TY - JOUR
T1 - Molybdenum reduction in a sulfidic lake
T2 - Evidence from X-ray absorption fine-structure spectroscopy and implications for the Mo paleoproxy
AU - Dahl, Tais W.
AU - Chappaz, Anthony
AU - Fitts, Jeffrey P.
AU - Lyons, Timothy W.
N1 - Funding Information:
Thanks to Stefanie Wirth, and Adrian Gilli (ETH Zürich), who arranged the field trip, provided lithological descriptions and to Moritz Lehmann (University of Basel) who collected the samples. At Harvard University, Peter Girguis and Colleen Hansel provided access to anaerobic chambers and Charles Langmuir and lab manager Zhongxing Chen helped us with Mo concentration analysis at the Center for the Environment ICPMS facility. We thank Trent Vorlicek for comments and improvements of an early version of the manuscript and three anonymous reviewers who greatly improved the manuscript. A.C. and T.W.L. are grateful for financial support from the Agouron Institute (fellowship) and the NSF-EAR Program (Grant 1124327). J.P.F. recognizes support from NSF (Grant 0919140). Brookhaven National Lab is supported by US Department of Energy, Office of Science under contract DE-AC02-98CH10886. T.W.D. thanks Andrew Knoll (NSF Grant EAR-0420592), Villum Kann Rasmussen foundation, Danish Council for Independent Research (FNU), and Danish National Research Foundation (NordCEE) for financial support.
PY - 2013/2/15
Y1 - 2013/2/15
N2 - Marine euxinic sediments, particularly organic-rich black shales, are important sinks for oceanic molybdenum (Mo), and the determination of Mo concentration and isotopic composition are used to constrain oxygenation state and specifically expansion of marine anoxic and sulfidic (euxinic) waters in ancient oceans. The use of Mo as a paleo-redox tracer is based on its distinct geochemical behavior in oxic and sulfidic environments. Mo removal from sulfidic waters starts with MoO42- reacting with aqueous H2S to form particle reactive thiomolybdates, MoO4-xSx2-, but the post-thiomolybdate steps and the ultimate Mo host in euxinic sediments are not well understood. We used X-ray absorption fine structure (XAFS) spectroscopy to determine the oxidation state and the molecular coordination environment of pristine, solid phase Mo in sediments from permanently euxinic Lake Cadagno, Switzerland. Samples were taken from a 9-meter piston core representing the last ten thousand years of deposition.The euxinic lake sediments provide unequivocal evidence that the post-thiomolybdate steps along the burial pathway involve Mo(VI) reduction to Mo(IV) before it is ultimately hosted in the sediments. Anoxic samples contain Mo(IV)-S compounds that, when oxidized upon air-exposure, are transformed into Mo(VI)-O, thus confirming our results. This observation is important to better understand the Mo burial pathway from oxic waters to euxinic sediments. Our results support a model where post-thiomolybdate MoVI reduction proceeds via reactions with zero-valent sulfur, S(0) (mainly S8, Vorlicek et al., 2004). In this model, Mo is scavenged from sulfidic waters as reactive Mo-polysulfide species, Mo(IV)O(S4)S2- or Mo(IV)S(S4)S2-, and not as thiomolybdate (MoOS32- or MoS42-) as has been previously assumed. This result can have important implications for how paleoenvironmental redox conditions are inferred from sedimentary Mo enrichments, Mo/TOC ratios, and δ98Mo, if the Mo accumulation rate is accelerated via the S8-assisted pathway in settings where partial oxidation of S is possible. For example, euxinic Mo/TOC ratios are predicted not only as a function of Mo concentration in overlying seawater. Instead, the ratio will be higher in sediments deposited under highly sulfidic waters where S8 is also abundant, such as in settings where the chemocline depth fluctuates considerably and/or oxygen intrusions occur to the overlying bottom waters. Partial oxidation of dissolved sulfide is an important step in polysulfide production. Finally, we conclude that XAFS measurements are a powerful way to study the Mo burial pathway in sediments and enhance our ability to infer past environmental conditions from the sedimentary record.
AB - Marine euxinic sediments, particularly organic-rich black shales, are important sinks for oceanic molybdenum (Mo), and the determination of Mo concentration and isotopic composition are used to constrain oxygenation state and specifically expansion of marine anoxic and sulfidic (euxinic) waters in ancient oceans. The use of Mo as a paleo-redox tracer is based on its distinct geochemical behavior in oxic and sulfidic environments. Mo removal from sulfidic waters starts with MoO42- reacting with aqueous H2S to form particle reactive thiomolybdates, MoO4-xSx2-, but the post-thiomolybdate steps and the ultimate Mo host in euxinic sediments are not well understood. We used X-ray absorption fine structure (XAFS) spectroscopy to determine the oxidation state and the molecular coordination environment of pristine, solid phase Mo in sediments from permanently euxinic Lake Cadagno, Switzerland. Samples were taken from a 9-meter piston core representing the last ten thousand years of deposition.The euxinic lake sediments provide unequivocal evidence that the post-thiomolybdate steps along the burial pathway involve Mo(VI) reduction to Mo(IV) before it is ultimately hosted in the sediments. Anoxic samples contain Mo(IV)-S compounds that, when oxidized upon air-exposure, are transformed into Mo(VI)-O, thus confirming our results. This observation is important to better understand the Mo burial pathway from oxic waters to euxinic sediments. Our results support a model where post-thiomolybdate MoVI reduction proceeds via reactions with zero-valent sulfur, S(0) (mainly S8, Vorlicek et al., 2004). In this model, Mo is scavenged from sulfidic waters as reactive Mo-polysulfide species, Mo(IV)O(S4)S2- or Mo(IV)S(S4)S2-, and not as thiomolybdate (MoOS32- or MoS42-) as has been previously assumed. This result can have important implications for how paleoenvironmental redox conditions are inferred from sedimentary Mo enrichments, Mo/TOC ratios, and δ98Mo, if the Mo accumulation rate is accelerated via the S8-assisted pathway in settings where partial oxidation of S is possible. For example, euxinic Mo/TOC ratios are predicted not only as a function of Mo concentration in overlying seawater. Instead, the ratio will be higher in sediments deposited under highly sulfidic waters where S8 is also abundant, such as in settings where the chemocline depth fluctuates considerably and/or oxygen intrusions occur to the overlying bottom waters. Partial oxidation of dissolved sulfide is an important step in polysulfide production. Finally, we conclude that XAFS measurements are a powerful way to study the Mo burial pathway in sediments and enhance our ability to infer past environmental conditions from the sedimentary record.
UR - http://www.scopus.com/inward/record.url?scp=84871371917&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2012.10.058
DO - 10.1016/j.gca.2012.10.058
M3 - Article
AN - SCOPUS:84871371917
VL - 103
SP - 213
EP - 231
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
SN - 0016-7037
ER -