Three-dimensional Modeling of Th-230 and Pa-231 in the Ocean


OCCI Project Funded: 2004

Proposed Research

What are the primary questions you are trying to address with this research?
The primary question we are trying to address is the sensitivity of an emerging proxy in paleoceanography to changes in the ocean general circulation - a major component of the climate system. This new proxy is the ratio between the activity (concentration) of two radiogenic isotopes in marine sediments: protactinium-231 and thorium-230. Both isotopes are naturally produced in ocean waters from the radioactive decay of uranium isotopes. Once they are produced, they become quickly attached to marine particles and removed to the sediment, a process referred to as “scavenging”. Importantly, Pa-231 is generally less particle-reactive than Th-230 and thus more readily transported by ocean circulation (to the lowest order Th-230, which is highly insoluble in seawater, is unaffected by the circulation). Thus, the ratio between the two isotopes can potentially provide quantitative information on the rate of past circulation, and our project is to assess this potential on the basis of a numerical model of the ocean general circulation.

What is the significance of this research for others working in this field of inquiry and for the broader scientific community?
For scientists working in this field of inquiry the significance of our research will be a quantitative assessment of the sensitivity of the sediment Pa-231/Th-230 ratio to circulation change. A very recent study has demonstrated that changes in this ratio observed in a sediment core from the North Atlantic occurred in concert with well-known climate oscillations during the last deglaciation (McManus et al., Nature, vol. 428, 834-837, 2004). A rigorous interpretation of sediment Pa-231/Th-230 records requires at least a conceptual understanding of the fundamental interaction between the scavenging process and the three-dimensional circulation, which is currently lacking. It is the very aim of our work to contribute to such an understanding on the basis of a state-of-the-art numerical model of the ocean general circulation.

For the broader scientific community the significance of our research will be to establish the sensitivity of a new proxy of the ocean paleocirculation. It is essential to document and understand the variability of such proxies to the extent that the ocean circulation - through its associated heat fluxes from the tropics to the high latitudes - constitutes a major component of the climate system.

What is the significance of this research for society?
The significance of our research for society is to refine the interpretation of marine sediment records encompassing periods of the recent geological past characterized by rapid climate changes, thereby providing a better understanding of the role of the ocean general circulation during such changes. Understanding this role is an issue of preeminent societal concern in the present context of a possible anthropogenic influence on ocean circulation and climate.

When and where will this investigation be conducted?
We are planning to start our project by the end of 2004/start of 2005. This model investigation will be conducted at WHOI.

What are the key tools or instruments needed to conduct this research?
The key tool will be a UNIX workstation at WHOI, which has the capability to run a numerical model of the ocean general circulation and to store the large amount of data generated by the model simulations.

What are the greatest challenges - physical or intellectual - to conducting this investigation?
The greatest challenge of our project will be to develop understanding of the behavior of Pa-231 and Th-230 in the ocean from a complex numerical model that simulates both the scavenging processes and the three-dimensional general circulation.

Is this research part of a larger project or program?

If you have conducted previous/similar work on this subject, please suggest any web links or citations that might help others better understand the background to your line of research. If appropriate and readily available, please suggest or provide photographs, illustrations, tables, and charts, as well.

Yu E.-F., Francois R., and Bacon M., Similar rates of modern and last-glacial ocean thermohaline circulation inferred from radiochemical data, Nature, 379, 689-694, 1996

Marchal O., Francois R., Stocker T., and F. Joos, Ocean thermohaline circulation and sedimentary Pa-231/Th-230 ratio, Paleoceanography, 15, 625-641, 2000

McManus J., Francois R., Gherardi J., Keigwin L. D., and Brown-Leger S., Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes, Nature, 428, 834-837, 2004