Publikationsdatum:
2019-07-17
Beschreibung:
Oceanic anoxic events (OAEs) were a frequent occurrence
in the Cretaceous greenhouse ocean. Based on a variety of
paleoredox indicators, euxinic water column conditions are
commonly invoked for these OAEs. However, in a high
resolution study of OAE3 deep sea sediments [1], revised
paleoredox indicators suggest that euxinic conditions
fluctuated with anoxic ferruginous conditions on orbital
timescales. Building upon this, we here present new data for a
continental shelf setting at Tarfaya, Morocco, that spans a
period prior to, and during, the onset of OAE2. We again find
strong evidence for orbital transitions from euxinic to
ferruginous conditions. The presence of this distinct cyclicity
during OAE2 and OAE3 in shallow and deep water settings,
coupled with its occurrence on the anoxic shelf prior to the
global onset of anoxia, suggests that these fluctuations were a
fundamental feature of anoxia in the Cretaceous ocean.
The observed redox cyclicity has major implications for
the cycling of phosphorus, and hence the maintenance and
longevity of OAEs. However, despite this significance,
controls on the observed redox cyclicity are essentially
unknown. Here, we utilize S isotope measurements (pyrite S
and carbonate-associated S) from the deep sea and shelf
settings to model oceanic sulphate concentrations across the
redox transitions. Perhaps surprisingly, we find no evidence to
suggest that ferruginous conditions arose due to extensive
drawdown of seawater sulphate (as pyrite-S and organic-S)
under euxinic conditions. Instead, S isotope systematics in the
deep sea imply increased sulphate concentrations during
ferruginous intervals. Based on these observations and other
major element data, we infer that the redox cyclicity instead
relates to orbitally-paced fluctuations in continental hydrology
and weathering, linking the redox state of the global ocean to
climate-driven processes on land.
[1] März et al (2008) GCA, 72, 3703-3717.
Repository-Name:
EPIC Alfred Wegener Institut
Materialart:
Article
,
notRev
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