Earth and Planetary Science Letters 391 (2014) 171–182
Contents lists available at ScienceDirect
Earth and Planetary Science Letters
www.elsevier.com/locate/epsl
Prolonged monsoon droughts and links to Indo-Pacific warm pool:
A Holocene record from Lonar Lake, central India
Sushma Prasad a,c,∗ , A. Anoop a , N. Riedel b , S. Sarkar c , P. Menzel d , N. Basavaiah e ,
R. Krishnan f , D. Fuller g , B. Plessen a , B. Gaye d , U. Röhl h , H. Wilkes a , D. Sachse c ,
R. Sawant i , M.G. Wiesner d , M. Stebich b
a
Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, D-14473 Potsdam, Germany
Senckenberg Research Institute, Research Station of Quaternary Palaeontology, Am Jakobskirchhof 4, D-99423 Weimar, Germany
c
Institute for Earth and Environmental Science, University of Potsdam, Karl-Liebknecht Straße 24-25, 14476 Potsdam, Germany
d
Universität Hamburg, Institute of Biogeochemistry and Marine Chemistry, Hamburg, Germany
e
Indian Institute of Geomagnetism, New Panvel, Navi Mumbai, India
f
Indian Institute of Tropical Meteorology, Pune, India
g
Institute of Archaeology, University College London, London, UK
h
MARUM – Center for Marine Environmental Sciences, University of Bremen, 28359 Bremen, Germany
i
Deccan College, Post-Graduate and Research Institute, Pune-411006, India
b
a r t i c l e
i n f o
Article history:
Received 15 August 2013
Received in revised form 24 January 2014
Accepted 26 January 2014
Available online xxxx
Editor: G. Henderson
Keywords:
Indian summer monsoon
ENSO
prolonged droughts
Holocene
Lonar Lake
a b s t r a c t
Concerns about the regional impact of global climate change in a warming scenario have highlighted
the gaps in our understanding of the Indian Summer Monsoon (ISM, also referred to as the Indian
Ocean summer monsoon) and the absence of long term palaeoclimate data from the central Indian core
monsoon zone (CMZ). Here we present the first high resolution, well-dated, multiproxy reconstruction of
Holocene palaeoclimate from a 10 m long sediment core raised from the Lonar Lake in central India. We
show that while the early Holocene onset of intensified monsoon in the CMZ is similar to that reported
from other ISM records, the Lonar data shows two prolonged droughts (PD, multidecadal to centennial
periods of weaker monsoon) between 4.6–3.9 and 2–0.6 cal ka. A comparison of our record with available
data from other ISM influenced sites shows that the impact of these PD was observed in varying degrees
throughout the ISM realm and coincides with intervals of higher solar irradiance. We demonstrate that
(i) the regional warming in the Indo-Pacific Warm Pool (IPWP) plays an important role in causing ISM
PD through changes in meridional overturning circulation and position of the anomalous Walker cell;
(ii) the long term influence of conditions like El Niño-Southern Oscillation (ENSO) on the ISM began only
ca. 2 cal ka BP and is coincident with the warming of the southern IPWP; (iii) the first settlements in
central India coincided with the onset of the first PD and agricultural populations flourished between the
two PD, highlighting the significance of natural climate variability and PD as major environmental factors
affecting human settlements.
2014 Elsevier B.V. All rights reserved.
1. Introduction
The ISM is a major component of the Asian monsoon system (Wang et al., 2005). While considerable data is available on
Holocene East Asian monsoon variability (e.g., Yancheva et al.,
2007; Dong et al., 2010; Wohlfarth et al., 2012; Chawchai et
al., 2013), not much is known about the ISM that affects climate throughout south Asia, and as such the livelihood of more
than a billion people in largely agriculture dependent societies
(Krishna Kumar et al., 2011) over a variety of timescales. The
*
Corresponding author at: Helmholtz Centre Potsdam, GFZ German Research
Centre for Geosciences, D-14473 Potsdam, Germany.
E-mail address: sprasad@gmx.de (S. Prasad).
0012-821X/$ – see front matter 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.epsl.2014.01.043
devastating socio-economic impacts of droughts (a deficit of 10%
below the long term mean rainfall) in the ISM realm are well documented (Mooley and Parthasarathy, 1982; Sinha et al., 2011), yet
little is known about the dynamical processes underlying these
natural catastrophes. ENSO is commonly linked to droughts in the
ISM realm – the weakening of this relationship in recent decades
has implications on the predictability of the ISM (e.g., Krishna Kumar et al., 1999). While indirect indicators of “ENSO-like” (ENSO-l)
activity during the Holocene over multidecadal to centennial scales
are available (Moy et al., 2002; Rein et al., 2005), testing the long
term stability of the link between ENSO-l conditions with the ISM
has not been possible as long term palaeoclimate reconstructions
exist mostly from the peripheral ISM regions (Enzel et al., 1999;
Staubwasser et al., 2003; Berkelhammer et al., 2012). There are no
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S. Prasad et al. / Earth and Planetary Science Letters 391 (2014) 171–182
Fig. 1. (a) Location of sites discussed in the text-1: δ 18 O from Oman Stalagmite (Fleitmann et al., 2003); 2: Marine core with catchment in Godavari delta (Ponton et al.,
2012); 3: Lonar Lake (our study); 4: δ 18 O from Mawmluh cave stalagmite (Berkelhammer et al., 2012); 5: Hyongong peat (Hong et al., 2003). (b) Geology of the Lonar Lake
(after Maloof et al., 2009) and position of the core.
continuous well-dated Holocene palaeoclimate records from central India, the core of the monsoon zone (CMZ) (Gadgil, 2003).
Here we present the first well-dated Holocene palaeoclimate
reconstruction from the CMZ of central India. Our archive, Lonar
Lake, is the only long term “rain gauge” for central India (Fig. 1a)
and its sedimentary record is used here to reconstruct the longterm regional palaeomonsoon variability in high-resolution. This
lake is the only natural lake in central India as most of this region
is covered by the Deccan basalts and lacks natural lakes, with the
exception of Lonar crater that was formed by a meteorite impact
ca. 570 ka (Jourdan et al., 2011) on the ∼65 Ma old (Fredriksson
et al., 1973; Milton et al., 1975) basalt flows of the Deccan Traps.
Currently, this crater contains a shallow (6 m deep, 1.2 km diameter) saline lake. We also compare the Lonar record with the
reconstructed ENSO-l, and sea surface temperature (SST) changes
in the IPWP to examine the forcing mechanisms behind extended
droughts in the ISM realm.
The Lonar Lake is a closed (endorrheic), hyposaline, and alkaline lake (Jhingran and Rao, 1958; Joshi et al., 2008). The lake is
stratified with an anoxic bottom layer below 4 m water (AD 2011)
depth with pH values between 9.5 and 10.4. The water level in
Lonar Lake fluctuates in response to ISM precipitation with higher
lake level during stronger monsoon years with input from both the
surface runoff and groundwater (Komatsu et al., in press). During
the dry season, the evaporation from the lake exceeds the input resulting in the significant drop of the lake level (Komatsu et al., in
press). The δ 18 O and δ D of the inflowing streams range from −2.1
to −3.1❤ and −15.4 to −21.4❤ respectively. However, the lake
waters showed relatively enriched values for δ 18 O and δ D fluctuating between +4.2 to +5.5❤ and +14.7 to +21❤ respectively.
Similarly, δ 13 CDIC was enriched (+11 to +14.8❤) in lake waters
and relatively depleted (between −9.9 and −12.6❤) in the inflowing streams indicative of biological productivity and evaporative
enrichment in lake waters (Anoop et al., 2013).
1.1. Study area
1.1.3. Modern vegetation
Lonar crater is located within the Southern Tropical Dry Deciduous Forest biome sensu Champion and Seth (1968). Our investigations indicate that the landscape outside the crater is
heavily influenced through grazing, fuel cutting and agriculture,
thus only shrubby thorn-vegetation occurs with Acacia spp., Annona squamosa, Senna auriculata, and Ziziphus mauritiana. Forestvegetation inside Lonar crater is roughly divided into three zones,
which form concentric belts. The crater slope between ca. 500 and
560 m asl is vegetated with tropical dry deciduous forest vegetation comprising teak (Tectona grandis), Azadirachta indica, Cassia
fistula, Wrightia tinctoria, and Butea monosperma. Vegetation on the
crater ground is dominated by Prosopis juliflora, with an admixture
of Alangium salviifolium, Ficus benghalesis, Trewia nudiflora, Ailanthus excelsa, etc. The lake shore is solely vegetated with Prosopis
juliflora. The north–eastern part of the crater ground is used for
agricultural purposes. Except in the dense and shady forest vegetation on the crater ground, the open character of vegetation on the
crater slopes and outside Lonar crater promotes a rich understory
dominated by Poaceae and various herbs of the Fabaceae, Asteraceae, and Acanthaceae families. The mouth of the Dhara rivulet
features extensive stands of swamp vegetation comprising Cyperaceae and Typha sp.
1.1.1. Geology
Lonar crater is a near-circular depression (Fig. 1b) with a rimto-rim diameter of 1.8 km and an average depth of around 135 m
from the rim crest to the lake level. The lake is fed by surface
runoff from ISM precipitation and three perennial streams (Dhara,
Sitanahani and Ramgaya). Since the lake has no outlet, the water
level is presently controlled by the balance between evapotranspiration and precipitation, plus discharge from the groundwater fed
springs. Tritium dating (Anoop et al., 2013) indicated a modern to
sub-modern age for the groundwater. This is consistent with the
local observations that a succession of below average rainfall results in partial or, as in AD 1982 (single year), a complete drying
of the lake.
1.1.2. Modern climate, hydrology, and hydrochemistry
The modern-day rainfall in the Lonar region is mostly provided
by the Arabian Sea branch of the southwest monsoon (Sengupta
and Sarkar, 2006). Temperatures during the pre-monsoon period are around 31 ◦ C, but may increase up to 45 ◦ C; southwestmonsoon and post-monsoon temperatures average 27 ◦ C and 23 ◦ C
respectively. The average summer monsoon rainfall is ca. 680 mm.
S. Prasad et al. / Earth and Planetary Science Letters 391 (2014) 171–182
173
Fig. 2. (a) Lonar litholog. Two prominent evaporite gaylussite horizons indicate drier conditions. Symbols adjacent to the lithology indicate the position of dated samples: blue
circles: terrestrial fragments, yellow circles: bulk organic matter, and brown circles: dated gaylussite. (b) Age model of composite Lonar profile, derived from the P_Sequence
depositional model implemented in OxCal 4.1 (Bronk Ramsey, 1999, 2008). The coloured shading represents the 2σ probability range. Individual AMS 14 C dates obtained
from bulk organic matter, terrestrial fragments, and gaylussite crystal are displayed as calibrated 2σ probability functions. (c) Correlation using marker layers. (d) Evaporitic
gaylussite crystal. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
2. Sample collection and methodology
2.1. Coring, documentation and correlation
Two long cores, with an offset of 50 cm were raised from the
Lonar Lake in May–June AD 2008 using a floating platform and a
UWITEC piston corer. Samples were collected in 1–2 m long plastic
liners with an inner diameter of 9 cm. The cores were opened and
the lithology documented in detail in the laboratory. The correlation between cores was achieved using a combination of at least
two of the three parameters: marker layers, magnetic susceptibility, and XRF scanner data. The overlap between cores ranged from
a few centimeters to tens of centimeters. A continuous composite core of 10.04 m is now available from the Lonar Lake (Fig. 2a)
and has been subdivided into 14 lithostratigraphic units (Table 1
in Supplementary Online Material (SOM)-1).
2.2. Thin section preparation
A continuous set of overlapping petrographic thin sections
(10 cm long) from selected core sections was prepared for microfacies analyses from the composite part following the method described by Brauer and Casanova (2001). Thin-section images were
obtained with a digital camera (Carl Zeiss Axiocam) and the software Carl Zeiss Axiovision 2.0.
2.3. Chronology
We have collected terrestrial wood samples throughout the core
to obtain a radiocarbon chronology (Fig. 2b, Table 2 in SOM-1).
We have also selectively dated the carbonate crystals, bulk organic matter, and leaf fragments along with the wood fragments
to ascertain the possibility of errors arising from reworked samples and/or any “hard water effect”.
For age modelling, we have used the P_Sequence method from
OxCal (e.g., Bronk Ramsey 1999, 2008; Blockley et al., 2008). This
method recognises that many processes are in fact a series of
events and can be modelled by using representative information
on the relationships between individual events. This model requires the estimation of the factor (k) which is the relationship
between the events and the overall stratigraphical process – a high
value for k would rigidly constrain the data and would be suitable for very simple sedimentary processes with little change in
the sedimentation rate, whereas a low k value would be the opposite.
2.4. Isotope analyses
The stable isotope compositions of bulk (gaylussite was removed by handpicking under the microscope) carbonates (δ 13 Ccarb
and δ 18 Ocarb ) were determined using a Finnigan GasBenchII with
carbonate option coupled to a DELTAplusXL mass spectrometer, following the analytical procedure described in Spötl and Vennemann
(2003). The TC, TN, and TOC contents and the δ 15 N and δ 13 Corg
isotopic compositions were determined using an elemental analyser (NC2500 Carlo Erba) coupled with a ConFlowIII interface on a
DELTAplusXL mass spectrometer (Thermo Fischer Scientific). Subsamples (0.5 cm thickness) were collected for isotope (organic and
carbonate) analyses representing a time resolution ranging from
<10 yr between 0 and 5.5 cal ka to multidecadal (ca. 10–80 yr) in
older sediments.
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S. Prasad et al. / Earth and Planetary Science Letters 391 (2014) 171–182
2.5. XRF core scanning
Continuous down-core X-ray fluorescence (XRF) scanning of the
sediment core surface was performed using the Avaatech XRF Core
Scanner III. The tube voltage was operated at 10 kV, with 5 mm
resolution, and the X-PIPS SXP5C-200-1500 detector from Canberra
to allow obtaining the elemental abundances of major elements
(Al, Si, Ca, K, Fe, Ti and Mn). The data was normalised using the
calculated sedimentation rate.
2.6. Lipid biomarker and stable isotope (δ 13 C) analysis
The sediment core was sub-sampled (70 samples) for extraction of lipid biomarkers. Subsamples (2 cm thickness) represented
a time resolution ranging from 8–20 yr until 5.1 cal ka to 60–120 yr
in older sediments. Ca. 2–4 g of samples, freeze-dried and homogenised, were extracted using an accelerated solvent extractor
(Dionex ASE 350) with a 9:1 mixture of dichloromethane and
methanol. After removal of elemental sulphur (passing through
activated copper pipette columns) and addition of internal standards the total lipid extracts (TLEs) were separated on SPE silica
gel columns into fractions of different polarity. Fractions (containing alcohols) were measured on an Agilent GC-FID/MSD system for
compound identification (comparison of mass spectra with published data) and quantification.
The stable isotope (δ 13 C) composition of tetrahymanol was
measured on a GC system (Agilent 6890N) coupled via combustion interface (GC-C III), to a Thermo Fischer Scientific (model 253)
isotope ratio mass spectrometer (GC-IRM-MS). Calibration of isotope values was performed by injecting several pulses of CO2 at
the beginning and at the end of each GC run and by measurement
of certified standards between sample runs. GC-IRM-MS analyses
were run in triplicate with standard deviations better than 0.5❤.
Isotopic ratios are expressed as δ 13 C values in per mil relative to
the Vienna Pee Dee Belemnite (V-PDB) standard.
2.7. Pollen analysis
Pollen assemblages were studied at a resolution of 1 cm between 878 and 870 cm composite core depth, 2 cm between
870 and 794 cm and 10 cm between 790 cm and the composite
core top. In total, 120 sediment samples were analysed, representing a time resolution between 150 and 40 yr. Samples were
treated with HF and hot acetolysis mixture and sieved on 200 µm
and 5 µm following the standard guidelines of Faegri and Iversen
(1989). All samples were spiked with Lycopodium marker grains
(Stockmarr, 1971). At least 600 pollen grains were counted per
sample. For identification of the pollen types, the reference collection of the Senckenberg Research Station Weimar, the pollen
atlas for Maharashtra (Nayar, 1990), and a web-based pollen atlas for the Australasian realm (APSA Members, 2007) were consulted.
511–612 cm lie on the trend formed by the dates on terrestrial
fragments. The magnitude of the “ageing” of bulk organics is not
constant throughout the profile and ranges from ca. 450 to 1500 yr.
The apparent “older” ages for the bulk sediments could be caused
by several factors, potentially the most important being hard water
effect (Fontes et al., 1996; Björck and Wohlfarth, 2001). However,
the absence of carbonate outcrops in the region eliminates the
overestimation of bulk 14 C due to hard water effect. Because the
Lonar Lake shows high salinity, stratification, and high pH, the apparent “ageing” could be most likely caused by lack of equilibration
with the atmosphere. The coincidences of some of the older ages
with zones of gaylussite crystals (high pH, stratification, and salinity) also support such a scenario.
We have excluded the dates that are obvious outliers (see Table 2 in SOM-1). For the remaining dates the calibration was
done using the OxCal programme (Bronk Ramsey, 2008) using the
INTCAL04 and NH3 curves. In the P_sequence method, we have
used three boundaries based on lithological changes and used a
k value of 0.1. The agreement index (AI) for single ages, a measure of the fit between the data and the model should be above
the threshold of 60 (Bronk Ramsey, 2008). The overall AI of 115.8
testifies to the suitability of this approach. The clay in the lower
section (904–980 cm) is similar in appearance and texture to the
clay found in the upper core, hence age extrapolation for this part
was done using the sedimentation rate for the uppermost section
(0–446 cm).
3.2. Lithology and evaporitic mineralogy
The lithology of the Lonar core is shown in Fig. 2a. Thin section
investigations reveal that the laminated calcareous clay (11.15–9
cal ka; Fig. 2c) comprises of seasonality controlled sub-laminations:
clay (monsoon rainfall erosion) with intercalations of organic (productivity) and calcareous (summer evaporation) layers, indicating
these laminae to be varves. Several terrestrial wood fragments and
one leaf were also found in this horizon indicating strong surficial
inflow into the lake. Calcareous silty clay with evaporitic gaylussite
crystals (Fig. 2), indicate highly saline conditions in two zones: mid
(4.6–3.9 cal ka) and upper (2–0.6 cal ka) parts of the core (Anoop
et al., 2013). Although no modern analogues are available, we
note that between 2002 and 2011 AD the average summer rainfall was ca. 20% below the long term mean (Indian Meteorological
Department). However, there was no gaylussite formation clearly
indicating that a substantially longer interval of reduced summer
monsoon is needed for the formation of these evaporites. Most of
the crystals in the core appear parallel to the original sediment
laminations and range in length from sub-mm to 8 mm. With a
few exceptions, the crystals are devoid of any sediment inclusions
(Fig. 2d). Their evaporative origin is also confirmed by isotope analyses (Anoop et al., 2013).
3.3. Geochemistry, isotope, and biomarker studies
3. Results
3.1. Chronology
The core chronology is based on AMS dating (Fig. 2b and
Table 2 in SOM-1) of wood, leaf, gaylussite, and bulk material
from the sediments. The radiocarbon dates on terrestrial fragments
(wood, twigs and leaf) and a gaylussite crystal (see Section 3.2)
are in stratigraphic order. Paired radiocarbon dates obtained on
bulk sediments and terrestrial fragments at the same depth show
similar ages (within errors) except for the single paired sample at
383.5 cm where the date on bulk material is significantly older
(Table 2 in SOM-1). Only two bulk organic matter dates between
XRF data (see SOM-2 for data) show that the elements K, Ti,
Fe and Si are significantly correlated with Al (r = 0.89, 0.81, 0.63,
0.93) for the whole core. Hence, Al was chosen as a representative for these elements. Our investigations of modern catchment
and lake surface sediments (Basavaiah et al., in press) show that
Al is contributed by catchment erosion and its distribution parallels the lithogenic content in surface sediments with higher values near the shoreline. Hence, Al can be used as an indicator of
detrital input into the lake and proximity to the shoreline. The
detrital input is highest between 11.4–11.15 cal ka and remains
consistently low until 5.2 cal ka when higher detrital inflow is seen
(Fig. 3).
S. Prasad et al. / Earth and Planetary Science Letters 391 (2014) 171–182
175
Fig. 3. Results of multiproxy investigations on the 10 m long Lonar core. In the first column (dotted grey box) are shown the prominent chalcolithic cultural periods (Kayatha,
Savalda, Malwa and Jorwe) (Misra, 2001) detected in this region. Arrow with (∗ ) indicates adoption of low rainfall crop patterns, and arrow with (∗∗ ) indicates decadal scale
drought induced famines. The Al (total counts) have been normalised with respect to sedimentation rate.
The Corg /N values of bulk organic matter remain consistently
high (>25) except for short intervals (11.4–11.2; 4.4–4.2, and from
1.4 cal ka to present) when they fall to <15 (Fig. 3).
The δ 13 Corg and δ 15 N values vary between −22❤ and −6❤
and between +4❤ and +15❤ respectively. The variability is low
between ca. 11 and 6.2 cal ka when a shift towards enriched values, most prominently between 6.2 and 5.3 cal ka is seen. Subsequently, a depletion is observed but the values remain higher than
during the early Holocene. The δ 13 Ccarb and δ 18 Ocarb values (after
removal of gaylussite) are high (δ 13 Ccarb = 6–20❤ and δ 18 Ocarb =
ca. 3–9❤) during the 11.2–5.8 cal ka. δ 13 Ccarb and δ 18 Ocarb co-vary
(r = 0.78) for the whole core indicating their evaporative origin
(Fig. 3).
The bacterial/ciliate community biomarker tetrahymanol makes
its first appearance at ca. 5.2 cal ka in exceptional concentrations
and persists until 3.9 cal ka and in lower abundance subsequently.
An unusual enrichment in δ 13 C (ca. 5.1–4 cal ka) is observed for
tetrahymanol (−17.2❤ to −7.2❤) (Fig. 3).
3.4. Pollen
Poaceae contribute between 63.5 and 96.5% to the terrestrial
pollen assemblage, while other herbs (e.g., Amaranthaceae and
Asteraceae) account for 0.6 to 14.2% of the terrestrial pollen spectrum. Arboreal pollen contribute between 1.2 and 32.6% to the
total terrestrial pollen assemblage. Pollen of woody plants representing moist forest communities appear with up to 7.7% between
8.9 and 5 cal ka (Fig. 4). Pollen of dry deciduous forest trees prevail steadily throughout the core, highest values of up to 30% are
found between 8.6 and 7.1 cal ka, 5.6 and 4.1 cal ka and again at
3.0 cal ka. Pollen percentages of xeric thorn shrub elements occur from 5 cal ka BP onwards, with maxima of 6.3% between 3.8
and 2.8 cal ka. Pollen of Ailanthus excelsa, a tree species considered
as disturbance indicator, appear between 4.6 and 3.0 cal ka, and
around 1.5 cal ka, with up to 10%.
4. Discussion
In our palaeoclimate reconstruction from a 10 m long sediment
core, we have used clastic influx, isotopic data from bulk organic
matter, pollen, biomarker flux and compound specific isotope data,
together with published data (Anoop et al., 2013) on evaporite
mineral assemblages and their respective isotopic composition, to
reconstruct past hydrological changes in the CMZ.
4.1. Environmental sensitivity of Lonar core proxies
The Corg /N ratio of sediments is used as an organic matter
source indicator. Based on the analysis of modern Lonar Lake sediments, it was inferred that a Corg /N ratio 25 represents a major terrestrial contribution and a Corg /N ratio below 25 a mixed
plankton and terrestrial signal (Menzel et al., 2013). Changes in
phytoplankton biological productivity are reconstructed here from
variations in carbon and nitrogen isotopes of bulk organic matter.
δ 13 C values of terrestrial organic material are mainly determined
by the contribution of plants using the C3 (wetter conditions;
δ 13 C = −25 to −30❤) or the C4 (drier; δ 13 C = −10 to −15❤)
pathway for CO2 uptake (Meyers, 1994). The δ 13 C record of the
phytoplankton component is governed by changes in organic productivity, salinity, and pH of the water (Stuiver, 1975). Photosynthetic organic productivity preferentially uses 12 C and 14 N, leaving the DIC and DIN pools enriched in 13 C and 15 N, respectively
(Swart, 1983; Talbot and Laerdal, 2000). Under CO2 deficient conditions (<0.01 mol/l), which are promoted in highly alkaline water
(Schelske and Hodell, 1991; Xu et al., 2006), phytoplankton may
be forced to change from CO2 to HCO−
3 based metabolism, which
produces relatively 13 C enriched organic matter (Talbot, 1990;
Leng and Marshall, 2004). Additionally, high evaporation during
drier (saline) conditions can cause supersaturation of carbonate in
the lake water, and consequently 12 CO2 degassing (Lei et al., 2012).
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S. Prasad et al. / Earth and Planetary Science Letters 391 (2014) 171–182
Fig. 4. Simplified pollen diagram for the Lonar Lake sediment sequence. The colours in the far right column have the same implications for climate as in Fig. 3. Arrows
indicate the major transition points in the catchment vegetation.
Increased or decreased productivity in a stratified lake therefore should be reflected in increasing or decreasing δ 13 C and δ 15 N
values of organic matter that was produced in surface waters
(Hodell and Schelske, 1998).
Within the core, the oxygen and carbon isotope values of
calcium carbonate (δ 13 Ccarb and δ 18 Ocarb ) correlate (r = 0.78)
indicating their evaporative origin (Li and Ku, 1997). The carbonates in surface sediments show less enriched values near
the stream inflow (shore proximity) as compared with deep
water due to the dilution by the isotopically depleted inflowing streams (Anoop et al., 2013). We note that due to the
damping of the drying signal by the stream inflow (5 yr of
age) and the high solubility of the evaporites (Anoop et al.,
2013), the Lonar isotopic record is sensitive to, and preserves,
only the evidence of weaker than normal decadal ISM fluctuations.
S. Prasad et al. / Earth and Planetary Science Letters 391 (2014) 171–182
The normalised aluminium (Al) (XRF, total counts) content indicates clastic influx – in the closed Lonar Lake, an increased detrital
influx could be indicative of increased catchment erosion and/or
shoreline proximity (lake level changes). However, since the stream
inflow is linked to recharge by monsoon, a combination of both the
proxies (δ 13 Ccarb and δ 18 Ocarb , and Al) can give a clear picture of
the lake level changes and monsoon inflow.
Pollen analysis is used for vegetation reconstruction in the
Lonar Lake catchment and the discrimination between different
vegetation types. Pollen of woody plants were assigned to three
forest types, with wet to moist forest today occurring under annual rainfall values of 5000 to 1800 mm and a 7–8 month-long
dry season, represented by Olea, Ligustrum Eleaeodendron, Trema
and Trewia, semi-arid dry deciduous forest (1500–800 mm/a rainfall, 7 month-long dry season), with Combretaceae, Phyllanthus,
Tectona, Lagerstroemia and Grewia, and arid thorn shrub vegetation (800–400 mm/a, 7–8 month dry season), comprising Acacia,
Prosopis, Rhamnaceae and Azadirachta. The classification follows
Gaussen et al. (1964, 1966, 1970) and Champion and Seth (1968).
Following these data, available moisture which controls the distribution of the different trees and forest types on regional scale
is primarily determined by the strong gradient in annual rainfall amounts. Grasses of the Poaceae family dominate the lower
stratum of most vegetation formations in tropical India, including moist forest types (Dabadghao and Shankarnarayan, 1973). An
identification of Poaceae pollen grains on higher taxonomic levels is usually not possible using light microscopy (Beug, 2004) and
thus grass pollen are not indicative for changes in rainfall amounts
at Lonar crater.
The identification of pollen grains of cultivated plants is commonly used to detect agricultural activity in the past (Bottema,
1992). However, based on the pollen morphology an identification of common cultivated cereals in S-India is not feasible
(Vishnu-Mittre, 1975). Moreover, pollen of cultivated pulses were
not identified in the Lonar pollen record, because of the large number of native Fabaceae herbs occurring in the vegetation. However,
we interpret increasing herb pollen values after ca. 1.3 cal ka as indirect indicators for artificial changes in the understory vegetation,
since herbs might be promoted over grasses through prolonged
grazing pressure. Additionally, many woody thorn shrub taxa may
also reflect anthropogenic degradation of dry deciduous vegetation
(Asouti and Fuller, 2008).
The lipid biomarker tetrahymanol is a pentacyclic triterpenoid
lipid that frequently occurs in lake sediments (e.g., Thiel et al.,
1997; Castañeda et al., 2011). It may have different biological origins among which predatory ciliates appear to be the most relevant in sedimentary records (Harvey and McManus, 1991). Other
documented sources of tetrahymanol are the phototrophic purple non-sulphur bacterium Rhodopseudomonas palustris (Kleemann
et al., 1990), the fern Oleandra wallichii (Zander et al., 1969) and
the rumen fungus Piromonas communi (Kemp et al., 1984). In
Lonar, tetrahymanol occurs in high amounts in surface sediments
and bacterial mats retrieved from various locations in Lonar Lake
(Sarkar et al., 2014) suggesting that bacteria/ciliate communities
are the most likely source of tetrahymanol in this ecosystem.
Sinninghe Damsté et al. (1995) proposed that tetrahymanol
may be synthesised by anaerobic ciliates living at or below the
chemocline, and thus may be indicative of water column stratification. Periods of increased abundance of tetrahymanol during
the Holocene in the saline and alkaline Lake Van (Turkey) have
been related to pronounced stagnation accompanied by the establishment of an anoxic water body (Thiel et al., 1997). In Lonar
Lake sediments we interpret tetrahymanol as originating from bacteria/ciliate communities and consider it as a marker for watercolumn stratification (Sinninghe Damsté et al., 1995) and its δ 13 C
is used here to assess alkalinity of the lake water.
177
4.2. Holocene proxy variability and climate reconstruction
In contrast to the lakes in NW India that dried out or became
seasonal after 4 cal ka (Prasad and Enzel, 2006), the Lonar Lake
has retained water level until today and provides the best preserved, high resolution record of Holocene climate variability from
the core monsoon region. The driest period (>11.4 cal ka) (Fig. 2a),
was marked by soil developing on the dry lake bed and followed
by a short period (∼300 yr) of intensive erosion, fluctuating salinity as indicated by the presence of evaporative gaylussite, and high
aquatic productivity (low Corg /N, high δ 13 Corg and δ 15 N) that resulted from a sudden increase in precipitation (Fig. 3). During
∼11–6.2 cal ka, wet conditions are indicated by the lowest δ 13 Corg
values (indicating C3 plants) and low clastic influx. We infer stratified, deep water conditions between 11–9 cal ka when seasonally
laminated (varved) sediments were deposited. Counterintuitively,
the early Holocene evaporative carbonates show less negative δ 18 O
values that could be related to reduce dilution by the isotopically
depleted stream inflow during periods of higher lake levels. However, the amplitude of spatial isotopic variability (8.5❤ for δ 13 C
and 4❤ for δ 18 O) in surface bulk carbonates (Anoop et al., 2013)
cannot alone explain the range of variability seen in the core bulk
carbonate isotopic composition. We exclude any contribution from
the winter westerlies as this region was under the influence of the
ISM during the Holocene (Prasad and Negendank, 2004) – the only
possible explanation for the apparently reverse trend in carbonate
isotope values is a change in source water composition or paths
of precipitation tracks related to shifts in the mean position of the
ITCZ (Haug et al., 2001).
Moist forests occurred from 8.8 cal ka onwards throughout the
early and lower middle Holocene, as evident from the pollen data.
Gradually decreasing pollen of dry deciduous forest elements after
ca. 8.5 cal ka, indicates further increasing rainfall amounts. A significant rise in moist forest pollen types around 7.1 cal ka attests to
the wettest phase.
From 6.2 cal ka a stepwise shift to a weaker summer monsoon
is evident from salinity and pH related enrichments of δ 13 Corg
and δ 15 N (Fig. 3) that precede the expansion (5.6–3.9 cal ka) of
semi-arid dry deciduous forest (Fig. 4). Lake shoaling, accompanied by an increase in clastic influx, sedimentation rate (Fig. 3,
>1.5 mm/yr compared to 0.18 mm/yr in lower sediments), and
less enriched evaporitic isotope values indicating increased proximity to inflowing streams, indicate onset of drier conditions
ca. 5.2 cal ka. The bacterial/ciliate community biomarker tetrahymanol makes its first appearance at this time in exceptional concentrations and persists until 3.9 cal ka and in lower abundance
subsequently. The unusual enrichment in δ 13 C (ca. 5.1–4 cal ka)
observed for tetrahymanol (−17.2❤ to −7.2❤) can only be explained fully by the utilisation of a 13 C-enriched carbon source
by the tetrahymanol-producing organism indicating increasing lake
water alkalinity. This shift to drier climate conditions is accompanied by a marked shift in the composition of the pollen assemblage at ca. 5 cal ka (Fig. 4), pointing to a significant reduction
of moist arboreal vegetation while dry thorn shrub elements become established. The drying trend beginning ca. 5.2 cal ka culminates in the formation of evaporative gaylussite (Anoop et al.,
2013) between 4.6 and 3.9 cal ka when lake salinity increased.
We refer to the highly saline, drier periods as prolonged droughts
(PD, centennial long intervals with weak summer monsoon) as
20% below the long term mean is needed for the formation
of gaylussite. Within this interval (PD1: 4.6–3.9 cal ka), tetrahymanol showed the least negative δ 13 C values, pollen of dry deciduous forest elements declined, and pollen of light demanding
species (Ailanthus excelsa) increased indicating a noticeable opening of the vegetation. Within PD1, between ca. 4.4 and 4.2 cal ka,
the Corg /N ratios drop to values 10. Although these values are
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S. Prasad et al. / Earth and Planetary Science Letters 391 (2014) 171–182
typical of planktonic organic matter (Meyers and Lallier-Vergès,
1999), and thus might indicate enhanced aquatic productivity, we
attribute them to reduced supply of terrestrial organic matter
due to drier conditions, as no corresponding δ 13 Corg enrichment
is seen, and the drop in Corg /N occurs during the cycle of continuous evaporation and reduced Al supply (Anoop et al., 2013)
– this short arid event led to a rapid reduction of the forest
vegetation, followed by a considerable expansion of thorn shrub
and savanna vegetation. Within dating errors, this short dry period coincides with the 4.2 ka event (Staubwasser et al., 2003;
Anoop et al., 2013), but our data demonstrate that the pronounced
drying began at least ca. 200 years earlier in the CMZ.
The disappearance of evaporitic gaylussite between 3.9 and
2 cal ka (Fig. 3) indicates reduced salinity. Nearly in parallel, a gradual humidification is seen in the change from arid thorn shrub vegetation to semi-arid deciduous forest between 3.8 and 3 cal ka with
the denser forest vegetation persisting until ca. 2 cal ka (Fig. 4).
The onset (ca. 2 cal ka) of PD2 is marked by the re-appearance
of gaylussite crystals that become abundant 1.4–0.6 cal ka when
enrichment in δ 13 Ccarb and δ 18 Ocarb , lowered Corg /N and higher
δ 13 Corg indicate increasing eutrophication of the lake. During PD2
the pollen (Fig. 4) of dry deciduous forest plants decline while
thorn shrub vegetation expanded after 1.2 cal ka. A marked increase in herb pollen values furthermore points to intensified
anthropogenic impacts on the vegetation from ca. 1.2 cal ka onwards. The occurrence of severe droughts (PD2) is also seen in
the Dandak cave record (Sinha et al., 2007) but the drier periods in the latter occur after (0.7–0.3 ka) those found in the
Lonar Lake. This may be due to either differing proxy sensitivities or spatial heterogeneity in ISM precipitation during the late
Holocene.
4.3. Possible climate-culture link?
An examination of archaeological data from the region reveals
that it is only around 4.5 cal ka that sedentary agricultural villages first occur in the northern and central Deccan, in response
to the ISM weakening in the CMZ (Fuller, 2011). The cultural development (Kayatha, see Misra, 2001) is significantly later than the
establishment of the early Indus Valley Civilisation (5.2–3.9 cal ka),
with Indus urbanism from 4.6 cal ka (Possehl, 1999). The majority of the northern Deccan sites dated to this period are close to
the rivers (Misra, 2001; Fuller, 2011) suggesting a need for a reliable source of water. After 4 cal ka there is a major increase in
the known archaeological sites (Savalda, Malwa, Jorwe), focused on
3.8–3.4 cal ka (Misra, 2001; supplementary text in Ponton et al.,
2012) and migration to locations distant from the rivers probably
in response to wetter climate. This expansion would have extended
and maintained thorn shrub vegetation (Asouti and Fuller, 2008).
The cultivation systems of this period incorporated winter crops,
like wheat and barley that had been adopted from the northwest,
as well as indigenous monsoon-grown millets (Fuller, 2011). Wheat
and barley cultivation was facilitated by the relatively wetter conditions of this period as they would have needed to be grown on
water retained by clay-rich soils for the northern peninsula or artificial irrigation. This is indicated in the higher presence of wheat
and barley during this era and on the northern peninsula, with the
representation of millets increasing after 3.5 cal ka and even further after 3 cal ka (Fuller, 2011). In central India, archaeological
evidence indicates adoption of low rainfall crop patterns beginning ca. 1.5 cal ka (Deotare, 2006), while decadal scale droughtinduced famines are documented in historical records from the
13th and 14th centuries AD (Dhavalikar, 1992; Maharatna, 1996;
Sinha et al., 2007).
Fig. 5. Comparison of Holocene reconstructions of the ISM. Location of sites is
shown in Fig. 1a. The colour bars have the same interpretation as in Fig. 3. (a) Oxygen isotope record from Oman (Fleitmann et al., 2003). The light grey bar shows
the range of modern stalagmite. (b) Carbon isotope data from biomarkers (C28) derived from the Godavari catchment (Ponton et al., 2012). (c) Oxygen isotope record
from NE India (Berkelhammer et al., 2012). (d) Carbon isotopic composition from
Hyongyang peat, eastern Tibet (Hong et al., 2003). (e) ENSO reconstruction from Laguna Pallacocha, Ecuador (Moy et al., 2002). (f) Percentage of sand in a core from
El Junco Lake, San Cristobal, Galápagos (Conroy et al., 2008). (g) A 10-year running
mean of the relative percentage of lithic sediments in a deep-sea core off the coast
of Peru (Rein et al., 2005). (h) The 14 C record from tree rings which largely reflects
changes in solar activity (Stuiver et al., 1998). Single point arrows indicate direction
of increase/decrease while double pointed arrow indicates no major change.
4.4. Regional correlation
Modern meteorological studies show differing climatic pattern
between regions in India (Hoyos and Webster, 2007). However,
the palaeoclimate data clearly show synchroneity of several events
throughout the ISM realm. The onset of ISM at ca. 11.4 cal ka, fol-
S. Prasad et al. / Earth and Planetary Science Letters 391 (2014) 171–182
179
Fig. 6. SST anomalies in the IPWP calculated with respect to averaged SST over last 2 cal ka during PD1 (a), in the less saline interval (D–D) sandwiched between the two
PD (b), and PD2 (c). See text for references. Only records covering all the three time slices are shown. Red circles and blue circles indicate warmer and cooler SST anomalies
respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
lowing the drier period in the Lonar Lake, is coincident with the
Arabian Sea (Sirocko et al., 1993), the Indus catchment (Limmer et
al., 2012), and the Bay of Bengal (Govil and Naidu, 2011) records.
The generally wetter phase recorded in Lonar between 11 and
6.2 cal ka is not seen in NW India (Prasad and Enzel, 2006) but
this could be related to the low precipitation/evaporation rates in
NW India. A change in Lonar hydrology beginning at ca. 6.2 cal ka
coincides with the final reduction in the monsoon rainfall contribution to the water balance in NW Indian lakes (Prasad and Enzel,
2006), Oman (Fleitmann et al., 2003), and NE India (Berkelhammer
et al., 2012), as well as eastern Tibet (Hong et al., 2003). A pivotal change in Lonar hydrology is seen ca. 4.6 cal ka with the
occurrence of two PD separated by a less saline phase (Fig. 3).
The impact of both these PD is seen to varying degrees at several
sites in the ISM realm (Fleitmann et al., 2003; Ponton et al., 2012;
Berkelhammer et al., 2012; Hong et al., 2003) (Fig. 5).
4.5. What could have caused the prolonged droughts?
The onset of weakening of the ISM at 6.2 cal ka recorded in
the Lonar data is coincident with the orbitally forced weakening of solar insolation. However, a comparison of available ISM
records (Fig. 5) with solar variability (Stuiver et al., 1998) show
that, contrary to the previously known studies (Neff et al., 2001;
Fleitmann et al., 2003; Gupta et al., 2005), the mid to late Holocene
prolonged droughts occurred during periods of stronger solar irradiance. Clearly, a simple model linking solar insolation, southward
shift of the mean position of the ITCZ (Haug et al., 2001) and reduced ISM strength in observed regions cannot explain the late
Holocene PD observed in the ISM realm (Fig. 5) and alternative
internal forcing mechanisms need to be explored. Variations in
ISM precipitation can also be driven by ENSO, which modulates
the regional monsoonal circulation through anomalous changes
in the planetary scale Walker circulation (e.g., Krishna Kumar et
al., 2006). We note that ENSO is an interannual phenomenon
and long, high resolution records that can provide information on
palaeo-ENSO activity currently are not available. We have therefore
used reconstructions of ENSO-l on centennial and millennial scales
(Moy et al., 2002; Rein et al., 2005) that resulted in precipitation
changes. However, during PD1, ENSO-l is moderately intense (Rein
et al., 2005) but less frequent as compared to the subsequent interval (Moy et al., 2002; Conroy et al., 2008) indicating some other
forcing mechanism for the ISM weakening. A late Holocene interval
with highly variable, intense ENSO-l activity occurred 3.5–2.5 cal ka
(Moy et al., 2002; Rein et al., 2005) and had a widespread impact
elsewhere (Moy et al., 2002; Rein et al., 2005; Langton et al., 2008;
Toth et al., 2012), but does not appear to have had any significant
impact in the CMZ where Lonar shows reduced salinity after the
PD1. Other ISM sites show little or no change (Fig. 5). The ISM and
ENSO-l link is established between 2 and 0.6 cal ka when PD2, coincident with increased solar activity, is recorded in the CMZ.
Climate model simulations indicate a likely intensification of
the Walker circulation with stronger easterly trade winds and enhanced cooling over the eastern Pacific during periods of increased
solar irradiance (Meehl et al., 2009). Therefore, it is rather intriguing to note the coincidence of PD associated with increased
solar irradiance (Fig. 5). We argue that during such periods, the
impact of the direct or indirect warming of the equatorial Indian Ocean (IO) and the IPWP can actually weaken the ISM. The
basis for this argument comes from understanding of the link between the Indian summer monsoon rainfall (ISMR) variability and
the Indian Ocean SST warming. It is important to mention that
the period of PD1 coincided with positive temperature anomalies in the western Pacific Warm Pool (WPWP) (Stott et al., 2004;
Linsley et al., 2010) and eastern IO (Govil and Naidu, 2011)
(Fig. 6a). Terrestrial archives indicate anomalous cooling in the
western IO (Thompson et al., 2002), fall in lake levels in eastern Africa (Garcin et al., 2012), and stronger monsoon in southern
Indonesia (Griffiths et al., 2009) – these temperature and precipitation anomalies strongly resemble those during the negative phase
180
S. Prasad et al. / Earth and Planetary Science Letters 391 (2014) 171–182
of the Indian Ocean Dipole (IOD, Saji et al., 1999) (warming in the
eastern equatorial Indian Ocean). The primary role of the eastern
IO in causing spatial climate heterogeneity is also supported by the
larger amplitude change in NE India (Berkelhammer et al., 2012) as
compared to Oman (Fleitmann et al., 2003). While the enhanced
SST warming in the eastern IO favours increased precipitation over
the equatorial IO, the regional equatorial anomalies actually tend
to weaken the boreal summer monsoon circulation by inducing
subsidence and rainfall suppression over the Indian subcontinent
(see Krishnan et al., 2006). In turn the weakened summer monsoon winds can amplify the SST warming in the eastern IO through
wind-thermocline feedback (Krishnan et al., 2006; Swapna et al., in
press).
The interval 3.5–2.5 cal ka, coincident with reduced solar irradiance is characterised by highly variable and intense ENSO-l
activity (Moy et al., 2002; Rein et al., 2005) with little impact over
CMZ where Lonar showed reduced salinity after the preceding PD1
(Fig. 3, 4). At this time, the eastern IO is cooler (Govil and Naidu,
2011). Between 2 and 0.6 cal ka (PD2) the stronger solar irradiance during PD2 is consistent with the enhanced SST warming of
the equatorial eastern IO and the southward expansion of positive temperature anomalies in the WPWP, with stronger impacts
over CMZ (Fig. 6c). A comparison of the SSTs in the IPWP during
the three time slices (Fig. 6), suggests that the ISM and ENSO-l
link on millennial scales is dependent on the SST anomalies in the
equatorial eastern IO and the southern part of the WPWP. Notwithstanding the mechanisms that control the position and magnitude
of temperature anomalies in the equatorial IO and WPWP (Newton
et al., 2011; Abram et al., 2009), it appears that the occurrence
of persistent droughts over India involve not only changes in the
Pacific east–west Walker circulation (Krishna Kumar et al., 2006),
but also regional changes in the meridional overturning circulation over the Indian Ocean (Krishnan et al., 2006; Swapna et al., in
press).
5. Conclusions
The high resolution Holocene palaeoclimate reconstruction
from the Lonar Lake in central India provides evidence of an extended dry period prior to 11.4 cal ka. This was followed by the
establishment of a shallow lake (for ca. 300 yr) marked by high
aquatic productivity and increased detrital input. The Holocene
wetter period lasted from 11 to 6.2 cal ka BP. Subsequently, two
prolonged intervals of drier conditions (PD) are indicated by the
presence of evaporative gaylussite (PD1: 4.6–3.90 cal ka) and (PD2:
2.03–0.56 cal ka) that are separated by calcareous clay sediments
indicative of lower salinity. Archaeological evidence indicates that
the first settlements in this region coincided with the onset of
the first PD and agricultural populations flourished between these
two prolonged droughts. A comparison of the Lonar record with
ENSO-l activity indicates that PD1 occurred during lower ENSO-l
activity. Our data show that the ISM and ENSO-l link, proposed for
the modern time, was established only ca. 2 cal ka. The Holocene
PD occur largely during periods of higher solar irradiance suggesting that the solar signal could have amplified and/or modified the
IPWP teleconnections through changes in sea surface temperatures.
While in recent decades the warming in the western Pacific may
be faster and result in more frequent ENSO (Hansen et al., 2006), it
is the warming of the eastern IO and southern part of the WPWP
that will crucially determine the long term monsoon rainfall activity over the subcontinent.
Acknowledgements
We thank all the people that have provided help during field
work including K. Deenadayalan and Md. Arif. Cooperation by
the Forest and Wildlife Department of Maharashtra State, India
is gratefully acknowledged. This work was funded by the German Research Foundation (FOR 1380) within the framework of the
HIMPAC project. This research used data acquired in the XRF Core
Scanner Lab at the MARUM – Center for Marine Environmental Sciences, University of Bremen, Germany, and was supported by the
DFG-Leibniz Center for Surface Process and Climate Studies at the
University of Potsdam. Additional support was provided through
the DFG Graduate School (GRK 1364). The hard work invested by
Richard Niederreiter for raising the core in 40 ◦ C summer temperatures is gratefully acknowledged.
Appendix A. Supplementary material
Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.epsl.2014.01.043.
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