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CPD
11, 2729–2762, 2015
Evidence of
a prolonged drought
ca. 4200 yr BP
J. Ruan et al.
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Clim. Past Discuss., 11, 2729–2762, 2015
www.clim-past-discuss.net/11/2729/2015/
doi:10.5194/cpd-11-2729-2015
© Author(s) 2015. CC Attribution 3.0 License.
This discussion paper is/has been under review for the journal Climate of the Past (CP).
Please refer to the corresponding final paper in CP if available.
Evidence of a prolonged drought
ca. 4200 yr BP correlated with prehistoric
settlement abandonment from the
Gueldaman GLD1 Cave, N-Algeria
J. Ruan1,5, F. Kherbouche2, D. Genty1, D. Blamart1, H. Cheng3,4, F. Dewilde1,
S. Hachi2, L. R. Edwards4, E. Régnier1, and J.-L. Michelot5
1Laboratoire des Sciences du Climat et de l’Environnement, Gif-sur-Yvette, France
2Centre National de Recherches Préhistoriques, Anthropologiqes et Historiques, Algiers,
Algeria
3Institute of Global Environmental Change, Xi’an Jiaotong University, Xi’an, China
4Department of Geological Sciences, University of Minnesota, Minnesota, USA
5Laboratoire Géosciences Paris Sud, UMR 8148, Université Paris-Sud, Orsay, France
Received: 12 June 2015 – Accepted: 15 June 2015 – Published: 03 July 2015
Correspondence to: J. Ruan (jiaoyangruan@gmail.com)
Published by Copernicus Publications on behalf of the European Geosciences Union.
2729
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11, 2729–2762, 2015
Evidence of
a prolonged drought
ca. 4200 yr BP
J. Ruan et al.
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Abstract
Middle Holocene cultures have been widely studied round the E-Mediterranean basin
in the last 30 years and past cultural activities have been commonly linked with regional
climate changes. However, in many cases such linkage is equivocal, in part due to ex-
isting climatic evidence that has been derived from areas outside the distribution of5
ancient settlements, leading to uncertainty from complex spatial heterogeneity in both
climate and demography. A few high-resolution well-dated paleoclimate records were
recently established using speleothems in the Central and E-Mediterranean basin,
however, the scarcity of such records in the western part of the Mediterranean pre-
vents us from correlating past climate evolutions across the basin and deciphering10
climate–culture relation at fine time scales.
Here we report the first decadal-resolved Mid-Holocene climate proxy records from
the W-Mediterranean basin based on the stable carbon and oxygen isotopes analyses
of two U/Th dated stalagmites from the Gueldaman GLD1 Cave in N-Algeria. Com-
parison of our records with those from Italy and Israel reveals synchronous (multi)15
centennial dry phases centered at ca. 5600, ca. 5200 and ca. 4200 yrBP across the
Mediterranean basin. New calibrated radiocarbon dating constrains reasonably well the
age of rich anthropogenic deposits (e.g., faunal remains, pottery, charcoal) excavated
inside the cave, which allows the comparison between in situ evidence of human oc-
cupation and of climate change. This approach shows that the timing of a prolonged20
drought at ca. 4400–3800 yr BP blankets the onset of cave abandonment shortly after
ca. 4403 cal yrBP, supporting the hypothesis that a climate anomaly may have played
a role in this cultural disruption.
1 Introduction
As drought in NW-Africa is a recurring phenomenon and prolonged dry conditions exert25
a significant impact on local social systems, it becomes important to accurately docu-
2730
CPD
11, 2729–2762, 2015
Evidence of
a prolonged drought
ca. 4200 yr BP
J. Ruan et al.
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ment the role of drought conditions on the area. For instance, the most recent drought in
Algeria began in 1998, as part of a widespread pattern of drying in the N-Hemisphere,
and brought considerable loss in regards to water resource and agricultural yields (Ho-
erling and Kumar, 2003). Increasingly dry sub-tropical conditions are predicted as one
potential consequence of anthropogenic climate change, but current general circula-5
tion models do not completely capture the magnitude and spatial extent of observed
drought conditions (Seager et al., 2007). To help understand recent climate anomalies,
paleoclimate studies are crucial to characterize the range of potential natural variability
in the past and to improve our understanding of the links between regional drought and
large scale forcing. Instrumental data from weather stations in NW-Africa report less10
than one hundred years. Tree ring based drought reconstructions in Algeria and Tunisia
have been extended back to the last nine centuries, which reveals large spatial hetero-
geneity of past climate evolutions in NW-Africa and concludes that the climate anomaly
1998–2002 appears to be the most severe in the last millennium (Touchan et al., 2008,
2011). Holocene paleoclimate studies in other regions, however, have suggested larger15
oscillations at centennial to millennial time scales highlighting the need for new records
from this area (Mayewski et al., 2004; Wanner et al., 2008).
A significant climate excursion ca. 4200 yr BP has been widely reported and is con-
sidered as an ideal case to study the causes and effects of a large-scale climate
anomaly that occurred against background conditions similar to those of today (Berkel-20
hammer et al., 2013; Booth et al., 2005; Roland, 2012). The climatic expression of the
4200 yr BP event differs around the world. For example, it has been documented as
droughts in much of mid-to-low latitudes, across Africa, Asia and N-America, wet and
stormy in N-Europe and cooler in N-Atlantic (Booth et al., 2005; Roland, 2012). More
recently, this climatic anomaly was characterized by extreme dry conditions on high-25
resolved speleothem isotope records from the Central (Drysdale et al., 2006; Zanchetta
et al., 2014) and E-Mediterranean basin (Bar-Matthews and Ayalon, 2011), but, until
now, such records have not available in the W-Mediterranean which prevents us the
correlation of past climate anomalies across the basin.
2731
CPD
11, 2729–2762, 2015
Evidence of
a prolonged drought
ca. 4200 yr BP
J. Ruan et al.
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Aside from its climatic interest, such an episode likely influenced numerous human
cultures. Major societal changes have been observed across the Mediterranean basin
during the Mid-Holocene, and in particular, a catastrophic desiccation ca. 4200 yr BP
has been suggested to trigger the collapse of the Akkadian Empire in Mesopotamia,
the Old Kingdom in Egypt and the Early Bronze Age civilizations of Greece and Crete5
(Weiss and Bradley, 2000; Weiss et al., 1993; Wiener, 2014). These studies have
been stimulating an increasing number of debates on climate–culture relationship (e.g.,
Coombes and Barber, 2005). Uncertainty regarding the societal impact of such an
event is still large, due in part that climatic evidence, in many cases, has been de-
rived from regions far from the distribution of ancient settlements (e.g., Cullen et al.,10
2000). Although the 4200 yr BP dry event has been observed in several mid latitude
sites, the database remains incomplete and conflicting observations of climatic con-
ditions between seemingly adjacent regions exist (Magny et al., 2013; Staubwasser
and Weiss, 2006). Additionally, a recent study demonstrated that the climatic impact
on many agricultural settlements in ancient Near East was diverse even within spatially15
limited cultural units (Riehla et al., 2014).
In N-Algeria, the extinction of large mammal species (e.g., S. antiquus) during the
Mid-Holocene was correlated with regional climate aridity, likely due to the competition
with pastoralists and livestock for increasingly scarce water (Faith, 2014). Similarly, the
evidence of the aridity (i.e., the termination of the African Humid Period) that provoked20
this extinction has been derived from the Sahara and its surroundings (deMenocal
et al., 2000), which is several hundred kilometres away, leaving this assertion ambigu-
ous and stimulating the search for new high resolution paleoclimate records in the
area.
In this study, we document the Mid-Holocene climate history in the Western Mediter-25
ranean by decadal-resolved stable carbon and oxygen isotopes analyses of two U/Th
dated stalagmites from the Gueldaman GLD1 Cave of N-Algeria. We compare the
records with those established earlier in the Central and E-Mediterranean basin. In ad-
dition, we describe archaeological deposits layers inside the cave whose ages have
2732
CPD
11, 2729–2762, 2015
Evidence of
a prolonged drought
ca. 4200 yr BP
J. Ruan et al.
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been reasonably well constrained due to new radiocarbon dating. Finally we test the
links between cultural changes and climate anomalies with a particular emphasis on
the 4200 yr BP event.
2 Samples and methods
2.1 Study site5
Gueldaman GLD1 Cave is one of a series of karstic caves formed within the SE-ward
slope of the Adrar Gueldaman ridge, western part of Babor mountains in N-Algeria
(Kherbouche et al., 2014). It is located close to the large Soummam River, 5–6 km from
the Akbou town, and approximately 65km southern inland from the W-Mediterranean
Sea (36◦260N, 4◦340E, 507 ma.s.l.) (Fig. 1). Gueldaman GLD1 is a relatively short cave10
(total extension of ∼80 m) that developed in Jurassic limestone. The entrance, facing to
the SE, is a semi-circular ∼6 m large arch, leading to a dome-shaped ∼10 m high and
6 m wide corridor which ends with the main chamber “Grande Salle” at a depth of 30–
40 m. The area is covered by a thin layer (<10cm) of soil derived from the limestone
bedrock, wind-blown silicate dust, and organic matter from local vegetation such as15
Pistacia lentiscus, Quercus ilex, Buxus sempervirens, typical Mediterranean Garrigue
type plant assemblage (C3 dominated).
Local climate is Mediterranean semi-arid type, characterized by hot-dry summers
and mild-wetter winters. From the ERA-interim reanalysis data between 1979 and 2013
(http://apps.ecmwf.int/datasets/) the annual total rainfall is 516 mm, and the annual20
mean temperature is 17.2 ◦C. Rainfall occurs rarely in the summer (37 mm) but rel-
atively evenly through the autumn (155 mm), winter (178 mm) and spring (147 mm).
Gueldaman GLD1 Cave is well ventilated with the outside atmosphere due to its larger
opening and shorter extension. Hobo logger data at 10 min resolution from Novem-
ber 2013 to April 2015 shows significant variations in cave temperature ranging from25
2733
CPD
11, 2729–2762, 2015
Evidence of
a prolonged drought
ca. 4200 yr BP
J. Ruan et al.
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13.7 to 19.5 ◦C. The relative humidity varies from 56 to 94 %. Carbon dioxide has not
been measured, but it is likely to be close to the atmospheric value.
2.2 Stalagmites analyses
Two stalagmites and three modern calcites samples were collected in 2012 and 2013
from the main chamber of Gueldaman GLD1 Cave. Stalagmite GLD1-stm2 is 350mm5
long and 100–200 mm wide; GLD1-stm4 is 203 mm long with a diameter of 50–120 mm
(Fig. 2). They were halved and polished along the longitudinal axis. Both stalagmites
show well-marked laminae with several shifts in the drip apex of the lower parts (Fig. 2).
Black bandings, with visible incorporations of charcoal particles, are found throughout
both stalagmites profiles.10
U/Th dating
Seventeen powder samples were drilled from the two stalagmites and dated by a multi-
collector inductively coupled plasma mass spectrometer. The procedure to separate
uranium and thorium were referred to Edwards et al. (1987) and Cheng et al. (2013).
The dating work was carried out at the University of Minnesota (USA) and the Xi’an15
Jiaotong University (China). One dating from the base part of stalagmite GLD1-stm2,
for the exploration of preliminary age frame, was done at the Laboratoire des Sciences
du Climat et de l’Environnement (LSCE, France). The U/Th dates were reported in
years before 2000 AD. (Fig. 2, Table 1). The age model for both stalagmites was devel-
oped using the StalAge program (Scholz and Hoffmann, 2011) where a linear interpola-20
tion between depth and age is made through each progressive triplet of adjacent U/Th
dates (Fig. 3). This procedure provides a quantitative estimate of age uncertainty con-
tinuously along the record despite having analytical constraints only at locations where
the U/Th dates exist. Stalagmite growth rates were calculated based on the StalAge
age model (Fig. 4).25
2734
CPD
11, 2729–2762, 2015
Evidence of
a prolonged drought
ca. 4200 yr BP
J. Ruan et al.
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Stable isotopes
Four hundred and thirty samples were drilled every 1 to 2 mm along the stalagmite cen-
tral growth axis (Fig. 2). Stable carbon and oxygen isotopes compositions of both sta-
lagmites and modern calcites were measured using a VG-OPTIMA mass spectrometer
at the LSCE. For each analysis, 60 to 80 µg calcite powder is reacted with phosphoric5
acid at 90 ◦C, and the resultant CO2is measured relative to a reference gas that has
been calibrated against a series of isotopic standards. Duplicates were run every 10
to 20 samples to check replicability. All values are reported in ‰ relative to the V-PDB
(Fig. 4). The error is 0.08 ‰ for δ18O and 0.05 ‰ for δ13C.
2.3 Archaeological analyses10
Archaeological excavations were carried out at two sectors S2 and S3 inside the Guel-
daman GLD1 Cave during the 2010–2012 campaign (Fig. 1). This work consisted
mainly in collecting, identifying, and referencing the archaeological materials found in
stratigraphic layers (refer to Kherbouche et al. (2014) for details). More than 7000 an-
thropogenic remains were collected, consisting mainly of faunal remains, ceramic, and15
lithic and bone tools. Besides, all sediments were water screened through 1.5 and 4mm
mesh and subjected systematically to flotation with collection in a 250 µm mesh yielding
a huge amount of charcoals. Initial radiocarbon dating of upper stratigraphic sequences
from S2 and S3 gave the median ages ranging from ca. 6800 to 1500cal yr BP (Kher-
bouche et al., 2014). In order to refine the chronology of these deposits, in this study,20
six new charcoal samples were collected from the key archaeological layers in exca-
vation area MN 47/48 of S2. These samples were dated using the AMS radiocarbon
method at the CEA Saclay (France). Detailed procedures of the chemical preparation
and the dating in the lab were referred to Cottereau et al. (2007). The dates were cal-
ibrated using the IntCal13 dataset (Reimer et al., 2013) and reported in years before25
2000 AD (Table 2).
2735
CPD
11, 2729–2762, 2015
Evidence of
a prolonged drought
ca. 4200 yr BP
J. Ruan et al.
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3 Results
3.1 Stalagmites U/Th dates and growth rates
The uranium contents of measured stalagmites samples are relatively high ranging
from 95 to 225 ppb (Table 1). The 2 sigma U/Th errors vary from 20 to 210 years with
an average of 77years (1.6 %). The U/Th date (5070 ±194 yr BP) of sample GLD1-5
stm4-47 was detected as a major outlier by the StalAge program, thus, it was not used
to calculate the final age model. Calculated StalAge age model for stalagmite GLD1-
stm4 shows large errors up to 500 years during ca. 4900–4200 yrBP (Fig. 3). Based
on individual StalAge age model, stalagmite GLD1-stm2 grew continuously from ca.
6200 to 4100 yr BP, whereas stalagmite GLD1-stm4 grew continuously from ca. 580010
to 3200 yr BP (Fig. 3).
Stalagmite GLD1-stm2 shows high and variable growth rates (mean =180 µm yr−1)
with higher values ∼400 µm yr−1at ca. 4800–4500 yrBP; whereas stalagmite GLD1-
stm4 shows relatively lower and less variable growth rates (mean =120 µm yr−1) with
higher values ∼200 µm yr−1at ca. 3800–3200 yrBP (Fig. 4).15
3.2 Stable carbon and oxygen isotopes
The isotopic compositions of modern calcite vary from −5.40 to −5.56 ‰ for the δ18 O
and from −8.43 to −10.34 ‰ for the δ13C. The δ18 O values from stalagmites GLD1-
stm2 and GLD1-stm4 range from −7.8 to −2.8 ‰ and from −7.3 to −0.6 ‰, respec-
tively; the δ13C values range from −10.6 to −3.3 ‰ and from −11.9 to −0.6 ‰, respec-20
tively. The δ18 O and δ13C significantly correlate in both stalagmites: R=0.87, P < 0.01
for GLD1-stm2 and R=0.92, P < 0.01 for GLD1-stm4. Albeit the different amplitudes,
the isotopic profiles of the two stalagmites show similarities during their common de-
velopment of ca. 5800–4100 yr BP: relatively elevated isotope values are found at ca.
5700–5400, ca. 5200, and ca. 4500 yr BP (Fig. 4). Two other isotopically enriched pe-25
riods in stalagmite GLD-stm2 are found at ca. 6200 and ca. 4900 yr BP (Fig. 4). There
2736
CPD
11, 2729–2762, 2015
Evidence of
a prolonged drought
ca. 4200 yr BP
J. Ruan et al.
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is a common isotopic enrichment trend since ca. 4800–4600 yrBP (depending on in-
dividual age model; abrupt in stalagmite GLD1-stm4 whereas more gradual in GLD1-
stm2). Toward the end of this trend, the most prominent anomaly occurs in stalagmite
GLD1-stm4 at ca. 4400–3800 yr BP during which the δ18O values are enriched by ap-
proximately 3.5 ‰ relative to the background values of that time as well as the modern5
calcite values for a period of ∼500 years (Fig. 4). Specifically within this anomalous
period, there is a mild depletion of δ18O at ca. 4200–4000 yrBP, followed by a second
enrichment toward its end during which stalagmite GLD1-stm2 stops growing (Fig. 4).
The last stage, ca. 3800–3200 yr BP, of GLD1-stm4, is characterized by a δ18O recov-
ery of about −3 ‰, synchronous with increased growth rates (Fig. 4).10
3.3 Anthropogenic deposits and 14C dates
Excavations inside Gueldaman GLD1 Cave revealed a large variety of archeological re-
mains and, among them, are numerous precious macro charcoals that have been used
for establishing the chronology of the deposits. In the ∼7 m2total excavated area of S2,
more than 7000 archaeological objects were identified and consisted of faunal remains,15
lithic artifacts and grinding equipment, potteries, bone tools, ornaments, and ochre. In
addition, a fragment of a human mandible and two isolated teeth were found during the
excavation 2010–2012 in Gueldaman GLD1 Cave (Kherbouche et al., 2014). These
deposits belong mainly to the Neolithic; only the top level of the sequence contains pot-
sherds of the historic period. In the lower Neolithic levels, identified domestic species20
(i.e. sheep and goats) represented ∼25 % of total faunal assemblages (N=2378) sug-
gesting a partly pastoral based economy. The potteries (N=825) are mostly related
to cooking vessels of 25–40 cm rim diameter. Hundreds of black charcoals (>1 cm)
were found and always associated with ceramic concentrations suggesting evidence of
cooking activities.25
Determined radiocarbon dates give the median ages of the sequence between 7002
and 1482 cal yrBP, with their 2σ-error intervals varying from 132 to 374 years (Table 2).
These dates provide a first chronology for the archaeological deposits excavated from
2737
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11, 2729–2762, 2015
Evidence of
a prolonged drought
ca. 4200 yr BP
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sector S2 (Fig. 6) (Kherbouche et al., 2014): anthropogenic remains (i.e. charcoals,
bones, teeth and potteries) are numerous during ca. 7002–6003 cal yrBP (depths of
∼150–120 cm), decreased at ca. 6003–4918 calyrBP (depths of ∼120–105 cm), most
abundant in the period of ca. 4918–4403 cal yr BP (depths of 105–75 cm), significantly
diminished during the long interval of ca. 4403–1484 calyr BP (depths of 75–60 cm),5
and finally, numerous again from ca. 1484 cal yr BP (depths of ∼60–50 cm). With an
overall decrease in archeological materials, there are two levels clearly marked by their
poverty in charcoal and pottery during the periods of ca. 6003–4918 cal yr BP and ca.
4403–1484 cal yr BP (Fig. 6).
4 Discussions10
4.1 Climatic significance of stalagmites proxies
Stalagmite growth requires humid climates allowing sufficient water infiltration into the
cave. In arid and semiarid areas, water availability is an essential controlling factor for
stalagmite growth, as well shown by Vaks et al. (2013) who correlated growth periods
with periods of effective rainfall regimes. The growth cessation of stalagmite GLD1-15
stm2 by ca. 4100 yr BP may suggest a phase of increased aridity, which is consistent
with the extreme dry condition inferred from the most elevated isotope values in sta-
lagmite GLD1-stm4 (see discussion in the following paragraphs) (Fig. 4). The fact that
stalagmite GLD1-stm4 does not stop growing at that time may be attributed to a dif-
ferent sensitiveness of the reservoir feeding the two speleothems (Fairchild and Baker,20
2012). Moreover, fast stalagmite growths together with wide diameters are usually as-
sociated with high drip rates suggesting humid conditions. A wetter period ca. 4800–
4500 yr BP is indicated by the high growth rates of stalagmite GLD1-stm2. The fact
that it is not seen in the growth rate change of stalagmite GLD1-stm4 is probably due
to the lack of dating between 5023 and 4197 yr BP (Fig. 4). Another wetter period ca.25
3800–3200 yr BP is suggested by fast growths of stalagmite GLD1-stm4 (Fig. 4).
2738
CPD
11, 2729–2762, 2015
Evidence of
a prolonged drought
ca. 4200 yr BP
J. Ruan et al.
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Under isotopic equilibrium precipitation, stalagmite calcite δ18O depends mainly on
the temperature of calcite-water fractionation and on the δ18O of drip water that is
controlled by local rainfall δ18O (Genty et al., 2014). Observations from the IAEA net-
work show that the rainfall δ18O at many Mediterranean stations (including one in Al-
giers, Algeria) are partly controlled by the amount of rainfall (IAEA, 2005), which is5
coherent with previous studies that most stalagmite δ18O records from the Mediter-
ranean regions were interpreted to primarily reflect changes in rainfall amount (e.g.,
Bar-Matthews and Ayalon, 2011; Bar-Matthews et al., 1997, 2003; Drysdale et al.,
2004, 2006; Zanchetta et al., 2014). The temperature effect on calcite-water fractiona-
tion, on the other hand, is partly counteracted by the condensation temperature effect10
on rainfall δ18O (Drysdale et al., 2006). We note that this interpretation may particularly
hold true for the present study because the regional temperature seems to has been
relatively constant since the Mid-Holocene (Martrat et al., 2004).
The rainfall signal imprinted in the Gueldaman GLD1 stalagmite δ18O is probably
enhanced by two other processes - evaporation and disequilibrium isotopic fractiona-15
tion (Mickler, 2006), partly due to the large cave entrance. It has been recently shown
that evaporation in semiarid caves could cause 4–5 ‰ δ18 O enrichments of a wide
range of drip waters (Cuthbert et al., 2014). The Hendy test (i.e. studying the isotopic
variation in contemporaneous layer, Hendy, 1971) made on three different levels in sta-
lagmite GLD1-stm2 show that the δ18O and δ13C correlate (R=0.86, 0.89, and 0.93)20
and increase by up to ∼1 ‰ from the center toward the edge, suggesting that stalag-
mite calcites precipitate out of isotopic equilibrium. These two processes may partly
explain the significant correlation of δ18O and δ13C profiles (R=0.87 for GLD-stm2
and R=0.92 for GLD1-stm4). Moreover, the larger amplitudes of isotopic variations in
stalagmite GLD1-stm4 (Fig. 4) can be explained by the likelihood of suffering from more25
evaporative and non-equilibrium enrichments due to lower drip rates, being indicated
by its smaller diameters (Fig. 2).
Stalagmite δ13C variations have several potential causes, the most likely, consider-
ing the studied location and time interval, being variations in soil CO2input and water
2739
CPD
11, 2729–2762, 2015
Evidence of
a prolonged drought
ca. 4200 yr BP
J. Ruan et al.
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flow rate (Genty et al., 2001; McDermott, 2004). Despite the fact that soil biogenic
CO2production varies according to both temperature and moisture level, moisture is
likely to be a major controlling factor due to low temperature variability of the consid-
ered time interval and limited water availability under semiarid climates. Moisture also
influences water flow rate and thus the CO2loss during the prior calcite precipitation5
(Fairchild et al., 2000). Low flow rate under diminished moisture condition enhances
CO2loss due to longer travel time and preferential 12C removing from solution, caus-
ing enrichments in stalagmite δ13C (Johnson et al., 2006; Mickler, 2004). Eventually,
atmospheric rainfall largely determines the moisture level and controls the δ13C vari-
ations. Therefore, the significant correlation between δ13C and δ18 O suggest not only10
a disequilibrium fractionation but a common control of rainfall.
Consequently, synchronous variations in two isotopes can be interpreted in terms
of water balance change: a prolonged severe drought can be inferred using the most
elevated δ18O and δ13C values during ca. 4400–3800 yrBP together with drier events
at ca. 6200, ca. 5700–5400, ca. 5200, and ca. 4500 yr BP (Fig. 4). The average δ18O15
value during ca. 4400–3800 yr BP is enriched by about 3.5‰ relative to the modern
calcite values, suggesting that the climatic conditions were likely drier during the Mid-
Holocene than at present. Thus, the current climate in N-Algeria appears to be within
the range of natural variability and the 4400–3800 yr BP climate anomaly may be con-
sidered analogous to end numbers of the most recent and ongoing drying.20
4.2 Mid-Holocene climate anomalies across the Mediterranean basin and their
dynamic implications
High-resolution absolute-dated Mid-Holocene climate records are rare in the W-
Mediterranean basin, however, there are a number of paleoenvironment studies using
sediment cores that documented large oscillations in vegetation ecology and indicated25
climate anomalies during the Mid-Holocene. A drying trend from ca. 4600 calyr BP on-
wards was inferred, based on the decreasing pollen ratio of deciduous broad-leaf vs.
evergreen sclerophyllous taxa at Capestang in the Mediterranean S-France (Jalut et al.,
2740
CPD
11, 2729–2762, 2015
Evidence of
a prolonged drought
ca. 4200 yr BP
J. Ruan et al.
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2000). At a nearby site in NE-Spain, the Mid-Holocene most arid condition at ca. 4800–
4000 cal yrBP was interpreted using maximum salinity values, more positive organic
carbon isotope values, and decreased algal productivity in Estanya Lake (Morrellon
et al., 2009). In the Mediterranean S-Spain, desertification phases at ca. 5200 calyrBP
and ca. 4100 cal yrBP were inferred using multiple palaeoecological indicators includ-5
ing pollen, microcharcoal, spores of terrestrial plants, fungi, non-siliceous algae, and
other microfossils in Siles Lake (Carrión, 2002). Similar environment changes have also
been observed in the Central Mediterranean basin, as shown by a synthesis study of
lacustrine palynological data which suggested a dryness peaking at ca. 4000 cal yr BP
(Sadori et al., 2011). Increasing aridity at ca. 5000–4000 calyr BP was suggested to10
explain the increases in non-tree pollen percentage and micro charcoal content in the
Lago di Pergusa Lake, Sicily (Roberts et al., 2011; Sadori and Giardini, 2007; Sadori
and Narcisi, 2001). Close to our site, a study at Preola Lake, in E-Sicily, documented
a significant low stand lake level at ca. 4500–4000 cal yrBP suggesting extreme arid-
ity (Magny et al., 2011). Although the sampling and dating resolutions in most of the15
above studies are low, they are in good agreement with the present study regarding the
5200 yr BP dry event, the drying trend from 4800–4600 yr BP onward, and the 4400–
3800 yr BP drought.
Recently, evidence of detailed Mid-Holocene climate change has been shown in
high-resolution U/Th-dated speleothem records from the Central and E-Mediterranean20
basin. Drysdale et al. (2006) demonstrated a severe Mid-Holocene drought through
multiproxy analysis on a flow stone from Renella Cave, Central Italy. The following
work at nearby Corchia Cave by Zanchetta et al. (2007) revealed similar climatic
condition during the Mid-Holocene (Fig. 5). In the E-Mediterranean, Bar-Matthews
and Ayalon (2011) explicitly discussed the Mid-Holocene climate by high-resolution25
dating and isotopic analysis on speleothems from Soreq Cave in Israel (Fig. 5).
Zanchetta et al. (2014) made comparisons between records from the Central and E-
Mediterranean; they identified coeval dry events at ca. 5600 and ca. 5200 yr BP based
on comparable enrichments in speleothem δ18O from Corchia Cave and Soreq Cave.
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Detailed comparisons of these speleothem records with the Gueldaman GLD1 stalag-
mite records reveal similar variations. In particular, elevated δ18O values in the Guel-
daman GLD1 stalagmites at ca. 5700–5400, ca. 5200 and ca. 4400–3800 yr BP are all
identified in speleothems from the Renella, Corchia and Soreq suggesting that anoma-
lous dry conditions synchronously developed across the Mediterranean basin (Fig. 5).5
These observations indicate that climates across the Mediterranean might have been
under an identical regional scale climate regime during the Mid-Holocene.
It has been suggested that climate change in mid-latitude Europe and Mediterranean
might arise from a perturbation of the Westerlies from a high-latitude trigger (i.e. the
North Atlantic) (Bond et al., 2001; Drysdale et al., 2006; Zanchetta et al., 2014) or10
from dynamics within the tropics (Booth et al., 2005; Hoerling and Kumar, 2003). The
three dry periods in the Mediterranean are broadly in phase with the ice rafting events
in the subpolar North Atlantic (Bond et al., 2001), which suggests some links with
the N-Atlantic circulation. Based on the coincidence with the elevated wind strength in
Iceland (Jackson et al., 2005), Zanchetta et al. (2014) argued that the dry events at15
ca. 5700–5400 and ca. 5200 yr BP might be caused by reduction of vapor advection
into the Mediterranean, due to the intensification and northward displacement of the
N-Atlantic Westerlies. However, lacking evidence of strengthened wind in the fourth
millennium BP argues for a different forcing of the 4400–3800 yr BP drought. The con-
siderably lower amplitude of the Bond ice rafting event at ca. 4200 yr BP than at the fifth20
millennium BP also indicates a varied ocean–atmosphere circulation state. The mod-
ern mid-latitude droughts (1998–2002) have been linked to the increased warmth in
equatorial oceans (Booth et al., 2005). During this event, SST changes lead to persis-
tent high pressure over the Northern Hemisphere’s mid-latitudes, causing widespread
synchronous drought (Hoerling and Kumar, 2003). However, the challenge in apply-25
ing the dynamics under the 1988–2002 drought toward an understanding of the 440–
3800 yr BP climate anomaly is that while the mechanism operate effectively on short
time scales, it has never been tested as to whether they could produce an anomalous
climate mode for several centuries (Berkelhammer et al., 2013). General circulation
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model simulations that begin with realistic boundary conditions and are perturbed with
a variety of forcings have been successfully undertaken to understand potential mecha-
nisms that lead to the 8200 yrBP event (Tindall and Valdes, 2011). Similar efforts would
be a useful starting point to produce hypotheses for the dynamical underpinnings of the
4200 yr BP event.5
4.3 Possible relations between climate anomaly and cultural change
A regional drought ca. 4200 yr BP has been widely linked to ancient cultural changes
in the E-Mediterranean and the Asia (Staubwasser and Weiss, 2006), though, in many
cases, climatic inferences have been derived from sites that are distant to these human
settlements. For instance, evidence of reduced precipitation from elevated δ18O of Irish10
stalagmites and increased dust input into the Gulf of Oman sediment core has been
suggested to contribute to the collapse of the Akkadian imperial in Mesopotamia (Bar-
Matthews and Ayalon, 2011; Cullen et al., 2000; Weiss et al., 1993). Similarly, a dry
period inferred from reduced discharge of the Indus river and elevated δ18O of a NE-
Indian stalagmite has been linked with the Indus Valley de-urbanization (Berkelhammer15
et al., 2013; Staubwasser et al., 2003; Staubwasser and Weiss, 2006). A recent study
in ancient Near East, however, revealed that the regional impact of the drought on
ancient civilizations, being influenced by geographic factors and human technology,
were highly diverse even within spatially limited cultural units (Riehla et al., 2014). This
highlights the need for caution when linking human activities from a site to the evidence20
of climate oscillations from another one.
The present study in the Gueldaman GLD1 Cave provides an opportunity to test
climate–culture relations by comparing in situ archeological sequences and high res-
olution paleoclimate records, thereby avoiding the uncertainty of inter-site correlation
arising from complex spatial heterogeneity in climate and demography.25
To facilitate the comparison, stalagmite-inferred climate changes at the cave site
during ca. 6200–3200 yrBP are separated into four stages 1–4 (Table 3):
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–Stage 1 (∼6200–5100 yr BP): wet, superimposed by several centennial-scale
drier events;
–Stage 2 (∼5100–4400 yr BP): wettest, ending with a ∼200-yr-long shift from the
wettest to extreme dry conditions;
–Stage 3 (∼4400–3800 yr BP): a drought-like climatic anomaly;5
–Stage 4 (∼3800–3200 yr BP): relatively wet.
In parallel, from the abundance of archaeological remains (especially bone, charcoal
and pottery) (Fig. 6), the temporal evolution of past cave occupations can be separated
into five phases 0–4 (Table 3):
–Phase 0 (∼7002–6003 cal yrBP): permanent and intensive occupation;10
–Phase 1 (∼6003–4918 cal yrBP): permanent but less intensive occupation;
–Phase 2 (∼4918–4403 cal yrBP): permanent and most intensive occupation;
–Phase 3 (∼4403–1484 cal yrBP): abandonment of the cave/occasional visits
–Phase 4 (∼1484 cal yrBP–): re-occupation of the cave
Correlations can be identified when comparing the climatic and archaeological records,15
though this does not necessarily mean that occupation of the cave depends merely on
climate (Table 3; Fig. 7). When the climate was wet and variable ca. 6200–5100 yr BP
(Stage 1), the Gueldaman GLD1 Cave preserved a few bones and rare charcoals and
potteries (Phase 1) (Fig. 7). When the climate was wettest ca. 5100–4400 yr BP (Stage
2), the most abundance of bones, charcoals and potteries suggests a permanent and20
more intensive occupation of the cave (Phase 2) (Fig. 7). More striking is the drought-
like climate anomaly that has been establishing ca. 4400–3800 yr BP (Stage 3), in
which the cave was abandoned for ca. 3000 years (indicated by a dramatic decrease in
anthropogenic remains, especially charcoal and pottery) (Phase 3) (Fig. 7). The rarer
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bones seen in this period imply that the cave might have been occasionally visited until
its re-occupation at ca. 1484 cal yrBP (Fig. 7). These observations argue for links be-
tween climate and settlement activity especially during the 4200 yrBP climate anomaly.
Water availability was likely crucial to maintain the Neolithic community at Gueldaman,
N-Algeria and the prolonged severe drought ca. 4400–3800 yr BP might have played5
a role in triggering the settlement abandonment, indicating that the pastoral economy
may not be as resistant, as commonly assumed, to climate anomaly in semiarid area.
Moreover, the sole piece of the bone from large ungulate (supposed to be elephant
or rhinoceros) found at the depth of ∼110 cm (Kherbouche et al., 2014) was anchored
by two calibrated 14C dates from present study between 6003 and 4913 calyr BP, which10
is in line with the latest survival of large mammal species (S. antiquus, E. Mauritanicus,
and E. melkiensis) at the proximate sites of N-Algeria during the Mid-Holocene (Faith,
2014 and references wherein). The extinction of the Mid-Holocene large mammal in N-
Algeria was attributed to the competition with pastoralists and livestock for increasingly
scarce water, corresponding with an abrupt climatic shift toward extreme aridity in the15
Sahara region ca. 5500 cal yr BP (i.e. the end of the Humid Africa Period, deMenocal
et al., 2000; Faith, 2014). Recently, the timing of this climatic transition was refined to
ca. 4900 yr BP ±200 yr (McGee et al., 2013). In addition, a paleoenvironmental study in
the Sahara revealed that the Mid-Holocene deteriorations of terrestrial ecosystem and
climate culminated at ca. 4200–3900 cal yr BP (e.g., Kröpelin et al., 2006). Therefore, it20
is more likely based on evidence from the Gueldaman GLD1 Cave and the proximate
sites (Faith, 2014) that extinction of large mammal/ungulate in N-Algeria occurred dur-
ing the prolonged drought ca. 4400–3800 yr BP.
5 Conclusions
It is increasingly clear based on a growing number of records spanning across much25
of mid-to-low latitudes, N-Europe, and the Atlantic ocean that there was a significant
large scale climate anomaly at around 4200 yr BP (Booth et al., 2005). The 4200 yr BP
2745
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aridity that had been suggested to affect the Early Bronze Age populations from the
Aegean to ancient Near East was recently characterized by high-resolution speleothem
records from the Central and E-Mediterranean basin (Bar-Matthews and Ayalon, 2011;
Drysdale et al., 2006; Zanchetta et al., 2014). The new record presented here from
the Gueldaman GLD1 Cave in N-Algeria provides increased evidence of a prolonged5
severe drought ca. 4400–3800yr BP, which suggests that the Mid-Holocene dryness
spread to the W-Mediterranean of N-Africa.
Radiocarbon dating made on charcoals constrains reasonably well the age of ar-
chaeological deposits excavated inside the cave (Kherbouche et al., 2014) and reveals
significant changes in human occupation during the last ca. 7000 years. Comparison10
of the stalagmite record with in situ archaeological sequence suggests synchronic-
ity between climate and settlement activity. Relatively wet/dry periods coincide with
the periods of more/less intensive human occupation. Particularly, the timing of the
prolonged drought at ca. 4400–3800 yr BP blanket the onset of the cave abandon-
ment event shortly after ca. 4403 cal yr BP, which argues a possible role of climate15
anomaly in this societal disruption. Further work on pollen-based reconstruction of
vegetation/environment change from the excavation sequence and on refinement of
the chronology of transitions between different occupation phases would potentially un-
cover the intrinsic relations among climate, environment and settlement. It is suggested
that the methodology and the findings from the present study at the Gueldaman GLD120
Cave be applied and tested at other sites.
Acknowledgements. Radiocarbon dating were analysed by Jean-Pierre Dumoulin at ARTEMIS
(LMC14, Saclay). We thank Edwige Pons-Branchu, Monique Pierre for the U/Th dating of
the base part of stalagmite GLD1-stm2 during the earlier stage of this study. We also thank
Lijuan Sha at the Xi’an Jiaotong University for assistance with the U/Th dating. Thanks25
to Cecilia Garrec for editing assistance. Funding is provided by the CNRS INSU program
PALEOMEX-ISOMEX, the NSFC grant 41230524 and the CSC scholarship.
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Table 1. U/Th dates from MC-ICP-MS analyses of stalagmites GLD1-stm2 and GLD1-stm4
from the Gueldaman GLD1 Cave. Analytical errors are 2σof the mean. U decay constants:
λ238 =1.55125 ×10−10 (Jaffey et al., 1971) and λ234 =2.82206 ×10−6(Cheng et al., 2013).
Th decay constant: λ230 =9.1705 ×10−6(Cheng et al., 2013). aδ234U=([234U/238 U]activity −
1) ×1000. bδ234Uinitial was calculated based on 230 Th age (T), i.e., δ234Uinitial =δ234Umeasured ×
eλ234×T. Corrected 230Th ages assume the initial 230 Th/232Th atomic ratio of 4.4 ±2.2 ×10−6.
Those are the values for a material at secular equilibrium, with the bulk earth 232Th/238 U value
of 3.8. The errors are arbitrarily assumed to be 50 %. cBP stands for “Before Present” where
the “Present” is defined as the year 2000 AD.
Sample 238U232Th 230 Th/232Th d234Ua 230Th/238U230 Th Age 230Th Age d234 UInitial
b 230Th Age Laboratory
(yr) (yr) (yrBP)c
Number (ppb) (ppt) (atomic ×10−6) (measured) (activity) (uncorrected) (corrected) (corrected) (corrected)
GLD1-stm2-7 169 ±0.1 396 ±0 136 ±1 863 ±2.3 0.105 ±0.001 6297 ±40 6228 ±72 863 ±2.3 6228 ±72 LSCE
GLD1-stm2-36 154 ±0.2 1922 ±39 137 ±3 910 ±2.2 0.103 ±0.000 6036 ±29 5848 ±136 925 ±2.3 5835 ±136 UM
GLD1-stm2-98 150 ±0.2 69 ±2 3077 ±73 776 ±2.4 0.086 ±0.000 5374 ±29 5366 ±29 788 ±2.5 5353 ±29 UM
GLD1-stm2-180 152 ±0.2 161 ±3 1157 ±25 644 ±2.0 0.074 ±0.000 5036 ±30 5018 ±33 653 ±2.1 5005 ±33 UM
GLD1-stm2-192 162 ±0.1 182 ±4 1158 ±24 807 ±1.7 0.079 ±0.000 4858 ±16 4840 ±20 818 ±1.7 4827 ±20 UM
GLD1-stm2-213 175 ±0.2 547 ±11 390 ±8 697 ±2.0 0.074 ±0.000 4841 ±33 4788 ±50 707 ±2.1 4775 ±50 UM
GLD1-stm2-286 169 ±0.2 207 ±4 980 ±21 756 ±1.8 0.073 ±0.000 4601 ±25 4581 ±28 765 ±1.9 4568 ±28 UM
GLD1-stm2-320 195 ±0.3 384 ±8 575 ±12 759 ±2.2 0.069 ±0.000 4321 ±26 4288 ±34 769 ±2.2 4276 ±34 Xi’an U
GLD1-stm2-340 194 ±0.3 354 ±7 612 ±13 800 ±2.9 0.068 ±0.000 4172 ±20 4142 ±29 809 ±2.9 4129 ±29 UM
GLD1-stm4-10 105 ±0.1 283 ±6 415 ±11 322 ±1.5 0.068 ±0.001 5734 ±90 5675 ±99 327 ±1.6 5662 ±99 UM
GLD1-stm4-24 126 ±0.1 983 ±20 135 ±3 347 ±1.9 0.064 ±0.001 5311 ±49 5143 ±128 352 ±1.9 5131 ±128 Xi’an U
GLD1-stm4-30 106 ±0.1 1362 ±27 80 ±2 298 ±1.9 0.062 ±0.001 5321 ±57 5035 ±210 302 ±1.9 5023 ±210 Xi’an U
GLD1-stm4-47 96 ±0.1 1104 ±22 88 ±2 290 ±1.9 0.062 ±0.001 5341 ±63 5082 ±194 294 ±1.9 5070 ±194 Xi’an U
GLD1-stm4-70 224 ±0.4 749 ±15 254 ±5 334 ±2.5 0.051 ±0.000 4282 ±27 4210 ±58 338 ±2.6 4197 ±58 UM
GLD1-stm4-113 155 ±0.2 130 ±3 910 ±20 363 ±2.1 0.046 ±0.000 3761 ±29 3743 ±32 367 ±2.1 3730 ±32 UM
GLD1-stm4-152 180 ±0.3 582 ±12 226 ±5 373 ±2.0 0.045 ±0.000 3590 ±25 3521 ±54 376 ±2.0 3508 ±54 UM
GLD1-stm4-195 175 ±0.1 929 ±19 131 ±4 369 ±1.7 0.042 ±0.001 3403 ±74 3290 ±108 372 ±1.8 3277 ±108 UM
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Table 2. Radiocarbon dates from AMS analyses of charcoals from excavation sector S2 inside
the Gueldaman GLD1 Cave. * Kherbouche et al. (2014). Ages are reported in years before
2000 AD.
Depth Z Square Lab No. Material 14C Age Median Age Cal. Interval Note
(cm) (SacA#) (±σ; yr) (yr) (2σ; yr)
60 M48 39 408 Charcoal 1600 ±30 1482 1385–1604 This study
65 N48 29731 Charcoal 1610 ±25 1484 1415–1547 *
84 N48 39410 Charcoal 4020 ±30 4495 4411–4785 This study
86 N48 39411 Charcoal 3975 ±30 4416 4290–4569 This study
91 M48 39 409 Charcoal 3945 ±30 4403 4244–4522 This study
108 N47 36982 Charcoal 4355 ±30 4918 4851–5032 This study
124 L48 23 883 Charcoal 5250 ±35 6003 5924–6178 *
132 L48 23 884 Charcoal 4260 ±30 6025 5933–6178 *
147 M47 36 981 Charcoal 6120 ±35 7002 6907–7157 This study
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Table 3. Summary of the features of climate and human activity in different climate stages and
occupation phases. Stages/phases 1–3 (bold) are presented in Fig. 7.
Climate stage Age Climate condition Occupation Age Human activity
(230Th yr) phase (14C cal yr)
– – – 0 ∼7002–6003 Permanent and inten-
sive occupation
1∼6200–5100 Wet & oscillatory 1 ∼6003–4918 Permanent but less
intensive occupation
2∼5100–4400 Wettest, ending with
a dramatic shift in
the last ∼200 years
2∼4918–4403 Permanent and most
intensive occupation
3∼4400–3800 Extremely dry 3 ∼4403–1484 Abandonment of the
cave/occasional visit
4∼3800–3200 Relatively wet 4 ∼1484– Re-occupation of the
cave
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Figure 1. The Gueldaman GLD1 Cave (36◦260N, 4◦340E, 507 m a.s.l.). The top left shows the
location of the Gueldaman GLD1 Cave, in the N-Algeria of W-Mediterranean basin; the low left
shows a photo of cave entrance and local vegetation cover; the right panel shows maps of inner
cave where stalagmites and archaeological deposits are collected.
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Figure 2. U/Th dating of stalagmites GLD1-stm2 and GLD1-stm4 from the Gueldaman GLD1
Cave. U/Th dates and 2σerrors are shown next to sampling positions.
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Figure 3. Age models of stalagmites GLD1-stm2 and GLD1-stm4 from the Gueldaman GLD1
Cave. The age models were calculated using the StalAge program (Scholz and Hoffmann,
2011). Note that the U/Th date of sample GLD1-stm4-47 was detected as a major outlier and
not used in the final age model of stalagmite GLD1-stm4. The 2σanalytical uncertainty of each
U/Th date (dot) is represented by the error bars, whereas the 95 % uncertainty assessed from
the model simulation is represented by thin curves.
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Figure 4. The δ18O,δ13 C and growth rate of stalagmites GLD1-stm2 and GLD1-stm4 from
the Gueldaman GLD1 Cave. U/Th dates with 2σerrors are presented at the top. The isotopic
ranges of modern calcites are also shown on the left (rectangles). Growth rates were calculated
from the StalAge age model. Note that the extraordinarily high and episodic growth rate at ca.
4600 yr BP in stalagmite GLD1-stm2 and at ca. 3800 yrBP in stalagmite GLD1-stm4 are likely
attributed to artificial simulations by the StalAge program and thus not discussed in terms of
climate in the text.
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Figure 5. Comparison of high-resolution Mid-Holocene stalagmite δ18O records across the
Mediterranean basin. From the top to bottom are stalagmite records from the Gueldaman GLD1
Cave in N-Algeria of W-Mediterranean basin (this study), the Corchia Cave (Zanchetta et al.,
2014) and the Renella Cave (Drysdale et al., 2006) in Central Italy of Central Mediterranean
basin, and the Soreq Cave (Bar-Matthews and Ayalon, 2011; Kaufman et al., 1998) in Israel of
E-Mediterranean basin. Different stalagmites from each area are represented in distinct colors.
U/Th dates with 2σerrors are shown at the top of each curve. Ages are reported in years before
2000 AD.
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Figure 6. Radiocarbon dating of anthropogenic deposits layers in excavation sector S2 in-
side the Gueldaman GLD1 Cave. From the left to right are 14C dates of charcoal samples,
anthropogenic deposit distribution, and a photo at depth across ∼75–88 cm showing a transi-
tion of layer from rich to rare anthropogenic deposits. Note that the gray colours highlight two
phases with diminished anthropogenic remains (especially pottery and charcoal) at ca. 4403–
1484 cal yr BP (depths of ∼75–60 cm) and ca. 6003–4918calyrBP (depths of ∼120–105 cm).
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Figure 7. Comparison between evidence of ancient human occupation and of past climate
change from the Gueldaman GLD1 Cave. Stages/phases 1–3 are same to those defined in
Table 3.
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