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| Titel |
Mo isotope record of shales points to deep ocean oxygenation in the early Paleoproterozoic |
| VerfasserIn |
Dan Asael, Clint Scott, Olivier Rouxel, Simon Poulton, Timothy Lyons, Emmanuelle Javaux, Andrey Bekker |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250093591
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| Publikation (Nr.) |
 EGU/EGU2014-8475.pdf |
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| Zusammenfassung |
| Two steps in Earth’s surface oxidation lie at either end of the Proterozoic Eon. The first step,
known as the Great Oxidation Event (GOE), occurred at ca. 2.32 Ga (1), when atmospheric
oxygen first exceeded 0.001% of present atmospheric levels (2). The second step, occurred at
ca. 0.58 Ga, resulting in the pervasive oxygenation of the deep oceans, a feature that
persisted through most of the Phanerozoic (3). The conventional model envisions two
progressive and unidirectional increases in free oxygen. However, recent studies have
challenged this simplistic view of the GOE (4, 5). A dramatic increase and decline
in Earth oxidation state between 2.3 and 2.0 Ga is now well supported (6–9) and
raises the question of how well-oxygenated the Earth surface was in the immediate
aftermath of the GOE. In order to constrain the response of the deep oceans to the
GOE, we present a study of Mo isotope composition and Mo concentration from
three key early Paleoproterozoic black shale units with ages ranging from 2.32
to 2.06 Ga. Our results suggest high and unstable surface oxygen levels at 2.32
Ga, leading to an abrupt increase in Mo supply to the still globally anoxic ocean,
and producing extreme seawater Mo isotopic enrichments in these black shales.
We thus infer a period of significant Mo isotopic Rayleigh effects and non-steady
state behaviour of the Mo oceanic system at the beginning of the GOE. Between
2.2-2.1 Ga, we observe smaller Mo isotopic variations and estimate the δ98Mo of
seawater to be 1.42 ± 0.27 oáºe conclude that oxygen levels must have stabilized at a
relatively high level and that the deep oceans were oxygenated for the first time
in Earth’s history. By ca. 2.06 Ga, immediately after the Lomagundi Event, the
Mo isotopic composition decreased dramatically to δ98MoSW = 0.80 ± 0.21 o
reflecting the end of deep ocean oxygenation and the return of largely anoxic deep
oceans.
References: [1] A. Bekker et al., 2004, Nature 427, 117–20. [2] A. Pavlov and J. Kasting,
2002, Astrobiology 2, 27–41. [3] C. Scott et al., 2008, Nature 452, 456–9. [4] C. Goldblatt et
al., 2006, Nature 443, 683–6. [5] L. Kump et al., 2011, Science 334, 1694–6. [6] A. Bekker
and D. Holland, 2012, Earth Planet. Sci. Lett. 317-318, 295–304. [7] N. Planavsky et al.,
2012, Proc. Natl. Acad. Sci. U. S. A. 109, 18300–5. [8] C. Partin et al., 2013, Chem.
Geol. 362, 82–90. [9] C. Scott et al., 2014, Earth Planet. Sci. Lett. 389, 95–104. |
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