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J Paleolimnol (2015) 53:35–45
DOI 10.1007/s10933-014-9805-3
ORIGINAL PAPER
Oxygen isotope analysis of multiple, single ostracod valves
as a proxy for combined variability in seasonal temperature
and lake water oxygen isotopes
Yama Dixit • David A. Hodell • Rajiv Sinha
Cameron A. Petrie
•
Received: 18 May 2014 / Accepted: 1 October 2014 / Published online: 11 October 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract Paleoclimate studies in lakes typically use
oxygen isotopic ratios in samples that consist of
multiple ostracod specimens, to obtain an average
d18O value that reflects the mean temperature and
d18O of lake water over the life spans of the combined
individuals measured. This approach overlooks potential information on seasonal climate variability that is
recorded in the single valves of short-lived ostracods.
Here we estimate seasonal variability in ostracod d18O
by measuring 10–30 individual carapaces of Cyprideis
torosa in selected stratigraphic levels of a sediment
core from paleolake Riwasa in Haryana, India. The
mean d18O values of ostracod populations show a
general decrease from 9.6 to 8.3 kyr BP, which was
interpreted previously as resulting from strengthening
of the Indian summer monsoon during the early
Holocene. The d18O measurements of single ostracods
within samples show a large range (up to *15 %) and
standard deviation (up to ±3.3), suggesting high
seasonal variability in the hydrology of this playa lake.
The great variability is ascribed to changes in both
seasonal temperature (16 °C) and d18O of lake water
in a drying water body. The latter is attributable to the
Rayleigh distillation process, described using a Craig–
Gordon model for isotopic fractionation during evaporation from an open water body. Our results suggest
that the range of d18O values measured in single
ostracod carapaces is useful to evaluate seasonal
changes in lake temperature and hydrology. Even with
great intra-sample d18O variability, however, the mean
d18O of multiple (more than 10) ostracods can be used
to infer long-term climate trends.
Keywords Single ostracod Paleolake Oxygen
isotopes Indian summer monsoon
Introduction
Y. Dixit (&) D. A. Hodell
Godwin Laboratory for Palaeoclimate Research,
Department of Earth Sciences, University of Cambridge,
Cambridge CB2 3EQ, UK
e-mail: [email protected]
R. Sinha
Department of Civil Engineering, Indian Institute
of Technology, Kanpur, India
C. A. Petrie
Department of Archaeology and Anthropology,
University of Cambridge, Cambridge CB2 3DZ, UK
Isotopic measurements of ostracods from lake sediment cores are used extensively for paleoclimate
reconstruction in continental settings. It is common to
measure samples containing more than ten ostracod
valves, depending on size, to obtain an average d18O
value for each stratigraphic level (Decrouy et al. 2011;
Pérez et al. 2013). Ostracod assemblages within a
single stratigraphic layer are composed of multiple
generations of shells that formed at different times,
and the d18O of multiple valves reflects the average
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conditions over which the individuals lived. Ostracods
molt and secrete new calcareous carapaces within a
short time, on the order of days to months (Horne et al.
2002). The range of isotopic values measured on
individual ostracod valves from a single stratigraphic
level can be used to estimate the seasonal range of
d18O variability (Schwalb et al. 2002; Holmes 2008;
Escobar et al. 2010).
Previous studies focused on the number of ostracod
valves per stratigraphic level needed to provide
reliable estimates of mean conditions (DeDeckker
1983; Chivas et al. 1986; Holmes 2008). A few studies
used intra-sample d18O and d13C variability for
seasonal paleolimnological reconstruction (Heaton
et al. 1995; Xia et al. 1997; Jones et al. 2002; Escobar
et al. 2010). Schwalb et al. (2002) collected valves of
dead ostracodes from ephemeral ponds and lakes in
Patagonia, Argentina and stable isotope data showed a
wide scatter, suggesting that each sample provides a
snapshot of the seasonal history of the host water.
Xia et al. (1997) studied single ostracod valve
geochemistry (d18O, Mg/Ca, and Sr/Ca) in Candona
rawsoni from lakes of the American Midwest and
suggested that 10–15 valves from each stratigraphic
level are required to remove high-frequency variability. Jones et al. (2002) studied the optimal sample size
required to obtain reliable estimates of the mean
climate signal and reported up to an 8 % range in d18O
within samples from lake sediments in Turkey.
Escobar et al. (2010) used ostracods in lacustrine
sediments from the Yucatan Peninsula to calculate
optimal sample size and assess the potential for using
intra-sample d18O variability in ostracod and gastropod shells to infer short-term climate change.
Here we present oxygen isotope measurements on
more than ten individual shells of the ostracod
Cyprideis torosa in 19 stratigraphic levels from the
early Holocene of paleolake Riwasa, located on the
semi-arid plains of Northwest (NW) India (Fig. 1).
The objective of this study was to measure d18O of
multiple, single ostracods in each stratigraphic level
and determine whether the data provide information
on seasonal climate variability. The d18O values of
individual ostracod valves within a stratigraphic
horizon showed very high variability, ranging from
4.3 to *15 %. This high within-sample variability
can be attributed to both changes in temperature and
hydrology. We used the Craig–Gordon model for
fractionation during evaporation, which is based on
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J Paleolimnol (2015) 53:35–45
the Rayleigh process of distillation, to explain the
seasonal changes in temperature and lake water d18O
in this playa. The study complements Dixit et al.
(2014b), who measured 10–20 specimens of C. torosa
in samples from a nearby well section at Riwasa to
reconstruct long-term changes in the intensity of the
Indian summer monsoon.
Study area
We collected sediment cores from paleolake Riwasa,
at the northeast edge of the Thar Desert in NW India
(Fig. 1). Paleolake Riwasa is represented by a lensoidal body of silty clay spread over more than 2,000 km2
of the Bhiwani District of Haryana. The Riwasa area is
principally composed of four geomorphic units:
granite-rhyolite inselbergs, sand sheets, sand dunes
and shallow lacustrine depressions (Saini et al. 2005)
(Fig. 1). The thickness of the lacustrine sediments
varies from 0.4 to 2.5 m (Saini et al. 2005). The lake
sediments contain abundant, well-preserved fossil
ostracod and gastropod shells.
Today, the region receives 300–500 mm of rainfall
annually, of which about 80 % falls during the summer
season (June to September) from the Bay of Bengal
arm of the southwest summer monsoon. The remaining
20 % falls during winter months (Saini et al. 2005).
Climate data from the nearest meteorological station at
Hissar, located 70 km northwest of Riwasa, indicate
mean annual precipitation is 446 mm. Mean of the
minimum monthly air temperatures over the last
50 years ranged from 17.4 °C in January to 32.8 °C
in May (Indian Meteorological Department 2000).
Riwasa was a closed-basin lake during the Holocene
that received water by direct rainfall on the lake and
runoff from the catchment. Greatest hydrologic loss
from the basin occurred by evaporation.
Materials and methods
In December 2008, two holes (25.1 and 34.4 m deep)
were drilled into the dry lakebed near Riwasa village
(N28°470 20.900 , E075°570 29.700 ) using a Calyx drilling
system. Complete recovery of the sequence was not
achieved because of the occurrence of a 12-cm-thick
hardground that was encountered at 1.67 m. This
hardground occurs at a level of 108–120 cm in a
nearby exposed well section located *60 m
J Paleolimnol (2015) 53:35–45
37
Fig. 1 a Map of northwest
India showing Riwasa
(black triangle), b Riwasa
region showing core
sampling location (star) and
well-section (circle). Also
shown in the map are the
villages of Riwasa and
Dhani Mahu (black shapes),
lacustrine deposits (blue)
and inselbergs (black with
white outlines). (Color
figure online)
northwest of the coring site (Dixit et al. 2014b). The
hardground was dated at 8.2 kyr BP, and formed when
Lake Riwasa dried completely (Dixit et al. 2014b). For
this study, we used sediment from the cored section
below the hardground at 167 cm. The sequence
between 237 and 167 cm was deposited in the early
Holocene and contains abundant, well-preserved ostracods that are suitable for isotopic analyses. We did
not measure individual ostracods below 237 cm
because of low ostracod abundance below this level.
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Between 10 and 30 well-preserved specimens of the
adult ostracod C. torosa were picked from each
stratigraphic horizon for oxygen isotopic measurement. C. torosa inhabits a wide range of aquatic
habitats with low Alk/Ca (Mezquita et al. 2005). This
species is mainly found in the meiobenthos on soft
sediments in shallow brackish environments where
large amounts of organic detritus are present. C. torosa
is euryhaline and tolerates salinities up to 60 % (Heip
1976). It requires permanent water to reproduce
because its eggs cannot withstand desiccation (Anadón et al. 1986).
Temperature plays an important role in the life
cycle of this species and the minimum temperature
required for the moulting stage is more than 15 °C for
complete development of adult C. torosa (Heip 1976).
In Belgian lakes, C. torosa has one generation
annually and moulting occurs only during the summer
season when temperatures are above 15 °C (Herman
et al. 1983; Marco-Barba et al. 2012). Because the
minimum mean monthly air temperature ranges from
17.4 to 32.8 °C, and for shallow lakes like Riwasa the
mean lake water temperature follows the ambient air
temperature closely (Haslett 2001), we assume C.
torosa moults throughout the year in this region. C.
torosa secretes its shell at near oxygen isotopic
equilibrium with lake water (Durazzi 1977; MarcoBarba et al. 2012). Males and females show no
difference in isotopic values (Marco-Barba et al.
2012).
The drill core was sampled at *4-cm intervals
between 237 and 167 cm, corresponding to
*9.6–8.3 kyr BP. Individual valves of adult specimens of the ostracod C. torosa were picked from the
sieved 425–500-lm fraction of each sample. Prior to
isotope analysis, ostracod valves were cleaned ultrasonically in methanol to remove organic material, and
rinsed in water before drying. Valves were subsequently checked for cleanliness using a binocular
microscope. Individual ostracod valves were weighed
and loaded into glass vials for oxygen isotope analysis.
The average weight of a C. torosa valve was *63 lg
and permitted isotope analysis of single valves using a
VG PRISM Mass Spectrometer and Multicarb preparation system in the Godwin Laboratory at the
University of Cambridge. Analytical precision for
d18O was estimated at ±0.1 % by repeated analysis of
Carrara Marble. Results are reported relative to
Vienna Pee Dee Belemnite (VPDB).
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J Paleolimnol (2015) 53:35–45
Table 1 Isotopic composition of oxygen and deuterium in a
few groundwater samples taken from wells near Riwasa and
adjacent areas
Location
d18O
dD
N28°45.8850 : E075°57.0240
-3.03
-28.73
N28°47.1360 : E075°57.3830
-7.76
-53.55
N28°48.0600 : E075°57.4480
-4.68
-37.23
0
-5.57
-39.9
N28°47.7840 : E075°57.8460
-4.58
-34.1
N28°48.3840 : E075°58.2920
-5.41
-48.93
0
N28°48.613 : E075°57.251
Carbonate content was measured on bulk sediments
that were dried at 60 °C and ground with a mortar and
pestle. Weight percent CaCO3 was determined by
coulometric titration (Engleman et al. 1985). Analytical precision was estimated by measurement of 94
reagent grade CaCO3 (100 %) standards that yielded a
mean and standard deviation (1r) of 99.97 ± 0.82 %.
Samples of groundwater from six wells around
Riwasa village were collected in April 2010 (Table 1).
Oxygen and hydrogen isotopes in groundwater samples were measured by cavity ringdown laser spectroscopy (CRDS) using an L1102-i Picarro water
isotope analyzer and A0211 high-precision vaporizer.
Each sample was injected nine times into the vaporizer. Memory effects from previous samples were
avoided by rejecting the first three analyses. Values for
the final six injections were averaged with in-run
precision of less than ±0.1 % for d18O and ±1 % for
dD (1 standard deviation). Calibration of results to
V-SMOW was achieved by analyzing internal standards before and after the six water samples. Internal
samples were calibrated against V-SMOW, GISP and
SLAP. All results are reported in parts per thousand
(%) relative to V-SMOW.
Results
Lithostratigraphy
The sediment core is characterized by yellow–brown,
well-sorted aeolian sand from the base up to 320 cm
(Unit I), which grades to grayish-brown shelly sand
(Unit II) from 320 to 229 cm. For this study, we used
sediments from 237 to 229 cm from Unit II, and from
229 to 167 cm in Unit III, the latter consisting of
J Paleolimnol (2015) 53:35–45
39
Fig. 2 Lithostratigraphy
and correlation of calcium
carbonate content between
the Riwasa drill core and
well section of Dixit et al.
(2014a, b). The highest
carbonate content at 167 cm
in the core correlates with
the base of the hardground in
the well section at 120 cm.
The hardground was poorly
recovered by the Calyx drill.
The mining of kankar and
backfilling of sediments led
to reworking of the top
1.5 m of sediments in the
drill cores, introducing an
offset of 47 cm between the
drill cores and well section.
The core depths were
corrected by 47 cm and the
age model of the well
section was applied
massive calcareous grayish-white lacustrine mud with
abundant gastropod and ostracod shells. Unit III in the
drill cores corresponds to the interval between 180 and
120 cm in the well section described by Dixit et al.
(2014b) (Fig. 2). The top of Unit III in the well section
is capped by a 12-cm hardground (Dixit et al. 2014b).
In the drill cores, a few calcretic remains of the
hardground were recovered at 167 cm, but the Calyx
drill did not recover this layer completely. The mining
of kankar and backfilling of sediments led to reworking of the top 1.5 m of sediments in the drill cores,
introducing an offset of 47 cm between the drill cores
and well section. Core depths were adjusted by 47 cm
by correlating the calcium carbonate content from the
well section to the drill cores (Fig. 2). The chronology
of the drill cores was established by correlation to the
well section, adopting the radiocarbon dates of Dixit
et al. (2014b) (Fig. 2).
Oxygen isotopes of ostracods
The data consist of 320 individual oxygen isotope
measurements from 19 stratigraphic horizons deposited during the early Holocene. The number of
individual ostracod (C. torosa) shell measurements
(n) from each horizon ranged from 10 to 30. The mean
d18O shows a general decrease from *?2.7 to *1.9 % up-section, between 9.6 and 8.3 kyr BP. The
d18O record of individual specimens of C. torosa
within each stratigraphic horizon shows very high
variability. At Riwasa, the average intra-sample d18O
range is 8.7 %, and varies from a minimum of 4.3 to a
maximum 14.9 % during this period, with the maximum range (-7.9 to ?7.1 %) at 9.2 kyr BP. The
intra-horizon single-shell oxygen isotope variability is
unrelated to sample size (Fig. 3). The standard deviation of d18O measurements within stratigraphic levels
is ±2.6 %, ranging from ±1.4 to 3.6 % (Table 2).
Oxygen isotopes of groundwater
Oxygen isotopic analyses of groundwater yielded a
mean value of -5.2 %. The best-fit line of d18O
versus dD for groundwater from Riwasa is
dD = 5.5 9 d18O - 12.148 (Fig. 4). The slope of
this line is significantly less than the local meteoric
water line (LMWL) calculated from the nearest
International Atomic Energy Agency (IAEA) station
at New Delhi, which is dD = 7.15 9 d18O ? 2.60
(Pang et al. 2004). The slope of Riwasa groundwater
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J Paleolimnol (2015) 53:35–45
Fig. 3 a Single ostracod
shell variability for each
stratigraphic horizon from
*9.6 to 8.2 kyr BP. Blue
diamonds represent d18O
measurements of single
ostracod valves of C. torosa,
red diamonds represent the
mean value for each sample,
and the errors bars are one
standard deviation. The
black line is a regression line
through the mean d18O
values. b The standard
deviation about the mean of
d18O measurements in each
sample. (Color figure
online)
samples is 5.5, which is indicative of evaporation
during rainfall and/or evapotranspiration from soils
during recharge (Bhattacharya et al. 1985; Darling
et al. 2006)
Lake water isotope modeling
Paleolake Riwasa was a closed basin during the
Holocene and the only significant water loss from the
lake was via evaporation. The change in oxygen
isotope ratios from a drying water body can be
described by the Craig–Gordon equation (1965),
which takes into account evaporation and the changing
isotopic composition between lake water and atmospheric water. Calculations were performed for a
temperature range of 17–33 °C. The assumption made
in the calculations is that water loss results from
evaporation and that the conditions of evaporation
(e.g. temperature, relative humidity) remained
unchanged. The evolution of oxygen isotope ratios
as a function of the fraction of residual lake water
undergoing progressive evaporation is calculated from
the following relationship (Gonfiantini 1986):
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d18 Olake ¼ ðd0 A=BÞ f B þ A=B
where d0 is the initial isotopic composition of water, f
is the fraction of residual water in the lake, A/B is the
isotopic composition that water attains in its final
evaporation stages when f approaches zero and A and
B are:
ðhda þ De þ e=aÞ
ð1 h þ DeÞ
ðh De e=aÞ
B¼
ð1 h þ DeÞ
A¼
da is the d18O of the atmosphere, which can be calculated
from the ambient temperature in Riwasa as,
d18O = 0.39T (°C)–22.8; given the average annual
temperature range at Riwasa, the calculations were
performed from 17 to 33 °C, with a step of 2 °C; aw is
the thermodynamic activity of water: aw = -0.000543/
f2 - 0.018521/f ? 0.99931; De, the kinetic enrichment
factor: De = 0.0142 (1-h/aw); a, equilibrium fractionation
factor:
a = exp
(1137/T2 - 0.4156/T
- 0.00207) (Majoube 1971); T is the temperature in
Kelvin; e is total isotopic enrichment factor: e = a - 1
J Paleolimnol (2015) 53:35–45
41
Table 2 Summary statistics for single-valve ostracod d18O
from 20 stratigraphic horizons of paleolake Riwasa
Depth (cm)
Age (kyr BP)
n
Cyprideis torosa d18O
Mean
SD
167
8.30
18
-1.9
1.9
172
8.37
18
-2.2
2.5
173
8.40
10
-1.4
1.4
177
8.44
20
-2.2
1.9
181
8.51
15
0.6
3.1
186
8.58
14
-0.6
2.7
189
8.63
10
-1.6
3.2
191
8.65
15
0.8
3.1
196
8.72
15
0.1
3.3
201
8.80
30
0.3
2.8
205
210
8.88
8.99
30
20
-0.2
1.3
2.0
2.4
215
9.09
24
1.2
2.7
220
9.20
25
1.5
3.3
221
9.24
10
0.7
2.5
225
9.31
15
1.3
2.1
229
9.42
10
0.6
2.7
234
9.52
10
0.0
2.4
237
9.59
10
2.7
1.9
The Riwasa best-fit line intersects the LMWL at
d18O = -8.9 % and dD = -60.7 %, and this value
is taken as the starting point for evaporation (Fig. 4).
Because Riwasa was predominantly a NaCl lake,
indicated by the abundance of C. torosa, which thrives
in NaCl-rich waters, and the thermodynamic activity
of NaCl solution (aw) is only valid up to 10 %
fractional residual water (Yadav 1997), calculations
for the evolution of lake-water d18O were carried out
until f = 0.2 and results are shown in Table 3.
Discussion
Single ostracod d18O as a proxy for seasonal
climate variability
Assuming the ostracod calcite is precipitated in
isotopic equilibrium with the lake water (Durazzi
1977; Marco-Barba et al. 2012), the d18O of C. torosa
reflects the temperature and d18O of lake water at the
time of moulting. C. torosa moults throughout the year
Fig. 4 dD–d18O plot of groundwater samples (black) collected
in the vicinity of paleolake Riwasa. Delhi MWL is the local
meteoric water line calculated using precipitation data from
New Delhi (Pang et al. 2004). The best-fit line (BFL) for Riwasa
samples falls below the Delhi MWL with r2 = 0.84 and is
described by dD = 5.5 9 d18O - 12.148; the slope of the line
(5.5) is typical of evaporating water bodies. The Riwasa BFL
intersects the Delhi MWL at -8.9 and -60.7 % for d18O and
dD, respectively, which equals the isotopic composition of
precipitation at Riwasa and the initial composition taken for
modeling of the lake water
in this region and therefore records the seasonal range
of both temperature and lake water d18O. The annual
temperature range is about 15 °C, which could only
account for up to 3.8 % variation in d18O, assuming C.
torosa undergoes continuous moulting. The d18O of
lake water is controlled by the amount and d18O of
rainfall and evaporation. Riwasa receives 80 % of its
total rainfall during the summer monsoon (June to
September), with a weighted mean of -10.7 %, and
the rest during winter, with a weighted mean of ?1 %.
Changes in monsoon timing and intensity affect the
lake water d18O by both altering the d18O of rainfall
and changing the hydrologic balance (evaporation/
precipitation) of the lake (Dixit et al. 2014a).
Results suggest that the d18O of lake water varies
greatly with changing lake volume and temperature.
Previously, Yadav (1997) measured the stable isotopic
composition of Sambhar Lake and adjacent ground
waters to study the evolution of the oxygen isotopic
composition of Sambhar Lake during the annual
precipitation and evaporation cycle. The study suggested that the d18O of atmospheric water vapor is the
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Table 3 Parameters used
to calculate the evolution of
d18O in drying lake Riwasa
at a range of temperature
and corresponding
atmospheric oxygen
isotopic compositions
following the Craig–Gordon
model for initial oxygen
isotopic composition (d0) of
-8.9 %, the intersection of
the evaporative and MWL
J Paleolimnol (2015) 53:35–45
Temperature (°C)
d18Oa
Lake water d18O at fraction residual lake water
1
0.8
0.6
0.4
0.2
17
-16.2
-8.9
-6.1
-3.8
2.5
21.2
19
-15.4
-8.9
-7.1
-4.3
1.4
18.1
21
23
-14.6
-13.8
-8.9
-8.9
-7.3
-7.5
-4.8
-5.3
0.2
-0.9
14.9
11.9
25
-13.1
-8.9
-7.7
-5.9
-2.2
8.7
27
-12.3
-8.9
-7.9
-6.4
-3.4
5.6
29
-11.5
-8.9
-8.1
-6.9
-4.5
2.5
31
-10.7
-8.9
-8.3
-7.5
-5.7
-0.6
33
-9.9
-8.9
-8.5
-7.9
-6.9
-3.8
Fig. 5 a Modeled changes in the d18O of lake water with
evaporation and temperature using the Craig–Gordon (1965)
equation and assuming an initial lake water d18O = -8.9 %.
b Modeled d18O of ostracod calcite as a function of both
temperature and changing lake water d18O with evaporation
most sensitive variable affecting the evolution of lake
water d18O and the back-condensation of isotopically
light atmospheric water vapor dominantly controls the
evolution of lake water d18O during the annual
evaporation cycle (Yadav 1997). For the temperature
range of 17–33 °C in the Riwasa region, the evolution
of d18O of lake water is shown in Fig. 5a. As the d18O
of ostracod calcite depends on both the temperature
and the isotopic composition of water in which the
shell was formed, the following relationship can be
used to calculate the d18O of ostracod calcite (Shackleton 1974):
dw, obtained from the Craig–Gordon equation and
various temperatures within the seasonal temperature
range 17–33 °C at a step of 2 °C (Fig. 5b). The
calculated d18O values of calcite encompass the range
of observed d18O variability of up to *15 % in a
single stratigraphic level. We conclude that the high
intra-sample d18O variability observed for paleolake
Riwasa is explained by its high surface to volume ratio
and the consequent large seasonal changes in temperature and evaporation/precipitation under a monsoon
climate.
T ¼ 16:9 4:38 ðdC dW Þ þ 0:1 ðdC dW Þ2
where dc is the oxygen isotopic value of shell calcite
relative to PDB and dw is the oxygen isotopic value of
the lake water relative to SMOW. We solved the
equation for dc, using the lake water isotopic values,
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Long-term changes in mean d18O of ostracods
Despite the great inter-shell d18O variability within a
sample, the mean d18O of all ostracod valves in a
sample shows a general decreasing trend from 9.6 to
8.3 kyr BP (Fig. 3). The regression line through the
J Paleolimnol (2015) 53:35–45
Fig. 6 The ostracod d18O variability and regression lines of
drill cores (red diamonds) and Riwasa well section (purple
circles) between *9.6 and 8.3 kyr BP. The slopes of both the
lines show a decrease in mean d18O during the early Holocene.
(Color figure online)
mean d18O has a slope of *?2.3 % per kyr. A similar
trend occurs in the well section, with a slope of
*?2.7 % per kyr, which was interpreted as resulting
from increased summer monsoon rainfall and
decreased E/P in the region (Fig. 6) (Dixit et al.
2014b). Intensification of the summer monsoon during
the early Holocene is also recorded in marine records
from the Arabian Sea (Overpeck et al. 1996; Schulz
et al. 1998; Anand et al. 2008), speleothems from Qunf
cave in Oman (Fleitmann et al. 2003), and speleothems and lake sediments from China (Hodell et al.
1999; Wang et al. 2005; Zhang et al. 2011; Cai et al.
2012).
Our results suggest that the mean d18O of [10
single ostracod valves from each stratigraphic level
records the mean climate state and provides a reliable
record of long-term climate variability. We used the
student t test to estimate the optimum sample size,
which is the minimum number of individual valves
that achieves acceptable error for each stratigraphic
level:
n ¼ ðt s=EÞ2
where t is the student-t value for a given level of
significance, s is the standard deviation, and E is the
acceptable error (Holmes 2008). We assume 10 %
acceptable error based on the observed d18O change
over the entire stratigraphic sequence, which is 4.6 %.
We estimate 10 valves as a minimum sample size to
achieve a 10 % error, which is consistent with
previous studies (Heaton et al. 1995; Xia et al. 1997;
Holmes 2008; Escobar et al. 2010).
43
To test whether the long-term decrease in d18O in
the Riwasa well section could occur merely by chance,
we tested if sample means are statistically different
from one another, given the observed d18O variability
of single valve measurements. We used the student-t
test to construct a 19 by 19 matrix of p-values of
significance for each set of pairs in the sample
population. The mean of the oldest sample at 9.6 kyr
BP is significantly different from sample means from
8.8 to 8.3 kyr BP. Similarly, the sample mean of the
youngest sample at 8.3 kyr BP is significantly different from the sample means at 9.6 to 8.65 kyr BP. We
further tested if the slope of the regression line through
the mean d18O is significantly different from zero. The
small p value (1.593 9 10-5) indicates a high probability that the slope is significantly different from
zero.
Holmes (2008) suggested that the ultimate test of
whether low-frequency trends are significant is to
replicate results in a parallel stratigraphic section. We
replicated the results of this study, in the well section
and drill cores. Despite the great intra-sample d18O
variability, the mean d18O of more than ten ostracod
valves reflects long-term trends in the mean hydrologic conditions of the lake (Fig. 6).
Summary and conclusions
We measured d18O of single ostracod valves from
selected stratigraphic levels in the early Holocene
sequence of paleolake Riwasa. Our results indicate
high d18O variability of single ostracods in each
sample, with a range of values from 4.3 to 14.9 %.
This high variability is superimposed on a longer-term
trend of decreasing mean d18O values between 9.6 and
8.3 kyr BP. The great within-sample d18O range can
be explained by seasonal changes in temperature and
lake water d18O in this playa lake. Despite this high
intra-sample variability, a long-term decrease in mean
d18O was discerned by measuring more than ten
ostracod valves per sample. This trend is related to
intensification of the Indian monsoon in the early
Holocene (Dixit et al. 2014b).
Acknowledgments This work was supported by the Natural
Environment Research Council (NE/H011463/1). Yama Dixit
was funded by the Gates Cambridge Trust and Learning and
Research Funds from St. John’s College, Cambridge. We thank
Mike Hall, James Rolfe and Jeannie Booth for analytical
123
44
assistance. Many thanks to Prof. R. N. Singh, (BHU), Vikas
Pawar and Sandeep Mallik for logistical field support. Ajit Singh
helped with sediment core sampling. Thanks also to Thomas
Guilderson for AMS radiocarbon dating at the Center for
Accelerator Mass Spectrometry (CAMS), Lawrence Livermore
National Laboratory (California, USA) and Ayan Bhowmik for
helpful discussions.
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