Aaron Kaufman

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.

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.

An evaluation of several methods for determining 230ThU ages in impure carbonates

Four different schemes have been used to correct the U-series ratios found in the leachates of impure carbonates so that an age may be calculated from them. The present work compares the performance of three of these, Schemes I, II and SL, with that of the fourth, Scheme L/L. Scheme L/L had been shown previously to yield valid ages in a comprehensive study of thirty sample sets comprising 170 samples of impure carbonates from the Dead Sea basin. This scheme performs the isotopic correction by comparing the ratios found in the leachates of several coeval samples. The validity of two of the other schemes, Scheme I and II, was tested by comparing their results with the Scheme L/L results on eight of the same thirty sample sets. Schemes I and II perform corrections by comparing the isotope ratios in the leachate and residue of a single impure carbonate; the two schemes differ in their assumptions regarding the leachate-residue relationship and, hence, in their mathematical treatments of the isotopic ratios. Comparison of Scheme L/L with I and II shows that the latter are invalid in more than half the cases. The fourth correction scheme, Scheme SL, performs the corrections on the isotopic ratios of a single leachate by assuming that the correction factor, R0 (the original detrital 230Th232Th ratio) has a more or less constant worldwide value, usually taken to be 1.5–1.7. The present study shows that R0 is more like 3.3 for the Dead Sea basin and evaluates the errors incurred if the incorrect R0 is used. Finally, an attempt is made to arrive at the best worldwide estimate of R0 and its uncertainty for any new unstudied area and a graph is presented by means of which one can estimate the consequent age uncertainty.

Reevaluation of the lake-sediment chronology in the Dead Sea basin, Israel, based on new 230ThU dates☆

The Dead Sea is surrounded by chemical and detrital sediments that were deposited in its larger precursor lakes, Lake Samra and Lake Lisan. The sedimentary history of these lakes was recon-structed by means of 230Th234U ages of 30 samples, mostly of argonite laminae, from 8 columnar sections up to 110 km apart. The general validity of the ages was demonstrated by subjecting them to tests of internal isotopic consistency, agreement with stratigraphic order, and concordance with 14C ages. In the south, only the part of the Samra Formation older than 170,000 yr is exposed, while the aragonite-detritus rhythmites found in the central and northern region are generally younger than 120,000 yr. The Lisan Formation started accumulating about 63,000 yr B.P., with the clay and aragonite beds in the south-central area reflecting a rise in water level to at least −280 m. The upper part of the Lisan Formation, the aragonite-rich White Cliff Member, started accumulating about 36,000 yr B.P. The lake probably reached its highest level sometime after this, based on the ages of Lisan sediments preserved in the southernmost reaches of the basin.

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.

Measurement of 129I concentrations in the environment after the Chernobyl reactor accident

Abstract The Chernobyl nuclear reactor accident which occurred on April 26, 1986 is known to have injected into the atmosphere a pulse of a large number of radionuclides. The activities of several radionuclides present in the subsequent fallout have been measured in different locations throughout Europe by gamma-ray and beta counting. We present here measurements of concentrations of the long-lived radionuclide 129I ( T 1 2 = 1.6 × 10 7 yr ) in environmental samples collected in Israel and Europe following the nuclear reactor accident. The measurements were performed by accelerator mass spectrometry, using the 14UD Rehovot Pelletron Accelerator. Concentrations of 129I in rainwater samples collected in the Munich (West-Germany) area and in Israel during the fallout period were measured to be 2.6 × 1010 and 1.2 × 109 atoms I respectively, while a 1982 rainwater sample from Israel shows a 129I concentration of 8.2 × 107 atoms I . Three measurements of the ratio 129I/131I gave a mean value of 21, from which an effective operating time of the reactor of 1.5 to 2 yr prior to the accident can be estimated. The possible use of anthropogenic 129I as a tracing tool for global environmental processes is discussed.

Calcium carbonate nodules in soils: 13O/16O and 13C/12C ratios and 14C contents

Abstract δ 18 O values, δ 13 C values and 14 C ages were determined in ten CaCO 3 , nodule populations collected from soil and paleosols in the Israeli coastal plain. The selected soils were carbonate-free when formed and the nodules in them represent either reprecipitation of carbonate illuviated from overlying horizons (descending mode) or precipitated from a raised brackish water table (ascending mode). The 14 C ages represent the times of migration (illuviation) or eustatic movement (both climate-related) and the stable isotopic conditions reflect the environmental conditions at such times. The δ 13 C values of most populations were found to have a wide range and cannot be used to reconstruct former climates. This wide range is attributed to microvariations of pCO 2 in the soil. Because the δ 18 O ranges in the same samples are usually comparable to analytical uncertainties and because the δ 18 O mean values follow an age-correlated pattern, they permit us to place the soils in one of three categories. In two cases where nodules were soft both turned out to be young (∼ 1000 years) and to have wide ranges in δ 18 O values; the latter are apparently due to continual re-equilibration with new soil solutions. In three cases of the descending mode of nodule formation, 12,000–14,000 year-old hard nodules had very narrow δ 18 O whose means reflect the rainfall value at that time. The period between 12,000–14,000 years B.P. is independently known to be one of higher precipitation/evaporation ratio and of more intensive soil formation. These ascending-mode populations of hard nodules had narrow ranges in δ 18 O (reflecting a mixture of sea water and rainfall) and all turned out to be 3,000–4,000 years old. At this period the sea reached its highest elevation, resulting in raising the nearcoast water table.

The distribution of 230Th234U ages in corals and the number of last interglacial high-sea stands

Abstract The 104 available [ 230 Th 234 U ] analyses of unrecrystallized corals from stable emerged terraces indicate that the last interglacial period occurred approximately 125,000 yr ago. An estimate is made of the exact duration of this period by accounting for the surprisingly small distribution width observed among the 80 most reliable analyses. This distribution width is compared with those obtained for model populations generated by assuming various characteristic analytical errors and various lengths of the last interglacial period. The results show that (1) if there was only a single rise in sea level, it probably lasted no more than 12,000 yr, in agreement with previous estimates; and (2) if there were two separate rises of sea level, the gap between them must have been less than 7500 yr and not 12,200 as proposed by some authors.

URANIUM CONCENTRATION AND ISOTOPE RATIO PROFILES WITHIN MERCENARIA SHELLS : GEOCHRONOLOGICAL IMPLICATIONS

Shells of nineteen specimens of Mercenaria from unconsolidated sediments of the southeastern USA coast, ranging in age from alive to Pliocene, were each separated into three to six sections parallel to the shell surface. The uranium concentration and isotope ratio of each subsample was determined. The results confirm those of previous studies that whole fossil shells have about 8 times as much uranium (300 vs. 40 ppb) and considerably higher 234U/238U ratios than do whole live-collected shells. However, it was also found that uranium concentrations fall very steeply (by factors of 300 and 15 in live and fossil shells, respectively) from the exterior of the shell to about two-thirds of the shell thickness. Between this depth and the interior surface, the uranium concentrations of live shells increase by about a factor of 5 while fossils have gradients which vary from moderately positive to moderately negative. The most surprising finding of the study is that the interior portions of the shells become closed chemical systems by no more than about 15 ka after their death. This is known from the observation that these portions of Late Pleistocene shells still have oceanic uranium isotope ratios while the exterior portions of the same shells have ratios which reflect an uptake of uranium from continental groundwaters (the water the shells have been in contact with for the vast majority of their existence). Thus, the U-series ages of the interior portions of Mercenaria shells may be valid after a small correction. Though shells older than about 200 ka seem less promising, firm conclusions cannot be drawn regarding them because of the great scatter in the data.