). To determine IRD fluxes
using
2 30
Th from our measurements, we used Equation (4). As a measurement of lateral sediment
redistribution, we calculated the focusing factor ('T) for each core:
230 Thxsavg*Pavg*(zsed,f
=
where
23 0Thxsavg
-zsed,O)
*zwater*(tf-to)
is the average non-detrital, non-authigenic Th activity (dpm/g) in the core;
Pavg is the average density of the core; zsed,f and zsedo are the depths of the bottom and top of
the core,
fl is the known
decay rate of 234U to
2 30
Th (0.0267dpm/m3 /yr); zwater is the depth of
the overlying water column; and tf and to represent the age at the bottom and top of the core.
The focusing factor indicates sediment redistribution as follows: 0 > 1 implies additional
lateral supply of sediment to the site (focusing);
site (winnowing);
4
4
< 1 implies net removal of sediment from the
= 1 implies all sediment at the site arrived vertically.
13
4. Results
4.1 Comparison of methods
TPC288
0.03
-
0.03
-
0.025
-
--
23Th-nomeIzedflux
*
LS-based fux
0.02 -
0.015 0.01
0.005 15
10
5
0
20
25
Age (ke)
TPC2900l78
0.07
0.06
--+
0.05
-
230Th-noimokied fux
LSRAeedfux
S0.04 -
-
0.03 0.02 0.01 0
15
10
5
Age
20
25
kca
Figure 2. Unsmoothed IRD fluxes in TPC288 (top) and TPC290/078 (bottom) calculated from
230
Th
normalization (red) and LSR (blue).
Within each core, the overall shape of the IRD flux histories calculated from
230Th
normalization and from the LSR-based method are similar. These similarities come from the fact
that both depend on the IRD weight percent of the sample, and in fact the curves track the IRD
weight percent fairly well.
230Th
normalization yields a curve with less dramatic peaks and
valleys than the curve produced by the LSR-based method. Still, the peaks and valleys match up
well within cores. In TPC288, we see peaks around 12 ka, 16 ka, and 18 ka; in TPC290/078, we
14
see peaks around 14 ka, 17 ka, and 21 ka. However, it is unclear whether these peaks represent
signal or noise, or whether the greater amplitude seen in the LSR-based curve suggests that they
may result from sediment redistribution. In our comparison of IRD histories between cores, we
smooth the data using a simple running average to better observe overall trends rather than
individual peaks and valleys.
4.2 Comparisonof cores
23OTh-BsedFID Fuxes
0.025
-+TPC288
-*TPC2901078
0.02 -
0.015 -
_0.01
0.0051
5
10
15
20
25
Age(ka)
LSR-BasedFID luxes
0.05
--
0.045 ----
TPC288
TPC290078
0.04 0.035 0.03 0.025 0.02
0.0 15
0.01
5
15
10
25
20
Agelk.J
Figure 3. IRD flux histories in TPC288 (red) and TPC290/078 (blue), calculated using
230
Th normalization
(top) and the LSR-based method (bottom). Records have been smoothed with a simple three-point running
average.
15
Although the absolute value of IRD flux differs between cores, which could be due to local
differences in core sites, the overall shape of the LSR-based IRD histories in both cores agree
fairly well. Both show a sustained high between around 20 ka and 10 ka, with a marked decrease
after 10 ka BP. However, the IRD histories calculated using
23 0
Th normalization do not agree
well; while the curve for TPC290/078 appears to tell a similar story as both the LSR-based
curves, the curve for TPC288 shows little variation over time, with a possible peak just before 10
ka and around 17 ka but little absolute change.
4.3 Focusingfactors
The average focusing factor for TPC288 was 1.364; for TPC290/078, it was 2.763. These
imply that some of the sediment from both cores arrived laterally instead of vertically, and that
this lateral sediment focusing was greater for TPC290/078 than for TPC288.
5. Discussion
5.1 Comparison to prior ice retreat reconstructions
Although a complete chronological history of the behavior of the Antarctic ice sheet since the
Last Glacial Maximum (LGM) has not been described, Mackintosh et al. (2013) hypothesize the
following retreat history based on geologic evidence from East Antarctica from their own and
previous studies. The LGM occurred around 27-20 ka BP, after which ice sheet retreat began as
early as 18 ka in the Lambert/Amery system on the east coast of the EAIS. At some sites,
possibly including the Antarctic Peninsula and West Antarctica, retreat began around 14 ka.
These retreats would have coincided with Meltwater Pulse 1 a (MWP 1 a), a rapid sea level rise of
about 20 m in around 500 years. In Mackintosh et al.'s hypothesis, the majority of ice sheet
retreat occurred around 12 ka, reaching its present-day extent by the middle Holocene (-7-5 ka).
In TPC290/078, both methods of calculating IRD flux indicate ice sheet behavior that agrees
fairly well with this hypothesis. We see high IRD fluxes at around 18 ka BP and between 15 and
12 ka BP. This could reflect the beginning and continuation of retreat during these times; if
Mackintosh et al.'s hypothesis is accurate, perhaps the high IRD fluxes around 18 ka come from
16
further afield from the core sites than the IRD fluxes around 15-14 ka, when ice may have been
shedding from the Antarctic Peninsula. Provenance studies on the IRD grains in the cores could
provide insight into potential differences in provenance, and thus add to a more complete picture
of changes at different places along the edge of the Antarctic ice sheet over time.
In TPC288, the LSR-based method appears to agree with previous studies better than
normalization, which gives fairly constant IRD fluxes between 22 and 5 ka. The
23 0
Th
23 0
Th record
does show small IRD flux increases around 17 ka and 10.5 ka. In the LSR-based record, peak
IRD flux comes around 17 ka, but high IRD fluxes sustain throughout 20-10 ka. Thus,
20Th
normalization does not appear to be a superior method for calculating IRD fluxes, though the
method merits future study based on the good agreement in TPC290/078.
5.2 Comparison between cores
Based on the difference in IRD flux history between the cores, it appears that local variations
play an important role in governing IRD flux. We observe differences in both the absolute fluxes
and the shape of the curves between the cores (Figure 2).
The record from TPC290/078 shows consistently higher fluxes than TPC288, with the
exception of the most recent point calculated by
23 0
Th normalization. The difference between the
cores is fairly consistent in the LSR-based flux histories. In contrast, the difference is highly
variable in the
230 Th-normalized
flux histories: the difference between the core histories is large
between 17 ka and 12 ka, after which we see a dramatic decrease in IRD flux from TPC290/078.
By 5 ka, the IRD flux from TPC290/078 has decreased to below the flux from TPC288.
Although both cores show an overall decline in IRD flux after 10 ka, their chronologies differ
before that point. These differences are perhaps due to the differences in location of the two
cores. TPC290/078 generally shows increases in IRD flux earlier than TPC288, which is located
closer to the Antarctic continent.
The reasons for the differences in IRD flux records between the two cores may be due to
differences in local conditions. Perhaps sea ice still covered the site of TPC288 while
TPC290/078 had an open water column above, so icebergs traveled over TPC290/078 and
dropped more sediment earlier. Furthermore, based on modern iceberg tracks (Figure 1),
TPC290/078 is located in a more densely traveled area than TPC288; this difference might help
17
to explain the generally higher fluxes seen in TPC290/078 and the more muted signal observed
in TPC288. TPC290/078 also has a higher focusing factor that TPC288, which suggests that its
core site was subject to greater sediment focusing. Changes in lateral sediment redistribution at
the site over time could explain the poor agreement between cores in the
23 0
Th-normalized flux
histories.
5.3 Role of lateral redistribution
The focusing factors calculated for each core, 1.364 for TPC288 and 2.763 for TPC290/078,
indicate that sediment was laterally supplied at both sites in addition to the sediments falling
from the overlying water column. The higher focusing factor for TPC290/078 implies that its
location was subject to greater sediment focusing. This could mean that the IRD flux record for
TPC290/078 is subject to greater error than the record for TPC288, since lateral redistribution
230Th-normalizated
fluxes. Sorting of laterally supplied sediments should not bias the LSR-
based IRD flux; although the net sediment flux would increase, the concentration of IRD
would decrease by the same proportional amount and the IRD flux would remain the same.
5.4 Limitation of " Th for IRD flux
Although
23 0
Th normalization was originally suggested as a way to decrease error from
lateral sediment redistribution, our results indicate that it may not offer improvement to the
traditional LSR-based method when applied to IRD fluxes. In fact, upon further reflection,
230Th
normalization may also be subject to error from lateral sediment redistribution. It is likely that
sediment redistribution involves some sediment sorting, as fine grains will travel more easily
than larger grains. However, we estimated IRD weight percent by taking the coarse fraction,
which is unlikely to be supplied during lateral transport. Therefore, if sediment focusing did
occur at the core sites,
2 30
Th concentrations would likely be greater, since the finer grains with
greater surface area would bring in a concentration of
23 0
Th higher than the average contained in
the vertically falling sediments. More importantly, the laterally transported sediments would
dilute the IRD concentration. Both of these factors would result in an underestimation of IRD
18
flux. We would also expect to see a greater difference between LSR-based and
fluxes in the core with the higher focusing factor Indeed, we can see from Figure 2 that
than LSR-based fluxes. Moreover, the
230Th-normalized
in this case, TPC290/078.
230 Th-normalized
IRD fluxes are consistently lower
230
Th-normalized flux records agree less well between the
two cores than the LSR-based flux records (Figure 3). Finally, it is not immediately clear from
Figure 2 which core shows better agreement between the two methods, but the records appear to
agree better in TPC288 (4 = 1.364) than in TPC290/078 (0 = 2.763). These comparisons
support our suggestion that 23 0Th normalization may be affected by sediment focusing at the core
sites.
The limitations that we outlined specifically affect coarse grains that are subject to sediment
sorting during lateral redistribution. However,
2 30
Th normalization could provide another,
indirect use: tracking the finer fractions. The reason that IRD weight percent alone is not
typically used to track IRD fluxes is due to the possibility that change in other sediment fluxes
would affect IRD weight percent. If 23 0Th normalization could be used to track changes in finer
grains than IRD, this assumption could be tested. If changes in non-IRD sediment fluxes are
small, IRD weight percent itself could be used to track IRD fluxes, potentially eliminating some
sources of error from the LSR-based calculation.
19
6. Conclusions
2 30
Th normalization may not be an ideal technique for determining IRD fluxes in locations
with high sediment focusing or winnowing, as it cannot account for sediment sorting during
redistribution. The problem of accurately reconstructing IRD flux records in such situations thus
remains a challenge.
23 0
Th normalization may still prove a useful addition to the various methods
of determining ice sheet dynamics, as it can provide flux measurements for each core sample and
is not subject to the trade-off between precision and resolution inherent to the LSR-based
method.
23 0
Th normalization could also be used to track changes in non-IRD sediment fluxes that
are less prone to sorting during lateral sediment distribution. Taken together, the two methods
may be more useful in constructing a chronology than either by itself.
The difference between the IRD flux records in TPC290/078 and TPC 288 indicate that core
location and local conditions affect the preserved record. The question remains, however, as to
whether the differences in the record accurately reflect the differences in vertical IRD fluxes to
each core location, or whether they are an artifact of differences in sediment redistribution at
each site. If IRD fluxes are to be used to help reconstruct the history of the Antarctic ice sheet,
every core must be placed within its local context, and many more cores must be taken and
analyzed to gain a complete picture of the ice sheet's behavior over time.
The LSR-based flux histories we constructed here are encouraging: they indicate that, as
expected, IRD fluxes do increase when the Antarctic ice sheet was disintegrating the fastest. This
agreement between LSR-based IRD flux histories and Quaternary ice sheet reconstructions from
prior work suggests that IRD fluxes can indeed be applied to the deeper past to reconstruct ice
sheet stability.
20
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