Damara Strong - MS Defense

July 5, 2017 - 1:00pm to 3:00pm

Allen Hall 321

Biogeochemical Analysis of Leaf Wax Fatty Acids from Lake Surface Sediments in Mesoamerica

Leaf wax hydrogen isotope (δD) values in paleoclimate records are a rapidly developing tool to determine how precipitation and aridity have changed through time. Previous studies have demonstrated a variation in fractionation of δD from precipitation and biomarker hydrogen isotope values due to biosynthesis and other environmental factors including elevation, precipitation patterns, and vegetation types, varying from region to region. This study focuses on identifying factors contributing to variations in leaf wax fatty acid abundances and meteoric δD across a Mesoamerican lake transect, as well as preliminary data identifying patterns in δD fractionation from meteoric waters to fatty acids.

Lake core top samples were taken from over 100 sites through the Trans-Mexican Volcanic Belt to the Yucatan Peninsula and south through Central America. These samples span environmental, elevational, and longitudinal gradients, and vary among lake types. All samples exhibited an even-over-odd predominance of fatty acid chain lengths across Mesoamerica, indicating little degradation of terrestrial input and successful recovery of leaf waxes for δD analysis.  Most samples exhibited a dominance in terrigenous fatty acid input into the lake with short-chained fatty acids in relatively low abundance, assuring little contamination to the lake sediment surface samples from petroleum byproducts and successful recovery of fatty acids for δD analysis.

δD from lake waters (δDwater) were analyzed for comparison with leaf wax δD for quantifying fractionation during incorporation of δD into leaf waxes.  This study found an overall decline in δDwater­ values further from the coast westward and an overall decline in δDwater­ values with increasing elevation, however the correlations were poor with R2 values of 0.1889 and 0.0769, respectively. The weak correlation is likely due to differing climate systems in the region, as well as a combination effect of continental and orographic effects on δDwater, as well as an evaporative effect of lake waters.

Preliminary leaf wax δD values along an elevational gradient in the Yucatan Peninsula into the highlands of Guatemala demonstrate the promising aspects for a broad calibration of the difference of δD leaf waxes and δD of meteoric waters for future interpretation in downcore lake sediments and paleoclimate studies.