Evidence from molybdenum and iron isotopes and molybdenum-uranium covariation for sulphidic bottom waters during eastern Mediterranean sapropel S1 formation

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doi: 10.1016/j.epsl.2014.02.054
Author(s): Azrieli-Tal, I.; Matthews, A.; Bar-Matthews, M.; Almogi-Labin, A.; Vance, D.; Archer, C.; Teutsch, N.
Author Affiliation(s): Primary:
Hebrew University of Jerusalem, Fredy and Nadine Hermann Institute of Earth Sciences, Jerusalem, Israel
Geological Survey of Israel, Israel
University of Bristol, United Kingdom
Volume Title: Earth and Planetary Science Letters
Source: Earth and Planetary Science Letters, Vol.393, p.231-242. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X CODEN: EPSLA2
Note: In English. Includes appendices. 89 refs.; illus., incl. 2 tables, sketch map
Summary: Redox exerts a critical control on organic carbon-rich sedimentation. This is particularly true for Eastern Mediterranean sapropels where seawater stratification is regarded as a major driving force for oxygen depletion, but in which sulphidic (euxinic) bottom waters occur only sporadically. Here we apply a powerful array of geochemical proxies (Fe and Mo stable isotopes together with Mo/U ratios and redox sensitive trace elements (RSTE)) to the determination of water redox evolution during the deposition of Holocene S1 sapropel and its underlying and overlying sediments (ODP core 967D; 2550 m depth). RSTE are asymmetrically distributed within the sapropel, with peak enrichments occurring in its lower (early) part. Negative correlations are found between δ57Fe and both Fe/Al and S wt% in the lower sapropel, and are consistent with the benthic Fe shuttle model Fe enrichment in euxinic basins. MoEF/UEF enrichment factor variations show well defined trends identical to those proposed for open marine settings, in which sub-oxic conditions in the background sediments give way to sulphidic waters at the RSTE peak in the lower sapropel. The most notable features of the Mo isotope profile are 'atypically' light values (δ98/95Mo < -0.7 ppm) in the lower sapropel. Such light Mo isotope values (relative to sea water δ98/95Mo =2.3 ppm) have been related to oxic remobilisation. However, negative correlations between δ98/95Mo and Fe/Al, Ba/Al, Mo/Al and S imply that the lowest Mo isotopic compositions are associated with peak reducing conditions. Taken in conjunction with the evidence from the other proxies for a sulphidic water column, the light Mo isotope values in the lower sapropel are best explained by a large isotopic fractionation between sea water molybdate and thiomolybdate species in mildly euxinic bottom waters ([H2S]aq <10 µM). The data from this study thus show that hitherto unrecognised euxinic conditions occur during the early stages of deposition of the Holocene sapropel S1. Molybdenum isotopes and Ba/Al ratios identify a short-lived sapropel re-ventilation event timed to coincide with the 8.2 ka cold climatic Event. Abstract Copyright (2014) Elsevier, B.V.
Year of Publication: 2014
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 24 Surficial Geology, Quaternary Geology; Actinides; Bottom water; Cenozoic; East Mediterranean; Eh; Euxinic environment; Holocene; ICP mass spectra; Iron; Isotope fractionation; Isotopes; Leg 160; Marine environment; Mass spectra; Mediterranean Sea; Metals; Molybdenum; ODP Site 967; Ocean Drilling Program; Organic compounds; Paleoenvironment; Quaternary; Sapropel; Sea water; Spectra; Stratification; Sulfidic composition; Sulfur; Trace elements; Uranium
Coordinates: N340411 N340411 E0324331 E0324331
Record ID: 2014046667
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands