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| Titel |
Cenozoic Climate Change: Geochemical Proxy Records from Deep Ocean Sediments |
| VerfasserIn |
K. Billups, N. Venti |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250023543
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| Zusammenfassung |
| Cenozoic climate evolved from a the warm Paleocene and Eocene (Â 16 Ma) to the relatively
cold conditions of the modern world via three major ice growth events first on Antarctica at
the Eocene/Oligocene boundary then during the middle Miocene and finally in the
Northern Hemisphere during the late Pliocene. Much of what we know about past
climate change comes from the oxygen isotopic composition of benthic foraminifera.
Although this proxy outlines large scale changes in the degree of polar glaciation, the
absolute magnitude and the relationship between ice extent and ocean temperature
cannot be uniquely determined. The recent development of foraminiferal Mg/Ca
ratios as a proxy for paleotemperatures provides an opportunity to improve our
understanding of climate change on both tectonic and orbital time scales. For example,
paired
delta18O and Mg/Ca deep water records show that expansion of ice at the Eocene/Oligocene
boundary occurred in steps and was accompanied by cooling of water temperatures by about
2-3C (Lear et al., 2008; Katz et al.2008). During the middle Miocene expansion of ice
predates cooling of Southern Ocean surface waters providing evidence for the importance
of heat and moisture transport in Antarctic ice growth (Shevenell and Kennett,
2007). Relatively few deep sea studies have focused on late Miocene climate, and
foraminiferal
delta18O records do not support major oceanographic and climatic changes. Although, the
late Miocene may have been a time of global cooling, especially in the circum-Antarctic
region, with the establishment of a grounded West Antarctic ice sheet. The early Pliocene, in
contrast, has been the focus of much research because of the relevance to understanding
intervals of sustained global climatic warmth with near modern-day tectonic configuration,
warm upwelling regions, and elevated CO2 levels with respect to the pre-industrial
atmosphere. The deep sea
delta18O record, however, suggests that Antarctic ice sheet size remained relatively stable
during this particular warm period.
Early Pliocene climatic warmth ended with the beginning of wide spread Northern
Hemisphere Glaciation at
sim3 Ma and enhanced sensitivity of ice sheet variations to obliquity forcing. One hypothesis
seeks to explain this climate transition by a change in oceanic heat balance calling for
coherent changes in eastern tropical and western subtropical Pacific sea surface hydrographic
changes after
sim3 Ma (Philander and Fedorov, 2003; Fedorov et al., 2006). Preliminary new isotope data
from the subtropical northwestern Pacific (Site 1208) show that, on obliquity time scales,
planktonic foraminiferal
delta18O maxima occur slightly before benthic foraminiferal
delta18O maxima (2-3kyr) but are close in time with minima in equatorial Pacific sea surface
temperatures in first order support for the hypothesis. |
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