Mid- to late Holocene Indian Ocean Monsoon variability recorded in four speleothems from Socotra Island, Yemen

Mid- to late Holocene Indian Ocean Monsoon variability recorded in four speleothems from Socotra Island, Yemen

Quaternary Science Reviews 65 (2013) 129e142 Contents lists available at SciVerse ScienceDirect Quaternary Science Reviews journal homepage: www.els...

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Quaternary Science Reviews 65 (2013) 129e142

Contents lists available at SciVerse ScienceDirect

Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev

Mid- to late Holocene Indian Ocean Monsoon variability recorded in four speleothems from Socotra Island, Yemen Maïté Van Rampelbergh a, *, Dominik Fleitmann b, c, Sophie Verheyden a, d, Hai Cheng e, f, Lawrence Edwards f, Peter De Geest g, David De Vleeschouwer a, Stephen J. Burns h, Albert Matter b, Philippe Claeys a, Eddy Keppens a a

Earth System Sciences Department, Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussels, Belgium Institute of Geosciences, University of Bern, Baltzerstrasse 1-3, CH-3012 Bern, Switzerland Oeschger Centre for Climate Change Research, University of Bern, Zähringerstrasse 25, CH-3012 Bern, Switzerland d Geological Survey of Belgium, Royal Belgian Institute of Natural Sciences, Jennerstraat 13, B-1000 Brussels, Belgium e Institute of Global Environmental Change, Xi’an Jiaotong University, Xi’an 710049, China f Department of Geological Sciences, University of Minnesota, 100 Union Street SE, Minneapolis, MN 55455, USA g Bijlokestraat 57, B-9070 Destelbergen, Belgium h Department of Geosciences, University of Massachusetts, Morrill Science Center 233, Amherst, MA 01003, USA b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 May 2012 Received in revised form 11 January 2013 Accepted 17 January 2013 Available online 16 February 2013

Four stalagmites covering the last 7.0 ka were sampled on Socotra, an island in the northern Indian Ocean to investigate the evolution of the northeast Indian Ocean Monsoon (IOM) since the mid Holocene. On Socotra, rain is delivered at the start of the southwest IOM in MayeJune and at the start of the northeast IOM from September to December. The Haggeher Mountains act as a barrier forcing precipitation brought by the northeast winds to fall preferentially on the eastern side of the island, where the studied caves are located. d18O and d13C and Mg/Ca and Sr/Ca signals in the stalagmites reflect precipitation amounts brought by the northeast winds. For stalagmite STM6, this amount effect is amplified by kinetic effects during calcite deposition. Combined interpretation of the stalagmites’ signals suggest a weakening of the northeast precipitation between 6.0 and 3.8 ka. After 3.8 ka precipitation intensities remain constant with two superimposed drier periods, between 0 and 0.6 ka and from 2.2 to 3.8 ka. No link can be established with Greenland ice cores and with the summer IOM variability. In contrast to the stable northeast rainy season suggested by the records in this study, speleothem records from western Socotra indicate a wettening of the southwest rainy season on Socotra after 4.4 ka. The local wettening of western Socotra could relate to a more southerly path (more over the Indian Ocean) taken by the southwest winds. Stalagmite STM5, sampled at the fringe between both rain areas displays intermediate d18O values. After 6.2 ka, similar precipitation changes are seen between eastern Socotra and northern Oman indicating that both regions are affected similarly by the monsoon. Different palaeoclimatologic records from the Arabian Peninsula currently located outside the ITCZ migration pathway display an abrupt drying around 6 ka due to their disconnection from the southwest rain influence. Records that are nowadays still receiving rain by the southwest winds, suggest a more gradual drying reflecting the weakening of the southwest monsoon. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Speleothems Indian Ocean Monsoon Socotra Paleoclimate Stable isotopes Trace elements

1. Introduction The seasonal migration of the Intertropical Convergence Zone (ITCZ) and coupled monsoon systems influences more than half of

* Corresponding author. Tel.: þ32 2 6293397; fax: þ32 2 6293391. E-mail address: [email protected] (M. Van Rampelbergh). 0277-3791/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.quascirev.2013.01.016

the world population. For the countries around the Arabian Sea, at the northern limit of the ITCZ-pathway, a good understanding of the Indian Ocean Monsoon (IOM) and its summer and winter subsystems is of major importance, especially considering the predicted further drying (Fleitmann et al., 2007; Kropelin et al., 2008). Around the northern Indian Ocean, information on changes in the IOM wind direction and strength during the late-Pleistocene and

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Holocene have mostly been extracted from Indian Ocean sediment cores (Sirocko et al., 1993; Gupta et al., 2003; Ivanochko et al., 2005) and from dune deposits in the south of the Arabian Peninsula (Radies et al., 2005; Parker et al., 2006; Lezine et al., 2010). However, in these archives, radiocarbon ages with relative high uncertainties impede the precise determination of the timing and duration of IOM variations. Tree rings, allowing higher resolution dates, have proved useful around the Indian Ocean, but they are limited to the last 1000 years (Cook et al., 2010). The lacking information on hydrological patterns can be retrieved from speleothems (Neff et al., 2001; Wang et al., 2001; Fleitmann et al., 2003a; Shakun et al., 2007). Speleothems have a relatively fast and often continuous growth over long time periods allowing the elaboration of long-term records at high resolution (McDermott, 2004; Fairchild et al., 2006). Most important, speleothems can be precisely dated both by counting annual layers (if present) and/or by using the U/Th-dating method, which makes them powerful archives to date important climatic, historical or cultural events precisely (Wang et al., 2001; Henderson, 2006; Zhang et al., 2008; Cheng et al., 2009a). Most of available Holocene palaeoclimatological records for the area reflect variations in the summer IOM subsystem because the highest amounts of precipitation are associated with the summer IOM, also known as the Asian summer monsoon. Consequently, the precipitation signals brought by the winter or northeast IOM subsystem are often overwritten by the southwest signal. In this study, we present four new high-resolution stalagmite records sampled on the eastern side of Socotra Island (Yemen) (Fig. 1). Rainfall on the eastern side of the island mainly consists of northeast winter monsoon precipitation due to orographic effects (Scholte and De Geest, 2010). Eastern Socotra constitutes therefore an ideal location to study the changes in the northeast or winter IOM subsystem (source, directions, amounts). Different studies around the Arabian Sea have already shown that speleothem d18O values reflect changes in palaeo-precipitation intensities (Fleitmann et al., 2003b, 2004a, 2007; Shakun et al., 2007). In this study, several proxies (d18O, d13C, Mg/Ca and Sr/Ca ratios) are measured on selected stalagmites to complete the current understanding of the northeast IOM subsystem around the Northern Indian Ocean during the mid- to late Holocene (7 kae0 ka). A better insight in the mid- to late Holocene palaeoclimatic and environmental evolution of Socotra constitutes an important step to understand regional climate dynamics.

2. Regional setting 2.1. Site location and present climate Socotra Island lies in the northwestern part of the Indian Ocean, between the horn of Africa and the Arabic Peninsula (Fig. 1). Mean annual rainfall and temperature measured by a network of 11 meteorological stations from 2002 to 2006, are 216 mm and 28.9  C respectively, referring to a semi-arid tropical climate (Scholte and De Geest, 2010) (see also Fig. 1). The present-day climate on Socotra Island is governed by the seasonal migration of the ITCZ and episodic passages of tropical cyclones as well as oceane atmosphere interactions, such as the Indian Ocean Dipole (IOD) and the El Nino-Southern Oscillation (ENSO) (Cheung et al., 2006). From the western Pacific to the Indian Ocean, the ITCZ differs from its traditional notion as a narrow well-defined cloud band. The ITCZ above the Indian Ocean is broader in latitude with significantly more horizontal spatial variation (Waliser and Gautier, 1993). Because of its atypical configuration, discussion can arise on the exact use of the term ITCZ in region around the northern Indian Ocean. In this study, the ITCZ forms part of the ascending branch of the Hadley cell and is defined as the convergence zone between southwest and northeast winds where precipitation is fairly high. This definition is similar to the ITCZ definition used in Fleitmann et al. (2003a, 2007). In boreal summer, the low-pressure cell centred above the southern foothills of the Tibetan plateau pulls the ITCZ north until it reaches its northernmost position in July. During boreal autumn, the pressure gradients reverse and the ITCZ retreats southward until it reaches its southernmost position in January (Fleitmann et al., 2004b). This annual migration of the ITCZ creates two wind patterns on Socotra, known as the southwest summer and the northeast winter monsoon period interrupted by two short transition periods when winds from all directions ensue. The southwest monsoon season begins in early May (Fig. 2a) with a stable southwesterly and moisture loaden wind (course w230 ) coming from the Indian Ocean. In July, the ITCZ reaches its northernmost position, wind speed increases and the airflow takes a more westerly path (w240 ) across the semi-desert of Somalia. Strong and dry southwest monsoonal winds blow over the island. In September at the end of the summer monsoon season, wind direction swings back to w230 , wind speed decreases and winds filled with moisture from the Indian Ocean deliver precipitation

Fig. 1. Location of the island of Socotra in the northern Indian Ocean. Black dots represent the location of the caves. The numbers represent the location of the 11 meteorological stations studied in Scholte and De Geest (2010). The watershed is represented by the dotted line.

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Fig. 2. a) The black star indicates the location of Socotra. Wind patterns and ITCZ location for the northern Indian Ocean during (1) northeast winter monsoon in January, (2) southwest rainy season in May, (3) southwest summer monsoon in July and (4) northeast rainy season in November (adapted after Fleitmann et al., 2004a). b) Socotra’s monsoon and precipitation intensities for every month derived from NASA’s Tropical Rainfall Measuring Mission. Purple bars indicate percentages of the mean annual rainfall of 100.3 mm. The black and dotted lines indicate the percentage of monsoon strength for both monsoon periods. Grey bars indicate the transition periods.

(Culek et al., 2006). After an autumn transition period starting in October, the northeast monsoon starts in the first half of November (Fig. 2b). The northeast monsoon onset, however, is not as sudden as that of the southwest summer monsoon due to the lower

pressure gradient between the high-pressure cell above the Tibetan plateau and the high-pressure cell near Madagascar. During this period, rainfall reaches a maximum averaging of 120 mm (or 42% of the mean annual rainfall) as winds transport moisture originating

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from the warm Arabian Sea towards Socotra Island (Scholte and De Geest, 2010). In early February, the ITCZ starts to migrate back north and the northeast monsoon weakens. The spring transition occurs at the end of March with winds blowing from both northwestern and southwestern directions. At the beginning of April, southwest winds become dominant again, cumulus clouds form and bring rain (Culek et al., 2006). In summary, Socotra experiences two distinct rainy seasons (typically from April to June and from November to December) interrupted by two dry seasons (typically from January to March and June to October) (Fig. 2b). Since the ITCZ is defined as the convergence zone between northeast and southwest winds, the two Socotran rainy seasons are associated with the passage of the ITCZ over the island. Based on detailed analyses of cloud cover satellite images for Socotra Island (Fig. 1), the northern and southern plateaus experience different precipitation regimes (Scholte and De Geest, 2010). The 400e600 m high limestone cliffs at the northern and southern coast and elevated plateaus around the Haggeher Mountains cause orographic uplift and induce two distinct rain areas on both sides of the mountain range. The northern and eastern parts receive most of their rain during the northeast rainy season in November (Fig. 1) whereas the western and southern coasts receive almost equal amounts of rain during both rainy seasons (Scholte and De Geest, 2010). Sporadically, during strong southwest spring rainy years, the northern regions, which are normally located in the rain shadow of the Haggeher Mountains, can be influenced by southwest rains (Culek et al., 2006). Based on the geology and geomorphology, Socotra Island is subdivided into three zones: the Quaternary alluvial coastal and inland plains, the PalaeoceneeEocene reef-limestone plateaus and the Precambrian granitic Haggeher Mountains (Cheung et al., 2006). The limestone plateaus, covering approximately half of Socotra’s surface, are strongly karstified and harbour numerous large cave systems (Cheung et al., 2006). The main karst areas are the Momi karst plateau in the east, the Diksam/Sibehon karst plateau in the centre and the Ma’alah karst plateau in the northwestern part of the island (Fig. 1). Three stalagmites were sampled in Hoq Cave (12 35011.900 N; 54 21015.4400 E, elevation 335 masl), located on the Momi karst plateau, 5 km from the northeast coast (Fig. 1). The entrance, fully facing the seaside, is around 45 m wide and 30 m high. The first stalagmite (specimen Hq1) was sampled in 2000, 200 m from the entrance in a chamber that often experienced strong ventilation and varying humidity, as observed during fieldwork. Two coeval stalagmites, STM1 and STM6, were sampled 1 m next to each other in 2003 and 2006 respectively, approximately 2 km from the only known entrance, where ventilation is minimal. At this site, cave air temperature remained constant throughout the year at 25  0.5  C (continuously monitored between January and December 2003). Relative humidity measured during six visits between 2003 and 2006 was always higher than 98%. Stalagmite STM5 was collected from Casecas Cave (12 330 20.0200 Ne54180 33.3400 E, elevation 542 masl) in January 2004, 6 km southwest from Hoq Cave (Fig. 1), also on the Momi karst, in an upper gallery where ventilation is expected to be minimal. Temperature and humidity measured during sampling were 29  0.5  C and above 95%. 2.2. Palaeoclimate of the region Changes in palaeorainfall in the areas located at the northern fringe of the Indian Ocean Monsoon domain are generally explained by variations in the strength of the IOM and the associated boreal summer position of the ITCZ (Fleitmann et al., 2003a, 2007). Various Late Pleistocene palaeoclimatic studies using

different archives from northeast Africa (Gasse, 2000), southern Arabia (Burns et al., 1998, 2001; Preusser et al., 2002; Fleitmann et al., 2003a, 2003b, 2004a; Parker et al., 2006; Fleitmann et al., 2007; Fuchs and Buerkert, 2008; Lezine et al., 2010), the Arabian Sea (Gupta et al., 2003) and China (Dykoski et al., 2005; Wang et al., 2005), demonstrate that changes in the position of the ITCZ are going along with variations in IOM rainfall intensity. During the Glacial to Lateglacial, the ITCZ was considered to be located south of the Arabian Peninsula and IOM rainfall did not reach southern Arabia as indicated by the absence of speleothem growth (Burns et al., 2001; Fleitmann et al., 2007), lack of lacustrine deposits (Lezine et al., 2010) and homogenous sedimentation rates of aeolian sediments (Fuchs and Buerkert, 2008). A northward shift in the mean latitudinal position of the summer ITCZ and orbitally forced intensification of the IOM during the early Holocene led to the onset of a humid period. This Holocene wet optimum is well documented in northeast Africa and southern Arabia and characterized by widespread formation of lakes (Gasse, 2000) and enhanced speleothems deposition (high effective moisture) (Burns et al., 1998, 2001; Fleitmann et al., 2003a, 2007). The termination of the Holocene wet optimum is associated with a southward displacement of the ITCZ to its present-day position along the coast of southern Arabia (Fleitmann et al., 2007). Discrepancies in the timing and duration of this humid period remain. Speleothems from Oman and Yemen suggest an onset at 10.5 ka whereas data from Yemen (Lezine et al., 2007) suggest an onset at 12 ka or in the UAE an even later onset at 8.5 ka (Parker et al., 2006). However, most of the studies from southern Arabia confirm a period of maximal rainfall at 8 ka. According to sediment records from northern Oman (Fuchs and Buerkert, 2008), speleothems from China (Dykoski et al., 2005; Wang et al., 2005), Oman (Fleitmann et al., 2003a, 2007) and the findings of Lézine et al. (2010) in Yemen, a gradual long term decrease in precipitation starts short after 8 ka. In contrast, Burns et al. (2001) identified a reduction in rainfall at ca 6 ka in northern Oman as well as in the UAE where Parker et al. (2006) placed the Holocene wet period termination at 6 ka. These partially contrary results from various studies might be due to differences in the sensitivity of the investigated archives and to a more complex pattern of atmospheric conditions at a local scale, as will also be shown further in this study. Contrary to the long term decrease in precipitation on the Arabian Peninsula, a speleothem from western Socotra (Dimarshim Cave; Fig. 1) shows increasing humid conditions since 4.4 ka (Fleitmann et al., 2007). This anticorrelation between western Socotra and the Arabian Peninsula is explained by Fleitmann et al. (2007) as a result of a progressing southward displacement in the mean latitudinal summer position of the ITCZ, implying a decrease in precipitation in the areas located at the northern fringe of the IOM, but an increase in areas closer to the equator (Fleitmann et al., 2007). 3. Materials and methods For analyses, a slab of 1 cm was cut from the middle of each stalagmite parallel to its growth axis using a diamond saw. The slabs were polished using carbide powder and finished with Al2O3powder. Twenty-six (13 for STM1, 7 for STM6 and 6 for STM6) Useries age determinations were carried out at the University of Minnesota (USA), using the procedures for uranium and thorium chemical separation and purification described in Edwards et al. (1987) and Cheng et al. (2000, 2009a, 2009b). Age determinations on Hq1 were carried out at the University of Bern. Details on analytical methods are given in Fleitmann et al. (2007). Age models are established using the StalAge algorithm (Scholz and Hoffmann, 2011) and are reported with their uncertainty. All ages are expressed in a B2011.

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Samples for d13C and d18O measurements in STM1, STM5 and STM6 were drilled along the stalagmites central axis with a Merchantec Micromill. Ethanol was used to clean the speleothem surface and drill bit prior to sampling. Between every sampling, the drill bit and sampling surface were blown clean with compressed air. Sample resolution in STM1 was 500 mm for the upper 32 mm, from 48 to 51 mm and from 177 to 248 mm. The remaining parts were sampled at a 1 mm resolution. Stalagmites STM5 and STM6 were sampled every 500 mm. Stable isotope sampling in the Hq1 stalagmite was carried out in the upper 305 mm every 600 mm. One Hendy test was carried out on STM1 and three on STM5 (Hendy, 1971) by drilling samples along an individual growth layer. To obtain samples of modern precipitated calcite, 6 glass slabs were placed in Hoq Cave for approximately one year between January 2003 and May 2005; 2 slabs rested on top of STM6 and 4 at the end of the cave. Stable isotope analyses on STM1, STM5, STM6 and on fresh calcite from the glass slabs were carried out at the Vrije Universiteit Brussel with a Kiel-III-device coupled on a Thermo Delta plus XL. Analytical uncertainties (1s) were 0.06& for d13C and 0.08& for d18O. For isotopic analyses on stalagmite Hq1 approximately 5 mg of powder was drilled from the sample and analysed with an on-line, automated, carbonate preparation system linked to a VG Prism II isotope ratio mass spectrometer at the University of Bern. Reproducibility of standard materials is 0.08& (1s). All isotopic values are reported in per mille (&) relative to Vienna Pee Dee Belemnite (VPDB). Samples for elemental concentration determination of Ca, Mg and Sr were taken every 5 mm along the growth axis of STM1 and STM6 using a carbide dental drill (1 mm diameter). A total of 111 samples (ca 15 mg) for STM1 were analysed by Atomic Absorption Spectrometry at the Vrije Universiteit Brussel with analytical uncertainties (2s) less than 5%. 32 samples (ca 5 mg) for STM6 were measured on an Element 2 HR-ICP-MS at the Royal Museum for Central Africa (Brussels, Belgium) with analytical uncertainties (2s) less than 5%. Three seepage water samples from Casecas Cave and 21 samples from Hoq were collected for d18O measurements. The water samples were prepared using the CO2/H2O-equilibration method described by Epstein and Mayeda (1953). Measurements were performed on a Finnigan Delta E mass spectrometer at the Vrije Universiteit Brussel. All values are reported in per mill (&) relative to Standard Mean Ocean Water (SMOW). Analytical uncertainties (2s) were less than 0.10&.

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Table 1 Location of the 6 glass slabs in the cave, period of calcite deposition and d18O and d13C values of the fresh present-day calcite. Fresh calcite from glass slabs Location

STM6 STM6 END END END END

Period Start

Stop

January 2003 November 2004 October 2003 October 2003 November 2004 November 2004

January 2004 January 2006 November 2004 November 2004 May 2005 May 2005

d13C

d18O

(% VPDB)

(& VPDB)

7.7 4.5 7.2 3.2 7.9 7.3

2.5 2.1 3.4 2.2 4.2 4.1

8.69& and 2.41& around an average of 0.50& and correlate significantly with the d18O values (r ¼ 0.83 and p ¼ 3.5758  10129). Two periods of very positive isotopic values occur from 0.7 to 1.5 ka and from 3.9 until 6.9 ka. Stalagmites STM1 (length 520 mm, sampled January 2003) and STM6 (length 160 mm, sampled January 2006) were actively dripping when sampled at the end of the cave. Calcite fabric in both stalagmites varies between dark compact and white porous parts, with lamination only visible in the white porous parts. In STM1, at 10 cm from the base, a shift in growth axis occurs (Fig. 4). Based on 13 U-series dates (Table 2b) and the StalAge age model STM1 grew constantly since 5.6 ka at a rate of around 87 mm/yr. In STM6, 7 U/ Th-ages (Table 2b) show that the stalagmite grew constantly from 4.5 ka (Fig. 4) with an average growth rate of 34 mm/yr, which is similar to the growth rate of stalagmite Hq1 located about 2 km away, and three times slower than stalagmite STM1 only 1 m away. The STM1 d18O values vary between 4.24 and 0.84&, around a mean of 2.97& (Fig. 5). Similar d18O values are found in STM6 varying between 5.06& and 0.60& (average at 2.27&). The d13C values of STM1 vary between 12.11& and 3.64&, average at 8.41&. For STM6, the d13C values are slightly more positive, varying between 10.58& and 3.54& around an average at 5.88&. A good correlation can be established between the d18O and d13C profiles in STM1 (r ¼ 0.70 and p ¼ 1.7988  1093) and in STM6 (r ¼ 0.55 and p ¼ 8.6530  1026). Compared to stalagmite Hq1, sampled close to the cave entrance, STM1 and STM6 display generally more negative d18O and d13C values. From the start of the records until 3.8 ka, the STM1 and STM6 isotopic records display

4. Results 4.1. Hoq Cave The stable isotopic compositions of calcite deposited on glass slabs in the deepest parts of the cave (near STM6) display large variations between 2.15& and 4.22& for d18O and between 3.18& and 7.86& for d13C (Table 1). No link can be established between the measured isotopic values and the slabs location in the cave. A similar large range in d18O values is also observed in the 21 seepage waters ranging between 1.36& and 4.26& (Fig. 3). Stalagmite Hq1, sampled only 200 m from the wide entrance, is a 381 mm tall stalagmite that was still actively dripping when collected in 2000. Nine U-series dates carried out on the upper 290 mm (Table 2a) indicate continuous growth at an average rate of 32 mm/yr. According the results given by StalAge, large age uncertainties occur after 6.9 ka (Fig. 4). Because the timing of events is largely insecure after 6.9 ka, no climatic interpretations are based on that part. The d18O values vary between 3.06& and 0.70& around an average of 1.69& (Fig. 5). The d13C values vary between

Fig. 3. d18O composition of the 21 Hoq Cave (black dots) and 3 Casecas Cave (white dots) seepage waters. Most negative values occur in November, when Socotra receives most of its rain, suggesting that the drip water d18O values are influenced by the ‘amount effect’.

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Table 2a U/Th measurements for stalagmite Hq1 (University of Bern). All ages were converted to before 2011 Sample number

Depth (mm)

c (U) (ppb)

Hq1-1 Hq1-2 Hq1-3 Hq1-4 Hq1-5 Hq1-6 Hq1-7 Hq1-8 Hq1-9

13.1 38.9 67.1 110.3 157.1 191.9 220.l 250.1 290.3

20,321.1 22,029.6 10,921.3 22,383.2 13,513.6 8725.9 8048.1 12,223.9 18,368.8

a

        

c (Th) (ppb) 52.51 58.77 28.55 58.84 35.45 22.46 21.26 31.18 47.14

0.3160 0.4610 0.3595 2.4010 6.5158 6.0375 6.1969 4.5306 1.0082

        

0.0061 0.0222 0.0027 0.0133 0.0354 0.0308 0.0445 0.0248 0.0060

234

U/238U

1.0000 0.9931 1.0136 1.0166 1.0284 1.0007 1.0123 1.0234 1.0216

        

230

0.0006 0.0007 0.0009 0.0009 0.0009 0.0009 0.0010 0.0006 0.0007

Th/232Th

349.6907 808.1328 1341.2558 602.1049 195.7649 167.6249 196.0517 514.2779 3933.7280

        

230

7.4303 39.1271 14.8675 6.4128 1.4870 1.2385 1.7976 4.0313 34.9869

Th/234U

0.0018 0.0056 0.0143 0.0210 0.0304 0.0384 0.0494 0.0615 0.0686

        

0.0000 0.0000 0.0001 0.0002 0.0002 0.0002 0.0003 0.0004 0.0005

Age (ka)a 0.198 0.610 1.572 2.319 3.357 4.227 5.524 6.927 7.760

        

0.02 0.02 0.03 0.03 0.03 0.05 0.06 0.06 0.09

Age (ka B2011) 0.207 0.619 1.581 2.328 3.366 4.236 5.533 6.936 7.769

        

0.02 0.02 0.03 0.03 0.03 0.05 0.06 0.06 0.09

Ages relative to AD 2002.

decreasing values with similar centennial and millennial variations. After 3.8 ka, the d18O signal of both stalagmites varies around 2.5& without significant trend. Between 0 and 0.6 ka and from 2.2 to 3.8 ka the STM6 record shifts to more positive values. The Mg/Ca ratios (103) of STM1 range from 7.03 to 13.83, with an average of 9.40 (Fig. 5). The STM1 Sr/Ca ratios (103) vary between 0.20 and 0.34 averaging 0.27 and correlate successfully with the Mg/Ca ratios (r ¼ 0.65 and p ¼ 6.5197  1015) (Fig. 5). For STM6, similar conclusions can be established; the Mg/Ca (103) values vary between 7.74 and 13.03 and average at 10.69. The Sr/Ca (103) values vary between 0.21 and 0.31 and average at 0.26 and correlate well with the Mg/Ca (103) profile (r ¼ 0.72, p ¼ 6.6556  106). For all three stalagmites, calcite colour, growth rate and isotopic signals appear to correlate, with darker compact calcite (indicative

Fig. 4. Age versus depth plots and average growth rate of the 4 studied stalagmites. Black line represent the StalAge (Scholz and Hoffmann, 2011) age models. Grey lines indicate the uncertainties as modelled by StalAge.

of slow growth) coinciding with more positive d18O and d13C values. Lighter calcite is formed during faster growth periods and is characterized by more negative d18O and d13C values. In the coeval stalagmites STM1 and STM6, higher trace elemental concentrations correspond to darker calcite, slower growth rate and more positive isotopic values. Due to the low-resolution trace elemental measurements in STM6, the covariation with the d18O and d13C values is less clear. 4.2. Casecas Cave Three seepage water samples were collected from Casecas Cave during the dry winter monsoon season in January 2003 and 2004. Their d18O values vary between 1.97& and 3.01& (Fig. 3) and are slightly more negative than the January d18O values of Hoq Cave. STM5 was continuously deposited between January 2004 (date of sampling) and 0.7 ka as is shown by the six 230Th ages (Table 2b).

Fig. 5. d18O (black line) and d13C (grey line) values in & VPDB for the studied stalagmites plotted against age in ka before 2011. Dots with error bars mark the U/Th ages with their uncertainty. For STM1 and STM6, the Mg/Ca  103 record is indicated in black and the Sr/Ca  103 record is indicated in grey.

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135

Table 2b U/Th measurements for stalagmites STM1, STM5 and STM6 (University of Minnesota). All ages were converted to before 2011. Sample number STM1 STM1-1 STM1-2 STM1-3 STM1-4 STM1-5 STM1-6 STM1-7 STM1-8 STM1-9 STM1-10 STM1-11 STM1-12 STM1-13 STM6 STM6-1 STM6-2 STM6-3 STM6-4 STM6-5 STM6-6 STM5 STM5-1 STM5-2 STM5-3 STM5-4 STM5-5 STM5-6

Depth (mm)

6 50 80 166 201 219 302 352 386 422 428 487 548 17 42 50 82 100 166 12 15 31 52 64 88

238

U (ppb)

Hoq Cave 213.6  0.4 161.4  0.3 289.3  0.6 202.7  0.4 307.7  0.5 192.0  0.4 185.3  0.3 197.1  0.3 239.8  0.4 227.0  0.4 197.9  0.3 180.3  0.4 148.8  0.3 Hoq Cave 122.6  0.1 275.8  0.3 201.2  0.4 111.3  0.1 225.4  0.4 185.4  0.5 Casecas Cave 790  2 741  1 871  2 689  1 595  1 617  1

232

Th (ppt)

230

/Th232Th (atomic  106)

d234Ua

230

(measured)

/Th238U (activity)

230

Th age (y) (uncorrected)

230

Th age (y)b (corrected)

d234UInitialc (corrected)

230

Th age (ka B2011)d (corrected)

42 86 322 35 204 38 58 33 119 132 806 197 253

            

22 24 24 13 13 17 13 14 20 16 16 21 19

18 140 152 1590 454 1816 1632 3877 1420 1342 210 816 558

            

30 44 13 570 31 800 378 1590 240 168 6 88 45

99.8 92.6 94.1 100.2 98.8 91.4 93.9 98.2 106.6 100.5 101.1 91.3 94.9

            

2.9 2.9 2.7 2.1 1.8 2.8 2.1 2.2 2.6 2.5 2.2 3.1 3.0

0.00041 0.00451 0.01025 0.01678 0.01821 0.02164 0.03074 0.03946 0.04259 0.04723 0.05189 0.0540 0.0568

            

0.00036 0.00059 0.00047 0.00054 0.00037 0.00068 0.00058 0.00070 0.00081 0.00080 0.00091 0.0011 0.0014

41 450 1027 1678 1824 2186 3112 3993 4283 4787 5268 5540 5816

            

36 60 47 54 38 70 60 70 83 83 95 120 150

36 440 998 1673 1807 2180 3104 3990 4270 4772 5160 5510 5770

            

36 60 49 54 39 70 60 72 84 84 109 120 150

99.8 92.7 94.4 100.7 99.3 92.0 94.7 99.3 107.9 101.8 102.5 92.7 96.4

            

2.9 2.9 2.7 2.1 1.8 2.9 2.1 2.2 2.6 2.5 2.2 3.1 3.1

0.044 0.448 1.006 1.681 1.815 2.188 3.112 3.998 4.278 4.780 5.168 5.518 5.778

            

0.036 0.060 0.049 0.054 0.039 0.070 0.060 0.072 0.084 0.084 0.109 0.120 0.150

46 29 324 46 249 506

     

1 1 13 1 12 17

376 2140 184 1082 527 281

     

19 74 8 31 26 10

105.6 100.1 97.4 94.4 93.7 94.2

     

1.9 1.7 2.3 1.6 2.2 3.0

0.0086 0.0138 0.0180 0.0272 0.03528 0.0465

     

0.0004 0.0002 0.0003 0.0004 0.00036 0.0005

848 1372 1803 2747 3579 4737

     

38 18 28 41 38 53

838 1369 1761 2736 3549 4664

     

38 18 36 41 40 64

106 100 97.9 95 94.6 95.4

     

2 2 2.3 12 2.2 3.0

0.838 1.369 1.766 2.736 3.554 4.669

     

0.038 0.018 0.036 0.041 0.038 0.053

7237 884 7333 3073 579 4490

     

30 18 147 35 12 28

5.5 33 10 23.4 104 24.6

     

0.3 2 1 1.1 3 0.7

173.7 231.7 203.2 165.4 185.9 70.7

     

1.6 2.2 2.4 1.9 2.4 1.6

0.00315 0.0024 0.0053 0.00631 0.0061 0.01083

     

0.00018 0.0001 0.0001 0.00029 0.0001 0.00032

293 212 484 593 566 1110

     

16 9 10 28 9 33

66 184 280 481 542 910

     

115 22 145 62 19 110

173.7 232 203 165.6 186 70.8

     

1.6 2 2 1.9 2 1.6

0.073 0.184 0.280 0.488 0.542 0.917

     

0.115 0.022 0.145 0.062 0.019 0.110

230 Th dating results. The error is 2s. l230 ¼ 9.1577  106 y1, l234 ¼ 2.8263  106 y1, l238 ¼ 1.55125  1010 y1. Corrected 230Th ages assume the initial 230Th/232Th atomic ratio of 14.4  2.2  106. Those are the values for a material at secular equilibrium, with the crustal 232Th/238U value of 3.8. The errors are arbitrarily assumed to be 50%. a d234U ¼ [234U/238U]activity  1000. b Ages relative to AD 2003 for STM1, AD 2006 for STM6 and AD 2004 for STM5. Samples STM6-1, STM6-2, STM6-4, STM5-2, STM5-3 and STM5-5 are relative to AD 2011. c d234Uinitial was calculated based on 230Th age (T), i.e., d234 Uinitial ¼ d234 Umeasured  el234 T . d Ages before AD 2011.

STM5’s growth rate averages 123 mm/yr making this stalagmite the fastest growing speleothem of our four samples (Fig. 4). The large U/ Th-error bars, especially in U/Th-samples STM5-1 and STM5-3, are due to the high amounts of detrital 232Th, leading to large uncertainties in the age model (Fig. 4). Therefore, this record will only be used to discuss multi-millennial variations. STM 5’s d18O values, averaging 3.24&, range from 4.24& to 1.41& (Fig. 5). As for the seepage waters, these d18O values in STM5 are more negative compared to the d18O values found in the Hoq Cave stalagmites. The d13C values of STM5 range from 7.40& to 2.33& and average at 5.26&. They are more positive compared to the d13C values of the Hoq Cave speleothems. A good correlation can be established between the d18O and the d13C signal (r ¼ 0.76 and p ¼ 4.9724  1034). 5. Discussion 5.1. Low d18O and d13C values indicate wetter conditions Provided that calcite formed under conditions of isotopic equilibrium, the d18O of speleothem calcite is governed by the watere calcite fractionation factor (0.25& decrease per 1  C increase), and by the d18O of the cave-seepage water that in turn is determined by the d18O of rainwater (Lachniet, 2009). In tropical and subtropical areas, such as Socotra, the “amount effect”, describing the inverse relationship between the amount of precipitation and its oxygen isotopic composition, is mainly responsible for changes in rainwater d18O (Dansgaard, 1964; Rozanski et al., 1992) and consequently for the d18O composition of cave seepage waters. In Socotra, changes in the d18O of stalagmites deposited in or close to

isotopic equilibrium with their seepage waters, reflect fluctuations in the amount of precipitation. In this study, the presence of the “amount effect” is clearly demonstrated by more negative d18O values of the seepage waters in November (Fig. 3), when most of the rain falls on the island. The rest of the year, rainwater d18O values display less negative values, independent of source of the rain indicating the absence of a strong ‘source effect’. Other studies using Socotran speleothems also interpreted the changes in d18O composition as reflecting the “amount effect” with more negative d18O values occurring during wetter conditions (Burns et al., 2003; Fleitmann et al., 2007; Shakun et al., 2007). The d13C of speleothems deposited in equilibrium is mainly determined by the isotopic composition of soil-CO2, which reaches the cave with the seepage water and normally constitutes the major carbon source (Genty et al., 2001). Variations in the d13C composition of soil-CO2 are mainly controlled by changes in the type of vegetation cover in terms of C3/C4/CAM-plants above the cave (Smith and Epstein, 1970; Frumkin et al., 2000). If no major vegetation changes occurred over the studied period, as is most likely the case in this study, variations in the soil-CO2 d13C are primarily related to the intensity of soil activity with heavier d13C values during drier periods (Genty et al., 2003). If the stalagmites were not deposited in full isotopic equilibrium, additional intra-cave mechanisms may have a distinct influence on d18O and d13C calcite values. A first way to investigate the equilibrium conditions of speleothem calcite can be done by a Hendy-test where several samples are drilled along a single growth layer across the stalagmite. If in one layer (1) a simultaneous enrichment in d18O and d13C occurs away from the growth axis and (2) a good

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correlation between both stable isotopic signals can be established, calcite precipitation was affected by kinetic fractionation (Hendy, 1971). One Hendy-test was carried out on STM1 and three on STM5 (Fig. 6). The Hendy test of STM1 displays constant d18O and d13C signals within 20 mm of the stalagmites apex and an increase further away. The three Hendy tests in STM5 are very different from each other which is partially due to the difficulty of sampling within the very thin layers. STM5(75 mm) fails the Hendy test by displaying increasing d18O and d13C values away from the apex. For the Hendy tests STM5(9 mm) and STM5(90 mm), d18O and d13C values remain constant within 10 mm of the stalagmites apex. Only the d18O signal in STM5 (90 mm) on the right side of the apex displays decreasing values before displaying an increase after 30 mm from the apex. Although, STM1 and STM5 theoretically fail the Hendy-test, the rather constant d18O and d13C values close to the stalagmites’ apex and the weak correlation suggest fragile equilibrium near the central axis. However, different studies have pointed out that even if stalagmites pass the Hendy test, speleothem calcite could still have been deposited out of isotopic equilibrium (Dorale and Liu, 2009; Mühlinghaus et al., 2009). Better is to look at the enrichment of d13C values along an individual growth layer as indicator of disequilibrium (Mühlinghaus et al., 2009). As discussed in the Hendy test, the d13C values remain rather constant within 20 mm of the apex for STM1 and within 10 mm of the apex for STM5 and increase towards the sides of the stalagmite suggesting fragile equilibrium conditions of the deposited calcite. The fragile equilibrium conditions and presence of kinetic effects in the studied stalagmites is also suggested by a strong correlation between the d18O and the d13C signals in each stalagmite, indicating that similar processes, most likely by kinetic effects, influence both proxies. A third test providing information on the degree of isotopic equilibrium is to calculate the expected d18O and d13C values based on the theoretical “equilibrium” waterecalcite fractionation factors for C and O (Kim and O’Neil, 1997), the present-day cave temperature and the isotopic compositions of seepage waters and to compare these results with the measured d18O and d13C values. For Hoq Cave, the large range (2.89&) in d18O values measured for the seepage waters (Fig. 3) hampers any meaningful modelling of this kind. Moreover, fresh calcite deposited on glass slabs also displays large variations in isotopic composition (Table 1). Such large variations do suggest that site specific differences such as different groundwater flow-paths and the water residence time in the epikarst (storage water versus event water) can partially be responsible for these large variations.

Also strong locally varying intra-cave factors such as degree of humidity and in particular ventilation may be responsible for these large variations in isotopic composition of the seepage waters and the present-day calcite within one cave. Also for Casecas Cave, no significant modelling of this kind is possible because the seepage waters were sampled in January only, which is the dry season and will consequently lead to too heavy calculated d18O values for the expected speleothem calcite. Taken the evidences together, stalagmites in Hoq Cave and Casecas Cave were deposited under fragile equilibrium conditions and the isotopic signals may partially be influenced by kinetic effects. To summarize, any above described mechanism can/will influence the d18O and d13C signal of the studied stalagmites in the same direction; higher d18O and d13C values will always occur during drier conditions. Similar conclusions have been established in previous work on speleothems in Yemen and Oman (Burns et al., 2001; Fleitmann et al., 2003a, 2004a, 2007; Shakun et al., 2007). 5.2. High Mg/Ca and Sr/Ca ratios indicate drier conditions Mg/Ca ratios and to a lesser extend Sr/Ca ratios can be used as hydrological proxies (Fairchild and Treble, 2009). In arid and semiarid areas, prior calcite precipitation (PCP) is considered to be a main reason for variable Mg/Ca and Sr/Ca ratios in speleothems (Fairchild et al., 2000, 2006). When downward percolating water encounters a zone with lower pCO2, degassing occurs and calcite can precipitate. Consequently, Mg and Sr become enriched compared to Ca in the residual water. During drier periods PCP is enhanced as aerated zones increase in the aquifer and residence time of the water becomes longer (Fairchild et al., 2000). The main evidence for PCP is covarying Mg/Ca and Sr/Ca ratios (McMillan et al., 2005; Johnson et al., 2006). Because of calcite precipitation in the epikarst during PCP, d13C calcite values increase in tandem with Mg/Ca and Sr/Ca ratios. The strong similarities between the Mg/Ca and Sr/Ca profiles in stalagmites STM1 and STM6 suggest that PCP is a primary control for trace elemental ratios in both stalagmites (Fig. 5). This assumption is further validated by the significant correlation between d13C and Mg/Ca (r ¼ 0.33, p ¼ 5.8112  104) ratios in STM1. Furthermore, d18O values also show a significant correlation with Mg/Ca (r ¼ 0.38, p ¼ 6.3180  105) and Sr/Ca (r ¼ 0.42, p ¼ 8.6029  106) ratios in stalagmite STM1. As mentioned before, the low resolution of the

Fig. 6. d18O (black line) and d13C (grey line) values in & VPDB for the Hendy tests carried out on stalagmites STM1 (b) and STM5 (a and d). Speleothem calcite of STM1 is deposited near equilibrium conditions within 20 mm from the stalagmites apex. Due to very thin layers in STM5, sampling was difficult explaining the less successful results. STM5(75 mm) fails the Hendy test while STM5 (9 mm) and STM5(90 mm), displays rather constant values within 10 mm of the stalagmites apex.

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STM6 Mg/Ca and Sr/Ca time series hampers a meaningful correlation with the d-signals. Nevertheless lower Mg/Ca and Sr/Ca ratios appear to correspond roughly to more negative isotope values. Based on our observations, we suggest that Mg/Ca and Sr/Ca ratios are sensitive hydrological proxies with higher ratios during drier periods when PCP is enhanced. 5.3. Kinetic effects as amplifier of the rainfall signal As indicated by the different equilibrium tests and by the strong correlation between the trace elements and the stable isotope signals, calcite deposition of the studied stalagmites is affected by kinetic fractionation. As discussed in Dreybrodt and Scholz (2011) and Dreybrodt (2011), the degree of kinetic isotopic enrichment of the deposited calcite is not caused by rapid CO2 degassing but mainly depends on drip rates and calcite precipitation rates, which in turn depends on the calcite supersaturation. Since the isotopic profiles of stalagmites STM1 and STM6 sampled at the end of the cave differ strongly from the isotopic signals of stalagmite Hq1, sampled near the cave entrance, we expect that different kinetic effects affect calcite deposition. At the end of 2 km-long Hoq Cave, changes in the d18O and d13C signals of the coeval stalagmites STM1 and STM6 are interpreted to be controlled by variations in effective moisture, with lower isotopic values indicating higher net precipitation. However, despite their close proximity (w1 m), the isotopic profiles of stalagmites STM1 and STM6 are not identical. The most striking difference occurs from 0 to 0.6 ka and from 2.2 ka to 3.8 ka when the STM6 isotopic profiles shift to positive values differing by 1.5& for d18O and 3& for d13C with the STM1 isotopic record (Fig. 7). A similar isotopic variation range is also observed in fresh calcite deposited on glass slabs (Table 1), in this case 2& for d18O and 4.5& for d13C. This suggests that despite the high humidity and reduced ventilation at the end of the cave, strong differences in isotopic composition occur between the different drip sites. During dry periods drip rates will decrease at the fast growing (STM1) and

137

slow growing (STM6) sites. However, calcite precipitation at the STM6 site will be more strongly affected by kinetic effects, as its growth rate is considerably lower compared to the one at the STM1 site. The effect of drip rate on the degree of kinetic isotopic enrichment of the deposited calcite is confirmed by different modelling (Dreybrodt, 2011; Dreybrodt and Scholz, 2011; Deininger et al., 2012) and laboratory experiments with synthetic carbonates (Polag et al., 2010; Day and Henderson, 2011). Stalagmite STM6 is thus more sensitive to small reduction in precipitation and effective moisture. Because of its faster growth rate, STM1 keeps growing during these slightly drier periods and calcite precipitation still occurs closer to isotopic equilibrium. Deep in the cave, the slower the growth rate, the stronger calcite deposition will be sensitive to small changes in kinetic effects and the more the rainfall signals induced by the amount effect will be amplified. Compared to the coeval stalagmites STM1 and STM6, the isotopic signals of Hq1 vary around much more positive values. Also, the isotopic records of Hq1 display no similarities on millennial and centennial scale with the records from the coeval stalagmites STM1 and STM6 (Fig. 7). These observations suggest very strong isotopic disequilibrium deposition of the Hq1 calcite that is further confirmed by its very strong correlated d18O and d13C signals. Compared to the coeval stalagmites sampled at the end of Hoq Cave, stalagmite Hq1 was sampled only 200 m from the entrance where relative humidity is low and ventilation effects are considerably stronger. The wide Hoq Cave entrance is located on the face of a cliff with a large opening towards the downhill seaside allowing strong air circulation in the first parts of the cave. The presence of air circulation in chambers near the entrance of Hoq Cave is validated by the preferential growth direction of helictites e a small variety of stalactites that are twisted and contorted with no apparent regard for gravity. The stronger the air circulation in the chambers near the entrance, the stronger the evaporation effects on the stalagmite surface and the further out of equilibrium the calcite will precipitate. The relationship between enhanced evaporation and disequilibrium deposition of the speleothem calcite has quantitatively been confirmed by Deininger et al. (2012). They demonstrated that loss of water on the solution layer due to evaporation increases the Ca2þ leading to higher precipitation rates and consequently larger kinetic fractionation effects. Higher isotopic values in stalagmite Hq1 therefore most probably reflect stronger ventilation near the cave entrance. To summarize, kinetic effects affecting the d18O and d13C signals in the Hoq Cave stalagmites are related to their location in the cave. For stalagmites STM1 and STM6, sampled deep in the cave where ventilation is minimal, the d18O and d13C signals are affected by changes in precipitation and effective moisture. Furthermore, the slower the stalagmite’s growth rate, the stronger the amplification of the precipitation signal by the kinetic effects. For stalagmite Hq1, sampled near the entrance, kinetic effects are related to enhanced ventilation. The Hq1 record provides information on the intensity of ventilation near the cave entrance. For reconstructing the eastern Socotra climate variability, the best records are given by the coeval STM1 and STM6 stalagmites. Stalagmite STM6 is even more sensitive to small climatic variations because kinetic effects amplify its climate signal. 5.4. Evolution of the IOM and its northeast subsystem since 7 ka

Fig. 7. Superimposed d18O and d13C values, both in & VPDB, of the Hoq Cave stalagmites STM1 (black) STM6 (grey) and Hq1 (dotted black) together with their U/Th points (large dots). Two drier northeast monsoon periods with lower cave ventilation occur from 0 to 0.6 ka and from 2.2 until 3.8 ka (indicated in grey), when stalagmites STM1 and STM6 break-up and stalagmite Hq1 shifts to more negative values.

Combined interpretation of the stalagmites STM1 and STM6 provides the best precipitation reconstruction for eastern Socotra. Both stalagmites reflect variations in precipitation brought by the northeast winds. From 6.0 ka until 3.8 ka, the STM1 and STM6 records suggest a gradual decrease in precipitation brought by the

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northeast monsoon (Fig. 7). After 3.8 ka, no long-term trend is visible and precipitation intensities brought by the northeast winds vary around a constant value. As suggested by the more sensitive STM6, two drier periods occur between 0 and 0.6 ka and from 2.2 to 3.8 ka. Superimposed shorter-term millennial and centennial variations are similar in both stalagmites but can differ up to 100 years due to age uncertainties in the age models. The exact forcing behind the variations in precipitation brought by northeast winds remains unclear. Its characterization would require comparison with other northeast precipitation records in the region. The first limitation that hampers such comparison is that records affected by only winter monsoon precipitation are difficult to find in the area. For most of the sites in Monsoonal Asia, the amounts of precipitation brought by the southwest monsoon are higher compared to those brought by the northeast winds. Consequently, the northeast signal is often overwritten by the southwest signal. Furthermore, most existing records such as ocean cores or sedimentary records do not provide the high resolution needed to compare centennial or millennial scale variations. The only record allowing comparison with our northeast precipitation records is a d18O stalagmite record from Hoti Cave, northern Oman (Fleitmann et al., 2007). This speleothem shows the evolution of rain brought by northeast winds for northern Oman at millennial and centennial scale covering the period between 6 ka until 5.2 ka and from 2.5 ka until present (Fig. 8). As for the coeval stalagmites STM1 and STM6, the d18O values in the northern Oman

Fig. 8. After 6 ka, the northern Oman record (Hoti Cave (3), Fleitmann et al., 2007) displays similar variations as the eastern Socotran records (STM1 and STM6 (2), this study). Values vary around a similar average of 2.3& (dotted line) with similar millennial scale variations. No similarities with the Greenland ice core record ((1), Rasmussen et al., 2006; Vinther et al., 2006) indicate that the northeast rains in northern Oman and eastern Socotra are not sensitive to Northern Hemisphere temperature variations. No similarities can be seen between the northeast monsoon signals from this study and a southwest monsoon precipitation signal ((4), Qunf Cave, Fleitmann et al., 2007) suggesting that both monsoons have different mechanisms. Values are reporten in & SMOW for (1) and in & VPDB for (2, 3 and 4).

stalagmite vary around 2.3& and display no significant long-term trends. The presence of an important hiatus in the Hoti Cave record, makes a long-term comparison with STM1 and STM6 difficult. However, similar millennial scale variations can be found between both regions (see also dotted lines in Fig. 8), confirming that northern Oman and eastern Socotra are affected by similar monsoon dynamics since 6.2 ka. Indeed, after 6.2 ka the southward retreating ITCZ shifted south of northern Oman, making winter precipitation brought by northeast winds the only moisture source (Fleitmann et al., 2007). Comparison of the records reflecting northeast rain variations with the Greenland ice core records (Rasmussen et al., 2006; Vinther et al., 2006) is difficult. On a long term, the Greenland ice cores records display a gradual decrease that cannot be found in our records (Fig. 8). This suggests that the long-term decrease in high latitude temperatures doesn’t influence the long-term evolution of the northeast monsoon precipitation. Also on a shorter time scale, millennial and centennial variations don’t correlate between our records and the Greenland ice core records. Also no links exists with the Bond events (Bond et al., 1997). In contrast to the evolution of the northeast monsoon, records reflecting variations in southwest-monsoon precipitation show similarities with Greenland ice core records. Southern Oman stalagmites (Fleitmann et al., 2003a, 2007) display a long term weakening of the monsoon and shorter-term variations that correlate with Greenland ice core variations (Fig. 8). Colder Northern hemisphere periods correspond to weaker and consequently drier southwest monsoon periods. Since our records do not match those of the Greenland ice core, we expect no similarities with precipitation variations of the southwest monsoon. Indeed, comparison between different southwest records (Sirocko et al., 1993; Cullen et al., 2000; Fleitmann et al., 2003a; Gupta et al., 2003) and the northeast records from this study display no similarities (Figs. 8 and 10). A speleothem d18O record from western Socotra is interpreted to reflect variations in the southwest summer monsoon since 4.4 ka (Fleitmann et al., 2007). Comparison with our records from the eastern side of the island, show that both sides of Socotra have a different long-term evolution (Fig. 9a). Whereas western Socotra gradually evolves towards wetter conditions (Fleitmann et al., 2007), eastern Socotra (this study) has a stable long-term precipitation trend since 3.8 ka. This emphasizes the important role of the Haggeher Mountains as a watershed creating two different precipitation areas on the island. The presence of two precipitation areas on Socotra is confirmed by the intermediate d18O values of stalagmite STM5 from Casecas Cave (Fig. 9a). The latter is located at the fringe between the northeast rain area in the east and the mixed southwest northeast rain area in the west (Fig. 1). Despite the different long-term evolution of both sides of Socotra Island, the shorter-term (millennial and centennial scale) variations between eastern and western Socotra display similar changes. Due to age uncertainties, 200-year offsets occur between the variations of western and eastern Socotra, causing an unsuccessful statistical correlation between the records. Such an offset is clearly visible for the positive peak around 2.6 ka where the STM6 record lags approximately 200 years (thus within age uncertainties) behind the peak in the western Socotran record (Fig. 9a). The observation that shorter-term variations are similar on both sides of the island can be explained in two ways. The southwest rains also affects the eastern part of Socotra or the northeast rains also affect western Socotra. Nowadays, western Socotra receives equal amounts of rain during the southwest and the northeast rainy season. For eastern Socotra, the rainfall amounts brought by the northeast rains are three times the precipitation amounts brought by the southwest rains (Scholte and De Geest, 2010).

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Fig. 9. (a) Whereas western Socotra becomes wetter over the last 4.4 ka (black line, Dimarshim cave, Fleitmann et al., 2007), conditions remain stable on eastern socotra after 3.8 ka as indicated by STM1 (light grey) and STM6 (dark grey) from Hoq Cave (this study). STM5 from Casecas Cave (dotted grey, this study) is influenced by both rainy seasons and has intermediate d18O values between east and west Socotra. Superimposed millennial variations are similar for all stalagmites but can display offsets of 200 years (within the age uncertainty ranges). (b) Possible hypothesis: southwest winds hitting western Socotra gradually come from more over the Indian Ocean due to the southward displacement of the summer ITCZ causing a wetter southwest rainy season.

Therefore, we conclude that the most plausible hypothesis explaining the similar millennial and centennial scale variations between east and west Socotra is that the northeast rains reach the western side of Socotra. Consequently, the millennial and centennial scale variations in the d18O record from western Socotra are created by variations in the northeast rainy season. The long-term trend of the western Socotra record displays an evolution towards wetter conditions that cannot be seen in eastern Socotra (Fig. 9a). Since the northeast monsoon does not display an increase in precipitation after 3.8 ka, the long-term trend on western Socotra is most likely linked to increased southwest monsoonal rainfall. This is apparently surprising considering that for Monsoonal Asia, the southwest monsoon is weakening from the beginning of the Holocene (Sirocko et al., 1993; Neff et al., 2001; Fleitmann et al., 2003a, 2004a, 2007; Gupta et al., 2003; Wang et al., 2005). Modern wind direction measurements carried out on the western side of Socotra indicate that when the southwest monsoon reaches its maximal intensity in July, the southwest winds gradually change direction from the south to southwest (Culek et al., 2006). The southwest winds are first passing over dry Somalia before hitting Socotra. This means that during the summer monsoon, thus when the ITCZ reaches its northernmost position, air reaching Socotra is drier than during other periods when air is directly coming from over the Indian Ocean. The more northern the ITCZ, the more the southwest winds are forced into a westerly path, and the drier the southwest rainy season on Socotra. Based on these observations, the following hypothesis for the increasing wettening of western Socotra can be established. After 8 ka, the mean latitudinal position of the summer ITCZ moved southward in response

to the decreasing boreal summer insolation (Fleitmann et al., 2003a). As a consequence, the southwest winds that were originally coming from over dry Somalia are forced into a more southerly path over the Indian Ocean (Fig. 9b). The resulting winds will therefore contain more moisture and lead to a wetter summer monsoon and thus an increase of precipitation on the western side of Socotra. This hypothesis suggests that the increasingly wetter southwest summer monsoon over Socotra reflects a local effect and is therefore not representative for the whole summer monsoon region. More research and comparison with currently other high resolution regional late Holocene precipitation reconstructions from around Socotra are necessary to confirm this hypothesis. However, so far such records are lacking. 5.5. End of the Holocene wet period The similar forcing behind the northern Oman and the eastern Socotra records sheds new light on the timing and characteristic of the termination of the Holocene wet period in southern Arabia. During the early to middle Holocene (6e10.5 ka), southern Arabia experienced clearly wetter conditions than today (Burns et al., 2001). Different proxy records such as from speleothems, lake and dune deposits and marine sediments from the Arabian Sea do not seem to agree on the timing and characteristics of the end of the Holocene wet period. Roughly, these records can be subdivided in two groups. The ones that suggest a gradual decrease in precipitation since 8 ka (Fig. 10, records 8e10) and those advocating an abrupt decrease around 6 ka (Fig. 10, records 1e7). The first group (Fig. 10, records 8e10) suggesting a gradual decrease since 8 ka

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Fig. 10. Compilation of mid Holocene records from the southern Arabian Peninsula and the northern Indian Ocean. Location of all records is shown on the map on top. The Hoq Cave STM6 record (7) correlates well with the northern Oman speleothem record (6) suggesting that over the last 6.0 ka northern Oman received rain only by northeast winds. Records currently located out of the ITCZ influence (1e4 and 6) display an abrupt end of the mid Holocene wet period around 6.0 ka (indicated by the dotted line) due to their disconnection of the influence of the southwest monsoonal rains. Records still located within the ITCZ migration pathway (8e10) display a gradual decrease in precipitation due to the southwest monsoon weakening. Sediment records from northwestern Yemen (5) still received rain after 6.0 ka due to orographic effects explaining the longer wet conditions for that area. The Holocene boreal summer insolation curve is established using Analyseries (Paillard et al., 1996). 1. Awafi, Parker et al., 2006; 2. Hajar Mountains, Fuchs and Buerkert, 2008; 3. Wahiba Sands, Radies et al., 2005; 4. Al-Hawa, Lezine et al., 2010; 5. Rada and Saada, Lezine et al., 2010; 6. N-Oman, Fleitmann et al., 2007; 7. E-Socotra, this study; 8. S-Oman, Fleitmann et al., 2007; 9. Arabian Sea, Gupta et al., 2003; 10. Arabian Sea, Sirocko et al., 1993.

includes the speleothems from southern Oman (Fleitmann et al., 2007) and sedimentary cores from the Arabian Sea (Sirocko et al., 1993; Gupta et al., 2003). To the second group (Fig. 10, records 1e 7) belong speleothems from northern Oman (Burns et al., 2001; Fleitmann et al., 2007), sediment records from the UAE (Parker et al., 2006) and the Wahiba sands in northern Oman (Radies et al., 2005), all of them place the termination of Holocene wet period at around 6 ka. All locations showing an abrupt end of the Holocene wet period at 6 ka are currently located outside the ITCZ migration pathway and receive their precipitation only once a year during the winter from the northeast winds. Around 6 ka the ITCZ shifted south of the UAE and northern Oman, disconnecting these areas from the southwest rains and explaining the clearly marked end of the wet period. Winter and spring precipitation brought by

northeast winds is the only moisture source for that region. Archives still located within the ITCZ migration pathway such as southern Oman, the Yemenite coast and the northern Indian Ocean show a rather gradual decrease in precipitation since 8 ka. These areas experience a gradual decrease in precipitation due to the gradual southward retreat of the summer ITCZ. In this matter, the two apparently opposite views can be reconciled. Two lake records from the Yemenite lowlands and the Yemenite Highlands in the west (Lezine et al., 2010) seem to form an exception to this hypothesis by showing an abrupt end of Holocene wet period at 7.2 ka (Fig. 10, record 4) and 5 ka (Fig. 10, record 5). A more gradual decrease would be expected for this area since these lakes receive no rain from northeast winds. These abrupt shifts correspond perhaps to a different timing of the threshold

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point for lake survival. At 7.2 ka BP rain brought by the southward migrating ITCZ was too weak to feed the Yemenite lowland lakes located in the rain shadow of the Yemenite Highlands, explaining the abrupt drying. Due to orographic effects, lakes located in the Yemen Highlands still received enough precipitation until the threshold value was reached at 5 ka, when the amount of rain brought by the southward migrating ITCZ also became insufficient to maintain the lake. A similar situation occurs with a playa-like sediment record from the Hajar Mountains (Northern Oman, Fuchs and Buerkert, 2008) (see also Fig. 10, record 2) that show an abrupt decrease in sedimentation rate at 8 ka while an abrupt decrease at 6 ka should be expected for this region. Although precipitation decreased gradually until the abrupt shift at 6 ka in northern Oman, the threshold point for these lakes was already reached at 8 ka. In summary, the end of the Holocene wet period at 8 ka in southern Arabia is related to the continuous southward retreat of the summer ITCZ in response to the decreasing Holocene boreal summer insolation (Fleitmann et al., 2003a). Currently, areas still reached by the summer ITCZ display a gradual end of the Holocene wet period starting around 8 ka. The areas that are not affected anymore by the ITCZ and currently receive rain only once a year only from northeast winds display an abrupt end at 6.2 ka. Around 6 ka, the summer ITCZ was located south of northern Oman and thus winter precipitation delivered by frontal depression systems from the Mediterranean Sea became the dominant source of moisture. Climate in the western parts of Socotra evolved gradually towards wetter conditions since 4.4 ka due to the trajectory of the southwest summer monsoon winds passing gradually more over the Indian Ocean. However, since the currently available western Socotra records do not cover the mid Holocene period, no robust conclusion can be established about the presence or absence of a mid Holocene wet period on Socotra.

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4. The long term wettening of the southwest monsoon on western Socotra since 4.4 ka (Fleitmann et al., 2007) is a local effect that relates to a changing wind path. A possible hypothesis could be that in response to the weakening of the southwest monsoon, the southwest winds are forced into a more southerly path over the Indian Ocean consequently causing a wetter southwest monsoon rainy season on western Socotra. 5. After 6.2 ka, similar precipitation signals can be found between eastern Socotra and northern Oman suggesting both regions are similarly affected by the northeast winter monsoon from then on. Areas on the Arabian Peninsula such as northern Oman currently receiving rain only once a year from northeast winds display an abrupt end of the Holocene wet optimum around 6 ka due to their disconnection from the southwest winds. In contrast, records from the southern Arabian Peninsula still located within the ITCZ migration pathway and receiving rain during both monsoon seasons display a gradual drying after the Holocene wet optimum due to the weakening of the southwest monsoon after 8 ka. Acknowledgements We thank the Yemen Ministry of Water and Environment and the Environment Protection Authority (EPA e Socotra Branch), the members of the Socotra Karst Project and of the Friends of Socotra group for their help during the fieldwork. Maïté Van Rampelbergh also want to thank Mr. R. Van Dierendonck for his interest and support, Kay Van Damme for sharing his knowledge on Socotra’s vegetation cover and Dirk Van Dorpe for providing additional information on the Hoq Cave dimensions. This work is support by the Hercules Foundation to Philippe Claeys, and Research Foundation Flanders (FWO) through project G-0422-10 to Philippe Claeys. References

6. Conclusions 1. Strong differences in the isotopic composition of the seepage water, modern calcite and between the isotopic profiles of the Hoq Cave stalagmites demonstrate that kinetic effects are site specific and affect the isotopic composition of contemporaneously deposited stalagmites significantly. A detailed understanding of the cave dynamics and waterecarbonate interaction by cave monitoring, combined with a multi-proxy and multi-stalagmite approach is necessary to derive solid climate reconstruction in semi-arid environments. At the end of Hoq Cave, kinetic effects are caused by variations in growth and drip rate whereas near the entrance, evaporation effects are responsible for out of equilibrium deposition of the calcite. 2. Understanding the present local climate is necessary to correctly interpret the obtained records. For Socotra, due to the watershed action of the Haggeher Mountains, records obtained from the southern and western parts reflect the long-term changes of the southwest rainy season with superimposed smaller scale variations of the northeast monsoon. Records obtained from the eastern and northeast parts document changes in northeast rains only. 3. The northeast winter monsoon displays a drying from 6.0 ka until 3.8 ka and remains stable after 3.8 ka. Two superimposed weaker northeast monsoon periods occur between 0 and 0.6 ka and from 2.2 until 3.8 ka. No correlation can be established with variations in the southwest monsoon and with the Northern Hemisphere climatic variations. More high resolution records are required to understand the exact forcing behind the northeast monsoon for this area.

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