Holocene Rock Varnish Microstratigraphy and its Chronometric
Application in the Drylands of western USA
Tanzhuo Liu and Wallace S. Broecker
Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, NY 10964, USA
Abstract
Analyses of hundreds of rock varnish samples from latest Pleistocene and
Holocene geomorphic features in the drylands of western USA reveal a regionally
replicable Holocene layering sequence. This sequence consists of twelve approximately
evenly-spaced weak dark layers interdigitated with thirteen orange/yellow layers.
Preliminary radiometric age calibration indicates that six dark layers in the upper portion
of the sequence deposited during the last 6000 years, diagnostic of the Little Ice Age and
late Holocene wet events; five dark layers in the lower portion of the sequence deposited
after the termination of the Pleistocene but slightly before 7000 years BP, indicative of
the early Holocene wet events; one dark layer in the middle portion of the sequence
deposited around 6500 years BP, suggestive of the middle Holocene wet phase. Our age
calibration further indicates that the Holocene wet events represented by those dark layers
largely correlate in time with the millennial-scale Holocene cooling events in the North
Atlantic region (Bond et a., 1997, 1999). This radiometrically calibrated and climatically
correlated Holocene varnish layering sequence was then used as a unique correlative
dating tool to determine the surface exposure ages of geomorphic and geoarchaeological
features in the western US deserts. The varnish layering dating of debris-flow fan deposits
in Death Valley, California yields minimum ages of 12 500, 12 500-11 100, 11 100, 10
300, 9400, and 2800 years BP for six debris-flow fan building events, suggesting that
such events are more likely to have occurred during relatively wet periods of the
Holocene. The varnish layering dating of a historic grinding stone from Chili of northern
New Mexico yields a minimum age of 900-1100 years BP for the abandonment of this
occupation site by the Anasazi Indians. The varnish layering dating of a prehistoric flaked
stone (i.e., a primary core) from Ocotillo, southern California yields a minimum age of 12
500 years BP for the making of this stone artifact, suggesting at least a Paleo-Indian
human occupation at Ocotillo during the terminal Pleistocene. These results indicate that,
when properly applied, varnish layering dating has the great potential to yield numerical
age assignments for surface stone tools, petroglyphs, and geoglyphs of both historic and
prehistoric age in the drylands of western USA.
Keywords: Rock varnish dating; Wet events; Holocene climate; Debris-flow fan; Petroglyph; Stone
artifact; The Great Basin
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1. Introduction
Rock varnish is a dark to dark-brown coating on subaerially exposed rock
surfaces. It is probably the world's slowest-accumulating sedimentary deposit, growing at
only a few to tens of microns per thousand years (Liu and Broecker, 2000). Its thickness
ranges from less than 5 up to 600 µm, with a typical thickness of 100 µm or so. Although
found in all terrestrial environments, rock varnish is mostly developed and well preserved
in arid to semiarid deserts of the world. It is composed of about 30% manganese and iron
oxides, up to 70% clay minerals, and over a dozen trace and rare earth elements (Potter
and Rossman, 1977, 1979). The building blocks of rock varnish are largely blown in as
airborne dust (Fleisher et al., 1999; Moore et al., 2001; Thiagarajan and Lee, 2004).
Microlaminations in rock varnish were first reported by Perry and Adams (1978),
who recognized their potential as a paleoenvironmental indicator in drylands.
Microlaminations can be observed when the varnish is shaved thin enough (5-10 µm) to
see through in ultra-thin section with a light microscope (Fig. 1). Electron microprobe
chemical mapping reveals that dark layers in varnish thin section are rich in Mn and Ba,
but poor in Si and Al, while orange and yellow layers are poor in Mn and Ba, but rich in Si
and Al (Fig. 1). These two types of layers are intercalated to form distinct
microstratigraphy in varnish.
A growing body of evidence indicates that varnish microstratigraphy carries a
climatic record (Dorn, 1984; Dorn, 1990; Cremaschi, 1996; Liu and Dorn, 1996; Liu et
al., 2000; Broecker and Liu, 2001). In the Great Basin of western USA, Mn-poor yellow
layers (usually containing 5-15% MnO) formed during dry periods of the Holocene and
the last interglacial, while Mn-rich black layers (usually containing 25-45% MnO)
3
deposited during wet periods of the last glacial time; Mn-intermediate orange layers
(usually containing 15-25% MnO) formed during periods of climatic transition between
extremely dry and extremely wet condition (Broecker and Liu, 2001). Furthermore, the
glacial-age black layers in varnish microstratigraphy appear to correlate in time with the
cold episodes of the Younger Dryas and Heinrich Events (Broecker, 1994; Bond et al.,
1993) in the North Atlantic region (Liu and Dorn, 1996; Liu et al., 2000; Liu, 2003).
Varnish microstratigraphy as a correlative dating technique (cf. terminology in
Colman et al., 1987) is relatively new and different in principle and independent from
both cation-ratio and AMS 14C methods (Dorn, 1983; Dorn et al., 1989). It was first used
by Dorn (1988) to study the chronostratigraphy of alluvial-fan deposits in Death Valley of
California. Subsequent studies by others (Liu, 1994; Liu and Dorn, 1996; Liu, 2003) have
greatly improved the usability of the technique. The basic assumption of this dating
approach is that the formation of varnish microstratigraphy is largely influenced by
regional climatic variations. Since climatic signals recorded in varnish have proven to be
regionally contemporaneous (Liu and Dorn, 1996; Liu et al., 2000), varnish
microstratigraphy has the potential use as a tephrachronology-like dating tool. Recently a
rigorous and completely blind test of this method has been conducted on late Quaternary
lava flows in the Mojave Desert, California (Liu, 2003; Phillips, 2003). The testing
results demonstrate that varnish microstratigraphy is a valid dating tool to provide surface
exposure age for late Pleistocene surficial geomorphic features in the Great Basin of
western USA (Marston, 2003).
The purpose of this paper is to extend the utility of varnish microstratigraphy
method to surficial geomorphic and geoarchaeological features of the Holocene. For this
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purpose, we need to: (1) systematically establish the Holocene varnish microstratigraphy
for the Great Basin of western USA; (2) radiometrically calibrate microlaminations in the
varnish microstratigraphy; and (3) demonstrate the potential use of this dating technique
in determination of surface exposure age for various geomorphic and geoarchaeological
features in the drylands of western USA.
2. Study region and methods
The Great Basin and its vicinities of western USA have been chosen as our study
region for several reasons (Fig. 2). First, previous studies (Liu, 1994; Liu and Dorn, 1996;
Liu, 2003) have established and radiometrically calibrated the varnish microstratigraphy
of late Pleistocene age for the region, providing a temporal framework for further
studying of the Holocene varnish microstratigraphy. Second, fine-grained and fastgrowing rock varnish of latest Pleistocene to late Holocene age is well developed in the
study region, facilitating generalization of high-resolution Holocene varnish
microstratigraphy. Third, the study region also hosts numerous latest Pleistocene and
Holocene geomorphic features that have been dated by radiometric methods (Table 1),
thus providing opportunities for radiometric calibration of Holocene varnish
microstratigraphy.
In order to establish a complete Holocene varnish microstratigraphy for the study
region, varnish samples were intentionally collected from well-dated geomorphic features
of latest Pleistocene age (Fig. 2; Table 1). These features include the highstand shorelines
of Summer Lake (~ 18 ka; see Fig. 3F of this study; Freidel, 1993), Bonneville Lake (~
14.4 14C ka; Cerling and Craig, 1994), Lahontan Lake (~ 13.6 14C ka; Benson et al.,
5
1995), Searles Lake (~ 14 14C ka; Benson et al., 1990), Silver Lake (10.5-15 14C ka;
Wells et al., 1987), Panamint Lake (~ 15 14C ka; Jayko et al., 2001); and the Owens River
Dry Falls in Owens Valley (~ 15.9 3He ka; Cerling, 1990). For some localities in central
Arizona (i.e., McDowell Mountain) and southern California (i.e., Blythe) where such
well-dated geomorphic features don’t exist, samples were collected from alluvial-fan
surfaces of latest Pleistocene age based on field observation and judgment (Fig. 2).
To radiometrically calibrate Holocene varnish microstratigraphy, samples were
collected from well-dated geomorphic features of Holocene age in the study region (Fig.
2; Table 3). These features include radiocarbon dated alluvial-fan surfaces in Las Vegas
Valley of southern Nevada (~ 9.9 14C ka; Bell et al, 1998a), Fish Lake Valley of southern
Nevada (0.5-6.5 14C ka; Reheis et al., 1993, 1995), Silurian Valley of southern California
(~ 1.8 14C ka; Anderson and Wells, 2003), Ajo Mountains of southern Arizona (~ 2.5 14C
ka; Pohl, 1995), cosmogenic 3He dated debris flow boulders in the Grand Canyon of
northern Arizona (~ 1.5 ka; Cerling, 1997, written comm.), cosmogenic 36Cl dated
Carrizozo lava flows (~ 5.2 ka; Dunbar, 2002), and radiocarbon dated McCartys lava
flows (~ 3.0 14C ka; Laughlin et al., 1994) in central New Mexico.
To demonstrate the potential use of varnish microstratigraphy method as a unique
geomorphic and geoarchaeological dating tool in the drylands of western USA, varnish
samples were collected from varied age surfaces of a debris-flow fan in Death Valley of
southern California; a grinding stone (i.e., metate) near Chili of northern New Mexico,
and a stone artifact (i.e., a primary core) near Ocotillo of southern California (Fig. 2;
Table 2).
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A key to this study is an innovative method of making ultra thin sections of rock
varnish (Liu, 1994; Liu and Dorn, 1996), which reduces failure rates from 80% to <5%
and permits rapid preparation and hence inter-comparison of many sections. The
conventional way of making varnish thin sections cannot be employed because the
resulting thin sections are too thick (25-30 µm) and do not reveal the microstratigraphy,
which is opaque in normal geological thin sections. Ultra-thin sections (5-10 µm) of
varnish, in contrast, allow for identification and analysis of late Pleistocene varnish
microstratigraphy (see Fig.1). Unlike pre-Holocene varnish microstratigraphy, however,
ultra-thin sections need to be made slightly thicker (10-15 µm) to see Holocene varnish
microstratigraphy. Otherwise, the entire Holocene microstratigraphy would be turned into
a single surface yellow layer in ultra-thin section (see the inset image of optical varnish
microstratigraphy in Fig. 1).
Varnish ultra-thin sections were then photographed on Kodachrome ASA 64 film
using a Leica DMLB light microscope, equipped with a Leica MPS 60 Photoautomat
system. A polarized light was utilized to increase slide contrast. The use of 10 eyepiece
in combination with 40 objective lens yields good quality of color slides with 400
magnification. Kodak Royal Gold ASA 100 color print films were used to make prints
of the color slides on a slide duplicator. Both slide and color print films were
commercially developed after exposure. Color slides and prints thus obtained provide
high-resolution (~ 1 µm) images of optical varnish microstratigraphy for layering pattern
analysis.
Varnish ultra-thin sections which contain clearly-defined microstratigraphies were
selected for further electron microprobe chemical analyses. X-ray elemental mapping and
7
line profile were obtained on a fully-automated five-spectrometer CAMECA SX100
electron probe, with the use of quantitative mode of wavelength dispersive x-ray
spectrometry (WDS). Probe mapping provides high-resolution (~ 2 µm) images of
chemical varnish microstratigraphy. Combination of both optical and chemical
microstratigraphy allows detailed detection of Mn, Ba, and other elemental fluctuations in
varnish.
In this study, we collected 138 varnish samples from 20 different localities that
cover most parts of the Great Basin and its vicinities in the western US drylands. A total
of 414 ultra-thin sections were made and 1078 spot chemical measurements were
obtained by electron microprobe along 25 individual line profiles. These chemical data,
together with 754 high-resolution (~ 1 µm) optical images of varnish microstratigraphy,
form the database used to generalize and correlate varnish layering sequences.
3. Results
3.1. Holocene varnish microstratigraphy
Our results indicate that, by carefully selecting fine-grained (< 0.1 µm in size) and
fast-growing (10-20 µm/ky) varnish and by making slightly thicker (10-15 µm) varnish
ultra-thin sections, we can detect and assess relatively weak signals of Holocene wet
phases in the western US drylands. For instance, a fast-growing varnish of Holocene age
from the ~ 16 800 yr BP (in calendar years before present; Table 1) highstand shorelines
of Searles Lake in the Mojave Desert displays eleven approximately evenly-spaced weak
black layers (Fig. 3A). Similar layering sequences were observed in varnish samples from
latest Pleistocene highstand shorelines of Summer Lake (OR), Lahontan Lake (NV), and
8
Panamint Lake (CA) and from the Owens River Dry Falls (CA) in the study region (Figs.
3B,D-F). One varnish sample from an alluvial-fan surface of latest Pleistocene age in the
McDowell Mountain of Arizona displays the most complete Holocene layering sequence
(Fig. 3C).
Based on examination of hundreds of varnish microstratigraphies from the study
region, we established a generalized Holocene layering sequence in varnish (Fig. 4). This
sequence contains twelve approximately evenly-spaced Mn- and Ba-rich weak black
layers interdigitated with thirteen Mn- and Ba-poor orange/yellow layers. Six of these
black layers, called here as WH1 to WH6 (WH stands for “Wet event in Holocene”),
occur in the upper portion of the sequence, likely indicative of the Little Ice Age and late
Holocene wet phases in the study region. Five of these black layers WH8 to WH12
appear in the lower portion of the sequence, probably indicative of early Holocene wet
phases. There is one relatively weak dark layer WH7 roughly in the middle portion of the
sequence, likely indicative of a middle Holocene weak wet phase. Other orange/yellow
layers in the sequence appear to be diagnostic of Holocene dry phases in the study region.
Table 2 presents a detailed description of characteristics of these black and orange/yellow
layers in the Holocene varnish layering sequence.
Figure 5 shows comparison between optical and chemical microstratigraphies in
varnish samples from three locales along an N-S traverse in southern California. It can be
seen that, for a given varnish microstratigraphy, chemical signals of Mn and Ba in the
Holocene portion of the layering sequence are generally much weaker than those in the
pre-Holocene sequence. Compared to the pre-Holocene black layers that contain 25-45%
MnO and 2-6% BaO, the Holocene black layers often contain only about 15-25% MnO
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and less than 1-4% BaO (Fig. 5). This observation is consistent with the fact that the
overall climate during the last glacial was much wetter than during the current interglacial
in the drylands of western USA (Broecker and Liu, 2000). Moreover, black layers WH1
to WH12 observed in the optical microstratigraphy can be clearly identified as minor Mnand Ba-rich peaks in the chemical microstratigraphy (Fig. 5).
Also, as seen in Fig. 5, there are regional variations of Mn and Ba content in the
Holocene varnish layers along the traverse. On average, the Holocene layers in varnish
from Death Valley contains <10% MnO and <0.5% BaO, while those from Blythe
contains about 10% MnO and 0.5-1.0% BaO, and those from Ocotillo contains about
>10-15% MnO and >1.0-1.5% BaO. Such variations of Mn and Ba content in the
Holocene varnish layers seem to reflect the present-day effective moisture gradient along
the traverse: from the most arid land with less summer monsoonal precipitation in Death
Valley of the Mojave Desert to a relatively less arid region with more summer monsoonal
precipitation in Ocotillo of the Yuma Desert.
Figure 6 presents chemical microstratigraphies of Ba for varnish samples of
different age and inter-sample correlation of Holocene dark layers WH1-WH12
represented by Ba peaks. We found that the chemical signals of Ba in the lower portion of
the Holocene microstratigraphy are generally stronger than those in the upper portion
(Figs. 6A,B), likely indicative of an overall wetter climate condition during the early
Holocene than during the late Holocene in the study region. We also noticed that the Ba
peaks of dark layers WH1-WH6 in varnish of late Holocene age are easily recognized and
correlated in the chemical microstratigraphy (Figs. 6C-E), and so are the Ba peaks of dark
layers WH8-WH12 in varnish of early Holocene age (Figs. 6A,B). Due to differential
10
varnish growth, however, the Ba peaks of dark layer WH1-WH6 in early Holocene
varnish are often more likely to be “squeezed up” than those in late Holocene varnish,
which may cause some ambiguity in inter-sample correlation of these dark layers (Figs.
6A,C). In most cases, the use of both optical and chemical microstratigraphies (see Fig. 5)
helps reduce such ambiguity and assures correct identification and inter-sample
correlation of the Holocene dark layers.
3.2. Climatic correlation
The Holocene wetness variations uncovered in varnish microstratigraphy of the
western US drylands appear to have a striking correlation with the Holocene climate
events in the North Atlantic region (Fig. 4). Bond et al. (1997, 1999) discovered a
pervasive millennial-scale cycle, with a cyclicity of 1470 ± 500 years, of Holocene events
and abrupt climate shifts during the last glaciation in the North Atlantic deep sea cores.
The discovery most interesting to this study is that the Holocene portion of the climate
record revealed a total of nine cooling events, with the Little Ice Age (LIA) being the
most recent cold phase in the series of millennial-scale cycles. For the sake of simplicity,
Bond et al. (1997, 1999) named these events (from oldest to youngest) as 8, 7, 6, 5, 4, 3,
2, 1 and 0 cooling events, and their reported ages are ca. 11 100, 10 300, 9400, 8100,
5900, 4100, 2800, 1400, and 650-300 yr BP, respectively (Fig. 4).
Several lines of evidence support a possible correlation of the varnish climate
record with the deep sea record. First, our previous studies in the Great Basin of western
USA have shown that the formation of pre-Holocene dark layers in varnish
microstratigraphy was contemporaneous with the abrupt climate shifts such as Heinrich
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events during the last glaciation (Liu and Dorn, 1996; Liu et al., 2000; Broecker and Liu,
2000; Liu, 2003). In the Las Vegas Valley of southern Nevada, black layer WP0 (WP
stands for “Wet event in Pleistocene”; also see Liu, 2003 for more discussion on this
topic) in the latest Pleistocene varnish microstratigraphy has proven to correlate in time
with the Younger Dryas (YD) cold snap in the North Atlantic (Liu et al., 2000). The YD
cooling was considered as one of those abrupt climate shifts that manifest the millennialscale climatic cyclicity (Bond et al., 1997, 1999).
Second, there is nearly a perfect match in the number of wet/cold phases between
the two climate records (Fig. 4). Our current study has documented a total of 12 black
layers in varnish microstratigraphy, each representing a Holocene wet phase in the
western US drylands. Four black layers of the early Holocene, namely WH12, WH11,
WH10 and WH9, presumably match four early Holocene cooling events 8, 7, 6, 5; and
four late Holocene black layers WH6, WH5, WH4, WH3 presumably match four late
Holocene cooling events 4, 3, 2, 1. Since the topmost black layer WH1 in the varnish
microstratigraphy represents the most recent wet phase during the Holocene, it most
likely match the LIA cooling. Although black layers WH8, WH7, and WH2 each have no
clear match to any of the nine cooling events reported by Bond et al. (1997, 1999), further
examination of the deep sea record suggests that these black layers may match some
minor cold peaks in the record, as indicated by a question mark (?) in Fig. 4. Specifically,
if such climatic correlation exists, the deposition of black layers WH8, WH7, WH2 in the
varnish microstratigraphy likely occurred around 7300, 6500, 1100-900 yr BP,
respectively, based on the chronology of the deep sea record (Bond et al., 1997, 1999;
Fig. 4).
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Third, there is a close tie-up in the cyclicity of the Holocene wet/cold events in
both records. Given a well-known duration of ~ 12 000 years for the Holocene, a total of
12 roughly evenly-spaced black layers in the Holocene varnish microstratigraphy
implicate, to the first degree of approximation, a 1000-year cyclicity of wet events in the
drylands of western USA. Such cyclicity overlaps, within ± 1 error, the reported
cyclicity of 1374 ± 502 years for the Holocene cooling events, or 1470 ± 500 years for
both the Holocene and the last glacial cold events in the North Atlantic region.
In summary, although still lack of a detailed radiometric chronology at this
moment, the evidence presented above seems to support a tentative correlation between
the Holocene varnish climate record in the western US drylands and the Holocene deep
sea record in the North Atlantic.
3.3. Age calibration
In order to make a firm correlation between the varnish climate record and the
deep sea record, we need to build a reliable radiometric chronology for the Holocene
varnish microstratigraphy. Unlike the pre-Holocene layering sequence that has been
radiometrically calibrated (Liu, 2003), it is extremely difficult to calibrate the Holocene
sequence for the following reasons. First, the Holocene sequence contains a total of 25
black and orange/yellow layers within a time period of ~ 12 000 years. This suggests an
average duration of nearly 500 years for each individual varnish layer and thus requires
much more accurate and precise radiometric age constraints for calibration. Secondly,
although it would be ideal to date each individual layers within varnish microstratigraphy,
such breakthrough has not yet been accomplished. So our approach to dating varnish
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layers was still indirect: examining samples from sites with radiometric age control for
the underlying landform. Such age control is maximum for the start of the layering
sequence but minimum for the age of the geomorphic feature, because the varnish started
to form only after the dated landform was created. Lastly, there are only few localities in
the western US drylands where varnished Holocene geomorphic features have been
accurately and precisely dated by radiometric means to provide unambiguous age
constraints in the study region (Table 1).
However, the results of our preliminary age calibration for the Holocene varnish
microstratigraphy are very promising (Figs. 7 and 8). Most of our calibration samples
were from radiocarbon dated Holocene alluvial-fan surfaces (500-7500 yr BP; Table 3) in
Indian Creek, Leidy Creek, and Trail Canyon of Fish Lake Valley, Nevada (Reheis et al.,
1993, 1995). Varnish from the youngest radiocarbon dated alluvial-fan surface (540 ± 30
yr BP; Table 3) in Trail Canyon displays a latest Holocene layering sequence LU-1
(WH1) (Fig. 7A), which stands for “Layering Unit 1 with a basal layer WH1” (see Liu,
2003 for terminology). If the lag time of varnish initiation on subaerially exposed rock
surfaces is assumed to be ~ 200 years (see discussion on this topic in Liu, 2003), the basal
dark layer WH1 in the varnish probably deposited around 300-400 yr BP, which is
synchronous with the LIA cold snap that culminated 300 years ago (Fig. 8). Varnish from
radiocarbon dated alluvial-fan surfaces in Leidy Creek of Fish Lake Valley (1510 ± 70 yr
BP) and Silurian Valley of the Mojave Desert (1700 ± 70 yr BP) displays two similar and
well-defined late Holocene layering sequences LU-1 (WH3) (Figs. 7D,E; Table 3). If the
varnish lag time of ~ 200 years is still assumed, the deposition of the basal dark layer
WH3 likely occurred around 1300-1500 yr BP, which largely correlates in time with the
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late Holocene cooling event 1 (ca. 1400 yr BP) in the North Atlantic region (Fig. 8).
Varnish from radiocarbon dated alluvial-fan surfaces (4280 ± 150 yr BP) in Leidy Creek
displays a late Pleistocene layering sequence LU-1 (WH5) (Fig. 7I). Given a varnish lag
time of ~ 200 years, the deposition of the basal dark layer WH5 likely occurred around
4000-4100 yr BP, which perfectly correlates in time with the late Holocene cooling event
3 (ca. 4100 yr BP) in the North Atlantic region (Fig. 8). Varnish from the Carrizozo lava
flows in central New Mexico displays a late Holocene layering sequence LU-1 (WH6)
(Fig. 7K). Dunbar (2002) reported three cosmogenic 36Cl ages for emplacement of the
flows: 4900 ± 500, 5400 ± 1000, and 5600 ± 900 yr BP, with a weighted average of 5200
± 700 yr BP. If the maximum flow age of 5600 ± 900 yr BP is assumed, the basal dark
layer WH6 in the varnish from the flows probably deposited around a time that is largely
close, within ± 1 error, to ca. 5900 yr BP as implicated by WH6 (Fig. 8). Further,
varnish from radiocarbon dated alluvial-fan surfaces (11421 ± 458 yr BP) in Las Vegas
Valley of southern Nevada (Liu et al., 2000) displays a nearly complete Holocene
layering sequence LU-1 (WH12) (Fig. 7N). Given a ~ 200 years of varnish lag time, the
basal dark layer WH12 in the varnish likely formed around 11 200-11 300 yr BP, a time
that is in accord with the reported age of ca.11 100 yr BP for the earliest Holocene
cooling event 8 in the North Atlantic (Fig. 8 ).
For dark layers WH2, WH7, and WH8 in the Holocene layering sequence, the
results of our preliminary age calibration are also very encouraging. One varnish sample
from radiocarbon dated latest Holocene alluvial-fan surfaces (1320 ± 190 yr BP) in Trail
Canyon displays a latest Holocene layering sequence LU-1 (WH2) (Fig. 7B). Given a
varnish lag time of ~ 200 years, the basal dark layer WH2 likely deposited around 110015
1200 yr BP, which is fairly consistent with the correlative age of ca. 900-1100 yr BP for
WH2 (Fig. 8). Another varnish sample from radiocarbon dated early Holocene alluvialfan surfaces (7450 ± 80 yr BP) in Indian Creek displays a clearly-defined layering
sequence LU-1 (WH8) (Fig. 7M). Given a varnish lag time of ~ 200 years, the basal dark
layer WH8 likely deposited around 7200-7300 yr BP, which perfectly matches the
correlative age of ca. 7300 yr BP for WH8 (Fig. 8). In this study, no age control was
obtained for WH7, a relatively weak dark layer at the middle point of the Holocene
layering sequence. However, one varnish sample from radiocarbon dated middle
Holocene alluvial-fan surfaces (7181 ± 120 yr BP) in Trail Canyon displays a clearlydefined layering sequence LU-1 (WH7+) (Fig. 7L); here WH7+ stands for the orange
layer directly underneath dark layer WH7 (Fig. 8). Given a varnish lag time of ~ 200
years, the basal orange layer WH7+ likely deposited around 6900-7000 yr BP, thus
predating dark layer WH7 that is presumably at ca. 6500 yr BP.
For those orange/yellow layers in the Holocene layering sequence, our preliminary
age calibration provides reasonable time constraints. Varnish from cosmogenic 3He dated
~ 1500 yr BP debris-flow boulders in the Grand Canyon of northern Arizona (Cerling,
1997, written comm.) displays a latest Holocene layering sequence LU-1 (WH2+) (Fig.
7C). Given a lag time of ~ 200 years for the varnish initiation, this layering sequence
suggests an age of 1300 yr BP for the basal orange layer WH2+, which predates dark
layer WH1 at ca. 900-1100 yr BP and postdates dark layer WH3 at ca. 1400 yr BP (Fig.
8). Varnish from radiocarbon dated alluvial terraces (2613 ± 110 yr BP) in the Ajo
Mountains of southern Arizona (Pohl, 1995) displays a late Holocene layering sequence
LU-1 (WH3+) (Fig. 7F). Given a lag time of ~ 200 years for the varnish initiation, this
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layering sequence suggests an age of 2400-2500 yr BP for the basal orange layer WH3+,
which postdates dark layer WH4 at ca. 2800 yr BP (Fig. 8). Similarly, varnish from
radiocarbon dated McCartys lava flows (3170 ± 140 yr BP) in central New Mexico
(Laughlin et al., 1994) and alluvial-fan surfaces (3630 ± 80 yr BP) in Leidy Creek of Fish
Lake Valley displays a late Holocene layering sequence LU-1 (WH4+) (Figs. 7G,H). This
suggests an age of 3000-3400 yr BP for the basal orange layer WH4+ that predates dark
layer WH4 at ca. 2800 yr BP and postdates dark layer WH5 at ca. 4100 yr BP (Fig. 8).
Furthermore, varnish from radiocarbon dated alluvial-fan surfaces in Leidy Creek (5750
± 90 yr BP) displays a well-defined late Holocene layering sequence LU-1 (WH5+) (Fig.
7J), suggesting an age of 5500-5600 yr BP for the basal orange layer WH5+ that predates
dark layer WH5 at ca. 4100 yr BP and postdates dark layer WH6 at ca. 5900 yr BP (Fig.
8).
In conclusion, although some of the twelve dark layers such as WH9, WH10, and
WH11 in the Holocene varnish layering sequence have not been calibrated due to lack of
appropriate radiocarbon age control in the study region, our preliminary age calibration
presented above indicates that there is generally a temporal correlation between the
varnish climate record and the deep sea record. In other words, dark layers WH12-WH9,
WH6-WH3, and WH1 in the Holocene varnish layering sequence most likely correlate in
time with the Holocene cooling events 8-5, 4-1, and LIA uncovered in the deep sea
record; and dark layers WH8, WH7, and WH2 likely correlate in time with minor cold
phases around ca. 7300, 6500, 900-1100 yr BP that were not qualified as the Holocene
cooling events in the deep sea record based on the criteria of Bond et al. (1997, 1999).
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3.4. Chronometric application
Once established and radiometrically calibrated, varnish microstratigraphy can be
used to estimate numerical ages for geomorphic and geoarchaeological features of interest
(Liu and Dorn, 1996; Bell et al., 1998b; Friend et al., 2000; Liu, 2003; Douglass et al.,
2005). For the purpose of age dating in this study, we tentatively assume that dark layers
WH1-WH12 in the Holocene varnish microstratigraphy deposited around 300-650, 9001100, 1400, 2800, 4100, 5900, 6500, 7300, 8100, 9400, 10 300, and 11 100 yr BP,
respectively, based on our preliminary age calibration and climatic correlation. Figure 8
gives a graphic presentation of these age assignments. We also like to point out that any
varnish-based age estimate should be interpreted as minimum surface exposure age for the
varnished geomorphic feature and only as good as its calibration could get. In the
following sections, we present several case studies to demonstrate the potential use of
varnish microstratigraphy as a unique dating tool in the western US drylands.
3.4.1. Geomorphic use
Debris-flow fan deposits, Death Valley
Debris-flow fans are common features in arid to semiarid environments. In Death
Valley of the Mojave Desert, some ten debris-flow fans of various sizes crop out within a
distance of about 1 km on the east side of the valley north of Mormon Point, as illustrated
by this most fantastic one that we studied (Fig. 9). Previous studies (Bull, 1991) classified
the deposits of this debris-flow fan into five age groups, namely Q2c, Q3a, Q3b, Q3c, and
Q4, based largely on differential development of rock varnish on the deposits (see Fig.
2.26B of Bull, 1991). Here, Q2c represents the oldest fan unit with dark varnish coverage,
18
probably formed during the terminal Pleistocene (70-12 ka); Q3a-c represents the
subsequent fan units with varying degree of orange varnish coverage, likely formed
during the early-to-late Holocene around 12-8, 8-4, 4-2 ka, respectively; and Q4
represents the modern debris-flow deposits that have no varnish coverage, likely formed
during the latest Holocene around 2-0 ka (see Table 2.13 of Bull, 1991). However, due to
the extreme difficulty in radiometric dating of debris-flow fan deposits, no specific
numerical chronology exists for the formation and evolution of this debris-flow fan.
In this study, varnish layering dating was used to provide numerical age estimates
for the debris-flow fan deposits. Based on fan morphology and varying degree of varnish
development, we identified seven debris-flow fan units (i.e., Df1 through Df7), with Df1
being the oldest and Df7 the youngest (Fig. 9). Varnish samples were collected from all
but the youngest (i.e., Df7) units for layering pattern analysis and age dating. Table 4
gives a summary of these debris-flow fan units, samples collected on each unit, oldest
varnish layering patterns observed, varnish-based age estimates, and their correlation to
Bull’s (1991) generalized alluvial-fan units (i.e., Q2c, Q3a-c, and Q4) in the Mojave
Desert. Varnish from Df1 and Df2 debris-flow fan units with the darkest/dark varnish
coverage (i.e., Q2c) displays, respectively, the oldest pre-Holocene LU-1/LU-2 (WP0)
and early Holocene LU-1 (WH12+) layering sequences, suggesting a minimum age of 12
500 yr BP for Df1 and 11 100-12 500 yr BP for Df2 (Figs. 9E,F; see Liu, 2003 for
terminology). Varnish from Df3 and Df4 debris-flow fan units with dark/orange varnish
coverage (i.e., Q3a) shows early Holocene layering sequences LU-1 (WH12) and LU-1
(WH11) (Figs. 9C,D), indicating a minimum age of 11 100 yr BP for Df3 and 10 300 yr
BP for Df4. Varnish from Df5 debris-flow fan units with light orange varnish coverage
19
(i.e., Q3b) displays an early Holocene layering sequence LU-1 (WP10) (Fig. 9B),
suggesting a minimum age of 9400 yr BP for Df5. Varnish from Df6 debris-flow fan unit
with the lightest orange varnish coverage (i.e., Q3c) shows a late Holocene layering
sequence LU-1 (WH4) (Fig. 9A), indicating a minimum age of 2800 yr BP for Df6. It is
clear from Table 4 that our preliminary varnish-based age estimates are in close
agreement with Bull’s (1991) correlative age assignments for these debris-flow fan units
and also have much better age resolution. More importantly, our dating results indicate
that debris-flow fan building events in Death Valley of the Mojave Desert are more likely
to have occurred during relatively wet periods of the Holocene, as evidenced by the basal
dark layers in varnish microstratigraphies from four (out of five) sampled Holocene
debris-flow fan units in this case study (Fig. 9).
3.4.2. Geoarchaeological use
Grinding Stone, Chili of northern New Mexico
Although varnish microstratigraphy method has been previously applied to dating
prehistoric rock arts in the Great Basin of western USA (e.g., Dorn,1992; Liu and Dorn,
1996), this dating technique was used for the first time in this study to determine the age
of historic stone artifacts in the western US deserts. One varnish sample was obtained
from a grinding stone (i.e., metate) near Chili of northern New Mexico (Figs. 2 and 10A).
The outcrop of hundreds of Anasazi pottery shards (Pueblo II-III pottery; Liu,
unpublished data) from a nearby hillslope implicates a maximum age of 700-1100 yr BP
(Lister and Lister, 1978) for the abandonment of this occupation site by the Anasazi
Indians. Varnish from the sampled grinding stone displays a latest Holocene layering
20
sequence LU-1 (WH2) (Fig.10B; also see Fig. 6E), suggesting a minimum age of 9001100 yr BP for the varnish initiation on the abandoned metate. This age estimate appears
to be consistent with the available archaeological evidence from the sampling site.
Stone Artifact, Ocotillo of southern California
In order to demonstrate the great potential of varnish microstratigraphy method in
dating old prehistoric stone artifacts in the western US deserts, one sample was also
obtained from a well-varnished flaked stone (i.e., a primary core) collected by
archaeologist Jay von Werlhof near Ocotillo, southern California (Figs. 2 and 10C). This
varnish displays a latest Pleistocene layering sequence LU-1/LU-2 (WP0) (Fig.10D; also
see Fig. 5C), indicating a minimum age of 12 500 yr BP for the making of this stone
artifact and thus suggesting at least a Paleo-Indian human occupation at Ocotillo during
the terminal Pleistocene.
4. Discussion
As illustrated above, varnish microstratigraphy method is a unique dating tool in
studying desert geomorphology and geoarchaeology in the western US drylands. It can be
very useful in dating surficial geomorphic features where other radiometric means such as
radiocarbon and cosmogenic radionuclide methods are not applicable or difficult to use.
However, like any other Quaternary dating techniques in earth sciences, it also has some
intrinsic problems that deserve further discussion.
4.1. Validity of age calibration
21
As a correlative dating tool, varnish microstratigraphy method depends largely on
radiometric age calibration of layering sequence. In this study, most of the age constraints
came from radiocarbon dates (with age uncertainty of less than 200 years), which are
accurate and precise enough to differentiate the Holocene millennial-scale wet events
recorded in rock varnish (see Figs. 7 and 8). However, there is an intrinsic uncertainty
related to our indirect way of radiometric age calibration. Because the dated charcoal or
organics are often buried 0.5-5 m underneath the sampled alluvial-fan surfaces (Table 3),
there must be a time gap between deposition of the dated materials and deposition of the
varnished alluvial boulders. In our age calibration model, this time gap plus a varnish lag
time is tentatively assumed to be about ~ 200 years (Liu, 2003). Such assumption may not
be true for buildup of large alluvial fans during the last glacial time when long-lasting (on
a time scale of ~ 103 years) and extremely wet climate was dominant in the drylands of
western USA (Bension et al., 1990; Oviatt et al., 1992). It is however probably reasonable
for buildup of small alluvial fans during the interglacial time of the Holocene when shortlived (on a time scale of ~ 102 years) and debris-flow fan building wet climate was
prevalent (Miller et al., 2001). One piece of circumstantial evidence from this study
appears to support such claim. For all of the radiocarbon dated Holocene alluvial-fan
surfaces in the study region, if we take 200 years of varnish lag time plus the depositional
time gap into consideration, the radiometric age constraints available for the basal dark
layers of Holocene varnish samples almost perfectly match the corresponding climatic
correlation-based age assignments of these dark layers; otherwise, the former would be
systematically older than the latter for about 200 years (Table 3). Therefore, this evidence
22
further indicates that our age calibration of the Holocene layering sequence is probably as
good as radiocarbon dating could get for the varnish climate record.
4.2. Validity of varnish climate record
As a correlative dating tool, varnish microstratigraphy method also depends on a
premise that climate variations are recorded in varnish. Based on our preliminary
radiometric age calibration, we found that the Holocene climate record in rock varnish is
generally valid and comparable to other climate records in the study region (Table 5). In
the Mojave Desert, for example, Enzel et al. (1989) reported three Holocene wet periods
in the lacustrine record of Silver Lake. The first wet period occurred around 10 540 ± 140
yr BP, fairly in accord with the early Holocene wet phase WH11 presumably at 10 300 yr
BP. The second one occurred around 3930 ± 100 yr BP, closely tied to the late Holocene
wet phase WH5 at 4100 yr BP. The third one occurred around 440 ± 100 yr BP, largely
overlapped the most recent wet phase WH1 around 300-650 yr BP. Miller et al. (2001)
documented a wet period in the middle Holocene at 6500 ± 500 yr BP (Infrared
Stimulated Luminescence or IRSL age) that was concurrent with paired debris-flow
building and a short-lived lake stand in the northern Silurian Valley of the Mojave Desert.
This wet period correlates favorably with the middle Holocene wet phase WH7
presumably at 6500 yr BP in the varnish record (Fig. 8). Other regional climate proxies,
such as a pollen record from San Joaquin Marsh (Davis, 1992), a flow discharge record
from Sacramento River (Meko et al., 2001; also see Yuan et al., 2004), lake sediment
records from Walker Lake (Adams, 2005), Owens and Pyramid Lakes (Benson et al.,
2002), black mat records from southern Nevada (Quade et al., 1998) and Silurian Valley
23
(Anderson and Wells, 2003), and morainal and lake sediment records of Holocene glacier
advances in the Sierra Nevada (Curry, 1969; Bowerman, 2005), provide complementary
Holocene wetness records in the western US drylands that nearly perfectly correlate in
time with the twelve Holocene wet phases in the varnish record (see Table 5 for interrecord correlation).
It is also interesting to mention that varnish microstratigraphy may have the
potential to carry a high-resolution (centennial scale) Holocene wetness record in the
western US drylands. As illustrated in Fig.11 (right panel), one fine-grained and fastgrowing varnish sample from radiocarbon dated alluvial-fan surfaces in Leidy Creek of
Fish Lake Valley (1510 ± 70 yr BP; Table 3) hosts a high-resolution optical
microstratigraphy. Here, dark layer WH1 contains three secondary dark layers (namely,
WH1a, WH1b, and WH1c) and dark layer WH2 contains two secondary dark layers
(namely, WH2a and WH2b); each of these secondary dark layers further consists of 2-4
narrowly-spaced dark bands. Given a lag time of ~ 200 years for varnish initiation on the
sampled alluvial boulder, the basal dark layer WH3 in the microstratigraphy likely
deposited around 1310 ± 70 yr BP. If these dark layers are interpreted as a climatic proxy
of wetness (Liu et al., 2000; Broecker and Liu, 2001; Liu, 2003), the varnish
microstratigraphy provides a centennial-scale wetness record of the past 1300-1400 years
for Fish Lake Valley of Nevada. A comparison of this varnish record with a nearby 2000year long Mono Lake record (Stine, 1990) suggests a fascinating correlation (Fig.11).
Specifically, three secondary dark layers WH1a, WH1b, and WH1c in the varnish record
likely correlate in time with three LIA highstands (HS) of Mono Lake (i.e., Clover Ranch
HS, Danberg Beach HS, and Rush Delta HS), one secondary dark layer WH2a with an
24
Medieval Warm Period (MWP) (ca. 800-1200 yr BP; Soon et al., 2003) highstand (i.e.,
Post Office HS), and one major dark layer WH3 with a pre-MWP highstand (i.e., Mill
Creek-East HS). The secondary dark layer WH2b in the varnish record may correlate in
time with a short-lived wet period (~ 1100 yr BP) during the MWP that has been well
documented in other regional climate records from the western US drylands (Table 5), but
such wet period is not discernible in the Mono Lake record (Fig. 11). Clearly, without
detailed radiocarbon age calibration of these dark layers, the above mentioned correlation
between the two climate records is at best conjectural. However, if such correlation
indeed exists, dark layers WH1a, WH1b, and WH1c are most likely to be deposited
respectively around ca. 300, 500, and 650 yr BP, and dark layers WH2a and WH2b
around ca. 900 and 1100 yr BP (Fig. 11; Table 5). More studies in the future are
desperately needed to ascertain this correlation.
5. Conclusion
Rock varnish microstratigraphy is a useful geomorphic and geoarchaeological
dating tool. Without age calibration, varnish microstratigraphy can stand along as a
regional correlation and mapping tool (Liu, 1994; Liu and Dorn, 1996). With radiometric
age calibration, it can provide numerical age constraints on surficial geomorphic and
geoarchaeological features in the drylands of western USA, with a possible age resolution
of 300-500 years for the Holocene and a few thousand years during the late Pleistocene
(Liu, 2003). Moreover, when detailed radiometric age calibration is accomplished for the
Holocene varnish layering sequence, the technique has the potential to yield numerical
age assignments for surface stone tools, petroglyphs, and geoglyphs that may be
25
correlated to the major cultural periods and phases of American prehistory in the western
U.S. deserts.
Acknowledgements
This research was funded by the US Department of Energy (DE-FG0295ER14572). The chemical analyses of rock varnish samples were conducted by a
CAMECA SX100 electron probe at the American Museum of Natural History in New
York City. We thank the late Gerard Bond for stimulating discussion on chronology of
Holocene cooling events; Charles Mandeville for assistance in obtaining the probe data;
George Kukla and Richard Alley for comments on the paleoclimatic aspects of this paper.
We thank Thure Cerling, Dorothy E. Freidel, and the late Jay von Werlhof for providing
varnish samples for microstratigraphic analysis. Thanks also go to Moanna St. Clair for
her decade-long support and help in managing this DOE project. Last but not least, the
first author likes to thank Ping Zhou and Marisa Liu for their assistance in numerous field
trips to the Great Basin and for their understanding of the importance of this work.
Without their continued appreciation and support, this work could have never been
completed. Lamont-Doherty Earth Observatory contribution no. xxxx.
26
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Figure Captions
Figure 1. Electron microprobe element maps (128 128 m) of rock varnish from Death
Valley, California. The inset image at lower right is an optical microstratigraphy seen
in varnish ultra-thin section under a polarized light microscope. As seen from the
probe maps, silica (Si), aluminum (Al), magnesium (Mg), and potassium (K) achieve
their highest concentrations in the yellow layer at the top of the varnish
microstratigraphy (interpreted to represent the Holocene; Liu, 2003). In contrast,
maganese (Mn), barium (Ba), calcium (Ca), and phosphorus (P) achieve their highest
concentrations in the dark layers (interpreted to represent the last glacial time; Liu,
2003). Iron (Fe) shows the smallest glacial to interglacial change in concentration.
With some exceptions, sulphur (S) is not generally associated with barium in varnish
microlaminae during both glacial and interglacial time. The line profile of barium was
taken along a vertical traverse and the probe mapped portion of the varnish
microstratigraphy is marked by a square. This varnish contains a layering sequence of
LU-1/LU-2/LU-3/LU-4 (WP6), suggesting a minimum varnish-based age estimate of
60,000 yr BP (see Liu, 2003 for more discussion on layering pattern interpretation of
rock varnish).
Figure 2. Location map of rock varnish sampling sites in the drylands of western USA.
Abbreviations of some localities are: Death Valley (DV), Owens Valley (OV),
Panamint Valley (PV), Searles Lake (SL), Silver Lake and Silurian Valley (SV).
34
Figure 3. Replicated Holocene varnish microstratigraphies in the drylands of western
USA. Varnish samples in (A), (B), (D), and (F) were from latest Pleistocene
highstand shorelines of Searles Lake, Panamint Lake, Lahontan Lake, and Summer
Lake. Varnish in (C) was from latest Pleistocene alluvial-fan surfaces in McDowell
Mountain of Arizona, and the one in (E) from the Dry Falls of Owens Valley,
California. These fast-growing and younger varnish samples were intentionally
collected from the older latest Pleistocene geomorphic surfaces for observation of a
complete Holocene varnish microstratigraphy (except for (F) that was collected by
D.E. Freidel). Age constraints are maximum-limiting ages for varnish (reported in cal
yr BP), and age sources are given in Table1. WH = Wet event in Holocene, and WP =
Wet event in Pleistocene. See Table 2 and text for discussion.
Figure 4. Rock varnish record of the Holocene wet phases (WH1-WH12) in the drylands
of western USA and its possible correlation with deep sea sediment record of the
Holocene millennial-scale cooling events (LIA and 1-8) in the subpolar North
Atlantic region. WH = Wet event in Holocene, WP = Wet event in Pleistocene, LIA =
Little Ice Age, and YD = Younger Dryas.
Figure 5. Optical (left) and chemical (right) microstratigraphies in rock varnish from
three localities along an N-S traverse in the drylands of western USA. Note that Mn
and Ba contents in the Holocene portion of each varnish microstratigraphy are much
lower than that in the terminal Pleistocene portion of the microstratigraphy. Also note
that the average Holocene Mn and Ba contents in the varnish exhibit a general trend
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along the traverse, from the lowest in Death Valley to the highest in Ocotillo, which
appears to reflect the present-day effective moisture gradient in southern California
(see text for discussion). WH1-WH12 and WP0 in the right panel identify Mn-rich
peaks on each probe line profile that correspond to the Holocene and terminal
Pleistocene dark layers seen in the optical microstratigraphies. WH = Wet event in
Holocene, WP = Wet event in Pleistocene.
Figure 6. Chemical microstratigraphies of Ba in rock varnish of different age and intersample correlation of Holocene dark layers WH1-WH12 as represented by Ba peaks.
Note that the vertical axis of varnish thickness is on the same scale for all varnish
samples except for (C) that is on a different scale, as labeled in the figure. Age
constraints for (C) and (D) are from Table 3, and those for (A), (B), and (E) are from
Figs. 9 and 10.
Figure 7. Optical microstratigraphies in rock varnish from radiometrically dated
geomorphic features in the western US drylands. Age constraints are reported in cal yr
BP (± 1) and sources of age control are given in Table 3. Letters in the varnish
images correspond to those in both Table 3 and Fig. 8. Note that black layer WH7 in
(N) is missed due probably to postdepositional modifications such as wind abrasion or
peeling (see Liu, 2003, for more discussion). The green lines in (C) and (L) identify
positions of probe line profile for Ba seen in Figs. 6C, D. WH = Wet event in
Holocene, WP = Wet event in Pleistocene.
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Figure 8. A generalized Holocene varnish layering sequence in the drylands of western
USA and its preliminary radiometric age calibration. The assumed age assignments
for dark layers in the varnish layering sequence are based on a possible correlation
between the Holocene wet events recorded in rock varnish and the Holocene
millennial-scale cooling events recorded in deep sea sediments from the subpolar
North Atlantic region (Fig. 4; Bond et al., 1999). Radiometric age constraints are from
Table 3 and the letters are keyed to those in both Fig. 7 and Table 3.
Figure 9. A debris-flow fan on the east side of Death Valley, California (upper panel;
creosote bushes of ~ 2-m high in the foreground for scale) and optical
microstratigraphies of rock varnish from the debris-flow fan deposits (lower panel).
Seven debris-flow fan units (Df1-Df7) were identified on the basis of fan morphology
and degree of varnish coverage (see Table 4). Varnish-based age estimates (reported
in cal yr BP) indicate that placement of these fan units are more likely to have
occurred during relatively wet periods of the Holocene. Note that while optically
similar due to slightly greater thickness of varnish ultra-thin sections, the Holocene
dark layers in these images often contain less barium than the pre-Holocene dark
layers, as illustrated in (C) and (F). The brown burning marks in (C) were made by
electron microprobe during line profiling.
Figure 10. Varnish layering dating of stone artifacts in the drylands of western USA.
(A) A varnished grinding stone from Chili of northern New Mexico, where numerous
Anasazi pottery shards were found. (B) Rock varnish from the grinding stone in (A)
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displays a latest Holocene layering sequence LU-1 (WH2), suggesting a varnish-based
minimum age of 900-1100 yr BP for abandonment of this occupation site by the
Anasazi Indians. (C) A varnished primary stone core from Ocotillo of southern
California (collected by Jay von Werlhof, Desert Museum of Imperial Valley
College). (D) Rock varnish from the primary core in (C) displays a latest Pleistocene
layering sequence LU-1/LU-2 (WP0), suggesting a varnish-based minimum age of 12
500 yr BP for the making of this stone artifact by the Paleo-Indians. A green line in
(B) identifies position of probe line profile for Ba seen in Fig. 6E. A camera cap in
(A) is for scale; arrows in (A) and (C) point to sites where varnish-filled microbasins
were cored with a mini-drill (4 mm in diameter) for dating.
Figure 11. A possible correlation of rock varnish climate record with Mono Lake record.
The varnish record was from Leidy Creek of Fish Lake Valley, NV, with radiometric
age control of 1310 ± 70 yr BP for the basal dark layer of WH3 (Fig. 7D; Table 3).
The Mono Lake record was from Stine (1990). Note that the LIA wet phases of
WH1a, WH1b, and WH1c recorded in varnish likely correlate in time with the Clover
Ranch HS, Danberg Beach HS, and Rush Delta HS of Mono Lake at ca. 350, 500,
650 yr BP, respectively. Similarly, wet phase WH2a likely correlates with the Post
Office HS at ca. 900 yr BP during the Medieval Warm Period, and wet phase WH3
with the Mill Creek-East HS at ca. 1375 yr BP. Wet phase WH2b may be concurrent
with the 1100-yr BP wet/cooling event in the western US drylands (see Table 5) that
is however not discernible in the Mono Lake record. HS = Highstand, LIA = Little
Ice Age, and WH = Wet event in Holocene.
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