Lipids preserved in soils and sediments are important proxies in paleoclimate research. However, various growth conditions that affect the organisms synthesizing the lipids can in turn affect the abundance and stable isotopic compositions of the lipids themselves, and, consequently, can introduce significant errors in the paleoclimatic inferences drawn from them. This work examines how two climate proxies based on lipids, namely, glycerol dialkyl glycerol tetraether (GDGT)-based paleotemperature proxies in soils, and paleohydrological proxies based on hydrogen isotopic composition of lipids, respond to variability in environmental and other growth conditions (e.g., carbon source).
In order to evaluate the role of annual precipitation amount on the distribution of soil GDGTs and on GDGT-based temperature proxies in soils, we studied GDGT distribution in soils collected from two environmental transects in the USA—a dry, western transect covering six western states and a wet, east coast transect from Maine to Georgia. Our results indicate a significant impact of precipitation amount on soil GDGT distribution, which is related to soil aeration that in turn depends on precipitation amount, and also to soil pH. Our results also indicate that below an annual precipitation of 700-800 mm yr–1 the MBT/CBT-temperature proxy based on soil GDGTs is not applicable. Furthermore, due to the distinct GDGT distributions in soils under arid conditions, soil input into lacustrine or marginal marine environments cannot be estimated using BIT index.
In order to estimate the effects of variability in environmental conditions and utilization of different substrates on D/Hlipid, we studied two heterotrophic organisms—Haloarcula marismortui, a halophilic archaeon and Tetrahymena thermophila, a ciliated protozoan, in pure cultures. Our results from experiments with H. marismortui indicate that metabolism of different substrates leads to formation of reducing agents (mainly nicotinamide adenine dinucleotide phosphate or NADPH) with distinct D/H signatures, which is reflected in the significant D/H variations in isoprenoidal lipids (ca. 100‰). Growth temperature affects growth rate as well as enzyme activities, and salinity of the growth media affects mainly growth rate of H. marismortui, and both cause similar variations in D/Hlipid (ca. 20-30‰) that are smaller compared to the substrate-effect. T. thermophila , on the other hand, responds to variations in growth temperature in a different manner. Isoprenoid and fatty acids synthesized by T. thermophila generally become more D-enriched with temperature increase. The isoprenoid ranges from being D-depleted to D-enriched relative to water with temperature increase, but the fatty acids do not display similar patterns. Our results from T. thermophila culture experiments indicate that temperature has a critical control on the D/H ratios of NADPH and possibly also intracellular water, due probably to temperature effects on processes that are related to growth and metabolism of T. thermophila.
|School Location:||United States -- Connecticut|
|Source:||DAI-B 75/05(E), Dissertation Abstracts International|
|Subjects:||Biogeochemistry, Paleoclimate Science, Geochemistry|
|Keywords:||Climate proxies, Geobiology, Lipid-based proxies, Organic geochemistry, Stable isotopes|
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