Rates and Cycles of Microbial Sulfate Reduction in the Hyper-Saline Dead Sea over the Last 200 kyrs from Sedimentary δ34S and δ18O(SO4)

Abstract

We report the δ 34 S and δ 18 O (SO4) measured in gypsum, pyrite, and elemental sulfur through a 456-m thick sediment core from the center of the Dead Sea, representing the last ∼200 kyrs, as well as from the exposed glacial outcrops of the Masada M1 section located on the margins of the modern Dead Sea. The results are used to explore and quantify the evolution of sulfur microbial metabolism in the Dead Sea and to reconstruct the lake's water column configuration during the late Quaternary. Layers and laminae of primary gypsum, the main sulfur-bearing mineral in the sedimentary column, display the highest δ 34 S and δ 18 O (SO4) in the range of 13-28 and 13-30 , respectively. Within this group, gypsum layers deposited during interglacials display lower δ 34 S and δ 18 O (SO4) relative to those associated with glacial or deglacial stages. The reduced sulfur phases, including chromium reducible sulfur, and secondary gypsum crystals are characterized by extremely low δ 34 S in the range of −27 to +7 . The δ 18 O (SO4) of the secondary gypsum in the M1 outcrop ranges from 8 to 14 . The relationship between δ 34 S and δ 18 O (SO4) of primary gypsum suggests that the rate of microbial sulfate reduction was lower during glacial relative to interglacial times. This suggests that the freshening of the lake during glacial wet intervals, and the subsequent rise in sulfate concentrations, slowed the rate of microbial metabolism. Alternatively, this could imply that sulfate-driven anaerobic methane oxidation, the dominant sulfur microbial metabolism today, is a feature of the hypersalinity in the modern Dead Sea. Sedimentary sulfides are quantitatively oxidized during epigenetic exposure, retaining the lower δ 34 S signature; the δ 18 O (SO4) of this secondary gypsum is controlled by oxygen atoms derived equally from atmospheric oxygen and from water, which is likely a unique feature in this hyperarid environment.

Intensified microbial sulfate reduction in the deep Dead Sea during the early Holocene Mediterranean sapropel 1 deposition

“…Sedom, with comparably high . Thus gypsum dissolution alone cannot explain the observed and values (Levy et al, 2019; Torfstein & Turchyn, 2017). Finally, there are no known natural water sources to the modern Dead Sea with such 34 S‐enriched SO 4 2− isotope composition (Torfstein et al, 2005).…”

“…Values for and are the lowest within the individual datasets unless stated otherwise. Datasets presented: shelf sediments from site PC‐6 in the Mediterranean Sea (open triangles) with = 20.2‰ and = 9.4‰ (Rubin‐Blum et al, 2014); primary gypsum layers deposited in and around the Dead Sea dating back to the last glacial period (crosses) with estimated initial Lake Lisan SO 4 2− values of = 10‰ and = 10‰ (Torfstein & Turchyn, 2017); groundwater at Ein Gedi along the Dead Sea coast (black diamonds) with = 15.9‰ and = 13.7‰ (Avrahamov et al, 2014); Eastern Mediterranean site NA‐80 methane cold‐seeps (plus signs) with = 20.3‰ and = 8.6‰ (Antler et al, 2015); Lake Kinneret stratified water column (open diamonds; LK4 expedition 10/2012) with = 12.6‰ and = 11.3‰ (Knossow et al, 2015); Yarqon estuary (gray squares) with = 21.1‰ and …”

“…Stable and from primary gypsum layers found in the Dead Sea sedimentary record were also used to investigate rates of MSR using a similar isotope plot given that the effect of isotope fractionation during gypsum precipitation has a negligible effect on the slope (Torfstein & Turchyn, 2017). Primary gypsum deposits dating ca.…”

“…Whether modern deep‐dwelling MSR communities in the hypolimnion and/or deep sediments respond to transient periods of increased freshwater runoff is unknown, however, the sediment record may be used to shed light on whether such a connection occurred in the past. There have been a number of studies on MSR in the paleo‐Dead Sea using the distribution and stable isotope composition of S‐bearing minerals, such as gypsum (CaSO 4 ·2H 2 O), pyrite (FeS 2 ), elemental sulfur nodules (S 0 ), greigite (Fe 3 O 4 ), and mackinawite (FeS) (Bishop et al, 2013; Thomas et al, 2016; Torfstein et al, 2005, 2008; Torfstein & Turchyn, 2017). Investigation of MSR in paleolimnological studies has typically focused on the distribution and isotope composition of authigenic S‐bearing minerals.…”

The Calcium Isotope Systematics of the Late Quaternary Dead Sea Basin Lakes

“…Although it was proposed initially that the fine‐crystalline secondary gypsum was also primary, its relatively low δ 34 S and δ 18 O (SO4) shows unequivocally that its formation stems from the oxidation of sedimentary sulfides after lake level drop and exposure of the Lisan Formation (Torfstein et al, , ; Torfstein & Turchyn, ). Further evidence of the nature of the secondary gypsum comes from the fact that it has not been found within a sediment core retrieved from the bottom of the modern Dead Sea during the recent Dead Sea Drilling Project core (Torfstein et al, ), where sedimentary sulfide minerals are found.…”