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| Titel |
Arctic Ocean circulation during the anoxic Eocene Azolla event |
| VerfasserIn |
Eveline Speelman, Jaap Sinninghe Damsté, Christian März, Hans Brumsack, Gert-Jan Reichart |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043927
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| Zusammenfassung |
| The Azolla interval, as encountered in Eocene sediments from the Arctic Ocean,
is characterized by organic rich sediments (Â 4wt% Corg). In general, high levels
of organic matter may be caused by increased productivity, i.e. extensive growth
of Azolla, and/or enhanced preservation of organic matter, or a combination of
both. Anoxic (bottom) water conditions, expanded oxygen minimum zones, or
increased sedimentation rates all potentially increase organic matter preservation.
According to plate tectonic, bathymetric, and paleogeographic reconstructions,
the Arctic Ocean was a virtually isolated shallow basin, with one possible deeper
connection to the Nordic Seas represented by a still shallow Fram Strait (Jakobsson et al.,
2007), hampering ventilation of the Arctic Basin. During the Azolla interval surface
waters freshened, while at the same time bottom waters appear to have remained
saline, indicating that the Arctic was highly stratified. The restricted ventilation and
stratification in concert with ongoing export of organic matter most likely resulted in the
development of anoxic conditions in the lower part of the water column. Whereas
the excess precipitation over evaporation maintained the freshwater lid, sustained
input of Nordic Sea water is needed to keep the deeper waters saline. To which
degree the Arctic Ocean exchanged with the Nordic Seas is, however, still largely
unknown.
Here we present a high-resolution trace metal record (ICP-MS and ICP-OES) for the
expanded Early/Middle Eocene section capturing the Azolla interval from Integrated
Ocean Drilling Program (IODP) Expedition 302 (ACEX) drilled on the Lomonosov
Ridge, central Arctic Ocean. Euxinic conditions throughout the interval resulted
in the efficient removal of redox sensitive trace metals from the water column.
Using the sedimentary trace metal record we also constrained circulation in the
Arctic Ocean by assessing the relative importance of trace metal input sources
(i.e. fluvial, eolian, and through seawater inflow). Excess vanadium accumulation
during the Azolla event (80 ppm), basin volume and surface area, average vanadium
sea (1.8 ppb) and river water (1.0 ppb) concentrations, together indicate that an
inflow of Nordic Sea water of 0.2 Sv is needed to sustain vanadium levels. The
same calculation using molybdenum gives an inflow of only 0.02 Sv. These low
inflow rates imply Arctic Ocean (deep) water residence times of 2000 - 20000 years,
respectively.
Based on climate modeling we calculated a summed net amount of precipitation for the
Eocene Arctic Basin (Precipitation – Evaporation + Runoff) of 0.46 Sv. Together these
notions indicate that a compensating inflow of saline North Atlantic water occurred,
accompanied by an outflow of more fresh waters, resulting in a bi-directional, two-layer flow
through the (proto-) Fram Strait. Consequently, the limited exchange of water through the
Fram Strait implies that a relatively low export productivity would have been sufficient to
render Arctic bottom waters anoxic.
Jakobsson, M., Backman, J., Rudels, B., Nycander, J., Frank, M., Mayer, L., Jokat, W.,
Sangiorgi, F., O’Regan, M., Brinkhuis, H., King, J., Moran, K. (2007). The early
Miocene onset of a ventilated circulation regimen in the Arctic Ocean. Nature 447,
986–990. |
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