Holocene environmental changes in Lake Bangong basin (western Tibet) .1. Chronology and stable isotopes of carbonates of a Holocene lacustrine core

A 12.4 m core collected from the eastern part of Lake Bangong is analyzed for mineralogy, radiometric chronology and stable isotope contents of authigenic, inorganic carbonate. Isotopic compositions of the modern lake waters and of its major tributaries are first presented. Radiocarbon activity of these waters show that equilibrium with the atmospheric CO2 is not reached, because of admixture of dead carbon in solution in the system. A model is thus developed to correct the radiocarbon ages of fossil material of aquatic origin for this ageing. The core provides a continuous Holocene record. Changes in mineralogy of bulk sediments, and in delta(13)C and delta(18)O contents of authigenic carbonates reveal extremely large changes in environmental conditions, induced by climate changes and/or local hydrological factors (e.g. closure/opening of the lake system controlled by the altitude of the outflow sill). The eastern basin of Lake Bangong was closed before approximate to 9.6 and after 1.2 ka B.P., in response to aridity on the catchment. Wet/warm conditions, attributed to the monsoon influence, were established suddenly at approximate to 9.6 ka B.P. and maintained until approximate to 6.2 ka B.P., although minor reversal events occurred between approximate to 8.6 and 7.7 ka B.P. The return towards aridity is non linear. Dry spells at approximate to 6.2, approximate to 3.9-3.2, and approximate to 1.3 ka B.P. are alternated with short-term periods with positive precipitation-evaporation balance.