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| Titel |
Mo isotopes in OAE 2 black shales |
| VerfasserIn |
Stéphane Westermann, Derek Vance, Vyllinniskii Cameron, Corey Archer, Stuart A. Robinson |
| 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 |
250094349
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| Publikation (Nr.) |
 EGU/EGU2014-9755.pdf |
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| Zusammenfassung |
| Sedimentary rocks, especially organic-rich deposits, have the potential to track change in the
oxygenation state of the ocean over geological time. Oceanic anoxic events (OAEs)
correspond to periods of profound and rapid environmental change, which have led to both
the widespread deposition of black shales and the development of widespread anoxia in the
ocean. Understanding the variations in redox conditions during these events is of primary
importance, since recent observations and modelling have shown that processes invoked to
explain the origin of OAEs are being observed today as a consequence of anthropogenic
change. Here, we compare redox-sensitive trace metal (RSTM) distributions and
molybdenum (Mo) isotope variations during a major Cretaceous OAE (OAE 2, Bonarelli
event). Whereas RSTM have the potential to provide insights regarding local depositional
conditions and processes in palaeoceanographic systems, Mo-isotope data can, under
certain circumstances, provide quantitative estimates of how the global extent of
seawater anoxia may have fluctuated in the past. We selected for study a series
sections within the western Tethys (La Contessa and Furlo, Italy) and in the northern
Atlantic (DSDP site 367, Cape Verde Basin and ODP site 1276, Newfoundland
Basin.
RSTM contents show similar trends through all the studied sections, characterized by
low concentration below and above the OAE interval and higher concentrations
within the Bonarelli interval. This suggests rapid variations in the redox conditions,
from suboxic to euxinic conditions during OAE 2. The RSTM enrichment factors
(EFs) indicate different depositional conditions and palaeoceanographic processes
between the Tethys and the North Atlantic. Whereas the North Atlantic sites show
evidence of weak watermass restriction associated with the action of a particulate
shuttle within the water column, the EFs of the Tethyan sections are characteristic
of unrestricted marine systems. Despite local differences in the redox conditions,
δ98Mo values show similar values and trends along the sections of La Contessa and
Furlo, and ODP site 1276. At the onset of OAE 2, an increasing trend in δ98Mo is
observed with values ranging from -0.6 to 0.6 oá¸uring the 2nd half of OAE 2, the
δ98Mo curve shows a progressive shift towards more negative values. This pattern is
contrasted with the Mo isotope record from DSDP site 367. Before the OAE 2
interval, the δ98Mo values fluctuate between 0.15 and 0.48oá¸uring OAE 2, an
increase in δ98Mo is observed, ranging from 0.15 to 1.15 oṪhen, the Mo isotopes
show two rapid fluctuations towards lower values (~0.70 ) and increase again to
relatively constant values, with values fluctuating around an average value of 1.10
o
Both the western Tethys and the Northern Atlantic sites show redox variations, reaching
anoxic/euxinic conditions. In the western Tethys, despite evidence of strongly reducing
conditions, the relatively light δ98Mo values suggest that redox conditions may not have been
fully euxinic. The light δ98Mo values may thus be related to non-quantitative removal of Mo
from the water column. At site 367, the Mo isotope variations and the RSTE concentrations
suggest fully euxinic conditions and the sediments deposited during OAE 2 may have
recorded the seawater signature. |
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