Deciphering the evolution of Oceanic Anoxic Event 2 (OAE2)

Abstract:

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 global carbon cycle. Although OAE2 is increasingly recognized, its detailed evolution including initiation, maintenance, and termination remains poorly understood. To unravel the evolution of OAE2, we have carried out a high-resolution magnetic and carbon isotope investigation of an expanded Cenomanian-Turonian boundary section at Tingri in southern Tibet, China. The studied section consists of a hemipelagic succession that was accumulated on the northern margin of India plate in eastern Tethys. Close-spacing sampling every 10 to 20 cm was conducted on the 76 m thick section. Magnetic susceptibility (MS) data of the samples display cyclic variations and spectral analysis of the MS data reveals dominant sedimentary cycles with cycle wavelength ratios similar to those of short eccentricity and precession. Thus, sedimentary cycles representing short eccentricity were used to establish an orbital timescale for the studied section. High-resolution carbon isotope data show a brief negative excursion preceding a relatiely prolonged positive CIE. Also, the new high-resolution carbon isotope data show that each stage of the positive CIE is not monotonic but contains stepwise, short-lived shifts, suggesting that carbon isotope variations were highly dynamic during OAE2. 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 duration 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 the evolution OAE2, especially for unravelling the complicated feedbacks among sedimentary, biotic, and geochemical processes during this geologically brief event.