Hydrogen Isotopes from Lipid Biomarkers: Purification, Field Calibration, and Application to Reconstructing Galapagos Paleohydrology

Abstract

The tropics are centrally important in determining global climate patters as the primary heat and moisture source for atmosphere and ocean circulation. Understanding of the range of natural variability and the sensitivity to external change is limited by the short period of direct observation and the relative scarcity of paleoclimate records. In this thesis I develop and apply new methods to determine rainfall changes in the tropics based on the hydrogen isotopic composition of lipids from phytoplankton. Specifically, I enhance the capacity to measure the hydrogen isotopic composition of organic compounds preserved in sediments, calibrate the sensitivity of these values to modern lake water and salinity, and apply these tools to reconstruct hydrologic changes in the Galápagos. Several streamlined methods for purifying dinosterol, taraxerol, brassicasterol, and individual alkenones are presented using high performance liquid chromatography (HPLC). These techniques permit quantification of the hydrogen isotope fractionation in dinosterol, brassicasterol, individual alkenones, and n-alkanoic acids, from a series of globally distributed lake surface sediments spanning a salinity range of 0 to 248 ppt. Dinosterol and brassicasterol hydrogen isotope fractionations decreased by approximately 1 //unit change in salinity, which agrees with previous localized environmental transect studies, but individual alkenones and n-alkanoic acids are found to be relatively insensitive to changing salinity. These fractionation factors are then applied to reconstruct hydroclimate variations in the Galápagos islands during the past 2,000 years. Using a combination of nC29-alkane, nC24-alkanol, taraxerol and dinosterol hydrogen isotope data, as well as concentration profiles of n-alkanes, n-alkanols, and taraxerol, along with calibration data sets for mangrove- and algal-lipids, from three different saline lakes in the southern part of Isabela Island I quantitatively reconstruct lake water hydrogen isotope and salinity changes. The data imply that the region experienced significant hydrologic changes, with those occurring between approximately 0 and 1200 AD driven by shifts in the El Nino-Southern Oscillation system, and those between 1200 AD and the present driven predominantly by changes in position of the Intertropical Convergence Zone.