The Cenomanian-Turonian anoxic event in southern Tibet

The Cenomanian-Turonian black shales in southern Tibet record a global oceanic anoxic event (OAE). A combined sedimentological, geochemical and micropalaeontological study shows: (1) increased total organic carbon (TOC: 0.5-1.7%) with a peak accumulation across the Cenomanian/Turonian boundary (CTB); (2) sulfur/carbon ratios (S/C) and high degree of pyritization (DOP) indicating that the depositional environment in the Gyangze area was oxygen-depleted and H2S-rich, while synchronous, shallower deposits in the Gamba area accumulated in an oxic environment (3) bulk-rock delta(13)C analyses of carbonates indicating a positive 2parts per thousand excursion across the CTB, similar to that observed in the western Tethys and Pacific; and (4) an extinction rate of planktic foraminifera across the CTB reaching 50-70%, while the extinction of benthic foraminifera was as high as 90%. A major extinction of benthic foraminifera indicates the development of an inhospitable environment associated with the presence of poorly oxygenated bottom waters. The CTB OAE in Tibet documents a fundamental change in Late Cretaceous palaeoceanography associated with increased dispersal of the southern continents. As the Indian plate moved northward, there was a change in ocean circulation which may have led to the development of a layer of poorly oxygenated bottom and intermediate waters within both the western and eastern Tethys. (C) 2001 Academic Press.