Geochemical proxies of North American freshwater routing during the Younger Dryas cold event
- Anders E. Carlson*,†,
- Peter U. Clark*,
- Brian A. Haley‡,
- Gary P. Klinkhammer§,
- Kathleen Simmons¶,
- Edward J. Brook*, and
- Katrin J. Meissner‖
- *Department of Geosciences, Oregon State University, Corvallis, OR 97331
- ‡The Leibniz Institute of Marine Sciences, University of Kiel, D-24148 Kiel, Germany
- §College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331
- ¶U.S. Geological Survey, Denver, CO 80225; and
- ‖School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, Canada V8W 3P6
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Edited by James P. Kennett, University of California, Santa Barbara, CA, and approved February 27, 2007 (received for review December 19, 2006)
Abstract
The Younger Dryas cold interval represents a time when much of the Northern Hemisphere cooled from ≈12.9 to 11.5 kiloyears B.P. The cause of this event, which has long been viewed as the canonical example of abrupt climate change, was initially attributed to the routing of freshwater to the St. Lawrence River with an attendant reduction in Atlantic meridional overturning circulation. However, this mechanism has recently been questioned because current proxies and dating techniques have been unable to confirm that eastward routing with an increase in freshwater flux occurred during the Younger Dryas. Here we use new geochemical proxies (ΔMg/Ca, U/Ca, and 87Sr/86Sr) measured in planktonic foraminifera at the mouth of the St. Lawrence estuary as tracers of freshwater sources to further evaluate this question. Our proxies, combined with planktonic δ18Oseawater and δ13C, confirm that routing of runoff from western Canada to the St. Lawrence River occurred at the start of the Younger Dryas, with an attendant increase in freshwater flux of 0.06 ± 0.02 Sverdrup (1 Sverdrup = 106 m3·s−1). This base discharge increase is sufficient to have reduced Atlantic meridional overturning circulation and caused the Younger Dryas cold interval. In addition, our data indicate subsequent fluctuations in the freshwater flux to the St. Lawrence River of ≈0.06–0.12 Sverdrup, thus explaining the variability in the overturning circulation and climate during the Younger Dryas.
Footnotes
- †To whom correspondence should be sent at the present address: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543. E-mail: acarlson{at}whoi.edu
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Author contributions: A.E.C. and P.U.C. designed research; A.E.C., B.A.H., G.P.K., and K.S. performed research; A.E.C., B.A.H., G.P.K., and K.S. contributed new reagents/analytic tools; A.E.C., P.U.C., E.J.B., and K.J.M. analyzed data; and A.E.C. and P.U.C. wrote the paper.
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The authors declare no conflict of interest.
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This article is a PNAS Direct Submission.
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See Commentary on page 6500.
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This article contains supporting information online at www.pnas.org/cgi/content/full/0611313104/DC1.
- Abbreviations:
- Sv,
- Sverdrup;
- AMOC,
- Atlantic meridional overturning circulation;
- SST,
- sea surface temperature;
- kyr BP,
- kiloyears B.P.
- © 2007 by The National Academy of Sciences of the USA
