Response of phytoplankton productivity to climate change recorded by sedimentary photosynthetic pigments in Lake Hovsgol (Mongolia) for the last 23,000 years

摘要

We analyzed photosynthetic pigments, total organic carbon (TOC) and total nitrogen (TN) in two radiocarbon-dated sediment cores of Lake Hovsgol, NW Mongolia, covering the period of the last glacial/post-glacial transition since 23 ~(14)C ka. The sedimentary profiles of total chlorophyll-a derivatives (TChl-a), TOC and TN reflect productivity increase during the last glacial/ post-glacial transition between 15 and 10 ~(14)C ka in response to regional climate warming and enhanced precipitation. Correlation between TChl-a and TOC was high (R~2 = 0.934, n = 74) with a zero intercept of a regression line suggests that total organic carbon in Lake Hovsgol sediments is of autochthonous origin. The depth profiles of TOC/TN ratio showed increases toward the core top in parallel with other paleoproductivity proxies, serving as an indicator of climatic transition, quite similar to Lake Baikal sedimentary record. The primary reason for the shifts of this parameter is still not clear, however. Chlorophyll-b derivatives and some algal carotenoids such as lutein, zeaxanthin diatoxanthin and canthxanthin, were detected, although in concentrations lower than that of TChl-a. These biomarkers indicate that green algae, cyanobacteria and diatoms were present in phytoplankton during the postglacial period, significantly pre-dating the beginning of the Holocene. Abrupt drops of biomarker profiles at ca. 1-2 and 10 ~(14)C ka due to decreased productivity in the lake or enhanced degradation of the biomarkers likely record short-term climatic events.