Molybdenum isotope fractionation in pelagic euxinia: Evidence from the modern Black and Baltic Seas

Nägler, T.F.; Neubert, N; Böttcher, M.E.; Dellwig, O; Schnetger, B (2011). Molybdenum isotope fractionation in pelagic euxinia: Evidence from the modern Black and Baltic Seas. Chemical geology, 289(1-2), pp. 1-11. Amsterdam: Elsevier 10.1016/j.chemgeo.2011.07.001

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Here we present stable isotope data for vertical profiles of dissolved molybdenum of the modern euxinic water columns of the Black Sea and two deeps of the Baltic Sea. Dissolved molybdenum in all water samples is depleted in salinity-normalized concentration and enriched in the heavy isotope (δ98Mo values up to + 2.9‰) compared to previously published isotope data of sedimentary molybdenum from the same range of water depths. Furthermore, δ98Mo values of all water samples from the Black Sea and anoxic deeps of the Baltic Sea are heavier than open ocean water.

The observed isotope fractionation between sediments and the anoxic water column of the Black Sea are in line with the model of thiomolybdates that scavenge to particles under reducing conditions. An extrapolation to a theoretical pure MoS42− solution indicates a fractionation constant between MoS42− and authigenic solid Mo of 0.5 ± 0.3‰. Measured waters with all thiomolybdates coexisting in various proportions show larger but non-linear fractionation. The best explanation for our field observations is Mo scavenging by the thiomolybdates, dominantly — but not exclusively — present in the form of MoS42−.

The Mo isotopic compositions of samples from the sediments and anoxic water column of the Baltic Sea are in overall agreement with those of the Black Sea at intermediate depth and corresponding sulphide concentrations. The more dynamic changes of redox conditions in the Baltic deeps complicate the Black Sea-derived relationship between thiomolybdates and Mo isotopic composition. In particular, the occasional flushing/mixing, of the deep waters, affects the corresponding water column and sedimentary data. δ98Mo values of the upper oxic waters of both basins are higher than predicted by mixing models based on salinity variations. The results can be explained by non-conservative behaviour of Mo under suboxic to anoxic conditions in the shallow bottom parts of the basin, most pronounced on the NW shelf of the Black Sea.

Item Type:	Journal Article (Original Article)
Division/Institute:	08 Faculty of Science > Institute of Geological Sciences
UniBE Contributor:	Nägler, Frank Thomas
ISSN:	0009-2541
Publisher:	Elsevier
Language:	English
Submitter:	Frank Thomas Nägler
Date Deposited:	04 Oct 2013 14:29
Last Modified:	02 Mar 2023 23:21
Publisher DOI:	10.1016/j.chemgeo.2011.07.001
Web of Science ID:	000296404700001
URI:	https://boris.unibe.ch/id/eprint/10598 (FactScience: 216499)