In the summer of 2007, I participated in a NSF-funded REU (research opportunity for undergraduates) at the University of South Florida. My research there focused on the Plio-Pleistocene boundary, which, in Florida, was a significant regional extinction event in the marine realm. I collaborated with Bill Weinlein under the supervision of Peter Harries. Our goal was to test the hypothesis that changes in ocean circulation resulted in changes in the amount of nutrients delivered to coastal ecosystems by altering seasonal upwelling. This is an idea that has been presented by a number of paleontologists.
Upwelled waters are rich in nutrients, and are oftentimes crucial to the health of coastal ecosystems. Deep waters collect and recycle nutrients into forms that photosynthesizers at the surface need to live. These organisms are the base of the food chain, so all other organisms are in some way dependant on them. When ocean currents are favorable, these nutrients can be delivered to the surface through upwelling, where deep, nutrient-rich water rises to the surface along the coast. Our hypothesis was that there would be a reduction in upwelling after the Pliocene that lead to high rates of extinction.
Our approach was to use stable isotopes (carbon and oxygen) in the carbonate of quahog shells to reconstruct their life history. By using oxygen as a temperature record, and carbon as a record of deepwater nutrient supply, we could test the idea of there was a strong ‘upwelling signal’ in the shells of Pliocene and Pleistocene quahogs. In our model, an upwelling signal would be annually recurring evidence of cold waters (high oxygen-18) and high nutrients (low carbon-13).
We sampled through their shells in a manner similar to counting tree-rings. Each season the quahog is alive, a new layer is added to the shell that records environmental information. If a reduction in nutrient supply were a reason for the high levels of extinction at this time, then we would expect there to be more evidence of upwelling in Pliocene shells over Pleistocene shells.
Our results suggested the opposite. We found almost no upwelling signal in Pliocene shells. The upwelling signature in Pliestocene shells, on the other hand, was found in a majority of shells. Our conclusion, then, was that either upwelling changes were unlikely to be a cause of the extinction at the Plio-Pleistocene boundary, or that the isotopic model we employed was flawed.
Gibson, JT., Sliko, JL., Harries, PJ., KASPRAK, AH., High Resolution Isotopic Sampling of Growth Bands of a Modern Mercenaria Campechensis Specimen and Implications for Paleoclimatic Analysis. Program with Abstracts: Southeastern Section Meeting, Geological Society of America. March 12th-13th, 2009
KASPRAK AH, Weinlein WA, Sliko JL, Harries PJ, Herbert GS, Oches RA, Portell RW. Stable Isotopic Investigation of Coastal Upwelling as an Explanation for High Pliocene Productivity on Florida’s West Coast. Program with Abstracts: Southeastern Section Meeting, Geological Society of America. April 10th-11th, 2008
Weinlein WA, KASPRAK AH, Sliko JL, Harries PJ, Herbert GS, Oches RA, Portell RW. The Role of Trace Metal Proxies as Upwelling Indicators on the Florida Platform During the Plio-Pleistocene. Program with Abstracts: Southeastern Section Meeting, Geological Society of America. April 10th-11th, 2008.