Publication Date:
2022-05-25
Description:
Author Posting. © The Author(s), 2002. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 67 (2003): 265-273, doi:10.1016/S0016-7037(02)01075-X.
Description:
Sedimentary records of redox-sensitive trace elements hold significant potential as
indicators of paleoceanographic environmental conditions. Records of Re can reveal the
intensity of past reducing conditions in sediments at the time of deposition (Crusius et al.,
1996), while records of Ag may record the magnitude of past diatom fluxes to the
seafloor. Confidence in paleoenvironmental reconstruction from records of either metal,
however, requires it to have experienced negligible redistribution since deposition. This
study examines diagenetic re-arrangements of Re and Ag that occur in response to
exposure to bottom-water O2 in environments of low sedimentation rate, including
Madeira Abyssal Plain turbidites and eastern Mediterranean basin sapropels. Authigenic
Re was remobilized quantitatively by oxidation, but poorly retained by the underlying
sediments. All records are consistent with previous work demonstrating that only a
limited re-immobilization of Re occurs preferentially in Corg-rich, reducing sediments
(Crusius and Thomson, 2000). Silver was also mobilized quantitatively by oxidation, but
was subsequently immobilized more efficiently in all cases as sharp peaks immediately
into anoxic conditions below active oxidation fronts, and these peaks remain immobile in
anoxic conditions during long-term burial. Comparison of Ag, S and Se records from
various cores suggests that Ag is likely to have been immobilized as a selenide, a
mechanism previously proposed for Hg in similar situations (Mercone et al., 1999). Coexisting
narrow peaks of Ag and Hg with Se offer a means of assessing whether oxidative
burndown has ever occurred at the top of Corg- and sulfide-rich sedimentary units. While
these results suggest that caution must be used when inferring paleoenvironmental
information from records of Ag and Re in cores with low sediment accumulation rates
(〈5 cm ka-1), they should not affect the promise that authigenic Ag and Re records hold
for paleoenvironmental reconstruction in sediments with higher accumulation rates and
where anoxic conditions have been maintained continuously.
Description:
The authors acknowledge financial support from the Japan Science and Technology Fund
(JSTF), the Natural Sciences and Engineering Research Council of Canada (NSERC), the
International Atomic Energy Agency (IEEA) and the EU Marine Science and
Technology Programme (project MAS3-CT97-0137 "Sapropels and
Palaeoproductivity").
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
Format:
application/pdf
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