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| Titel |
The Ocean δ¹⁸O Record, Ice Volume, and Sea Level: Four Million Years of Natural Climate Variability (Milutin Milankovitc Medal Lecture) |
| VerfasserIn |
Maureen Raymo |
| 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 |
250100511
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| Publikation (Nr.) |
 EGU/EGU2014-16485.pdf |
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| Zusammenfassung |
| How have the interactions between orbitally-controlled variations in insolation and
geologically and biologically controlled variations in atmospheric CO2 influenced
Earth’s climate over the last four million years? Evidence is growing that the late
Pliocene intensification of northern hemisphere glaciation was likely driven by a
modest decrease in atmospheric CO2 levels possibly driven by enchanced chemical
weathering on the continents. CO2 levels during the mid-Pliocene warm period
appear to have been between 350-450 ppm and the Antarctic ice sheet appears to
have been far more dynamic than at present. Exactly how much of the south polar
ice sheet melted during the warmest intervals of the mid Pliocene is unknown;
geochemical proxies for ice volume are fraught with uncertainty due to the possible
influence of diagenesis and changing ocean chemistry; geomorphologic evidence
for sea level highstands are complicated by the influence of dynamic topography
and glacial isostasy. I will present some recent results of the Pliomax project and
discuss how we are using predicted global patterns of glacial isostatic adjustment
(GIA) and dynamic topography to guide field efforts aimed at extracting the eustatic
component of sea level change for the mid Pliocene. I also discuss how our field
data is helping, in turn, to constrain uncertainties in models of both GIA and the
long-term convective evolution of the Earth (uncertainties in mantle viscosity, for
instance) as well as eustatic sea level during more recent warm extremes of the
Quaternary.
By 2.7 Ma ago, large changes in polar ice volume were also occurring in the northern
hemisphere, however the timing, physics, and amplitude of the ice response remain uncertain,
especially for the early Pleistocene. How, for instance, do we explain the lack of significant
precession variability in early Pleistocene climate records (e.g. the “41kyr world”)? This
observation has proven to be a conundrum for many decades because the canonical
Milankovic hypothesis (as well as most climate-ice sheet models) predict that polar ice
volume is most sensitive to high-latitude northern summer insolation, which is dominated by
the 23 kyr precession period. However, the possibility of a more dynamic Antarctic ice sheet
also opens the possibility that the δ18O record is not, in fact, providing us with an accurate
representation of the history of northern hemisphere ice sheets. We showed that
if one allowed for the possibility of Antarctic ice volume change between 3 and
1 Myr, ice volume changes in both the northern and southern hemispheres, each
controlled by local summer insolation, would result in the anti-phased precession
component cancelling in the globally integrated marine δ18O record. Only a modest
ice mass change in Antarctic ice volume, at the precession frequency, is required
to "hide" a much larger northern hemisphere ice volume signal at this frequency. |
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