Avner Ayalon

The Eastern Mediterranean paleoclimate as a reflection of regional events: Soreq cave, Israel

The climate of the Eastern Mediterranean region of the last 60 ky was determined by a high resolution study of the oxygen and carbon isotopic composition (1500 measurement pairs) of speleothems from the Soreq cave, Israel, with chronology provided by 53 precise ^(230)Th–^(234)U (TIMS) ages. The high precision of the speleothem TIMS ages permits us to determine the timing of regional climatic events in the Eastern Mediterranean region and to see if they correlate with global events. During the period from 60 to 17 ky, the δ^(18)O and δ^(13)C values were generally 2–2.5‰ higher than during the period from 17 ky to present. This is consistent with the climatic transition from glacial to interglacial. Within the 60 to 17 ky period, the Soreq cave stable isotope profile includes four cold peaks (at 46, 35, 25 and 19 ky) and 2 warm peaks (at 54 and 36 ky). In addition, the period <17 ky has two more cold peaks at 16.5 and from 13.2 to 11.4 ky. The ages of four of the six cold peaks correlate well with the ages of three Heinrich events (H1, H2, H5) and with the age of the Younger Dryas. However, the other two Heinrich events are not reflected in the Soreq cave record. Several other isotope peaks which appear during the last 7 ky are contemporaneous with regional climatic events in the Middle East and North Africa. In addition to the drop in δ^(18)O and δ^(13)C observed between the last glacial and the Holocene, sharp simultaneous drops in (^(234)U/^(238)U)_0 ratios, Sr concentrations and in ^(87)Sr/^(86)Sr are also observed, suggesting that the latter are climate related. These variations are interpreted in terms of major changes in the temperature, the mean annual rainfall and its isotopic composition, the isotopic composition of the Mediterranean vapor source, the soil moisture conditions, and in the mixing proportions of sources with different ^(87)Sr/^(86)Sr ratios (sea spray, dust particles and dolomitic host rock).

Timing and hydrological conditions of Sapropel events in the Eastern Mediterranean, as evident from speleothems, Soreq cave, Israel

This paper explores the connection between the timing of sapropel events in the Eastern Mediterranean Sea and the time of low δ18O events in speleothems of the Soreq cave (Israel). A new well-dated δ18O and δ13C profile of the Soreq cave speleothems for the last 140 kyr reveals five prominent low δ18O events dated at: 124 to 119 kyr, with the main peak at 122 kyr (event #V); 108 to 100 kyr with peaks at 107 and 102 kyr (event #IV); 85 to 79 kyr with peak at 80 kyr (event #III); 55 to 52 kyr with a peak at 54 kyr (event #II) and 8.5 to 7 kyr with peaks at 8.5 and 7 kyr (event #I). These events are characteristic of period of enhanced rainfall in the Eastern Mediterranean area, particularly so for events #V and #I, where the marked decrease in δ18O and the correspondingly marked increase in δ13C indicate very wet conditions. The chronology of the low δ18O events #V, IV, III and #I match the ages estimated for the formation of sapropels S5, S4, S3 and S1 which are also associated with high hydrological activity. Thus, it is suggested that the low δ18O events in the Soreq cave speleothems constrain the maximum duration of the sapropel formation. Pollen data from the sapropel layers and the isotopic pattern of the speleothems indicate that sapropels S5 and S1 were deposited during very wet periods when there were major decrease in the sea surface salinity, coupled with frost-free winters and drought-free summers. Sapropels S4 and S3 were formed during periods of increased precipitation, but ones in which the general climate was Mediterranean semi-arid.

Carbon and oxygen isotope study of the active water-carbonate system in a karstic Mediterranean cave: Implications for paleoclimate research in semiarid regions

Abstract In a semiarid climatic zone, such as the Eastern Mediterranean region, annual rainfall variations and fractionation processes in the epikarst zone exert a profound influence on the isotopic compositions of waters seeping into a cave. Consequently, the isotopic compositions of speleothems depositing from cave waters may show complex variations that need to be understood if they are to be exploited for paleoclimate studies. This is confirmed by a four-year study of the active carbonate-water system in the Soreq cave (Israel). The δ18O (SMOW) values of cave waters range from −6.3 to −3.5%.. The highest δ18O values occur at the end of the dry season in waters dripping from stalactites, and reflect evaporation processes in the epikarst zone, whereas the lowest values occur in rapidly dripping (fast-drip) waters at the peak of the rainy seasons. However, even fast-drip waters are about 1.5%. heavier than the rainfall above the cave, which is taken to reflect the mixing of fresh with residual evaporated water in the epikarst zone. δ13C (PDB) values of dissolved inorganic carbon (DIC) vary from −15.6 to −5.4%., with fast-drip waters having lower δ13C values (mostly −15.6 to −12%.) and higher DIC concentrations relative to pool and stalactite-drip water. The low δ13C values of fast-drip waters and their supersaturation with respect to calcium carbonate indicates that the seepage waters have dissolved both soil-CO2 derived from overlying C3-type vegetation and marine dolomite host rock. The δ18O (PDB) values of various types of present-day low-magnesium calcite (LMC) speleothems range from −6.5 to −4.3%. and δ13C values from −13 to −5.5%. and are not correlated with speleothem type. An analysis of δ18O values of present-day calcite rafts and pool waters shows that they form in oxygen isotope equilibrium. Similarly, the measured ranges of δ13C and δ18O values for all types of present-day speleothems are consistent with equilibrium deposition at cave temperatures. The δ13C–δ18O range of contemporary LMC thus reflects the variations in temperatures and isotopic compositions of the presentday cave waters. The 10%. variation in the δ13C values in waters can be modeled by a simple Rayleigh calculation of the carbon isotope fractionation accompanying CO2-degassing and carbonate precipitation. These variations may obscure the differences in the carbon isotopic composition of speleothems that could arise when vegetation cover changes from C3 to C4-type plants. This consideration emphasizes that it is necessary to characterize the full range of δ13C values associated with contemporaneous speleothems in order to clarify the effects of degassing from those due to differing vegetation types. Isotopic studies of a number of different types of fossil LMC speleothems show many of them to exhibit isotopic trends that are similar to those of present-day LMC, but others show both higher and lower δ18O ranges. In particular, the higher δ18O range has been shown by independent age-measurements to be associated with a period of drier conditions. The results of the study thus indicate that it is necessary to work on a well calibrated cave system in semiarid climates and that the fossil speleothem record should be obtained from different types of contemporaneous deposit in order to fully characterize the δ18O–δ13C range representative of any given climatic period.

Rainfall-recharge relationships within a karstic terrain in the Eastern Mediterranean semi-arid region, Israel: δ 18O and δD characteristics

Annual rainfall variations and processes in the upper vadose zone exert a profound influence on the chemical and isotopic compositions of waters of carbonate aquifers in semi-arid climatic zones. In order to define these processes we have studied the surface temperatures during rainfall events, the isotopic composition of rain, infiltrating and groundwaters. This study was carried out within a karstic terrain (Soreq Cave), Israel, located in Cenomanian dolomitic rocks, approximately 40 km inland of the Mediterranean Sea, and 400 m above sea level. The climate is typical of the Eastern Mediterranean semi-arid conditions, with a rainy winter and dry summer and the average annual rainfall in the area is ∼500 mm. Close monitoring indicates that the δD and δ 18O values of individual rainstorm events decrease with increasing rainfall. Annual average isotopic values of years with rainfall of 500–600 mm do not vary systematically. Years with extreme rainfall values define a negative covariation between the δ 18O and rainfall. The δD-δ 18O relationship of all rain events of more than 20 mm fall on the Mediterranean Meteoric Water Line (MMWL) with a slope of ∼8 and d-excesses of 20–30%. These rain events occur when mid-winter surface temperatures are 5 to 10°C. Rainfall events of less than 20 mm, mainly occurring at above 10°C, have slopes of less than 8 and smaller d-excess as a consequence of evaporation processes beneath the clouds. Two main water-types infiltrating into the cave are recognized: slow- and fast-drip. Slow-drip occurs from the tips of stalactites and takes place throughout the year; these waters represent seepage water that remains in the upper vadose zone for up to several decades. Fast-drip emanates from fissures in the cave roof during the winter seasons; these waters represent vadose flow with a short residence time of less than 1 year. The infiltration of the fast-drip water into the cave depends on the fracture system of the rock cover and on the intensity of the rainstorms. In the inner parts of the cave, where the rock cover is thick, fast-drip starts only after several massive (>20 mm) rainstorms, whereas below a thin roof the response time of fast-drip is a few hours to a few days after intensive rainstorms. Both fast- and slow-drip water fall on the MMWL, indicating that they are derived mainly from the relatively intensive rainstorms that compose about 23 of the annual rainfall. Although fast-dri water represents massive rainstorms, their δ 18O and δD values are nonetheless higher by ∼1% and 10% respectively, relative to the average rainwater compisition. This indicates that the fast-drip water consists predominantly of water derived from intensive rainstorms admixed with a minor component of isotopically 18O- and D-enriched water, which is represented by the slow-drip waters. The fast-drip waters are derived mainly from massive rain events at surface temperatures of 5 to 10°C, which thermally equilibrate with the host rock and upon reaching the cave they have already acquired the mean temperature (∼20°C) of the cave.

Mid-Holocene climate variations revealed by high-resolution speleothem records from Soreq Cave, Israel and their correlation with cultural changes

Mid-Holocene (7000—4000 yr BP) paleoclimate conditions were reconstructed for the eastern Mediterranean region through a high-resolution (3—20 yr) oxygen and carbon isotopic record in a speleothem from Soreq Cave, Israel. Mid-Holocene climate change is characterized by sinusoidal cycles lasting ~1500 years, that represent changes of ~400 mm (between ~700 and ~300 mm) in annual rainfall, coincident with major cultural changes. Two major cultural changes occurred during wet events at 6550—6450 yr BP and 4800—4700 yr BP, associated with the transitions from mid-Chalcolithic to late-Chalcolithic period and from the early Bronze II to early Bronze III, respectively. The transition from late Chalcolithic to early Bronze I occurred during a dry period at 5700—5600 yr BP. Superimposed on these cycles, several short-lived decadal- to centennial-scale climatic events were identified. Dry events occurred at 6650—6600 yr BP, 6250—6180 yr BP, 5700—5600 yr BP, 5250—5170 yr BP and 4200—4050 yr BP. The last two events coincide with the cultural collapse of the Uruk society in Mesopotamia and the Akkadian Empire. Short climatic wet events occurred at 6700—6680 yr BP, 6170—6100 yr BP, 5760—5740 yr BP and 5500—5450 yr BP.

U-TH ISOTOPE SYSTEMATICS FROM THE SOREQ CAVE, ISRAEL AND CLIMATIC CORRELATIONS

Precise ^(230)Th-^(234)U ages were obtained on thirty-one growth laminae in speleothem samples which are self-consistent with the detailed layer stratigraphy. Samples with low ^(232)Th/^(238)U ratios give ages with analytical uncertainties of 40 years at 2 ky and 1000 years at 90 ky. Some growth zones with high but variable ^(232)Th/^(238)U were dated by intermal isochrons. This permits the determination of the initial ^(230)Th/^(232)Th assuming equilibrium of ^(232)Th and ^(238)U series in the source of the high ^(232)Th component. This shows initial (^(230)Th/^(232)Th) (in activity units) of from 1.3 to 2.9. The calculated atomic ratios of ^(232)Th/^(238)U for the high ^(232)Th component range from 1.08 to 2.4 which is well below the average crustal value. Speleothem materials with high ^(232)Th/^(238)U are found to exhibit clear correlations of ^(232)Th with Si, Al and Fe, while ^(238)U correlates with Sr and Ba. Analyses of Soreq cave drip waters show that the particulates in the waters have high ^(232)Th concentrations and a ^(232)Th/^(238)U ratio much lower than that found in the high ^(232)Th component in speleothems but with ^(230)Th/^(232)Th) = 1.0 to 2.4. We infer that the trapped high-Th component in speleothems is from particulate matter in water with a large concentration of adsorbed U and not simply from detrital material. The speleothems have only small234U excess The initial ^(234)U/^(238)U)0 show a range of 1.02 to 1.14 that was found to correlate with age over the past 25 ky. The youngest samples have values in the same range as the modern drip waters. There appears to be a correlation of ^(234)U/^(238)U)0 with the δ^(18)O values. There is a drop of δ^(18)O in the time interval 20 to 15 ky which then remains relatively constant to recent times. As the high δ^(18)O values have been related to rainfall and associated climatic conditions, we suggest that the ^(234)U/^(238)U in the speleothem reflects the effects of rainfall and soil weathering conditions on drip-water composition and may provide a proxy for climate change.

Petrography, strontium, barium and uranium concentrations, and strontium and uranium isotope ratios in speleothems as palaeoclimatic proxies: Soreq Cave, Israel

The reconstruction of the palaeoclimate of the eastern Mediterranean region for the last 60 ka BP is based on theδ18O andδ13C variations of speleothems from Soreq Cave, Israel. Climatic conditions during most of the time interval between 60 and 17 ka BP (the period equivalent to the last glacial) were relatively cold and dry, while they were warmer and wetter from 17 ka BP to the present. At ~17 ka BP, there was a major climatic change with a sharp increase in annual rainfall and temperature and a very wet period occurring between 8.5 and 7.0 ka BP. During the colder and drier period, large, detritus-free, preferentially oriented calcite crystals were deposited from slow-moving water. As a result of a sharp change in the hydrological regime at ~17 ka BP, fast-moving water started entrainment of the soil and carrying detrital material into the cave, and the calcite crystals deposited became small and anhedral. Coinciding with the petrographic and isotopic changes, a sharp drop occurred in the concentrations of strontium, barium and uranium, and in the ratios 87Sr/86Sr and (234U/238U)0, which reached minimum values during the wettest period. This drop reflects enhanced weathering of the soil dolomite host rock. During colder and drier periods, higher trace-element concentrations and higher isotopic ratios reflect an increase in the contribution of salts derived from exo genic sources (sea spray and aeolian dust), and a reduced contribution of weathering from the host dolo mites.

Desert speleothems reveal climatic window for African exodus of early modern humans

One of the fi rst movements of early modern humans out of Africa occurred 130–100 thousand years ago (ka), when they migrated northward to the Levant region. The climatic conditions that accompanied this migration are still under debate. Using high-precision multicollector– inductively coupled plasma–mass spectrometry (MC-ICP-MS) U-Th methods, we dated carbonate cave deposits (speleothems) from the central and southern Negev Desert of Israel, located at the northeastern margin of the Saharan-Arabian Desert. Speleothems grow only when rainwater enters the unsaturated zone, and this study reveals that a major cluster of wet episodes (the last recorded in the area) occurred between 140 and 110 ka. This episodic wet period coincided with increased monsoonal precipitation in the southern parts of the SaharanArabian Desert. The disappearance at this time of the desert barrier between central Africa and the Levant, and particularly in the Sinai-Negev land bridge between Africa and Asia, would have created a climatic “window” for early modern human dispersion to the Levant.

Middle to Late Holocene (6,500 Yr. Period) Paleoclimate in the Eastern Mediterranean Region from Stable Isotopic Composition of Speleothems from Soreq Cave, Israel

Stalactites and stalagmites in the Soreq Cave, a well known tourist attraction in Israel, provide a very detailed climatic record for the Eastern Mediterranean region during the last 25,000 years (Bar-Matthews et al., 1997a) and 58,000 years (Bar-Matthews et al., 1997b). These studies indicate that the isotopic compositions of speleothems that are older than about 6,500 BP are significantly different from those of present-day speleothems; and that about 6,500 years ago the isotopic composition of the speleothems became similar to that of today. Thus, the climatic conditions that prevailed in the Eastern Mediterranean area before ~ 6,500 BP were very different from now. Only from that time have the conditions become similar to those of the present day. Soreq Cave is one of a series of karstic caves situated within the steeply westward dipping flank of the Judean Hill anticline. Its geological and hydrological setting and its environment are summarized by Asaf (1975), Even et al. (1986) and Bar-Matthews et al. (1991). The cave is located approximately 40 km inland from the Israeli Mediterranean coast, and is 400 m above sea level. Presently, the climate in the Soreq Cave area is typical of the semi-arid Mediterranean type, with an average annual air temperature of ~20.5°C, a cave water temperature varying between 18.0 and 20.5°C (and a mean annual rainfall of ~500 mm). The area is located in a narrow transition zone between humid and arid climates; and slight climatic variations, such as changes in the annual rainfall, would have affected the desert boundary and the inhabitants in the past.

Climatic conditions during marine oxygen isotope stage 6 in the eastern Mediterranean region from the isotopic composition of speleothems of Soreq Cave, Israel

At several times during marine oxygen isotope stage 6, the eastern Mediterranean region was influenced by two extreme climatic systems: the large ice sheet over northern Europe and the wet tropics associated with African monsoons. During this interval, two major climatic events occurred in the region; the sapropel S6 layer formed ca. 176 ka in the eastern Mediterranean basin owing to the increase in the African monsoon, and another event, although not large enough to form sapropel, occurred ca. 151 ka. The isotopic composition of Soreq Cave speleothems seems to record these events as very low δ 18 O-δ 13 C values dated as ca. 178 and 152 ka. The very low δ 18 O-δ 13 C values of −6‰ and −11‰ to −12‰, respectively, are typical of interglacial intervals, but here they were recorded during a glacial interval. Such low peaks indicate that in this part of the eastern Mediterranean region, i.e., Israel, the rainfall amount increased dramatically. Moreover, the isotopic record of the speleothems also shows that during the entire stage 6, although the climate was as cold as much of the last glacial, the conditions were never as dry.