Abstract:
The largest carbon extraction event of the past 100 m. y. occurred during the transition from the Cenomanian to the Turonian (C/T). This event was associated with ocean anoxia (OAE2), a period of global elevated carbon isotopic values in marine carbonate and organic carbon, and a thick black shale/organic rich interval (the Bonarelli Bed). Studies at Bass River, NJ (ODP Leg 174AX) previously demonstrated that: 1) a sharp carbon isotopic increase occurred across the C/T boundary; 2) long-term elevated carbon isotopic values persisted into the early Turonian, associated with high sedimentary organic carbon; 3) the extraction event lasted approximately 500 k.y.; 4) there is no relationship with sea-level change on the m.y. or 100 k.y. scales; and 5) a cooling was associated with carbon extraction. Based on recent observations that the initiation of OAEs may be associated with transient carbon isotopic excursions (e.g., OAE1 and the latest Paleocene), we resampled the C/T boundary section at 15 cm sampling interval (nominally 7-8 k.y.). In contrast to OAE1 and the latest Paleocene, there was no short-term carbon isotopic excursion associated with the initiation of OAE2/Bonarelli. However, we discovered the largest carbon isotopic excursion of the Phanerozoic (in excess of 8 per mille in both Gavelinella [calcite] and Epistomina [aragonite]) that occurred immediately following the C/T carbon isotopic increase. The carbon isotopic events consist of: 1) an extremely large, rapid (<8 k.y.), decrease in benthic carbon isotopic values; 2) an abrupt return to pre-excursion values after 15-20 k.y.; 3) a subsequent rapid transition back to low values that again persisted for less than 20 k.y.; and 4) a return to pre-excursion values. The carbon isotopic excursions are associated with large (3 per mille) variations in oxygen isotopes, indicating warming during carbon increases. The entire duration of these abrupt, transient events was approximately 50 k.y. (assuming no hiatuses). Because the Bass River site was deposited in less than 30 m water depth, these large excursions may primarily reflect variations in atmospheric chemistry rather than whole ocean carbon fluctuations. Nevertheless, the only mechanism that can explain these abrupt, transient variations is catastrophic and repeated release of methane hydrates.