HR: 08:15h
AN: PP41B-02    [Abstracts]
TI: Evaluating Climatic Controls of Proxy Indicators: A Calibration Study Using Annually Laminated Sediments From a Minnesota Lake
AU: * Tian, J
EM: jiantian@uiuc.edu
AF: Department of Geology University of Illinois, 245 Natural History Building 1301 W. Green St., Urbana, IL 61801 United States
AU: Hu, F
EM: fshu@life.uiuc.edu
AF: Department of Geology University of Illinois, 245 Natural History Building 1301 W. Green St., Urbana, IL 61801 United States
AU: Hu, F
EM: fshu@life.uiuc.edu
AF: Department of Plant Biology University of Illinois, 265 Morrill Hall 505 S. Goodwin Ave, Urbana, IL 61801 United States
AU: Hu, F
EM: fshu@life.uiuc.edu
AF: Program in Ecology and Evolutionary Biology University of Illinois, 265 Morrill Hall 505 S. Goodwin Ave, Urbana, IL 61801 United States
AB: The reliability of paleoclimatic inferences from sedimentary proxy data depends on a solid understanding of what environmental factors control the proxy indicator. Despite the rapid accumulation of paleoclimatic data, few rigorous calibration studies exist that compare proxy records with instrumental weather data. We analyzed annually laminated sediments of the past 100 years from Steel Lake (46o58' N, 94o41' W), Minnesota, for a suite of commonly used climatic proxies, including varve thickness (VT), grey-scale intensity (GSI), oxygen and carbon isotopic compositions (d18O and d13C), biogenic silica (BSi), inorganic carbon (IC), and organic carbon (OC). These proxy data were compared with time series of weather variables, including precipitation (P), temperature (T), P minus evapotranspiration (P-ET), and the Palmer drought severity index (PDSI), as well as with indices of the climate modes thought to influence the regional climate, especially the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO). We confirmed the annual nature of laminations using radiocesium (137Cs) dating and petrographic thin sections. The time series of d18O and the mean annual T share little similarity, but d18O covaries with both effective moisture (based on P-ET or PDSI) and PDO, with high d18O values generally corresponding to low P-ET and warm phase PDO. During the warm PDO phase, the climate in the midwestern United States is generally warm and dry (low P-ET) especially during the winter months, which would elevate d18O. Varves are typically thick during the period of low d18O and high P-ET, suggesting that thick varves tend to form under wet weather conditions that promote sediment deposition through increased production and/or formation of amorphous iron minerals. The accumulation rates of various sediment components (e.g., BSi, IC) show similar trends to VT. The time series of GSI and weather parameters share little similarity. d13C does not covary with d18O, but it exhibits trends similar to those of %BSi, the inverse of %IC, and to a less extent, %OC. These proxies are likely influenced by lake productivity, sediment diagenesis, and land-use history, which may compromise their comparisons with weather time series. Our calibration results demonstrate the utility of some of the proxies (especially d18O and VT) for reconstructing drought and large-scale climatic modes, but they also illustrate that not all the proxies commonly used for paleoclimatic reconstructions are climatically sensitive. In conjunction with the findings of several other recent studies, our data suggest that the controlling factors of a proxy indicator are region and potentially site specific, which further underscores the importance of proxy calibration.
DE: 4825 Geochemistry
DE: 4870 Stable isotopes
DE: 3344 Paleoclimatology
DE: 4239 Limnology
DE: 4267 Paleoceanography
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting