1989 Theses Doctoral

Pliocene-Pleistocene paleoceanography of the North Atlantic Ocean: nature and causes of climate variation of 10⁴-10⁶ year time scales

Raymo, Maureen E.

Information about past climates is obtained from analysis of foraminifera preserved in ocean floor sediments. This thesis focuses on the paleooceanographic record of the North Atlantic Ocean, for the interval spanning the last three million years, and examines potential climate forcing mechanisms using general circulation models and steady-state box models.

High resolution oxygen and carbon isotopic records from the late Pliocene North Atlantic are presented, a time scale developed, and oxygen isotope stages formalized. Variations in these records are dominated by the 41,000 year component of orbital obliquity, however, transient increases in variance at eccentricity and precessional frequencies are noted. Prior to 2.4 million years, ice volumes are estimated to be 1/4 to 1/2 as large as in late Pleistocene. Between 2.4 and 1.6 million years, ice sheets appear to be, on average, 1/2 as large as those of the late Pleistocene.

Carbon isotope records are used to trace the history of North Atlantic deep water circulation in the Pliocene and Pleistocene. Relative reductions in North Atlantic Deep Water production occur during times of ice growth for the last 2.5 million year, but a decoupling of circulation and ice volume patterns at low frequencies indicates that, in addition to ice volume, another climate forcing must exist.

The influence of Arctic sea ice on climate is investigated using a computer model of the Earth's atmosphere. Large reductions in sea ice limits result in significant atmospheric warming at high latitudes in winter. In addition, model results predict warming of sea-surface temperatures in the North Atlantic Ocean. These results are evaluated as an analog for Pliocene times.

Steady-state box models are used to estimate rates of global erosion and input fluxes of dissolved salts to the sea. Oceanic records of strontium, carbon, and calcium are consistent with a significant increase in river dissolved fluxes since the Miocene. This increase is ascribed to accelerated mountain uplift in Asia and South America The late Neogene cooling of global climate may be linked to a decrease in atmospheric carbon dioxide levels driven by enhanced weathering in these tectonically active regions.