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The hypersaline Dead Sea and its sediments are natural laboratories for studying extremophile microorganism habitat response to environmental change. In modern times, increased freshwater runoff to the lake surface waters resulted in stratification and dilution of the upper water column followed by microbial blooms. However, whether these events facilitated a microbial response in the deep lake and sediments is obscure. Here we investigate archived evidence of microbial processes and changing regional hydroclimate conditions by reconstructing deep Dead Sea chemical compositions from pore fluid major ion concentration and stable S, O, and C isotopes, together with lipid biomarkers preserved in the hypersaline deep Dead Sea ICDP-drilled core sediments dating to the early Holocene (ca. 10,000 years BP). Following a significant negative lake water balance resulting in salt layer deposits at the start of the Holocene, there was a general period of positive net water balance at 9500-8300 years BP. The pore fluid isotopic composition of sulfate exhibit evidence of intensified microbial sulfate reduction, where both δ34S and δ18O of sulfate show a sharp increase from estimated base values of 15.0‰ and 13.9‰ to 40.2‰ and 20.4‰, respectively, and a δ34S vs. δ18O slope of 0.26. The presence of the n-C17 alkane biomarker in the sediments suggests an increase of cyanobacteria or phytoplankton contribution to the bulk organic matter that reached the deepest parts of the Dead Sea. Although hydrologically disconnected, both the Mediterranean Sea and the Dead Sea microbial ecosystems responded to increased freshwater runoff during the early Holocene, with the former depositing the organic-rich sapropel 1 layer due to anoxic water column conditions. In the Dead Sea prolonged positive net water balance facilitated primary production and algal blooms in the upper waters and intensified microbial sulfate reduction in the hypolimnion and/or at the sediment-brine interface.

Rationale Since their introduction more than a decade ago, isotope ratio infrared spectroscopy (IRIS) systems have rapidly become the standard for oxygen (δ18O) and hydrogen (δ2H) isotope analysis of water samples. An important disadvantage of IRIS systems is the well-documented sample-to-sample memory effect, which requires each sample to be analyzed multiple times before the desired accuracy is reached, lengthening analysis times and driving up the costs of analyses. Methods We present an adapted set-up and calculation protocol for fully automated analysis of water samples using a Picarro L2140-i cavity ring-down spectroscopy instrument. The adaptation removes memory effects by use of a continuously moisturized nitrogen carrier gas. Water samples of 0.5 μL are measured on top of the water vapor background, after which isotope ratios are calculated by subtraction of the background from the sample peaks. Results With this new technique, single injections of water samples have internal precisions (1σ) below 0.05‰ for δ18O values and 0.1‰ for δ2H values, regardless of the isotope ratio of the previous sample. Precision is worse, however, when the isotope difference between the sample and background water is too large (i.e., exceeding approximately 9‰ for δ18O values and 70‰ for δ2H values). Isotope ratios show negligible drift across the four weeks within which the experiments were performed. The single-injection 1σ precision for 17O excess (Δ′17O) determined with this method is 60 per meg. Conclusions Our experiments demonstrate that by removing sample-to-sample memory effects with a moisturized carrier gas, the time for measurement of δ18O and δ2H values using an IRIS system can be reduced markedly without compromising the analytical precision and accuracy. Thorough replication is needed to achieve sufficiently low uncertainties for Δ′17O., Rapid Communications in Mass Spectrometry, 35 (8), ISSN:1097-0231, ISSN:0951-4198

Rationale Online oxygen (δ18O) and hydrogen (δ2H) isotope analysis of fluid inclusion water entrapped in minerals is widely applied in paleo‐fluid studies. In the state of the art of fluid inclusion isotope research, however, there is a scarcity of reported inter‐technique comparisons to account for possible analytical offsets. Along with improving analytical precisions and sample size limitations, interlaboratory comparisons can lead to a more robust application of fluid inclusion isotope records. Methods Mineral samples—including speleothem, travertine, and vein material—were analyzed on two newly setup systems for fluid inclusion isotope analysis to provide an inter‐platform comparison. One setup uses a crusher unit connected online to a continuous‐flow pyrolysis furnace and an isotope ratio mass spectrometry (IRMS) instrument. In the other setup, a crusher unit is lined up with a cavity ring‐down spectroscopy (CRDS) system, and water samples are analyzed on a continuous standard water background to achieve precisions on water injections better than 0.1‰ for δ18O values and 0.4‰ for δ2H values for amounts down to 0.2 μL. Results Fluid inclusion isotope analyses on the IRMS setup have an average 1σ reproducibility of 0.4‰ and 2.0‰ for δ18O and δ2H values, respectively. The CRDS setup has a better 1σ reproducibility (0.3‰ for δ18O values and 1.1‰ for δ2H values) and also a more rapid sample throughput (, Rapid Communications in Mass Spectrometry, 34 (16), ISSN:1097-0231, ISSN:0951-4198

The chemical composition and δ 37 Cl of pore fluids from the ICDP core drilled in the deepest floor of the terminal and hypersaline Dead Sea, and halites from the adjacent Mount Sedom salt diapir, are used to establish the dynamics of halite precipitation and dissolution during the last interglacial and glacial periods. Between ∼132 and 116 thousand years ago (ka) halites precipitated in the lake resulting in the expulsion of Na+ and Cl− from the residual solution. Over 50% of the Cl− reservoir was removed, resulting in a decrease in the Na/Cl ratio from 0.57 to 0.19. This process was accompanied by a decrease in δ 37 Cl values in the precipitating halites and the associated residual Cl− in the lake. The observed decrease fits a Rayleigh distillation curve with a fractionation factor of Δ(NaCl–Dead Sea solution) = +0.32‰ (±0.12) determined in the present study. This behavior implies negligible contribution of external sources of Cl− to the lake during the main peak of the last interglacial, MIS5e. Subsequently, during the last glacial (ca. 117 to 17 ka) dissolution of halite took place, the Na+ and Cl− inventory were replenished, accompanied by an increase in Na/Cl from 0.21 to 0.55 and in the δ 37 Cl values from −0.46‰ to −0.12‰. While the lake underwent significant dilution during that time, the decrease in salinity was somewhat suppressed by the dissolution of the halite which was mostly derived from Mount Sedom salt diapir.

Thick halite intervals recovered by the Dead Sea Deep Drilling Project cores show evidence for severely arid climatic conditions in the eastern Mediterranean during the last three interglacials. In particular, the core interval corresponding to the peak of the last interglacial (Marine Isotope Stage 5e or MIS 5e) contains ∼30 m of salt over 85 m of core length, making this the driest known period in that region during the late Quaternary. This study reconstructs Dead Sea lake levels during the salt deposition intervals, based on water and salt budgets derived from the Dead Sea brine composition and the amount of salt in the core. Modern water and salt budgets indicate that halite precipitates only during declining lake levels, while the amount of dissolved Na+ and Cl− accumulates during wetter intervals. Based on the compositions of Dead Sea brines from pore waters and halite fluid inclusions, we estimate that ∼12–16 cm of halite precipitated per meter of lake-level drop. During periods of halite precipitation, the Mg2+ concentration increases and the Na+/Cl− ratio decreases in the lake. Our calculations indicate major lake-level drops of ∼170 m from lake levels of 320 and 310 m below sea level (mbsl) down to lake levels of ∼490 and ∼480 mbsl, during MIS 5e and the Holocene, respectively. These lake levels are much lower than typical interglacial lake levels of around 400 mbsl. These lake-level drops occurred as a result of major decreases in average fresh water runoff, to ∼40% of the modern value (pre-1964, before major fresh water diversions), reflecting severe droughts during which annual precipitation in Jerusalem was lower than 350 mm/y, compared to ∼600 mm/y today. Nevertheless, even during salt intervals, the changes in halite facies and the occurrence of alternating periods of halite and detritus in the Dead Sea core stratigraphy reflect fluctuations between drier and wetter conditions around our estimated average. The halite intervals include periods that are richer and poorer in halite, indicating (based on the sedimentation rate) that severe dry conditions with water availability as low as ∼20% of the present day, continued for periods of decades to centuries, and fluctuated with wetter conditions that spanned centuries to millennia when water availability was ∼50–100% of the present day. These conclusions have potential implications for the coming decades, as climate models predict greater aridity in the region.

Pore fluids extracted from a 456 m sediment core, recovered within the framework of a multinational and International Continental Scientific Drilling Program (ICDP) co-sponsored effort at the bottom of the terminal Dead Sea, recorded the chemical variations in the deep lake over the past 220 k.y. Mg 2+ and Br – were shown to be conservative in the pore fluids, increasing in concentration during interglacial periods, diluting during glacials, and providing excellent proxies for deep lake net water balance changes. Furthermore, the Na/Cl ratio recorded the process of halite precipitation and dissolution induced by these hydrological changes. Mg 2+ and Br – records follow a glacial-interglacial pattern, such as observed in atmospheric CO 2 concentrations and global sea-surface temperatures, albeit with a phase offset. At the end of the last interglacial (ca. 116 ka), there is a delay in onset of dilution of the deep lake, most likely due to the limnological transition from holomictic to meromictic conditions. The increase in deep lake concentrations at Last Glacial Termination I is delayed as a result of freshwater input into the deep lake during the cooler Younger Dryas period. There is a persistent relationship between precipitation in the watershed and North Atlantic sea-surface temperatures, similar to conditions observed over the past instrumental record. Deviations from the long-term trends occurred during interglacial periods, Marine Isotope Stages MIS 5e and MIS 1, when the deep Dead Sea was significantly diluted, and coincided with Mediterranean sapropel layers S5 and S1.

The main comment by Charrach (2019) on the Levy et al. (2019) paper is that the Sedom salt diapir could not have been a source for sulfate to Lake Lisan (last glacial Dead Sea), arguing that the dissolution of the salt diapir started following the onset of the Holocene Dead Sea. We refute the comment for the following reasons: (1) The nature of the unconformity between the salt diapir and the overlying sediments indicates that it emerged from the surface prior to the last glacial and was submerged in Lake Lisan; (2) The formation of a ∼40 m thick layer of dissolution residue (caprock) sitting on an almost flat dissolution unconformity surface (salt mirror) suggests that 600 m-to-800 m thick layer of Sedom Fm. was dissolved under phreatic settings. During most of the Holocene, the diapir has been subjected to vadose type dissolution which formed karst cutting through the caprock, salt mirror and rock-salt below; (3) Based on the Charrach (2019) hypothesis, estimated diapir uplift rates during the early Holocene would have required to be an order of magnitude higher than the measured and calculated rates for the present and Holocene, respectively, provided by other studies; (4) Na/Ca ratios from primary aragonite in the Lisan Fm. found at the vicinity of Mt. Sedom, as well as Na/Cl ratios, saturation state of halite and isotopic composition of chloride in the pore fluids from the deep Dead Sea Deep Drilling Project (DSDDP) core 5017-1-A, suggests significant dissolution of halite from Mt. Sedom into Lake Lisan. In addition to halite, dissolution of the next abundant evaporite mineral, anhydrite, resulted in mobilization of sulfate to the lake.

This study presents geochemical evidence for biogenic methane formation (methanogenesis) in the shallow sediments of the oligotrophic SE Mediterranean continental shelf at water depths between 46 and 88 m. Depth-profiles of methane concentrations and related chemical parameters such as dissolved sulfate, dissolved inorganic carbon (DIC), and the stable carbon isotope composition of DIC and methane (δ 13 C DIC , δ 13 C CH4 , respectively) were measured in six sediment cores (each 4.2–5.4 m long) in order to characterize the processes that involve methane production and decomposition. All the sediment cores reached the consumption depth of the entire sulfate pool and the in-situ microbial methane production (methanogenesis) zone. Methane concentrations reached saturation levels in one of the cores, but not in the others, probably because the zone of maximum methanogenesis was at a greater depth. Although the sediments exhibit a low TOC content of ~1%, the biogenic methane formation indicates a relatively high organic carbon lability capable of sustaining all redox microbial activity potential. Anaerobic oxidation of methane (AOM) was also evident in the sulfate–methane transition zone, showing a distinct isotope signature in diffusion limited conditions.

The geological evolution of the unique Dead Sea Ca-chloride brine has been the focus of many research efforts for several decades. These studies relied on the information obtained from sedimentary exposures of the marginal terraces of the modern Dead Sea, mostly documenting the history of the surface lake brine during its high stands periods. The present study is the first attempt to establish the history of the deepest part of the lake by direct measurements of the chemical and isotopic composition of pore-fluids that were extracted from cores drilled during 2011 by ICDP in the deep basin of the Dead Sea at water depth of 300 m. The vertical profiles of chloride (Cl−) and oxygen isotopes ( δ O 18 ) in pore brines reveal a substantial decrease in the salinity of the hyper-saline lake during the last glacial and particularly during MIS2 ( ∼ 31 – 17 ka BP ). The Cl− concentration of the deep brine in the lake decreased gradually, reaching a minimum of less than 2/3 of its present value while the δ O 18 increased to maximum of ∼ 7 ‰ (3‰ higher than today). The low Cl− indicates significant dilution of the bottom water mass (hypolimnion) of Lake Lisan (the last glacial predecessor of the modern Dead Sea) during its highest stand period. Beforehand, during the interglacial and later during the post-glacial and the Holocene the Cl− concentrations and δ O 18 values were similar to those of the modern Dead Sea. The slow dilution of the deep Ca-chloride brine was caused probably by continuous turbulent mixing of the hypolimnion with the less saline high δ O 18 epilimnetic brine, across the epilimnion/hypolimnion interface (EHI). While the increase in δ O 18 during the salinity decrease of Lake Lisan is a result of “normal” evaporation of the less saline epilimnetic brine, the post-glacial δ O 18 decrease (contemporaneous with salinity increase) is attributed to the “reversed” behavior of δ O 18 during evaporation of high salinity brine. During the long freshening period the hypolimnion was enriched with dissolved sulfate supplied by the freshwater and transported by the turbulent mixing across the EHI until reaching gypsum saturation that commenced massive gypsum deposition at the end of this period, when full overturn took place.