Carbonate stable isotope signals in the 1-Ma sedimentary record of the HDP-04 drill core from Lake Hovsgol, NW Mongolia

摘要

Oxygen and carbon stable isotope ratios in carbonates from the HDP-04 drill core from Lake Hovsgol, NW Mongolia, show an overall covariant relationship suggesting that for the most of the past 1 Ma Hovsgol remained a closed-basin lake. Carbonate δ~(18)O ratio is responsive to regional climate change: a ca. +1.5‰ basinwide δ~(18)O shift has occurred with the onset of Bolling-Allerod warming (sensu lato), followed by a ca. 0.8‰ depletion during the Younger Dryas. The post-glacial δ~(18)O shift of the same magnitude is recorded in bulk carbonates, shells of two ostracod species and in wet-sieved fine fraction <63 μm. Associated with the lake-level rise and correlative with the post-glacial warming in the northern hemisphere, the observed δ~(18)O shift is nevertheless positive. This argues against changes in local temperature and hydrology as key driving mechanisms. Most likely, Lake Hovsgol δ~(18)O reflects a climate-driven shift in the composition of regional precipitation. Tied into a distinct lithologic succession, the radiocarbon-dated late glacial δ~(18)O shift apparently represents a 'template' of the lake's response to glacial-interglacial transitions: a similar pattern of parallel changes in lithology and carbonate stable isotope composition is observed in at least 10 more intervals in the 1-Ma record, including the MIS 20/ MIS19 transition at the Brunhes/Matuyama paleomagnetic reversal boundary. The comparison of carbon stable isotope ratios of untreated and in vacuo roasted bulk sediment with those of detrital carbonates suggests that clastic input of carbonates by lake tributaries does not affect the geochemistry of bulk carbonates in the HDP-04 section. The profiles of bulk carbonate δ~(18)O and δ~(13)C in the Pleistocene section of the HDP-04 drill core suggest at ca. 15.4 ka, at ca. 100 m below today's level, Lake Hovsgol still stood relatively high as compared with prior extended periods of time during late Matuyama and early Brunhes. Isotopically heavy δ~(18)O and δ~(13)C ratios during the mid-late Brunhes, particularly, in carbonate crusts and oolites, are suggestive of past episodes of dramatic evaporative ~(18)O-enrichment of lake waters. Despite the expectation of muted amplitudes of temperature- and precipitation-related isotope signals, the sedimentary record from the sensitive 'water gauge' basin of Lake Hovsgol has high potential for providing important constraints on past hydrologic evolution of continental interior Asia during the Pleistocene.