Enhanced aridification across the Eocene/Oligocene transition evidenced by geochemical record in the Tajik Basin, Central Asia

摘要

The Tajik Basin is an intermontane depression between the mountain ranges of Tian Shan, Pamir, and Hindukush, which is located in the northwestern part of the India-Asia collision zone. Due to the westward retreat of the Neotethys Sea, the paleoenvironment of the Tajik Basin has underwent change from a shallow-sea to a continental semiarid land during the Cenozoic era. This implies that the long-term climatic record in the Tajik Basin can potentially reflect the environmental effects of the changed land-sea distributions controlled by plate collision and global eustatic sea-level changes. In this paper, we present new results of stable oxygen and carbon isotopes of carbonates and chemical weathering indices of bulk samples from the Eocene-Oligocene terrestrial strata (ca. 39-30 Ma) in the center of the Tajik Basin. The temporal variations of oxygen and carbon isotopes of carbonates indicate an isotopic shift across the Eocene-Oligocene transition (-34 Ma), characterized by a relatively positive shift due to the isotopic enrichment in authigenic carbonates in an enhanced aridification episode in the Tajik Basin. This shift was also mirrored by the weakened chemical weathering indicated by the ratios of major oxides across the Eocene-Oligocene transition. Such a regional climatic shift was generally coincident with a remarkable global scale sea-level drop. We suggest that the isolated Paratethys Sea, which was mainly driven by global cooling due to the initial formation of the Antarctica Ice-sheet, together with the decreased seawater evaporation due to lower sea surface temperature, reduced water vapor transport by westerlies to the downwind region, and thus intensified the aridification in the Tajik Basin began at -34 Ma.