Hydroclimate forcing and asynchronous ecosystem responses since the late glacial from the northern margin of the Tibetan Plateau

Abstract:

High altitude ecosystems react sensitively to climatic changes. To evaluate hydroclimate dynamics and ecosystem responses since the late glacial in a typical glacier influenced, high-altitude, lacustrine ecosystem, we analysed concentrations and hydrogen isotopic composition of different aquatic (alkenones, nC_23:1, HBIs) and terrestrial (nC₃₁) biomarker groups in Hala Hu. The lake is located on the northern margin of the Tibetan Plateau, i.e. in the transitioning zone between the East Asian summer monsoon (EASM) and westerlies influence. According to species association of ostracods, it experienced a lake level rise of ca. 45 m during the late glacial and early Holocene and reached its highest stand during early mid-Holocene (8 - 6.1 cal. kyr BP) followed by lake level decline.

The organic geochemical data reveal asynchronous responses of the aquatic and terrestrial ecosystem in Hala Hu and its catchment to hydroclimatic forcing. We interpret δD values of terrestrial biomarkers as indicative for δD of growing season precipitation, with a potential contribution of surface meltwater runoff. Hence, most negative values during 9 - 7 cal. kyr BP indicate changes in moisture source, possibly related to EASM intensification. In contrast, rapid appearance and disappearance of aquatic biomarkers and shifts in theirs δD values between ca. 8 – 5 kyr BP, revealed distinct mid-Holocene aquatic regime-shifts and changes in lake hydrology. Maxima of terrestrial biomarker concentrations also occurred between 8 - 5 cal. kyr BP, suggesting expansion of terrestrial grasses and eventually increased biomarker flux via surface runoff.

These results give evidence that neither shifts of vapour source nor increased precipitation amounts were the major triggers of ecosystem responses. Instead we hypothesize, that warmer temperatures during the Holocene climate optimum, receding glaciers and consequently increased meltwater discharge into the lake, caused the pronounced ecological changes. The rapid regime-shifts within phytoplankton communities in the mid-Holocene illustrate the sensitivity of high altitude mountain ecosystems within the marginal Asian monsoon region to climatic changes. This is of relevance in context with future changes as response to global warming.