Earth Science Student Research Grants Recipients 2006

Paleoclimate and Shifts in Storm Intensities/Frequencies During the Late Holocene for the Gulf Coast, U.S.A

Joe Lambert

The southeast United States is severely lacking of high-resolution paleoclimate reconstructions that would aid in the understanding of past, present, and future local and global climate teleconnections. Researchers often use proxy information from tree rings, pollen, speleothems, lake levels, etc. to infer past continental climate conditions for multiple regions around the Earth. Current research at the Alabama Stable Isotope Laboratory located in the Department of Geological Sciences of the University of Alabama is aimed at deriving a climate (precipitation) record for the Southeast spanning the past 30,000+ years. The proxy record of choice is speleothems collected from DeSoto Caverns in Childersburg, Alabama.

Cave deposits, such as stalagmites, are widely accepted as proxies for records of precipitation (and potentially temperature and tropical storm) variations by means of interpreting changes in stable carbon and oxygen isotopes. In many cases these carbonate deposits form with annual bands that represent the mean climate of the environment above the cave. High humidity levels inside caves often allow for the stalagmites to form in isotopic equilibrium with the drip-water (sourced in precipitation). Variability of 18O in the drip-water and stalagmite is influenced by temperature, amount, and source of precipitation. Changes in 13C, on the other hand, are influenced by the type of vegetation above the cave or varying microbial activity, which in turn is related to the climatic conditions. Therefore, cave deposits have the potential of providing a continuous annual record of continental climate conditions based on the interpretation of carbon and oxygen isotope variations.

Because of the close proximity of the study area to the University of Alabama campus, extensive sampling and cave monitoring is possible. A data logger is currently recording cave air temperature and relative humidity values at 15-minute increments to ensure the cave environment is stable. Trips are made to the cave every two months to collect drip water and to measure the rate of water dripping into the cave. Once brought back to the laboratory, the drip water is analyzed for isotopic and chemical composition. Local precipitation is also collected at the University of Alabama campus and is monitored for both amount of precipitation and isotopic composition. Such monitoring of both the precipitation and drip water is crucial to properly interpreting a climate record derived from speleothems collected for this study.

To date, two stalagmites have been collected from DeSoto Caverns. Each will undergo isotope analysis, multiple dating techniques, and petrographic examination. A 5.5-cm sample, that was actively forming when collected in 2002, contains approximately 800 years of proxy climate information. A more recently collected 50-cm stalagmite began forming over 30,000 years ago and stopped "growing" near 6,590 B.C. based on initial radiocarbon ages. The large stalagmite will provide precipitation variability data spanning the peak of the last major glaciation, the deglaciation, up to the mid-Holocene Optimum. A high-resolution continental climate record for the Southeast during this time span does not currently exist. Future plans may involve collecting a third stalagmite to fill in the missing time interval of ~7,000 B.C. to present. Additionally, such a sample could potentially provide a multi-millennia record of tropical systems to impact the Gulf Coast.

Aside from radiometric dating, all sample analysis is being performed at the University of Alabama. The Alabama Stable Isotope Laboratory houses a suite of equipment specifically developed for the complete analysis of cave material. Data and climate interpretations will be published in professional scientific journals as the research project progresses.