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

摘要

delta(34)S 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 "Lower Gypsum unit" (deposited at similar to 56 ka) showing delta(34)S values of 18-20 parts per thousand, and the "Upper Gypsum unit" (deposited at 17 ka) displaying significantly higher delta(34)S values of 26-28 parts per thousand. Laminated and disseminated gypsum, residing within the aragonite, exhibit delta(34)S values in the range of -26 parts per thousand to 1 parts per thousand.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 "sulfur pump" mechanism and its effect on delta(34)S in the water can be modeled by a modified Rayleigh distillation equation. Steady state delta(34)S values (similar to 40 parts per thousand) 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 delta(34)S enriched gypsum precipitated.The negative delta(34)S 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. (c) 2005 Elsevier B.V. All rights reserved.