Isotopic Tracers of Past Ocean Circulation: Turning Lead to Gold

摘要

The broad framework of modern ocean circulation is reasonably well understood. Surface water of the Gulf Stream evaporates, cooling the current on its way to the Arctic. When its density increases above that of the underlying Atlantic waters, the surface water sinks and then begins its journey as deep water, first to the south along the east coast of the Americas, then into the circum-Antarctic current, and finally into the Pacific (see figure). A return flow of surface and intermediate waters from the Pacific and Indian oceans replenishes the lost Atlantic deep waters. This "conveyor belt" spins with a period on the order of 1600 years, and because it controls the longitudinal transfer of heat, it is thought to be one of the major factors of climatic modulation. Major issues relative to oceans and climates in the Quaternary and even further back in time hinge on our understanding of how ocean circulation has evolved throughout geological history. On page 913 of this issue, Christensenet al. (I) take a step forward by using laser mass spectrometry to reveal some surprises about lead isotope variation and past oceanic flow.