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Dynamic Tracer Proxies of Interglacial Thermohaline Circulation Variability

OCCI Project Funded: 2001

Proposed Research

Changes in ocean thermohaline circulation may have a strong impact on regional climate through associated changes in heat transport to high latitudes. Abundant existing evidence suggests that changes in thermohaline circulation and dramatic climate variability were linked during the last ice age. Currently, the best reconstructions of past thermohaline circulation regimes rely on nutrient-based proxies such as carbon isotopes and trace metals incorporated in the shells of microfossils, primarily benthic foraminifera. This study is designed to complement the nutrient-proxy identification of water masses with two dynamic-tracer proxies that yield information about the local and basin-wide movements of deep and intermediate waters. Protactinium and thorium, produced by the decay of uranium in seawater, will be measured in deep-sea sediments from the Atlantic ocean. The measured Pa/Th will provide an estimate of the advective removal of the less particle-reactive Pa, and thus a measure of the vigor of export of deep waters. Sediment particle-size distributions will also be measured, providing estimates of the strength of deep and intermediate flows along the Reykjanes Ridge and western boundary of the Atlantic. The goal of combining these approaches is to obtain information about the overall rates of circulation, and thus to quantify the heat transport and likely climatic impact of change in the thermohaline circulation. Refinements in these combined techniques will allow them to be applied to assess the subtle variability that may characterize interglacial intervals such as the modern one, and also to estimate the magnitude of climate changes that may be associated with any future variability in thermohaline circulation.

Final Report

The funds for this project were expended during the study period from 2001 until July 14, 2003. They were used to provide partial salary and analytical support for the efforts of the PI, technical staff, and three students to develop new approaches to assessing the strength of deep circulation in the past. Any changes to this system may have important climatic implications, as a result of the heat transport associated with the return flow of warm surface waters that compensates for the production and export of deep water from high latitudes. In particular, the study focused on "dynamical proxies", physical and radiochemical evidence preserved in sediment cores that can be used to infer and perhaps quantify water movements in the deep ocean. Because past changes during interglacial times may have been relatively small, we used the support from the Institute to make progress on honing the methods as well as developing new records.

Core samples were taken from three locations in the North Atlantic. Preliminary data were generated for the most recent interglacial interval, the Holocene, as well as for the two most prominent past interglacial intervals of the last half million years, Marine Isotope Stage 5 (MIS 5) and MIS 11. For two of the sites, we measured naturally occurring uranium, protactinium, and thorium in the sediments, since the 231Pa/230Th ratio is a promising measure of the residence time of seawater in the basin, and thus inversely related to the rate of deepwater production and export. At a site in the subtropical Sargasso Sea, we found evidence for a weakening of the deep overturning circulation in the mid-Holocene that appears to support the recent inference based on the carbon isotopic nutrient proxy, even though that proxy may not always provide a dynamical assessment of circulation strength. At a tropical site, we determined that the standard approach for measuring naturally occurring uranium and its radioactive daughter products was not adequate for the examination of MIS 5, even though 231Pa and 230Th are relatively long-lived, because much of the original signal has already decayed away in the more than 100,000 years since deposition of MIS 5 sediments. This led us to begin investigations into methods that would optimize the corrections required for this analysis, and to modify the sample preparation to maximize signal to noise ratios in the final measurements. At a site farther north, much nearer to the source of deep-waters that overflow the Iceland-Scotland Ridge into the deep Atlantic, we used grain size measurements. This is a more local approach that does not integrate the overturning rate in the entire basin but rather indicates the vigor of bottom currents at a specific key location. We measured sediment grain size at this site for the Holocene and MIS 11. The measurements indicate similar strength and variability in the overflows during these two interglacial intervals. OCCI support thus helped advance both the methods and the science. Much of the funding from the research award went to Marti Jeglinski, for technical support. Three students, Jeanne Gherardi, Luna Federici, and Sharon Hoffmann, also worked on these projects, one at each location. The students contributed to the following papers presented at national and international conferences.

References

Federici, L., and McManus, J. F. (2003). Ocean circulation and climate during the stage 11 Interglacial. Geophysical Research Abstracts 5, 04712.

Federici, L., and McManus, J. F. (2003). Ocean circulation and climate during the stage 11 Interglacial interval. XVI INQUA Congress Program with Abstracts, 138.

Gherardi, J., McManus, J. F., and Francois, R. (2001). New sedimentary 231Pa/230Th records of variable thermohaline circulation rates associated with deglacial climate oscillations in the North Atlantic. EOS 82, S217.

McManus, J. F., and Federici, L. (2002). Absolute chronology for the last climate cycle using excess 230Th in marine sediments. EOS 83, F409.

McManus, J. F., Hoffmann, S. S., Oppo, D. W., Johnsen, S. J., Cullen, J. L., and Cosier, S. (2003). The Last Interglacial in the Northern and Southern Hemispheres. EOS.

Originally published: January 1, 2001