COI Funded Project: A History of Paleo-Hurricane Activity Recorded in the Stable Isotope Composition of Coral Skeleton

Anne Cohen, Geology and Geophysics
Graham Layne, Geology and Geophysics

Project Duration: 6/1/99-12/31/00
Key Words: natural hurricane cycles, geochemical indicators, oxygen isotopes, coral reefs, Dominican Republic

Proposed Research

Hurricanes often inflict catastrophic property damage and loss of human life. In the light of recent predictions of increased hurricane activity in the Atlantic basin, as well as explosive coastal population growth, it is important to determine how these powerful storms could change in response to changes in global climate (Knutson et al. 1998, Bove et al. 1999). One approach is to examine natural cycles in hurricane frequency over several centuries and during times when the world climate was different from that of today. Reliable and comprehensive hurricane records are sparse prior to the 1940's, which is too short a time-period to allow analysis of decadal and centennial-scale trends. However, some of the numerous environmental and geochemical changes effected by hurricanes remain preserved in the geologic record. Using proxy techniques, we can extract this evidence and extend the current record of hurricane activity back through time.

This proposal describes a novel idea and a new technique to identify intense, landfalling hurricanes in the geologic record. It is based on the fact that hurricanes produce a large amount of rainfall with a very different oxygen isotope composition from that of normal summer rainfall. Hurricane rainfall is also isotopically distinct from water in lakes, rivers and streams, groundwater and the sea surface. It therefore comprises a natural isotope spike which is likely to be incorporated into the calcium carbonate skeletons of aquatic organisms. Although the isotope spike in seawater will quickly dissipate through mixing and dilution, if incorporated into skeletal carbonate it will remain preserved for centuries or even millennia as a record of each hurricanes' passage.

We predict that the CaCO₃ skeletons of massive, long-lived corals inhabiting shallow reefs in the Caribbean and Gulf of Mexico contain this isotope spike and therefore preserve a record of many of the hurricanes which have made U.S. landfall over the past several centuries. We will test this prediction by measuring the isotope composition of corals from the Dominican Republic, an island hit by several intense hurricanes over the past few decades. Oxygen isotope measurements will be made on the new large format, high-resolution, high-transmission Carneca IMS 1270 ion microprobe at WHOI, an unconventional instrument for this type of analysis but ideally suited to the purpose of tracking hurricane signals which are large but of extremely short duration. Once we have demonstrated that our technique works, the project will be expanded to include other Atlantic sites and ocean basins, and extended back through the past several centuries or more. In future projects we will look for other geochemical and structural changes which together with the isotope signal will comprise a multi-variate, unambiguous "hurricane stamp" in different biological and inorganic carbonate accretions.