RECONSTRUCTING HOLOCENE CLIMATE BASED ON ALKENONES AND ISOTOPES FROM WEST GREENLAND LAKE SEDIMENTS: TEMPERATURE OR EFFECTIVE MOISTURE AS A DRIVER?

Sediment cores were recovered from Limnaea and Braya Sø lakes in the Kangerlussuaq region of West Greenland to reconstruct climate during the Holocene epoch. These closed-basin lakes are ideal for studying regional changes in effective moisture (P−E). Variations in past lake conditions are recorded by lake sediment authigenic carbonates and alkenones. Cores were analyzed by x-ray, magnetic susceptibility, and bulk density for stratigraphic correlation between cores and to determine changes in sedimentation rate and lithology. After sectioning, a series of samples taken 20 cm apart were dated by AMS ¹⁴C and found to range from present day to ~9.5 ka (cal. age). Based on the rate of deposition, these sediments were sectioned at 0.5 to 1 cm intervals to achieve ~100-year resolution. Samples were analyzed for carbonate δ¹³C and δ¹⁸O. The δ¹⁸O data show a long-term trend in isotopic enrichment interspersed with periods of abrupt isotopic changes (e.g. 8.2 ka), indicating either shifts in the hydrologic regime and/or changes in temperature. We analyzed some of these abrupt periods using saturation index of alkenones and clumped-isotope thermometry to determine whether the carbonate δ¹⁸O isotopic variations are primarily a response to summer lake-water surface temperature variations or shifts in the lake water balance. While water isotope values from Greenland ice cores show relatively stable temperatures during the Holocene epoch, recent lake sediment alkenone-based temperature reconstructions by D’Andrea et al., (2011) from Braya Sø have suggested that summer temperatures may have been more variable than previously thought (as much as 5.5˚C), with a mid-Holocene warming of ~4−5˚C. Over the past ~2,000 years, our alkenone temperature estimates from Limnaea Sø show comparable mean lake water temperatures as Braya Sø, but a reduced magnitude of temperature variability. These preliminary results show modest temperature fluctuations, suggesting that lake carbonate δ¹⁸O variations are primarily recording shifts in the hydrologic cycle. The clumped isotope data show no statistically significant change in temperatures over the abrupt δ¹⁸O shifts. To our knowledge, this is the first application of clumped isotope paleothermometry in Greenland lake sediments to estimate temperatures during carbonate formation.