Abstract:
The Paleocene-Eocene Thermal Maximum (PETM; a.k.a. LPTM; 55 Mya) was characterized by an abrupt, large negative carbon isotope excursion (CIE) of 2.5 to 5.0 per mil in both marine and continental carbon reservoirs. One hypothesis attributes this isotopic aberration to the rapid dissociation of 1200 to 2000 Gt of marine methane hydrate. In this case, most of the methane (delta (super 13) C = -60 per mil), assuming complete oxidation, would have dissolved in the ocean as CO (sub 2) . Numerical simulations show that that in addition to lowering mean ocean delta (super 13) C (sub DIC) , the rapid (<10 (super 3) to 10 (super 4) y) dissolution of this large mass of CO (sub 2) lowers seawater pH and [CO (sub 3) ], thereby triggering a rapid shoaling of the carbonate compensation depth (CCD) and widespread dissolution of seafloor carbonates. Complete restoration of the CCD (to pre-excursion levels) would occur within approximately 150 kyr, primarily through dissolution of silicates (on land) and the subsequent deposition of carbonates. To test this hypothesis, Ocean Drilling Program Leg 198 drilled 4 sites (1209-1212) in a depth-transect between 2.4 and 3.1 km on the flanks of Shatsky Rise in the north Pacific. The P-E boundary was recovered within a sequence of carbonate rich ooze at each site. High resolution ( approximately every 3 to 10 cm) bulk stable C and O-isotope stratigraphies were constructed for 3 m long sections spanning the boundary at each site. Pronounced isotope excursions occur in each record coincident with the benthic foraminifera extinction horizon, an excursion taxa acme, and a carbonate "dissolution" interval. The delta (super 13) C excursions vary in magnitude from 2.5 to 3.0 per mil, and are abrupt occurring in less than a few cm. Recovery is gradual spanning as much as 75 cm. The delta (super 18) O records are characterized by negative excursions of approximately 0.5 per mil consistent with a approximately 2 degrees C warming. The thickness of the C-isotope recovery interval decreases with depth, and is actually step-like at the deepest site, 1211, indicating a brief depositional hiatus. The spatial and temporal patterns of change is consistent with a gradual recovery of the CCD. These findings, together with existing evidence of coeval carbonate dissolution in the deep Atlantic, demonstrate this horizon to be global in extent, thereby supporting the methane dissociation mechanism as the primary cause of the PETM.