Radiogenic isotopes in Paleoclimate/Paleochemistry Studies
In addition to applying radiogenic isotopes to igneous and metamorphic terrains, the SU laboratory has been used for studies focusing on hydrogeology (lakes, groundwater, and most recently surface water), marine stratigraphy, and sedimentology, both by researchers at SU and at other institutions. This aspect of the laboratory will likely increase as the make-up of the Earth Sciences faculty continues to evolve, and as collaboration with researchers from other institutions continues to increase. A very brief summary of some of those studies are given here.
87Sr/86Sr of paleoseawater, Eocene-Oligocene transition
The number of paleoclimate studies concentrating on the Quaternary – Recent interval has increased exponentially during the last decade. In contrast, there have been far fewer studies exploring changing paleoclimates and faunal responses in deeper Earth history. One particularly intriguing deep-time interval for exploring climate change is the transition from the Eocene into the Oligocene Epoch. During this time interval the Earth’s climate appears to have shifted from warm and equable to one that was cooler and glaciated. The earliest Oligocene marks a time of the first major expansion of Antarctic ice in the Cenozoic (Zachos et al., 1992). A significant question is the extent of this initial pulse of glaciation – direct physical evidence has been limited to cores from East Antarctica. Clear glacial deposits on the Antarctic peninsula were known only from mid to late Oligocene (Troedson and Smelli, 2002). Recently, however, our colleague Linda Ivany and her co-workers described glacial marine sediments and a glacial diamicton from Seymor Island, Antarctic Peninsula. They considered these to be earliest Oligocene in age, suggesting the onset of Antarctic glaciation was much more profound than previously thought. This view is controversial, as it hinges on the age of the sedimentary sequence and there are no ash beds, or other easily radiometrically dateable units in the section. However, there are unaltered aragonitic bivalves immediately below the base of the diamicton; fossils perfectly suited for Sr isotopic analysis as seawater proxies. And because the 87Sr/86Sr ratio of seawater increased dramatically in the latest Eocene, small changes in 87Sr/86Sr permit good age control. We analyzed (in duplicate) bivalves from a restricted vertical interval and determined ratios of 0.70779 – 0.70780 for the lowest interval (Ivany et al., 2006). These values correspond closely to the E-O boundary, based on the published marine seawater curves of McArthur et al. (2001). An instrument capable of high precision Sr measurements will allow us to refine this type of work.
Eocene sea surface temperature
Based on our earlier success, Ivany and Samson plan to continue work on the paleoclimate shift during Eocene-Oligocene time, this time examining the rich and well-preserved Paleogene faunal record in the US Gulf Coastal Plain. A major goal is determining linkages between paleoclimate and evolution, thus it is critical that we establish variations in δ18O as paleotemperature based, and not on other factors such as salinity variations. Sr isotopes thus will play a critical role, as the Sr isotopic composition of the fauna will help to test for possible temporal influxes of fresh water into the marine environment. Introduction of nonmarine waters would significantly affect the interpretation of the δ18O based paleotemperature record. Freshwater in the region would have significantly higher 87Sr/86Sr than seawater, thus if the aragonitc fossils in particular stratigraphic sections have ratios > 0.70850 it is likely that conditions were brackish rather than purely marine during those intervals. Dr. Ivany currently has a newly funded NSF project (Exploring the Links among Climate, Ecology, and Evolution in Paleogene Marine Faunas of the U.S. Gulf Coastal Plain) that will include this isotopic research. Preliminary work has already demonstrated the importance of this combined approach: an early Eocene shell shows significant seasonal δ18O variation interpreted to reflect a combination of temperature and salinity (Ivany et al., 2004). Recent Sr isotopic measurements suggests this hypothesis is correct, with more radiogenic carbonate being precipitated in the spring-summer in association with unusually negative δ18O values. Continued analysis, particularly of very small samples derived from the selective microsampling of mollusk aragonite, is a key to the success of resolving debates about Eocene sea surface temperatures generated from open-ocean planktonic microfossils. The ability to collect isotopic ratios on nanogram levels of Sr is critical in reconstruction of sea surface temperatures uncompromised by diagenetic cement or salinity variation.