DETAILED EVOLUTION OF OCEANIC ANOXIC EVENT 2 (OAE2) REVEALED BY A REFINED CHEMOSTRATIGRAPHIC RECORD, SOUTHERN TIBET, CHINA

Oceanic anoxic event 2 (OAE2), which occurred at the Cenomanian/Turonian Boundary (CTB, ~93.9 Ma), is characterized by enhanced burial of organic matter in ocean basins that was accompanied by a pronounced positive carbon isotope excursion (CIE) in both marine and terrestrial realms, thus representing a major perturbation to the global carbon cycle. Although OAE2 is increasingly widely recognized, the details of how it evolved from its inception to termination remain poorly understood. To unravel the evolution of OAE2, we have carried out a high-resolution magnetic and carbon isotope investigation of a stratigraphically expanded Cenomanian-Turonian boundary section at Tingri in southern Tibet, China. This hemipelagic succession was deposited on the northern margin of the Indian plate in the eastern Tethys. 10 to 20 cm resolution sampling over a 76 m thick interval permitted development of high resolution proxy time series that document orbital signals. Magnetic susceptibility (MS) data exhibits cyclic variability and spectral analysis of the MS data reveals dominant sedimentary cycles with cycle wavelength ratios similar to those of short eccentricity and precession. Sedimentary cycles representing short eccentricity were used to establish an orbital timescale for the section. High-resolution carbon isotope data show a possible brief negative excursion preceding a prolonged positive CIE and documents short-lived, stepwise shifts throughout OAE2 suggesting highly dynamic C cycling throughout the C perturbation event. With the new orbital timescale, the OAE2 interval in the Tibetan section is estimated to last for ~870 kyr, which is in striking similarity to the estimate of 847 to 885 kyr from the CTB stratotype section. Correlation of our new high-resolution data with other OAE2 records permits detailed examination of its evolution, especially for unraveling the complicated feedbacks among sedimentary, hydrological, biotic, and geochemical processes during this geologically brief event.