Gypsum as a monitor of the paleo-limnological-hydrological conditions in Lake Lisan and the Dead Sea

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Geochimica Et Cosmochimica Acta
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May 15
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The isotopic composition and mass balances of sources and sinks of sulfur are used to constrain the limnological-hydrological evolution of the last glacial Lake Lisan (70-14 ka BP) and the Holocene Dead Sea. Lake Lisan deposited large amounts of primary gypsum during discrete episodes of lake level decline. This gypsum, which appears in massive or laminated forms, displays delta S-14 values in the range of 14-28 parts per thousand. In addition, Lake Lisan's deposits (the Lisan Formation) contain thinly laminated and disseminated gypsum as well as native sulfur which display significantly lower delta S-34 values (-26 to 1 parts per thousand and -20 to -10 parts per thousand, respectively). The calculated bulk isotopic compositions of sulfur in the sources and sinks of Lake Lisan lacustrine system are similar (delta S-34 10 parts per thousand), indicating that freshwater sulfate was the main source of sulfur to the lake. The large range in delta S-34 found within the Lisan Formation (-26 to +28 parts per thousand) is the result of bacterial sulfate reduction (BSR) within the anoxic lower water body (the monimolimnion) and bottom sediments of the lake.Precipitation of primary gypsum from the Ca-chloride solution of Lake Lisan is limited by sulfate concentration, which could not exceed similar to 3000 mg/l. The Upper Gypsum Unit, deposited before ca. 17-15 ka, is the thickest gypsum unit in the section and displays the highest delta S-34 values (25-28 parts per thousand). Yet, our calculations indicate that no more than a third of this Unit could have precipitated directly from the water column. This implies that during the lake level decline that instigated the precipitation of the Upper Gypsum Unit, significant amounts of dissolved sulfate had to reach the lake from external sources. We propose a mechanism that operated during cycles of high-low stands of the lakes that occupied the Dead Sea basin during the late Pleistocene. During high-stand intervals (i.e., Marine Isotopic Stages 2 and 4), lake brine underwent BSR and infiltrated the lake's margins and adjacent strata. As lake level dropped, these brines, carrying 34 S-enriched sulfate, were flushed back to the shrinking lake and replenished the water column with Sulfate, thereby promoting massive gypsum precipitation.The Holocene Dead Sea precipitated relatively small amounts of primary gypsum, mainly in the form of thin laminae. delta S-34 values of these laminae and disseminated gypsum are relatively constant (15 +/- 0.7 parts per thousand) and are close to present-day lake composition. This reflects the lower supply of freshwater to the lake and the limited BSR activity during the and Holocene time and possibly during former and interglacials in the Levant. (C) 2008 Elsevier Ltd. All rights reserved.


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DOI 10.1016/j.gca.2008.02.015