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| Titel |
Sedimentary cobalt concentrations track marine redox evolution |
| VerfasserIn |
Elizabeth Swanner, Noah Planavsky, Stefan Lalonde, Jamie Robbins, Andrey Bekker, Olivier Rouxel, Kurt O. Konhauser, Stephen J. Mojzsis |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250080257
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| Zusammenfassung |
| Oxygen production by photosynthesis drove the redox evolution of the atmosphere and
ocean. Primary productivity by oxygenic photosynthesizers in the modern surface ocean is
limited by trace nutrients such as iron, but previous studies have also observed high Co
uptake associated with natural cyanobacterial populations. Constraining the size and variation
of the oceanic reservoir of Co through time will help to understand the regulation of primary
productivity and hence oxygenation through time. In this study, Co concentrations from iron
formations (IF), shales and marine pyrites deposited over nearly 4 billion years of Earth’s
history are utilized to reconstruct secular changes in the mechanisms of Co removal from the
oceanic reservoir. The Co reservoir prior to ~2 Ga was dominated by hydrothermal inputs
and Fe(III)oxyhydroxides were likely involved in the removal of Co from the water column.
Fe(II) oxidation in the water column resulted in the deposition of IF in the Archean
and Paleoproterozoic, and the Co inventory of IF records a large oceanic reservoir
of Co during this time. Lower Co concentrations in sediments during the Middle
Proterozoic signify a decrease in the oceanic reservoir due to the expansion euxinic
environments, corresponding to the results of previous studies. A transition to an
oxidized deep ocean in the Phanerozoic is evidenced by correlation between Co and
manganese (Mn) concentrations in hydrothermal and exhalative deposits, and in
marine pyrites. This relationship between Co and Mn, signifying deposition of Co in
association with Mn(IV)oxides, does not occur in the Precambrian. Mn(II) oxidation
occurs at higher redox potentials than that required for Fe(II) oxidation, and the
extent of Mn redox cycling prior to full ventilation of the oceans at the end of the
Neoproterozoic was likely limited to spatially restricted oxic surface waters. In this
regard, Co is another valuable redox proxy for tracking the growth and decline
in oxygenated settings, complementary to other metals that are characteristically
enriched under euxinic conditions (i.e. Mo, Zn and Cu) and anoxic (i.e. U, Re and Os)
conditions, even as the availability of Co may have influenced primary productivity. |
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