Central Asian Paleoclimatology (Lake Karakul, Pamir) of the last 30,000 Years
Wednesday, 17 December 2014
The goal of this study is to deepen the understanding of past changes in Central Asia, a climate sensitive region located at the intercept of large scale atmospheric circulation systems of which no pre-Holocene records exist so far. A ca. 10 m sediment core with a basal age of ca. 30 ka BP was drilled at Lake Karakul (Tajikistan), a large closed brackish lake situated in a tectonic basin at an altitude of 3,928 m. The lake catchment may be classified as alpine steppe to alpine desert with mean annual temperature and precipitation of ca. -3.9 °C and 82 mm, respectively. We applied a multi-proxy approach which combines inorganic and organic geochemical parameters. δ13C and δ18O values of authigenic carbonates show comparable trends with two pronounced episodes of depleted values at ca. 17.5 ka BP and 24 ka BP. These are indicative for fluctuations within the hydrological cycle and periods of low primary productivity which could have occurred synchronous to Northern Hemispheric climate events H1 and H2. Total organic carbon contents and organic biomarker concentrations are low during the glacial and rapidly increase to Holocene levels between ca. 14 and 11 ka BP. Biomarker fingerprints of aliphatic compounds are mostly dominated by mid-chain n-alkanes with δ13C values up to -14‰ which suggest a primarily aquatic origin. Terrestrial long-chain n-alkanes are mainly abundant during the late glacial to Holocene transition, possibly introduced by enhanced meltwater input during deglaciation. Their hydrogen isotopic variability is ca. 50‰ with constant depletion from ca. 19 to 10 ka BP. This is reflected by generally higher average δD values of aquatic n-alkanes throughout the glacial compared to Holocene values. Effective moisture variations alone cannot explain this offset. We suggest that changes in atmospheric circulation dynamics, origin of water vapour and source water for lipid synthesis (i.e. meltwater vs. precipitation) are the reasons for the observed isotopic shift at the late glacial to Holocene transition.