DEPTH DEPENDENT VARIATION IN THE PALEOCENE-EOCENE BOUNDARY CARBON ISOTOPE EXCURSION HORIZON IN THE N. PACIFIC

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‰ 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 (d¹³C = -60‰), assuming complete oxidation, would have dissolved in the ocean as CO₂. Numerical simulations show that that in addition to lowering mean ocean d¹³C _DIC, the rapid (<10³ to 10⁴ y) dissolution of this large mass of CO₂ lowers seawater pH and [CO₃], 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 ~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 (~ 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 d¹³C excursions vary in magnitude from 2.5 to 3.0‰, and are abrupt occurring in less than a few cm. Recovery is gradual spanning as much as 75 cm. The d¹⁸O records are characterized by negative excursions of ~ 0.5‰ consistent with a ~2°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.

*This abstract is coauthored with Scientific Party ODP Leg 198.