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Article Microbial sedimentary imprint on the deep Dead Sea sediment THOMAS, Camille, et al. & DSDDP Scientific Team Abstract A study of an International Continental Drilling Program core recovered from the middle of the modern Dead Sea has identified microbial traces within this subsurface hypersaline environment. A comparison with an active microbial mat exhibiting similar evaporative processes characterized iron-sulphur mineralization and exopolymeric substances resulting from microbial activity. Exopolymeric substances were identified in the drilled sediment but unlike other hypersaline environments, it appears that they have a limited effect on the precipitation of calcium carbonate in the sedimentary column. Sulphate reduction, however, plays a role in all types of evaporative facies, leading to the formation of diagenetic iron sulphides in glacial and interglacial intervals. Their synthesis seems to occur under progressive sulphidation that generally stops at greigite because of incomplete sulphate reduction. The latter may be caused by a lack of suitable organic matter in this hypersaline, hence energy-demanding, environment. Pyrite may be found in periods of high lake productivity, when more labile organic matter is available. The [...] Reference THOMAS, Camille, et al. & DSDDP Scientific Team. Microbial sedimentary imprint on the deep Dead Sea sediment. Depositional Record, 2016, vol. 2, no. 1, p. 118-138 DOI : 10.1002/dep2.16 Available at: http://archive-ouverte.unige.ch/unige:84446 Disclaimer: layout of this document may differ from the published version. 1 / 1 The journal of the International Association of Sedimentologists ORIGINAL RESEARCH ARTICLE Microbial sedimentary imprint on the deep Dead Sea sediment CAMILLE THOMAS*,1, YAEL EBERT†, YAEL KIRO‡, MORDECHAI STEIN§, DANIEL ARIZTEGUI* and THE DSDDP SCIENTIFIC TEAM *Department of Earth Sciences, University of Geneva, rue des Maraichers 13, Geneva CH 1205, Switzerland (E-mail: [email protected]) †Institute of Earth Sciences, Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem, IL 91904, USA ‡Lamont-Doherty Earth Observatory, Columbia University, 61 Rt. 9W, Palisades, NY 10964, USA §Geological Survey of Israel, 30 Malkhe Israel St., Jerusalem, IL 95501, USA Keywords ABSTRACT EPS, geomicrobiology, hypersaline, iron-sulphur mineralization. A study of an International Continental Drilling Program core recovered from the middle of the modern Dead Sea has identified microbial traces within this 1Present address: UMR 42 CARRTEL, Alpine subsurface hypersaline environment. A comparison with an active microbial Research Center on Lake Food Webs, mat exhibiting similar evaporative processes characterized iron-sulphur mineral- University of Savoie Mont-Blanc, 73376 ization and exopolymeric substances resulting from microbial activity. Exopoly- Le Bourget du Lac, France meric substances were identified in the drilled sediment but unlike other hypersaline environments, it appears that they have a limited effect on the pre- Manuscript received: 24 September 2015; Accepted: 28 April 2016 cipitation of calcium carbonate in the sedimentary column. Sulphate reduction, however, plays a role in all types of evaporative facies, leading to the formation doi: 10.1002/dep2.16 of diagenetic iron sulphides in glacial and interglacial intervals. Their synthesis seems to occur under progressive sulphidation that generally stops at greigite because of incomplete sulphate reduction. The latter may be caused by a lack of suitable organic matter in this hypersaline, hence energy-demanding, environment. Pyrite may be found in periods of high lake productivity, when more labile organic matter is available. The carbon and sulphur cycles are thus influ- enced by microbial activity in the Dead Sea environment and this influence results in diagenetic transformations in the deep sediment. INTRODUCTION organic matter produced by the autotrophic eukaryotes would have been immediately degraded by blooms of The Dead Sea is one of the most saline lakes in the Halobacteria (Oren, 1983). world. In addition, it also has extremely high levels of The microbial influence on the Dead Sea subsurface is divalent cations Ca2+ and Mg2+, making it even harder less well-known. The recovery of a 457 m long core from for life to cope with its chemistry (Nissenbaum, 1975). the deepest part of the lake via the International Conti- While few microbes have adapted to such environments, nental Drilling Program (ICDP)-funded Dead Sea Deep successful colonists include Archaea members of the Drilling Project (DSDDP) has shown a potential for cli- extreme halophilic class Halobacteria, as well as a few matic and palaeoenvironmental reconstructions (Lazar halophilic Bacteria (Bodaker et al., 2010; Rhodes et al., et al., 2014; Neugebauer et al., 2014; Torfstein et al., 2012). 2015). In order to constrain the impact of microbial In such an environment, salinity gradients encourage activity on this pristine archive, a geomicrobiological and support life. Submarine freshwater springs host investigation was performed. Initial results revealed the numerous and diverse microbial mats benefiting from importance of assessing the role of microbial activity in local dilution of the brine (Ionescu et al., 2012). In the the early diagenesis of the Dead Sea sediment, highlight- recent past, rainy winters have also brought enough fresh- ing differential distribution of microbes with diverse func- water to the lake to dilute the shallow layers by 70%, tional potential along the core (Thomas et al., 2014; enabling blooms of the halophilic algae Dunaliella parva Ariztegui et al., 2015). The DSDDP project also provides (Oren & Shilo, 1982; Oren et al., 1995). The highly labile a unique opportunity to explore the diagenetic processes ª 2016 The Authors. The Depositional Record published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists. 1 This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. 2 C. Thomas et al. occurring in the sediment of the Dead Sea before they freshwater with the calcium-chlorine brine of the Dead become exposed and fully desiccated. Sea. Anoxic conditions developed in the resulting hypo- Here, we investigate the potential traces of past or cur- limnion, leading to high hydrogen sulphide concentration rent microbial activity in the Dead Sea depositional and the occurrence of iron sulphide explained by bacterial record, and qualify early diagenesis in its deep sediment. sulphate reduction activity (Nissenbaum, 1975; Nishri & In order to do so, the sedimentology and mineralogy of Stiller, 1984). These conditions disappeared quickly from an active microbial mat from the western shore of the the water column after complete overturn in the winter Dead Sea have been examined and identified microbial of 1978–1979 (Steinhorn et al., 1979). Exceptionally rainy traces were searched for in the cored material. winters in 1980 and 1991 led to the renewed formation of Hypersaline environments often allow for the growth temporal meromictic conditions. The diluted epilimnion of microbial mats as their salinity prevents the develop- constituted a less extreme environment in which the halo- ment of grazing communities (Des Marais, 1995). Being philic Green Algae Dunaliella could develop and bloom, often close to saturation with respect to halite, gypsum imparting a red colour to the lake (Oren & Shilo, 1982; and calcium carbonates, it can be difficult to differenti- Oren et al., 1995). ate abiotic mineralization from organomineralization. An array of evidence suggests the presence of relatively The role of EPS as a matrix for mineral nucleation, similar conditions supporting the influence of life on the particle binding or cation concentration is a cornerstone geochemical record of the Lake Lisan sediment. For of the calcium carbonate mineralization in hypersaline example, within the Lisan Formation in the Perazim Val- sediments (Dupraz et al., 2004; Dupraz & Visscher, ley (west of the Dead Sea southern basin), a general 2005). We address EPS recognition and its role in the increase in the d13C of the aragonite laminae deposited Dead Sea realm, which is known to precipitate arago- during the last glacial period has been interpreted as a nite abiotically from the lake water column (Stein et al., result of increasing autotrophic activity in the palaeo-epi- 1997). In addition, the iron-sulphur mineralization pre- limnion (Kolodny et al., 2005). viously recognized as diagenetic phases in palaeomag- Sulphur isotopes from gypsum intervals also support netic studies (Ron et al., 2006) is further investigated. the occurrence of active sulphate reduction in the water Studies of the subsurface waters of the Dead Sea Basin column and bottom sediment of the lake (Torfstein et al., have highlighted major redox transformations affecting 2005, 2008). Negative d34S values from disseminated gyp- carbon, iron and sulphur among other elements sum and thin gypsum laminae of the Lisan Formation (Avrahamov et al., 2010, 2014; Kiro et al., 2013). It is (from À26& to 1&) are interpreted as being a result of therefore necessary to assess the extent of such changes bacterial sulphate reduction in the anoxic hypersaline and whether they reach the deepest region of the Dead palaeo-hypolimnion. Sulphur concretions from the Sea Basin, or if they are constrained to

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