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| A close-up view of a SeaBeam bathymetry
(sea depth) survey conducted in 1999 around the researchers'
coring area, off Baja California. The black dot represents
the location of the 15m-long core, where the evidence
for the new proposed theory was found. Image credit:
Barry Eakins, USGS |
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.
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| A photo showing the dark/light
banding pattern characteristic of the area off Baja
California, indicative of a lack of bioturbation.
Photo credit: Allan Horneman/LDEO |
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.
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