HR: 10:50h
AN: PP42A-03    [Abstracts]
TI: Hydrogen Isotopic Ratios of Lacustrine Algal and Terrestrial Organic Matter as a Quantitative Proxy for the Reconstruction of Relative Humidity and Source Water Composition in Continental Settings
AU: Cross, E
EM: cross@seas.marine.usf.edu
AF: College of Marine Science, University of South Florida, 140 7th Ave. S.,, St. Petersburg, FL 33701 United States
AU: * Hollander, D
EM: davidh@seas.marine.usf.edu
AF: College of Marine Science, University of South Florida, 140 7th Ave. S.,, St. Petersburg, FL 33701 United States
AU: Huang, Y
EM: Yongsong_Huang@brown.edu
AF: department of Geological Sciences, Brown University, Prividence, RI 02912 United States
AU: VanVleet, E
EM: vanvleet@seas.marine.usf.edu
AF: College of Marine Science, University of South Florida, 140 7th Ave. S.,, St. Petersburg, FL 33701 United States
AB: Sedimentary studies have indicated that hydrologic conditions in low latitude continental environments have varied significantly during the late Quaternary and throughout the mid to late Holocene in association with sea level variations and major climatic phenomena such as the migration and mean position of the ITCZ. However, a quantitative approach to understanding hydrologic variability (i.e. variations in source water composition and relative humidity) is needed to accurately reconstruct changes in moisture balance and paleoclimatic conditions. Lake Tulane, located in Central Florida, is a subtropical, groundwater-fed acidic lake with a high sedimentation rate and well-preserved organic matter. This study utilizes the hydrogen isotopic composition (dD) of organic molecules (fatty acids) associated with algal (C16) and terrestrial (C28) materials in a lacustrine system to provide a modern calibration that quantifies the isotopic behavior associated with 1) changes in the chemistry of source waters and 2) variability in relative humidity. Over an annual cycle, the dDC16 of modern algal material shows little variability reflecting a constant dD of lake water coincident with the groundwater-fed nature of the lake and relatively constant dD of precipitation. Terrestrial biomass shows a seasonal variability of ~15% with more enriched values occurring in winter when higher rates of evapotranspiration lead to isotopic enrichment. Seasonal variations in the magnitude of isotopic offset between the algae and terrestrial markers, (DdD(C16-C28) ), removes the variations in the source waters and, is quantitatively related to seasonal changes (~6%) in the relative humidity. Together, dDC16 and dDCc16-c28) from lacustrine archives can serve as proxies for the quantitative reconstruction of source water composition and relative humidity from sedimentary records preserved in continental settings. These newly developed dD molecular-hydrologic proxies are applied to the sedimentary record in Lake Tulane spanning the past 80 Kyr, including the glacial-interglacial interval and a high-resolution study of the late Holocene (Little Ice Age (LIA) and Midieval Warm Period (MWP)). During the glacial-interglacial transition, dD of lake water showed a gradual +15% trend whereas relative humidity (DdD(C16-C28)) showed an abrupt +20% change at the deglaciation suggesting that sea level fluctuation was the dominant control on regional relative humidity. At times of relatively constant sea level, such as during the LIA and MWP, variations in the dD of lake water of 8% and significant changes in regional relative humidity of 15% are correlated with long-term migration and mean position of the ITCZ and the Bermuda High, two climatic phenomena largely responsible for supplying moisture to subtropical North America.
DE: 4805 Biogeochemical cycles (1615)
DE: 4215 Climate and interannual variability (3309)
DE: 4239 Limnology
DE: 1040 Isotopic composition/chemistry
DE: 1055 Organic geochemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting