of 10
ARTICLE
Recurring summer and winter droughts from
4.2-3.97 thousand years ago in north India
Alena Giesche
1
, David A. Hodell
1
, Cameron A. Petrie
2
, Gerald H. Haug
3
, Jess F. Adkins
4
,
Birgit Plessen
5
, Norbert Marwan
6
, Harold J. Bradbury
1,7
, Adam Hartland
8
, Amanda D. French
8
&
Sebastian F. M. Breitenbach
9
The 4.2-kiloyear event has been described as a global megadrought that transformed multiple
Bronze Age complex societies, including the Indus Civilization, located in a sensitive transi-
tion zone with a bimodal (summer and winter) rainfall regime. Here we reconstruct changes
in summer and winter rainfall from trace elements and oxygen, carbon, and calcium isotopes
of a speleothem from Dharamjali Cave in the Himalaya spanning 4.2
3.1 thousand years ago.
We
fi
nd a 230-year period of increased summer and winter drought frequency between 4.2
and 3.97 thousand years ago, with multi-decadal aridity events centered on 4.19, 4.11, and
4.02 thousand years ago. The sub-annually resolved record puts seasonal variability on a
human decision-making timescale, and shows that repeated intensely dry periods spanned
multiple generations. The record highlights the de
fi
cits in winter and summer rainfall during
the urban phase of the Indus Civilization, which prompted adaptation through
fl
exible, self-
reliant, and drought-resistant agricultural strategies.
https://doi.org/10.1038/s43247-023-00763-z
OPEN
1
Godwin Laboratory for Palaeoclimate Research, Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, United Kingdom.
2
Department of Archaeology, University of Cambridge, Cambridge CB2 3DZ, United Kingdom.
3
Climate Geochemistry Department, Max Planck Institute
for Chemistry, Mainz 55020, Germany.
4
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.
5
Section Climate Dynamics and Landscape Evolution, Helmholtz Centre Potsdam, German Research Centre for Geosciences GFZ, Potsdam 14473, Germany.
6
Potsdam Institute for Climate Impact Research (PIK), Potsdam 14473, Germany.
7
Department of Earth, Ocean, and Atmospheric Sciences, University of
British Columbia, Vancouver, BC V6T 1Z4, Canada.
8
Environmental Research Institute, School of Science, Faculty of Science and Engineering, University of
Waikato, Hamilton 3216, New Zealand.
9
Department of Geography and Environmental Sciences, Northumbria University, Newcastle upon Tyne NE1 8ST, UK.
email:
alena.giesche@gmail.com
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1
1234567890():,;
M
ost regions in the world are dominated by either a
winter or a summer rain regime. One of the few
exceptions is the Indus River Basin and surrounding
regions in northwest South Asia that are climatically and
environmentally diverse, and feature abrupt transitions between
a dominant summer versus winter rain regime (Supplementary
Fig. S1 and Supplementary Table S1). Here, we compare the six
months of the summer monsoon (May through October, i.e.,
summer rain) with the remaining winter and spring months
(November through April, hereafter referred to as winter rain).
For agricultural societies, the seasonal timing and intensity of
precipitation are more relevant than total annual rainfall,
because a reliable and somewhat predictable timing of water
supply is vital for agricultural planning
1
,
2
. With a robust multi-
seasonal water supply during the mid-Holocene
3
6
,theIndus
River Basin emerged as a favorable area for the development
of the complex society known as the Indus Civilization
(5000
3600 years ago). However, the intensity of seasonal
rainfall in this region appears to have changed considerably
since the mid-late Holocene transition c. 4200 years ago. The
impact of these changes in seasonal water supply on rainfed
agriculture warrants detailed inv
estigation, particularly within
the context of archaeological research about societal resilience
and decline
7
12
.
The seasonality component of precipitation variability has
increasingly garnered attention in studies from South Asia
13
,
14
.
Still, the contribution of the Indian Winter Monsoon (IWM) is
often overlooked because the Indian Summer Monsoon (ISM)
provides >80% of annual rainfall where most studies have been
undertaken. Yet, IWM precipitation is particularly important in
the Indus River Basin, where precipitation from the winter wes-
terlies dominates the snowpack
15
and late dry season intensity
affects agricultural practices and yields. Water availability on the
fl
oodplain is also in
fl
uenced by snow and ice melt during
the summertime, which contributes a considerable portion of the
annual runoff that feeds the headwaters of the massive Indus/
Punjab and Ganges River systems
16
. In the western extent of the
Indus River Basin, for example, the ratio of winter to summer
rainfall exceeds 1:1 (Fig.
1
), emphasizing the need to better
understand past changes in hydroclimate seasonality of this
region.
A consensus is emerging that some form of increased drought
affected the landscape of northwest South Asia during the mid-
late Holocene transition (i.e., the 4.2 ka event)
17
20
, although the
exact timing and magnitude of the drier period(s) remain
uncertain. Paleoclimatic reconstructions suggest a drier than
normal mid-late Holocene transition in the winter-rain domi-
nated parts of the Middle East
21
23
, as well as in parts of the
Indian and Asian Summer Monsoon domains of eastern India
and China
1
,
24
27
. With previous studies exploiting paleoclimate
records from either the westerlies-dominated parts of Western
Asia or the ISM domain, little is known about climate dynamics
in the region of interaction between these two climate regimes. In
this study, we focus on the bimodal rainfall regime of northwest
South Asia, including the timing and duration of multi-season
drought that has been often hindered by large age uncertainties in
available paleoclimate records from the region and an over-
emphasis on the ISM. Speci
fi
cally, we aim to reconstruct indi-
cations of both the winter and summer rainy seasons from our
multi-proxy time series and evaluate the impact these changes
may have had on the Indus Civilization.
Although several speleothem records from the Himalaya and
northeast India track Holocene ISM strength, and some even
cover the mid-late Holocene transition at 4.2 ka BP (kilo
annumbefore present, with respect to 1950 CE), e.g., refs.
25
,
27
,
very few are located in the western Indian domain
28
that
simultaneously receives substantial precipitation from the IWM.
Furthermore, none of these records incorporate trace element
data to independently check conclusions based on speleothem
δ
18
O. Perhaps due to the dif
fi
culty of interpretation,
δ
13
C data are
also often omitted from discussion although this proxy can
inform our understanding of local environmental conditions
29
,
30
.
Consequently, there is a great need for seasonally-resolved, multi-
proxy paleohydroclimatic reconstructions from the Indus River
Basin. Here, we aim to characterize hydrological seasonality
during the millennium after the mid-late Holocene transition
using a speleothem record from the west-central Himalayas. We
track the evolution of summer and winter moisture in a sea-
sonally resolved, multi-proxy stalagmite record that begins at
4.2 ka BP from Dharamjali Cave (29.5°N, 80.2°E).
Dharamjali Cave is a shallow (<14 m deep), climatically-
responsive cave system with sub-seasonal in
fi
ltration dynamics
Fig. 1 Local proportion of winter:summer precipitation.
Large Indus cities are labeled and shown with the site distribution during the Mature Harappan (c.
4.6
3.9 ka BP, orange) and Late Harappan (c. 3.9
3.6 ka BP, red) periods. Contour lines represent the ratio of IWM (Nov
Apr) to ISM (May
Oct)
precipitation based on the 0.25° GPCC v2018 dataset from 1951
2000 rain gauge data
86
.
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(see Supplementary Discussion for additional information). The
25-cm-long DHAR-1 stalagmite was located at the far end of the
cave. The stalagmite is predominantly aragonite and variations in
oxygen isotope values (
δ
18
O
stal
) are used here to reconstruct
regional hydrological changes. The
δ
18
O
stal
record of DHAR-1
re
fl
ects seasonal variations in the
δ
18
O of rainfall, which is
dominated today by the ISM (Supplementary Fig. S2). In
northwest South Asia, the moisture source region is the
fi
rst-
order control on ISM
δ
18
O
precip
, which includes the highly eva-
porated surface waters of the Arabian Sea (0.4 to 0.9
) and the
Bay of Bengal (
2 to 0.5
)
31
,
32
, as well as
18
O-enriched rainfall
recycled by evapotranspiration from the continent
33
(Supple-
mentary Fig. S3). Strong ISM seasons are generally characterized
by low
δ
18
O
precip
resulting from an increased transport path
length (Rayleigh distillation) and reduced sub-cloud
evaporation
34
,
35
. In contrast to the ISM, the
δ
18
O of IWM
rainfall is high because it draws more of its moisture from the
proximal Arabian Sea
36
,
37
and it loses
16
O through partial re-
evaporation of rainfall in dry air. Winter precipitation is pre-
dominantly carried by western disturbances (WDs) that originate
over the Mediterranean Sea or mid-Atlantic and travel
eastward
38
,
39
.
Local environmental conditions can also be traced using spe-
leothem carbon isotopes (
δ
13
C
stal
). Shifts in
δ
13
C
stal
are driven by
a combination of factors, including: overlying vegetation, soil
respiration, bedrock dissolution regime, cave ventilation, drip
rate, and prior calcite or aragonite precipitation (PCP/PAP)
during the driest season or intervals
30
. The dense C
3
-type forest
above Dharamjali Cave today
40
may well have once featured
more drought-resistant C
4
-type vegetation, as reconstructed for
parts of the northwestern Himalaya c. 4 ka BP
41
,
42
. In particular,
the growth and resilience of the Chir pine species (
Pinus rox-
burghii
) dominantly present above Dharamjali cave today
40
is
known to be particularly sensitive to winter and spring
rainfall
43
,
44
. Thus, we might expect increasing
δ
13
C
stal
values in
response to long-term drought-induced vegetation changes, par-
ticularly driven by winter and spring moisture limitation, e.g.,
refs.
43
46
. Other factors in
fl
uencing
δ
13
C
stal
are relevant at the
seasonal to interannual scale, and
fl
uctuate more dynamically
during the winter season. For example, in winter the cave air
temperature exceeds that of the outside air (promoting enhanced
ventilation), drip rate slows to 3
8 drops/min compared to 20
30
drops/min in summer
40
, and PAP is most likely to occur in air-
fi
lled voids in the epikarst as potential evapotranspiration exceeds
precipitation during the drier winter months. Taken together,
increased cave ventilation, lower drip rates, and PAP in winter-
time all lead to higher
δ
13
C
stal
and more bedrock-like values,
though the colder winter season also features minimal soil
respiration that acts to lower
δ
13
C
stal
. Cave CO
2
monitoring data
are not available at Dharamjali Cave, but the sum of mechanisms
suggests that cooler, longer, and/or drier winters lead to higher
δ
13
C
stal
values (see Supplementary Discussion for a more exten-
sive analysis of
δ
13
C). Seasonal cave monitoring would be
required to disentangle the primary effects responsible for
δ
13
C
stal
values and provide more certainty about their paleoclimatic
interpretation.
Shifts in speleothem calcium isotopes, speci
fi
cally
δ
44/40
Ca, are
similarly useful aridity proxies via PCP/PAP processes in caves,
and have been used to interpret past rainfall amount more
quantitatively than is possible with
δ
18
O
47
49
. During carbonate
precipitation,
40
Ca is preferentially incorporated into the solid
phase relative to the heavier isotope
44
Ca. Thus, water that
in
fi
ltrates more slowly and is given more time to degas and
precipitate before reaching the stalagmite under drier conditions
will yield a smaller difference between the two isotopes and a
resulting higher
δ
44/40
Ca
stal
. This concept underlies the
interpretation of any PCP or PAP proxy, and directly connects
this proxy to rainfall amount
50
. Importantly, an aridity threshold
is required before any PCP/PAP effects are noticeable
therefore,
such proxies tend to mainly re
fl
ect changes in the length or
severity of the driest season because only an extreme change in
wet season rainfall amount would reduce cave saturation to an
appreciable degree to cross the threshold for this proxy
14
,
48
.
Additionally, we use trace element changes of DHAR-1 as
independent tracers of past hydrologic changes
50
52
. The dis-
tribution of U
2
+
,Sr
2
+
, and Ba
2
+
in aragonite speleothems reacts
to PAP dynamics in the epikarst
53
,
54
. With intensi
fi
ed and/or
prolonged dry-season (winter) aridity and associated PAP, con-
centrations of uranium (D
U(Ar)
» 1) and strontium (D
Sr(Ar)
>1)
are expected to decrease in the resulting speleothem, while bar-
ium (D
Ba(Ar)
~ 1) appears to be a less reliable indicator, although
it may also decrease
54
. Research in calcite stalagmites further
suggests that transition metals such as Zn
2
+
,Ni
2
+
,Cu
2
+
, and
Co
2
+
increase with lower drip rates due to the dominance of
ligand-bonded metal dissociation over prior calcite precipitation
(PCP)
55
,
56
. We investigate preliminary transition metal relation-
ships in the DHAR-1 data to understand whether such drip rate
effects may also relate to ISM rainfall intensity or wintertime PAP
in this aragonite speleothem.
Results and discussion
Stalagmite age model and mineralogy
. The age model for the
multi-proxy record from stalagmite DHAR-1 is based on twelve
U-series dates (Supplementary Table S2) that span 4.2
2.55 ka BP
with an average 2
σ
error of ±18 years. Depth-age modeling was
completed for each proxy using the COPRA routine
57
. Low
growth rates of 90 μm/year in the older section (4.2
3.1 ka BP)
were followed by higher deposition rates of 200 μm/year for
3.1
2.55 ka BP (Supplementary Fig. S4). The higher growth rate
after 3.1 ka BP mainly re
fl
ects an increase in porosity associated
with changes in trace element concentration and crystal growth
fabric. The simultaneous shift in growth axis at 3.1 ka BP suggests
a change in drip
fl
ow path, related to a climatic or tectonic trigger
(Supplementary Discussion), and we therefore focus our climatic
interpretation on the earlier undisturbed interval between 4.2 and
3.1 ka BP (all data are available in the online repository).
At the base of DHAR-1, X-ray diffraction analysis con
fi
rms a
mineralogical transition from a 3-mm basal calcite layer to
primary aragonite for the rest of the sample over 4.2
2.55 ka BP.
Trace element data corroborates this shift with distinct changes of
several element/calcium ratios, where Mg/Ca is higher in calcite
compared to aragonite and Sr/Ca, U/Ca, and Ba/Ca are all lower
in calcite than aragonite (Fig.
2
a). This calcite-to-aragonite
transition in itself supports a shift towards drier conditions
because aragonite is preferentially deposited with more extensive
evaporation and PCP or PAP
58
60
. The darker, dirty layer
between the calcite-aragonite transition marks a hiatus between
the two deposition phases (Supplementary Fig. S5).
Oxygen, carbon, and calcium stable isotopes
. A pro
fi
le of 750
oxygen and carbon stable isotope samples was resolved at
100
300 μm (annually) over the entire sample 4.2 to 2.55 ka BP
(GFZ Potsdam), and extended with 876 samples milled at 50 μm
(sub-annual) resolution between 4.2 and 3.6 ka BP at the Uni-
versity of Cambridge (Fig.
2
). Laser ablation element data was
obtained at c. 25 μm resolution at the University of Waikato
(Supplementary Fig. S6). Both the low and high resolution
DHAR-1 isotope series show excellent agreement in the region of
overlap. Furthermore, DHAR-1 replicates the lower resolved and
less-well dated DH-1 record from the same cave
61
(Supplemen-
tary Fig. S7).
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3
The majority of the
δ
44
Ca
stal
measurements (49 out of
61 samples) focused on the period of greatest interest from 4.2
to 3.6 ka BP, yielding a nearly decadal resolution (1 sample per
12.6 years). Values during this period ranged from
1.06 to
0.27
(mean
=
0.84
). The prominent positive excursion in
the
δ
44
Ca
stal
results closely mirror both
δ
18
O
stal
and
δ
13
C
stal
between 4.2 and 3.97 ka BP (Fig.
2
b), a strong indication for a
lengthy, multi-season period of lower rainfall. The data from the
following period between 3.97 and c. 3.4 ka BP shows more
negative values in all proxies, though with divergent trends:
δ
18
O
stal
trends negative,
δ
44
Ca
stal
has no discernible trend, and
δ
13
C
stal
trends positive. This pattern suggests a less dominant
PAP forcing for
δ
44
Ca and
δ
13
C after crossing a moisture
threshold (e.g., an extended ISM season could ameliorate PAP
processes)
14
,
48
. The positive trend in
δ
13
C
stal
may relate to
strengthening winter cave ventilation driven by cooling tempera-
tures outside the cave and related CO
2
dynamics, decreasing cave
drip rates and thus enhanced CO
2
degassing in winter (summer
cannot be a candidate because
δ
18
O
stal
shows the ISM rainfall
increased), and/or a long-term shift to more winter-drought-
resistant C
4
-type vegetation above the cave, e.g., refs.
43
46
(see Supplementary Discussion for additional
δ
13
C analysis).
Trace elements tracking prior aragonite precipitation, drip
rate, and redox conditions
. The trace element pro
fi
les (seasonal
resolution at 25 μm) add further evidence to our interpretation of
the DHAR-1 proxies. A notable grouping of U
2
+
,Sr
2
+
, and Ba
2
+
in PC2 of a Principal Component Analysis (PCA) on the trace
element pro
fi
le over 4.2
3.1 ka BP (Fig.
3
), and their positive
correlation (Supplementary Fig. S8), suggest that PAP in
fl
uences
the concentration of these elements. In particular, U
2
+
and Sr
2
+
consistently follow the other PAP proxy,
δ
44
Ca (Fig.
2
a). The
cluster of transition metals Zn
2
+
,Ni
2
+
, and Cu
2
+
in PC3 (Fig.
3
)
highlights elements that may respond to drip-rate changes in
aragonite stalagmites
55
(see Supplementary Discussion for addi-
tional transition metal discussion
)
. Indeed, the transition metals
all indicate lower drip rates during the period of less rainfall from
4.2
3.97 ka BP. By extension, low drip rates also occur around
3.6 ka BP and again after 3.3 ka BP.
Furthermore, the redox-sensitive ions of S
2
+
,Mn
2
+
,Fe
2
+
,and
Cr
2
+
,togetherwithMg
2
+
and Si
2
+
, can be separated from the other
two groups in PC3 (Fig.
3
). While higher Mg and Si likely re
fl
ect
increased input of detrital material or enhanced weathering of fresh
bedrock material, the grouping of re
dox-sensitive elements suggests
thatthesoilabovetheepikarstmaybesensitivetosaturated
conditions that enable a series of redox processes
62
.Notably,theS/Ca
record deviates after 3.72 ka BP (marking a period of higher and
more variable sulfur content), and occasionally aligns with the PAP
(U/Ca, Sr/Ca) and drip rate (Zn/Ca,
Ni/Ca) proxies (Supplementary
Figs. S6 and S9). This dynamic suggests that another environmental
factor emerges after 3.72 ka BP
possibly related to redox conditions
(e.g., waterlogging) in the overlying soils. Concurrent peaks in the
redox-sensitive element
s (Supplementary Fig. S6) are thus interpreted
as periods when a more saturated drip
fl
ow path promoted water
Fig. 2 DHAR-1 proxies from 4.2 to 3.1 ka BP. a
4.27
3.1 ka BP DHAR-1 record with high-resolution
δ
18
O,
δ
44
Ca, and
δ
13
C stable isotope time series (note
reverse axes) and Ba/Ca, Sr/Ca, U/Ca, Ni/Ca, Zn/Ca, and Cu/Ca trace element data shown as 20-year LOESS smoothing curves. Dominant processes for
the proxies are indicated with arrows on the right (Indian Summer Monsoon, prior aragonite precipitation, and drip rate).
δ
13
C is not speci
fi
ed because it is
affected by a (variable) combination of prior aragonite precipitation, cave ventilation, and long-term soil carbon changes driven by vegetation ab
ove the
cave.
b
4.27
3.85 ka BP in more detail covering the 4.2 ka event, with
δ
18
O and
δ
44
Ca stable isotope time series, and U/Ca and Ni/Ca with 10-year LOESS
smoothing curves. The asterisk represents the transition from calcite to aragonite after a hiatus. For each series, shaded envelopes represent the 2
.5 and
97.5% proxy con
fi
dence intervals, and horizontal dashed lines show the mean value over 4.2
3.1 ka BP. Vertical shaded bars denote relatively dry (yellow)
and wet (blue) intervals between 4.27 and 3.85 ka BP. U-series dates with ±2
σ
error bars are shown at the bottom of the graphs above the lower age axes.
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retention
suggestive of a hydrological (and perhaps ecological) shift
associated with increased soil moisture and change in the seasonal
timing of precipitation.
Multi-proxy interpretation
. Correlations between individual
proxies (
δ
44
Ca,
δ
13
C,
δ
18
O, Sr/Ca, S/Ca, Ba/Ca, Zn/Ca, and Ni/
Ca) were examined for three discrete time periods over
4.14
3.61 ka BP (Supplementary Fig. S9). This analysis highlights
incongruous relationships between several pairs of proxies after
3.94 ka BP or 3.72 ka BP (e.g.,
δ
13
C with
δ
18
O, Sr/Ca, and U/Ca;
δ
18
O with Zn/Ca and Ni/Ca; Sr/Ca with Zn/Ca and Ni/Ca),
suggesting that certain proxies display threshold behavior (e.g.,
PAP occurs only during extreme drought). A nuanced inter-
pretation may be appropriate in some periods; for example, c. 3.6
and 3.3 ka BP when trace elements indicate a decreased drip rate
but also less PAP, which could result from a lengthened ISM
season that distributes the same amount of moisture over a longer
wet season. After 3.4 ka BP, the
δ
13
C and drip rate proxies (Cu/
Ca, Zn/Ca, and Ni/Ca) consistently indicate wetter and/or
warming conditions (with
δ
13
C suggesting either vegetation that
thrives in wetter conditions, less CO
2
degassing due to wet con-
ditions, or less ventilation from warming), and this appears
related to winter precipitation because the ISM proxies show a
decrease in summer precipitation. Drip rate could re
fl
ect both
ISM and IWM precipitation, and thereby requires reliable ISM or
IWM proxies to distinguish a seasonal interpretation. In contrast,
the correlation between
δ
13
C and the drip rate proxies breaks
down from 3.7 to 3.4 ka BP. Given that
δ
13
Cisin
fl
uenced by a
number of factors
30
, it is plausible that ventilation or CO
2
degassing could be the primary in
fl
uence on
δ
13
C during some
periods, while gradual shifts in soil carbon above the cave could
in
fl
uence the overall (centennial-scale) trends. For example, if
winter precipitation decreased over decades to centuries, it is
possible that winter- and spring-drought sensitive trees
43
46
would be increasingly replaced by C
4
-type plants, leading to
higher
δ
13
C (as seen between 3.9 and 3.4 ka BP).
Of the entire DHAR-1 record, the 230-year period after 4.2 ka
BP stands out for its above-average
δ
18
O,
δ
44
Ca,
δ
13
C, Ni/Ca, Zn/
Ca, and Cu/Ca, while Ba/Ca, Sr/Ca, and U/Ca values are all
below-average (Fig.
2
a). This agreement across all proxies reveals
three distinct dry periods lasting 25
90 years each, which
correspond to the general timing of the
4.2 ka event
that is
associated with the mid-late Holocene transition (Fig.
2
b). After
3.97 ka BP, some divergent trends emerge in ISM (
δ
18
O), PAP
(
δ
44
Ca,
δ
13
C, U/Ca, Sr/Ca) and drip rate (Zn/Ca and Ni/Ca)
proxies, suggesting that the drought threshold was not con-
sistently passed in both seasons and other environmental factors
assumed a more prominent role. While ISM rainfall increases
after 3.97 ka BP, PAP proxies indicate a more muted recovery (or
simply stabilization) of annual precipitation, with drip rate
recovering until a decrease around 3.6 ka BP, and
δ
13
C trends
point to cooling (enhanced ventilation) and/or drying conditions
in wintertime (either via more degassing from lower drip rate, or
a shift in soil carbon). Such a shift in seasonality towards more
pronounced rainfall contrast between summer and winter may
have contributed to long term changes in vegetation composition
(particularly away from winter-drought-sensitive C
3
-type trees to
more drought-tolerant C
4
-type grasses) that ultimately altered the
water retention and drainage properties of the soil above the cave
(redox conditions indicated by S/Ca, Fe/Ca, Mn/Ca, Cr/Ca). We
note that the
δ
13
C proxy is susceptible to multiple in
fl
uences,
while the Ni/Ca, Zn/Ca, and Cu/Ca seem to be reliable proxies for
drip rate, even though they are novel and untested in aragonite
speleothems. Keeping this caveat in mind, we cautiously interpret
the multi-proxy record as an increase in seasonality with stronger
ISM seasons and longer or drier winters between 3.97 and 3.4 ka
BP, followed by decreased seasonality as the ISM weakens and
winter moisture picks up again from 3.4 to 3.1 ka BP.
Fig. 3 Principal component analysis from 4.2-3.1 ka BP. a
Principal components 2 and 3 of the LA-ICP-MS dataset for the aragonite-only bottom half of
DHAR-1B (4.2
3.1 ka BP) with a 95% con
fi
dence ellipse (black circle), showing distinct groups of the drip-rate-sensitive transition metals Ni, Zn, and Cu,
the PAP-sensitive trace elements Ba, Sr, and U, and the redox-related trace elements of Fe, S, Cr, and Mn.
b
Loadings for PC2 (12.4% of variance) and PC3
(9% of variance). Statistics were calculated and the
fi
gure was generated using PAST 4.10 software
87
. Note that PC1 (28.8% of variance) is not shown
the loadings indicate a dominance of U/Ca over other elemental ratios.
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5
Since Dharamjali Cave sits in a transition zone in
fl
uenced by
winter westerlies and the ISM, it is likely that multidecadal shifts
in Northern Hemisphere insolation, the Atlantic Meridional
Oscillation (AMO) and North Atlantic Oscillation (NAO), as well
as the Paci
fi
c Decadal Oscillation (PDO) and El Niño-Southern
Oscillation (ENSO) would play a role in modulating the strength
of the dry and wet season rainfall systems and total annual
rainfall over Dharamjali Cave, as it does in northeast India
63
.
Along with negative NAO conditions, atmospheric blocking
patterns over Europe can translate to drier westerlies in the
Mediterranean and western Asia, which is one mechanism
proposed for the 4.2 ka event
1
,
64
,
65
. Furthermore, warm PDO
or ENSO phases have been linked to a weaker ISM with less
rainfall in northeast India
14
,
66
,
67
.
4.2 ka event
. DHAR-1 provides vital insight into the seasonality of
the 4.2 ka event and the millennium thereafter. The unambiguous
agreement between all moisture proxies (ISM, PAP, drip rate) from
4.2 to 3.97 ka BP provides a convincing set of evidence for a multi-
season drought that overprinted all other environmental forcings in
the DHAR-1 cave system. Although we lack early-Holocene sta-
lagmite records older than 4.2 ka BP from Dharamjali Cave, we are
still able to put the DHAR-1 reconstruction into perspective with
post-4.2 ka BP climate conditions. Our multi-proxy time series
allows us to characterize seasonality changes during the 4.2 ka event
(dry in both seasons), the duration of the dry periods (25
90 years),
the timing of ISM recovery (after 3.97 ka BP), and environmental
and seasonality changes after 3.97 ka BP.
We compare DHAR-1 with nearby records to provide a more
(pan)regional view of mid-Holocene climate change (see locations
of studies from Fig.
4
in Supplementary Fig. S1). A highly
resolved and well dated stalagmite ML.1 from Mawmluh Cave
shows a decrease in rainfall closer to 4.0 ka BP that does not
recover over the next centuries
27
(Fig.
4
a), whereas another
stalagmite record (KM-A) from Mawmluh suggests lower rainfall
at 4.1 ka BP and subsequent recovery by 3.9 ka BP
25
. The
discrepancy between both Mawmluh records may be due to
dating and dissolution issues in the KM-A record
27
, thus we refer
to the more recent and well-dated ML.1 reconstruction from this
cave. The discrepancy in timing of drought between the similarly
well-dated and highly-resolved ML.1 and DHAR-1 records is
most likely due to regional variation in ISM behavior. The
distance between Mawmluh and Dharamjali caves is approxi-
mately 1200 km (east-west), where Mawmluh cave sits much
closer to the Bay of Bengal moisture source for the ISM and is
completely removed from the westerlies/IWM in
fl
uence in
winter. Therefore, it would be reasonable to expect a reduction
in ISM rainfall to affect more distant locations like Dharamjali
more severely. We also cannot exclude the possibility that
changes to the Arabian Sea branch of the ISM might have
decreased ISM moisture in the western regions of India (and
thereby primarily impact the Dharamjali record), while the Bay of
Bengal branch may have been impacted differently
32
,
68
.
Similar to Dharamjali, the ISM record of marine core 63KA
from the northeast Arabian Sea shows a double-peaked increase
in
δ
18
O values centered around 4.1 and 4.0 ka BP that coincides
with reduced Indus River out
fl
ow, which is followed by a recovery
after 3.9 ka BP
18
,
69
(Fig.
4
b). In the Red Sea, high-salinity
conditions are observed over the same period
70
(Fig.
4
c). The
4.2 ka event is typically characterized as an undifferentiated
multi-century-scale drought, but the DHAR-1 record provides
considerably
fi
ner-grained detail over the 4.2 ka event and reveals
three major phases of lower rainfall peaking at 4.19, 4.11, and
4.02 ka BP, each lasting 25
90 years and separated by 20
30-year-
long recovery phases (Figs.
2
b and
4
d, e).
Notably, relatively wet winter conditions are apparent for
several centuries preceding the 4.2 ka event, as demonstrated by
the Gol-e-Zard speleothem record from Iran
23
(Fig.
4
f) as well as
enhanced upper ocean mixing and more evaporative, windy
winter conditions in the NE Arabian Sea inferred from the 63KA
marine record
69
(Fig.
4
g). During the mid-late Holocene
transition, upper ocean mixing was subdued
69
and regionally
arid and dusty conditions affected Western Asia for 290 years
after 4.26 ka BP
23
.
In contrast, the Sahiya Cave speleothem record (SAH-2),
located c. 250 km further west than Dharamjali Cave, shows
relatively low
δ
18
O values between 4.2 and 3.5 ka BP that would
indicate a stronger ISM or IWM
28
. However, a prominent growth
rate minimum (2
3 μm/year) in the SAH-2 record during this
interval suggests that a reduced water supply to the stalagmite
may have affected its growth and signal a depositional hiatus,
which could mask a drought and explain the discrepancy with the
DHAR-1 record (Fig.
4
h). Consequently, additional records with
multiple proxies from this cave or nearby sites would be useful
additions to clarify its interpretation.
Overall, the dry winter and summer conditions between c.
4.2 ka BP and 3.9 ka BP resulted in aeolian dust spikes in the
Arabian Sea
71
(Fig.
4
i) and drying of lakes in continental India,
e.g., refs.
19
,
20
(Fig.
4
j). While ISM strength was already slowly
deteriorating prior to the 4.2 ka event due to decreasing Northern
Hemisphere summer insolation, e.g., refs.
4
,
69
, the shift from
exceptionally wet to markedly dry winter conditions would be
most perceptible in regions such as the Indus River Basin that
receive a high proportion of winter rain.
Cultural implications
. Importantly, DHAR-1
s precise age model
allows us to sub-divide the 4.2 ka event into separate severe arid
phases within a 230-year drier-than-normal period. Records of
the 4.2 ka event often portray it as a single mega-drought that
lasted around 100
200 years. The high resolution of the DHAR-1
record advances our understanding by revealing at least three
major dry periods within this period lasting 25
90 years each.
Since the 4.2 ka event is signi
fi
cant in part because of its impact
on large, complex Bronze Age civilizations, DHAR-1
s level of
temporal resolution is applicable to the human decision-making
timescale. While farmers and traders may be able to temporarily
adjust practices in the face of a multi-year drought, a severe
multi-decadal dry period affecting several generations of people
would prompt more far-reaching and permanent adaptations or
even population movement
particularly after the peak of the
fi
nal and longest 90-year drought by 4.02 ka BP.
Paleoclimate data suggest that the Early Harappan phase
(c. 5.0
4.6 ka BP) and the
fi
rst half of the Mature Harappan
phase (c. 4.6
3.9 ka BP) of the Indus Civilization were
accompanied by relatively strong winter westerlies and associated
IWM precipitation
23
,
69
, but concurrently declining ISM
precipitation
69
,
72
74
. After 4.2 ka BP, Harappa (an urban center
within the winter-rain dominated region) began to show signs of
decline such as disease outbreaks and deteriorating urban
systems
8
,
9
. Based on the DHAR-1 record, the three major phases
of lower rainfall after 4.2 ka BP each lasted >25 years over a c. 230-
year period, and would have had long-term environmental
impacts on daily and year-round access to water, predictability
of rainfall, and the extent, timing, and recurrence of river
fl
ooding.
Moreover, the adverse effects of droughts on rainfed and
fl
oodplain agriculture would have been ampli
fi
ed if both rainfall
seasons weakened or failed entirely. These periods of lower rainfall
are particularly long in human timescales, and would have
impacted multiple generations of individual populations and
in
fl
uenced their subsistence practices. The diversity of crops and
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farming practices of some populations of the Indus Civilization
made them more resilient to such changes
10
,
11
,
75
.
Populations of the Indus Civilization were already adapted to
cope with unpredictable climate conditions, and had the capacity
to utilize a range of agricultural strategies involving both summer
and winter crops
10
,
75
. However, through an extended, multi-
generational drought, such adaptation strategies would have
become necessary measures of last-resort, leading to reduced
surpluses, decreased margins of error, and elevated vulnerability
to environmental hazards and rapid hydrological changes
7
,
11
,
76
.
Such changes are re
fl
ected in a reduction in craft activities and
innovation in Indus urban centers, the decline of long-distance
exchange and trade, and a trend towards deurbanization and the
proliferation of smaller and more
fl
exible rural settlements
8
,
11
.
The DHAR-1 record suggests that both summer and winter crops
would have become increasingly challenging to grow after 4.2 ka
BP, though an ISM recovery by 3.7 ka BP would have favored
summer crops in ISM-dominated regions (Fig.
4
k). Aridity-
adapted crops like winter barley and summer millet would have
been the most successful under these changed circumstances,
while crop diversity (including rice, which continued being used
in the Late Harappan and post-Indus period) would have helped
mitigate risk. The likelihood that cropping strategies were
designed to mitigate risk is supported by the available archae-
obotanical evidence
7
,
8
,
11
,
75
,
77
,
78
. The observed decreasing reliance
on winter wheat after 4.2 ka BP and increasing presence of more
drought-tolerant summer crops bene
fi
ted smaller communities
that were self-reliant
11
,
79
,
80
, and perhaps even encouraged
pastoralism
8
,
81
. This socio-economic transformation was com-
bined with a spatial displacement of population towards
settlements in the ISM-dominated northeastern and southeastern
Indus regions, which also offer a higher total annual rainfall
10
.In
this respect, the stressors of a climatic shift with associated
environmental changes over a multi-generational period would
have led to sustainability through tactical and strategic sub-
sistence choices, as well as adaptation through movement away
from cities into new parts of the rural hinterland. The side effect
of such strategic choices, however, was a transition away from an
urban way of life that had seen the
fl
oruit of new technologies.
As the Late Harappan period transitioned into the Painted
Grey Ware period after 3.5 ka BP, the DHAR-1 record suggests a
state of increased seasonality between 3.97 and 3.4 ka BP, where
summer crops may have been favored over winter crops (Fig.
4
k),
but there is little archaeobotanical data available for this period as
yet. From 3.4 to 3.1 ka BP, the trend may have reversed to a state
of decreased seasonality with a weaker ISM and warmer/wetter
winters. It was during this period that larger settlements began to
appear at Charsadda, Taxila, and in the Bannu region, which all
lie along the western edge of the Indus River Basin
82
. Larger
settlements would develop in a range of locations in the Ganges
Fig. 4 Comparison of northwest South Asian records over the 4.2 ka event. a
Indian Mawmluh stalagmite ML.1
δ
18
O
27
,
b
Arabian Sea marine core 63KA
δ
18
Oof
G. ruber
400
500
μ
m
69
,
c
Red Sea marine core GeoB 5836-2
δ
18
Oof
G. ruber
70
,
d
Indian stalagmite DHAR-1
δ
18
O (this study),
e
DHAR-1
δ
13
C (this
study),
f
Iranian Gol-e-Zard stalagmite GZ-14-1
δ
18
O
23
,
g
marine core 63KA
Δδ
18
Oof
N. dutertrei
G. sacculifer
69
,
h
Indian Sahiya stalagmite SAH-2
δ
18
O
28
,
i
Gulf of Oman marine core M5-422 CaCO
3
71
,
j
Kotla Dahar lake
δ
18
O
19
, and
k
Early Harappan (EH, c. 5.0
4.6 ka BP), Mature Harappan (MH, c. 4.6
3.9 ka
BP), and Late Harappan (LH, c. 3.9
3.6 ka BP) periods, shown with interpreted favorability for summer and winter crop cultivation based on the climate
records in the
fi
gure. Dates are shown above each record with ±2
σ
error bars.
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7
River Basin after 3.0 ka BP. The nature of adaptations in these
later periods remain to be explored in detail, but together with the
example of the Indus Civilization, they provide informative
analogs for the modern situation where we again face a changing
climate, albeit one that is largely being impacted by human
actions.
Conclusions
The DHAR-1 paleoclimate record indicates that the 4.2 ka event
consisted of three distinct dry phases that involved a decrease in
both the Indian Summer Monsoon (ISM) and Indian Winter
Monsoon (IWM). Prior studies have focused exclusively on either
ISM or winter rainfall depending on their location, so additional
multi-proxy studies from the overlapping region (particularly the
western region, covering 5.0
3.0 ka BP) are critically needed to
better understand the interaction of these two rainfall systems.
The impact of the 4.2 ka event is especially detectable in north-
west South Asia because the ISM and IWM domains overlap in
this region, and both seasons were affected by decreased rainfall
during the mid-late Holocene transition. The role of winter
aridity is particularly noteworthy as it enhances and prolongs the
driest growing season. The DHAR-1 record shows a 230-year
period from 4.2 to 3.97 ka BP where all moisture proxies (ISM,
PAP, and drip rate) unambiguously indicate aridity, punctuated
by three 25
90 year-long phases of lower rainfall that lasted long
enough to affect multiple generations of individual populations
and their subsistence strategies. After 3.97 ka BP, diverging trends
in these proxies suggest yearlong aridity was replaced by more
complex shifts in rainfall seasonality and environmental condi-
tions
including a recovery of the ISM by 3.7 ka BP alongside
drier and/or cooler winters that may have also promoted a long-
term ecological shift towards more drought-resistant grassy
vegetation. Eventually, this may have increased waterlogging of
soil above the cave to the point of altering local redox conditions.
Shifting seasonality of precipitation appears to be a key factor
in
fl
uencing the populations in the Indus River Basin over the
mid-late Holocene, encouraging cropping adaptations and shift-
ing population centers based on the availability of food and water
throughout the year.
Methods
Dating and mineralogy
. U-series dating was performed at Caltech on 22 samples
(Supplementary Table S2). No date has yet been analyzed from the calcite segment
older than 4.14 ka BP due to a lack of specialized equipment needed for such small
and very likely Th-rich samples. Twelve U-series ages (between 2.55 and 4.14 ka
BP) were used to construct the
fi
nal age model (Supplementary Fig. S4), which was
built using ensembles of 2000 Monte Carlo simulations for each proxy using the
MATLAB-based COPRA routine that explicitly considers individual proxy
uncertainties
57
. The Piecewise Cubic Hermite Interpolating Polynomial (PCHIP)
interpolation method was used for all proxies. COPRA output has the distinct
advantage of showing both the age uncertainties as well as con
fi
dence envelopes for
all the proxy time series. X-ray diffraction (XRD) was used to check for miner-
alogical changes of aragonite v. calcite. Four 20 mg samples were milled from
DHAR-1A, mixed with ethanol and evenly smeared onto a glass plate. Samples
were loaded onto a Bruker D8 XRD instrument equipped with a MoK
α
source
Lynxeye XE-T PSD detector. Measurements spanned 0
40° angle at 0.037° steps,
206.5 s per step. Phases were identi
fi
ed using the PDF2 (Powder Diffraction File)
database in Eva V10.0 software.
Stable isotopes
. Using the DHAR-1A half of the speleothem, 750 samples were
milled for stable isotope analysis (
δ
18
O and
δ
13
C) at 100
300 μm resolution and
analyzed at GFZ Potsdam. Further high-resolution stable isotope analysis at the
University of Cambridge included 876 samples from the bottom 4 cm of the
mirroring slab DHAR-1B, covering c. 4.2
3.6 ka BP. In the high-resolution series,
74 samples were taken from the basal section of DHAR-1A because this portion of
the speleothem is better represented and preserved on the DHAR-1A slab. As the
mirror image of DHAR-1A, slab DHAR-1B is minimally offset (Supplementary
Fig. S5). The curvature of the speleothem and the high-resolution nature of the
stable isotope dataset from DHAR-1B (50 μm) renders some inevitable offsets, and
requires adjustment prior to comparison of the high-resolution and lower-
resolution pro
fi
les. A master depth scale was created based on the LA-ICP-MS
transects from slab DHAR-1B.
A Sherline micromill with a Ø 1 mm drill bit was used for stable isotope sub-
sampling. Sampling lines were selected near the central drip point of the
speleothem, where the laminations showed minimal curvature. To eliminate as
much cross-sample integration error as possible during the high-resolution milling
process, we
fi
rst milled a 1 mm deep trench along the sampling lines, as well as a
parallel trench 5 mm away from the sample line following established procedures
29
(Supplementary Fig. S5). All sampling equipment was cleaned with ethanol before
each sample. An air duster was used to remove residual dust. This trenching
process ultimately resulted in a 4-mm-wide section between the trenches that was
sampled at 50 μm resolution, at 0.75 mm depth.
For each sample, c. 200 μg of material was milled, of which c. 100
200 μg were
sealed in a Borosilicate glass exetainer vial with a silicone rubber septum, and
loaded onto the Thermo Gasbench autosampler in batches of 40 samples. Each
batch of samples included 10 reference carbonates of the in-house standard Carrara
Z (calibrated to VPDB using the international standard NBS 19) and 2 control
samples of Fletton Clay. Samples and standards were
fi
rst
fl
ushed with helium and
then acidi
fi
ed with 104% orthophosphoric acid for 1 h at 70 °C, and
fi
nally
analyzed with a Thermo Delta V mass spectrometer in continuous
fl
ow mode.
Precision of Carrara Z was ±0.06
(1
σ
) for
δ
18
O and
δ
13
C.
To merge the isotope data from the aragonitic and calcitic parts of the
stalagmite, a
+
1.16
carbon isotope correction and a
+
0.81
oxygen isotope
correction
83
was applied to the measurements from the basal section of speleothem
(247
250.3 mm) that consists primarily of calcite instead of aragonite. The original,
uncorrected calcite data are reported in data
fi
les, but the corrected-to-aragonite
data are plotted in all
fi
gures.
The
δ
44/40
Ca measurements were made on 60 samples of aragonite and
1 sample of calcite milled along the stalagmite growth axis between 4.2
2.8 ka BP.
Of these samples, 23 were initially milled at point locations on the DHAR-1A slab,
followed up by 38 high-resolution measurements on the DHAR-1B slab focused on
the period 4.2
3.6 ka BP (achieving a resolution of one sample per 12.6 years in this
period). In addition, cave host rock (
0.36
), drip water (
0.43
), and modern
carbonate samples (mean of
0.78
) were measured. The
δ
44/40
Ca measurements
were made on a ThermoFisher Scienti
fi
c Triton Plus Multicollector Thermal
Ionization Mass Spectrometer (MC-TIMS) at the University of Cambridge
following established methods
84
. Carbonate samples were dissolved in 2% nitric
acid, and a double spike of
42
Ca and
48
Ca (1:1) was added at a 10:1 sample to spike
ratio. A dose of c. 4 μg of Ca was loaded onto double rhenium
fi
laments and
activated with phosphoric acid. The NIST 915 A or 915B standard was measured
about every 10 samples, and yielded a 2
σ
error of 0.1
for 11 total measurements.
Trace element analysis
. The elemental composition of DHAR-1B was determined
using laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS).
Elemental data was measured at the University of Waikato under supervision of
Dr. Amanda French. The speleothem was ablated using a RESOlution SE series
193 nm excimer laser ablation system. Helium was used as the carrier gas to move
the aerosol to an Agilent 8900 Triple-Quad ICP-MS. DHAR-1B was ablated using a
50
μ
m diameter laser spot size, which traversed the speleothem parallel to the
growth axis at 24.3
μ
m/second to achieve a
fi
nal spatial resolution of c. 25
μ
m.
Before the measurement, each line was traversed by a rapid 100
μ
m spot size laser
ablation cleaning sweep to remove potential contaminants. For the base of slab
DHAR-1B, sampling was done parallel to the sampling tracks generated for the
stable isotope milling, and beyond this the laser ablation analysis was continued
along the growth axis in 8 segments (Supplementary Fig. S5). The glass standards
NIST610 and NIST612 bracketed measurement transects at least every 15 min to
correct for instrumental drift. Raw data was processed using the IOLITE data-
processing software
85
, and trace element/Ca mass ratios were calculated using Ca
as an internal standard assuming stoichiometry (40% Ca in CaCO
3
).
The LA-ICP-MS data was further post-processed in MATLAB to remove any
obvious outliers by identifying points ±4
σ
away from a 5-point running mean of the
dataset. The software program PAST 4.10 (Hammer et al., 2001) was used for the
principal component analysis (Fig.
3
). We used 16 of the element ratios over the time
interval 4.2
3.1 ka BP in the aragonite portion of the speleothem (Ba/Ca, Co/Ca, Cr/
Ca, Cu/Ca, Fe/Ca, K/Ca, Mg/Ca, Mn/Ca, Ni/Ca, Pb/Ca, S/Ca, Si/Ca, Sr/Ca, Ti/Ca, U/
Ca, and Zn/Ca). The correlation matrix was used to compare standardized variables,
and a Kaiser
Meyer
Olkin value of 0.84 indicates that the dataset and sampling
resolution is well-suited for principal component analysis. PC1 explains 28.8% of the
variance, highlighting the unique properties of U/Ca compared to the rest of the
elements (a strong effect of PAP). PC2 explains 12.4% of the total variance
(highlighting the PAP v. transition metal element clusters), and PC3 explains 9% of
the variance (distinguishing the detrital/weathering/redox indicators).
Data availability
The data
fi
les for producing the charts and graphs of this manuscript are deposited in the
Apollo repository at the University of Cambridge (
https://doi.org/10.17863/CAM.
95036
). The data are also available in the PANGAEA repository (
https://doi.pangaea.de/
10.1594/PANGAEA.956928
).
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Received: 21 December 2021; Accepted: 17 March 2023;
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Acknowledgements
The authors would like to thank Dr. Giulio Lampronti for assistance with XRD mea-
surements at the University of Cambridge. N.M. received
fi
nancial support from the
German Science Foundation (DFG projects MA4759/9-1 and MA4759/11-1). A.G.,
D.A.H., and C.A.P. received support from the European Research Council (ERC) under
the European Union
s Horizon 2020 research and innovation program (grant agreement
no 648609). A.G., A.H. and S.F.M.B. received support from the European Union
s
Horizon 2020 program (QUEST project, grant agreement no 691037), C.A.P. received
funding from the Global Challenges Research Fund
s TIGR2ESS project (BB/P027970/1),
and A.H. was further supported by Rutherford Discovery Fellowship (RDF-UOW1601)
awarded by Royal Society Te Ap
ā
rangi. Calcium isotope analyses were supported
through NERC NE/R013519/1 (H.J.B.). No permissions were required for sampling, but
we gratefully acknowledge the support of the local village leaders and
accompanying guide.
Author contributions
A.G., D.A.H., C.A.P., and S.F.M.B. designed research; A.G., S.F.M.B., A.D.F., H.J.B.,
G.H.H., J.F.A., B.P., and N.M. performed research; A.G., A.H., D.A.H., and S.F.M.B.
analyzed data; A.G. and S.F.M.B. prepared
fi
gures; A.G., D.A.H., C.A.P., H.J.B., and
S.F.M.B. wrote the paper.
Competing interests
The authors declare no competing interests.
Additional information
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and requests for materials should be addressed to Alena Giesche.
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Communications Earth & Environment
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