Changes in climate and biota on geologic time scales.

The 1980s saw the emergence of a new approach that is both enhancing our understanding of climatic change on geologic time scales and providing insight into the impact of these climatic changes on the continental and marine biota. This approach (loosely termed 'data-model comparisons') has been applied both to glacial-interglacial changes occurring during the last three million years and driven by variations in the earth's orbit around the sun, and to the slower climatic changes occurring over much longer time scales and driven by tectonic processes. The strength of this approach lies in comparing the results of two independent ways of studying the climate system, and thus providing insight into the strengths and weaknesses of each. Comparisons are made between climatic changes simulated by experiments with General Circulation Models and those reconstructed from fossils. Close agreement between the results of these two independent techniques implies that the key physical processes acting within the climate system have been identified; fundamental mismatches motivate re-examination of both the models and the fossil biotic data.

Oceanic mechanisms for amplification of the 23,000-year ice-volume cycle.

Situated adjacent to the largest Northern Hemispher ice sheets of the ice ages, the mid-latitude North Atlantic Ocean has an important role in the earth's climate history. It provides a significant local source of moisture for the atmosphere and adjacent continents, forms a corridor that guides moisture-bearing storms northward from low latitudes, and at times makes direct contact along its shorelines with continental ice masses. Evidence of major ice-ocean-air interactions involving the North Atlantic during the last 250,000 years is summarized. Outflow of icebergs and meltwater initially driven by summer insolation over the ice sheets affects midlatitude ocean temperatures, summer heat storage, winter sea-ice extent, and global sea level. These oceanic responses in turn influence the winter moisture flux back to the ice sheets, as well as ablation of land ice by calving. Spectral data indicate that the oceanic moisture and sea-level feedbacks, in part controlled by glacial melt products, amplify Milankovitch (insolation) forcing of the volumetrically dominant mid-latitude ice sheets at the 23,000-year precessional cycle.

Warmth of the subpolar north atlantic ocean during northern hemisphere ice-sheet growth.

Two 10,000-year periods of Northern Hemisphere continental ice-sheet growth stand out prominently within the last full interglacial-to-glacial cycle. During the first half of each rapid ice-growth phase, the subpolar North Atlantic from 40 degrees N to 60 degrees N maintained warm sea-surface temperatures comparable to those of today's ocean. The juxtaposition at latitudes 50 degrees N to 60 degrees N of an "interglacial" ocean along-side a "glacial" land mass, particularly along eastern North America, is regarded as an optimal configuration for delivering moisture to the growing ice sheets.

North atlantic ice-rafting: a major change at 75,000 years before the present.

During the last interglacial-to-glacial climatic cycle [127,000 to 10,000 years before the present (B.P.)], the fundamental geographic shift in the main axis of ice-rafting deposition occurred at 75,000 years B.P. An earlier meridional depositional maximum along the Greenland-Newfoundland coasts was superseded by a nearly zonal and much stronger axis some 1500 kilometers to the south along 40 degrees N to 50 degrees N. Both depositional patterns are best explained by cyclonic flow in the subpolor gyre, with the depositional shift related to the retreat of warm, ice-melting North Atlantic drift water from the northwestern half of the gyre. Similar shifts must have characterized preceding interglacial-glacial cycles.

Recent planktonic foraminifera: dominance and diversity in north atlantic surface sediments.

Foraminferal dominance values above 50 percent and associated diversity minimums in surface sediments of the North Atlantic coincide with past extremes of temperature, productivity, or salinity in overlying surface waters. These parameters delimit a cold PolarSubpolar water mass and an impoverished, saline Southern Sargasso Central water mass for the late Recent. Anticipated pole-to-equator diversity and dominance gradients in the open ocean are virtually eliminated by the stronger trends of the vigorous subtropical North Atlantic gyre.

Shaping of the continental rise by deep geostrophic contour currents.

Geostrophic contour-following bottom currents involved in the deep thermohaline circulation of the world ocean appear to be the principal agents which control the shape of the continental rise and other sediment bodies.