For years, researchers have examined climate records indicating that millennial-scale climate cycles have linked the high latitudes of the Northern hemisphere and the subtropics of the North Pacific Ocean. What forces this linkage, however, has been a topic of considerable debate. Did the connection originate in the North Pacific with the sinking of oxygen-rich waters into the interior of the ocean during cool climate intervals, or did it originate in the subtropical Pacific with the transfer of heat between the ocean and the atmosphere?
New research, led by scientists from the Lamont-Doherty Earth Observatory of Columbia University and publishing in the June issue of the journal Geology, shows that over the last 52,000 years, these millennial-scale climate shifts linking the high latitudes of the Northern hemisphere with the subtropical Pacific have been associated with large variations in marine productivity off the coast of western North America. Changes in marine fertility in this area probably arose from local changes in subsurface nutrients concentrations that were driven remotely by wind patterns at low latitude. These millennial-scale climate cycles may have been driven by forces similar to El Niño-Southern Oscilations originating in the tropical Pacific on shorter time scales.
"Such records provide an indication of the way climate has changed in the past and, therefore, hopefully also an indication of the way climate may change in the future," says Lex van Geen, Doherty Senior Research Scientist, Lamont-Doherty Earth Observatory, and Chief Scientist of the oceanographic expedition that collected the sediment cores used for this study.
These findings overturn the previously held theory that bottom-water oxygen levels in Santa Barbara Basin off California were primarily linked to changes in ventilation of the North Pacific rather than changes in marine productivity. The evidence for the newly proposed theory was found in a 15m-long sediment record, spanning the past 52,000 years, collected off Baja California, Mexico in 1999.
The core comes from a site with high sedimentation rates (30 cm/thousand years). The sediments indicate a remarkably consistent teleconnection between changes in marine productivity at this eastern Pacific location, as indicated by several independent proxies, with millennial-scale climate change over the North Atlantic, as recorded by the oxygen isotopic composition of Greenland ice.
"The strength of the connection between the temperature record over Greenland and marine production off Baja California really challenges our understanding of the linkage between high and low latitude climate processes. Our results imply that variations in the workings of the equatorial heat engine may be as important as high latitude processes at millennial timescales," states Joseph Ortiz, Assistant Professor of Geology at Kent State University and first author of the Geology research paper. Dr. Ortiz began working on this research project as a Doherty Associate Research Scientist at the Lamont-Doherty Earth Observatory of The Earth Institute at Columbia University.
A possible interpretation of the new data is that the current balance of El Niño/La Niña conditions in the Pacific Ocean, which favor the supply of nutrients to the surface ocean and therefore high marine productivity today and during warm climate intervals of the past 52,000 years, was altered towards El Niño-like conditions and a lower supply of nutrients during cool climate intervals.
Researchers on this paper include A. van Geen, Columbia University, J.D. Ortiz, Kent State University, S.B. O'Connell and J. DelViscio, Wesleyan University, W. Dean, U.S. Geological Survey, J.D. Carriquiry, Universidad Autonoma de Baja California, T. Marchitto, University of Colorado (formerly at Columbia University), and Y. Zheng, Queens College of the City University of New and Columbia University. This collaboration was funded primarily by the National Science Foundation.
The same team of scientists is currently working on a sediment core collected during the same 1999 expedition from the Soledad Basin (25 °N), an anoxic basin with many of the same desirable properties as the Santa Barbara Basin, but located closer to the tropics, a region that some scientists consider to be an important driver of climate change in the past. Sedimentation rates in this basin are 110 cm/thousand years.