The sources and evolution of sulfur in the hypersaline Lake Lisan (paleo-Dead Sea)

dS values in gypsum are used to evaluate the fate of sulfur in the hypersaline Lake Lisan, the late Pleistocene precursor of the Dead Sea (70–14 ka BP), and applied as a paleo-limnological tracer. The Ca-chloride Lake Lisan evolved through meromictic periods characterized by precipitation of authigenic aragonite and holomictic episodes characterized by enhanced gypsum precipitation. The lake deposited two major gypsum units: the bLower Gypsum unitQ (deposited at ~56 ka) showing dS values of 18–20x, and the bUpper Gypsum unitQ (deposited at 17 ka) displaying significantly higher dS values of 26– 28x. Laminated and disseminated gypsum, residing within the aragonite, exhibit dS values in the range of 26x to 1x. The isotopic composition of the gypsum was dictated by freshwater sulfate input that replenished the upper layer of the lake (the mixolimnion), bacterial sulfate reduction (BSR) that occurred under the anoxic conditions of the lower brine (the monimolimnion), and mixing between these two layers. During meromictic periods, the sulfate reservoir in the lower brine was replenished by precipitation of gypsum from the upper layer, and its subsequent dissolution due to sulfate deficiency induced by BSR activity. This process describes a bsulfur pumpQ mechanism and its effect on dS in the water can be modeled by a modified Rayleigh distillation equation. Steady state dS values (~40x) were reached in the lower brine after long meromictic periods. Following overturn episodes, induced by diminishing freshwater input and lake level decline, large quantities of dS enriched gypsum precipitated. The negative dS values in laminated and disseminated gypsum provide evidence for BSR activity in the lower brine that removed isotopically depleted sulfides from the water column, causing significant isotopic enrichment of remaining sulfate. Following the lake desiccation, the sediments were exposed and the latter sulfides oxidized and re-crystallized as gypsum. D 2005 Elsevier B.V. All rights reserved.