A HOLOCENE CENTENNIAL-SCALE TEMPERATURE AND PRECIPITATION RECONSTRUCTION FOR WESTERN GREENLAND USING LEAF WAX AND GDGT BIOMARKERS

The Arctic responds strongly and rapidly to climate change. The Greenland Ice Sheet is especially vulnerable, and changes in ice extent there directly impact global sea level. Therefore, researchers are working to understand the climate dynamics which drive ice sheet change—especially at the ice sheet margin. Changes in ice sheet margins over the past 150 years are well documented, but prior to that, records of ice margin changes are not available.

We present an ~8000 year climate record from proglacial-threshold lake N3 in western Greenland. We conducted analyses at centennial resolution, examining leaf waxes and branched GDGTs.

Leaf waxes—long-chain hydrocarbons that vary compositionally according to environmental factors—are well preserved in lake sediments and have shown considerable promise in the past ten years for reconstructing temperature, precipitation and ecological variability in the lake catchment. We measure chain length distribution, which varies with plant type and therefore reflects changes in catchment ecology, and leaf wax hydrogen isotopes (δD_wax), which are related to precipitation hydrogen isotopes (δD_precip). Due to permafrost, plants do not have access to an extensive groundwater system, so the majority of plant growth uses precipitation as the source water. The apparent fractionation between δD_wax and δD_precip, caused by preferential use of the lighter isotope during biosynthesis, differs between plant species, but is systematic on the catchment-scale. In the Arctic, δD_precip is positively correlated to temperature, so δD_wax can be used to reconstruct surface temperature in the catchment.

Likewise, branched GDGTs (glycerol dialkyl glycerol tetraethers), a component of bacterial membranes produced in soils and lakes, vary compositionally with catchment temperature and pH. The MBT index measures the degree of methylation, which correlates positively with air temperature during synthesis, and the CBT index measures cyclization, which correlates positively with soil pH.

We compare our paleoclimate reconstruction to reconstructions of Holocene ice sheet behavior, with the goal of better understanding ice sheet response to climate variability. Samples are currently being processed, and will be presented at the meeting.