Tropical South Pacific Paleohydrology from Hydrogen Isotopes in Algal Lipids

Abstract

The tropics play a central role in global atmospheric moisture transport, however paleoclimate records of tropical precipitation are relatively scarce. Reconstructing pre-instrumental hydrological change requires the use of indirect indicators of rainfall such as hydrogen isotope ratios (2H/1H) of phytoplankton lipids preserved in sediments. In this thesis, I examine mechanisms of 2H/1H fractionation in phytoplankton lipids, develop a modern spatial calibration of biomarker hydrogen isotope ratios with instrumental precipitation rates, and quantitatively reconstruct Late Holocene precipitation across the tropical South Pacific. The lipid-water 2H/1H fractionation response to salinity in nutrient-replete continuous cultures of the centric diatom Thalassiosira pseudonana decreased linearly as salinity increased by 1.3‰ ppt-1 in fatty acids (C14:0, C16:0, C16:1) and by 1.0‰ ppt-1 in the sterol 24-methyl-cholesta-5,24(28)-dien-3β-ol. A steady state flux balance model allowed further examination of the controls on lipid 2H/1H fractionation. The dinoflagellate biomarker dinosterol (4a, 23, 24-trimethyl-5a-cholest-22E-en-3β-ol) purified from tropical South Pacific freshwater lake surface sediments was correlated with precipitation rates, yielding a sensitivity of -12.1±2.6 ‰ (mm d-1)-1. This empirical relationship, developed in the context of known controls on the isotopic composition of tropical precipitation, lake water, and algal lipid isotopes, provides a means of quantitatively reconstructing past precipitation. Sediment cores were collected from ten freshwater lakes on six islands in the Solomon Islands, Vanuatu, Wallis, and Samoa. Down-core dinosterol 2H/1H measurements show wet Modern (1850-present) hydroclimate conditions and widespread dry conditions during the Little Ice Age (1450-1850) and Medieval Climate Anomaly (950-1250).