Use of Branched Glycerol Dialkyl Glycerol Tetraethers and the MBT/CBT Proxy: a Paleothermometer for the Islands of Hawaii and Oahu

Branched glycerol dialkyl glycerol tetraethers (GDGTs), produced by a yet unknown phyla of bacteria, have previously been found to be ubiquitous within soils. Through quantification of abundance of 4 varying types of GDGTs within a soil sample, the methylation of branched tetraethers (MBT) and the cyclization of branched tetraethers (CBT) can be determined. While prior researchers have deduced that CBT closely correlates to paleo-pH of soil and the MBT proxy corresponds to both paleo-temperature and paleo-pH of soils, it has been explicitly and unanimously stated that local calibrations are necessary for application of the MBT/CBT proxy to be viable. Additionally, there is very limited terrestrial paleo-temperature data for the Pacific subtropical gyre. In order to develop a calibrated MBT/CBT proxy for Hawaii, 8 sites of varying elevation (800-1600m a.s.l.) along the Laupahoehoe region of Mauna Kea as well as a diversity of sites around the island of Oahu were chosen and samples of the soil-surface were taken. Sites around Oahu include both the windward and leeward side, accounting for the orographic effect, as well as sites of varying elevations. Analysis and quantification was done using liquid chromatography-mass spectrometry (LC-MS). Preliminary results have thus far confirmed the hypothesized outcome that temperature and pH do in fact have a direct affect on the abundance of GDGTs in Hawaiian surface-soils. Although consistent with previous research in that abundance of GDGTs is linked to both temperature and pH, the results show that effectiveness of the MBT proxy at finding minute changes in temperature in Hawaii is limited, (as previous research has been conducted mainly on higher latitude soils with more variant temperature distributions). Therefore, it is pertinent to use the calibration for mainly larger scale temperature changes. Results of the calibration are currently being applied down-core to detect temperature changes during the last glacial-interglacial cycle. Developing this proxy will aid in upcoming research conducted in Hawaii to determine paleo-temperature change through time and will in turn aid in determining future implications for climate change.