PALEOCEANOGRAPHY

Abstract

Paleoceanographic records are of special significance in the Quaternary sciences because of their continuous nature and widespread geographic distribution. Comprised primarily of terrigenous sediment (mostly clay) and the remains of planktic and benthic plankton, marine sediments offer a diverse array of proxy evidence of past climate and environmental change in the oceans and adjacent continents. The development of oxygen-isotope stratigraphy and the correlation of oxygen-isotope stages to orbital cycles in solar radiation has provided a method to date marine sediments extending back 5 myr with an uncertainty of a few thousand years during the Quaternary. These long records indicate the nonstationary nature of Quaternary climate change. Gradual cooling at high latitudes and increasing ice-sheet extent (and ice-sheet variability) indicated by oxygen isotopes and other proxies led to the emergence of the 41-kyr glacial cycles during the early Quaternary. Following the mid-Pleistocene transition 850 ka, the dominant period of variance changed from 41- to 100-kyr cycles. Throughout, variance in many aspects of climate at 100, 41, and 23 kyr can be attributed to pacing by the Earth orbital cycles in eccentricity, obliquity, and precession. Understanding how these small variations in the seasonal and geographical distribution of solar radiation are amplified to produce the Quaternary ice ages remains a key puzzle. The glacial world reconstructed on the basis of marine sediment cores reveals the largest differences from today at mid- to high latitudes, where the sea surface cooled by 10 °C or more and sea ice expanded in extent. In the tropical ocean, sea-surface temperatures (SSTS) cooled by several degrees (0–3 °C). Changes were smallest in the mid-latitude gyres. The deep circulation also changed. The Atlantic meridional overturning circulation slowed and became more shallow in the glacial North Atlantic. The deep Pacific became less corrosive with respect to calcium carbonate and the deep North Atlantic more corrosive, another indicator of altered deep circulation. Consistent with ice-core records of atmospheric carbon dioxide, paleoceanographic records reveal that the surface–deep ocean gradient in carbon increased during glacial times. All of these changes point to the roles of carbon dioxide, land and surface ice-sheet extent and vegetation change, and ocean circulation acting in different ways to amplify the pacemaking provided by the orbital cycles. Paleoceanographic records reveal that glacial times were unusually variable compared to interglacial times, marked by abrupt changes between cold stadial events and warm interstadial events, termed Dansgaard–Oeschger (D–O) cycles. These events are most pronounced in the Greenland ice cores and the North Atlantic sediments, but have been found in records as far away as the California Margin and Asian monsoon regions. Most theories proposed to explain these abrupt climate changes involve the interaction between unstable ice sheets surrounding the North Atlantic and the surface and deep circulation. In contrast to glacial times, the Holocene appears relatively stable apart from trends in the monsoon and tropical circulation attributed to decreasing seasonality in insolation between 11 kyr BP and the present. Superimposed on the Holocene trend are small-amplitude millennial-scale cycles in the North Atlantic and monsoon regions that can be correlated with centennial-scale solar radiation changes indicated by radiocarbon and 10 Be, supporting a role for even small variations in solar radiation on Quaternary climate.