Publication Abstracts

LeGrande 2006

LeGrande, A.N., 2006: The Climate Imprint on the Oxygen Isotopic Composition of Water: Observations, Proxies, and Coupled Isotopic Model Simulations. Ph.D. thesis. Columbia University.

The isotopic composition of oxygen (δ18O) provides an excellent tool for examining the hydrologic cycle and climate in general; δ18O variability is explored in seawater observations, foraminiferal calcite, and model simulations for both modern and past climate to understand the imprint of climate variability on this important tracer.

A 3-dimensional gridded data set for the annual mean δ18Oseawater is created and imposed as surface δ18Oseawater on the GISS atmosphere-only general circulation model (GCM) yielding a more realistic distribution of δ18Oprecip compared to data and quantifying the "source" effect of δ18Oseawater on δ18Oprecip.

Temperature and δ18Oseawater (regionally linearly related to salinity) determine δ18Ocalcite; since temperature and salinity also control seawater density, δ18Ocalcite can proxy density. Intermediate depth horizontal density gradients are associated with the vertical shear of upper-ocean flows. We find that δ18O in the test of Globorotalia truncatulinoides can proxy intermediate depth density, and spatial patterns of core top δ18Otrunc in the Atlantic Ocean mimic the upper-ocean density gradients associated with the major currents.

During the last ice age, the density gradient across the Florida Current was reduced, implying a reduction in flow of the Gulf Stream through the Florida Straits. Using the above technique, δ18Otrunc from down-core samples suggests that this flow, like today, was confined within the Florida Straits during the last glacial maximum.

Isotope, aerosol, and methane records document an abrupt cooling event across the Northern Hemisphere 8200 years before the present, while separate geologic evidence documents the catastrophic drainage of glacial Lakes Agassiz and Ojibway into the Hudson Bay. The coincidence of this freshwater release suggests it was the catalyst; however, debate remains concerning whether alterations in ocean circulation caused this cooling event.

The climate response to this freshwater pulse are simulated in the GISS coupled and atmosphere-only GCM versions that explicitly track water isotopes, calculate atmospheric aerosol deposition, and determine wetland methane emissions and find that the simulated short period of significantly diminished North Atlantic Deep Water quantitatively matches proxy observations. This direct comparison with multiple paleoclimate records provides compelling evidence that changes in ocean circulation played a major role in this abrupt climate change.

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BibTeX Citation

@phdthesis{le01200l,
  author={LeGrande, A. N.},
  title={The Climate Imprint on the Oxygen Isotopic Composition of Water: Observations, Proxies, and Coupled Isotopic Model Simulations},
  year={2006},
  school={Columbia University},
  address={New York, N.Y.},
}

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RIS Citation

TY  - THES
ID  - le01200l
AU  - LeGrande, A. N.
PY  - 2006
BT  - The Climate Imprint on the Oxygen Isotopic Composition of Water: Observations, Proxies, and Coupled Isotopic Model Simulations
PB  - Columbia University
CY  - New York, N.Y.
ER  -

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