EVIDENCE FOR CARBONATE COMPENSATION DEPTH SHOALING AT THE PALEOCENE-EOCENE THERMAL MAXIMUM ON SHATSKY RISE, ODP LEG 198

The Paleocene-Eocene Thermal Maximum (PETM) was a transient interval of global warming approximately 55 million years ago. The event is associated with a prominent carbon isotope excursion that can best be explained by rapid dissociation of methane hydrate. This massive release of methane in conjunction with its subsequent oxidation to carbon dioxide likely resulted in a shoaling of the lysocline and carbonate compensation depth (CCD). Ocean Drilling Program Leg 198 on Shatsky Rise in the central Pacific recovered the PETM at five sites (Sites 1208 to 1212) spanning a modern depth range of almost 1000 meters from 2387 m at Site 1209 to 3346 m at Site 1208. These sites provide an ideal opportunity to test the response of the Pacific lysocline and CCD to events at the PETM.

In the shallowest four sites (Sites 1209-1212), the PETM corresponds to an 8 to 23 cm-thick layer of clayey nannofossil ooze with a sharp base and a gradational upper contact. The clay-rich layer is generally yellowish brown in color and is often bioturbated into the underlying sediment. An extremely thin (1 mm) dark brown clay seam lies at the base of the PETM in several locations. The deepest of the four shallowest sites (Site 1211) shows a greater sedimentological response to the PETM indicating that it was at a depth close to the lysocline that was more sensitive to changes in carbonate solubility. At the deepest site on Shatsky Rise (Site 1208), the PETM is highly condensed (<3 cm), lies in a dark claystone with few nannofossils and almost no foraminifers, and was clearly close to the CCD before and after the event.

Detailed counts of planktonic and benthic foraminifers as well as other grains (fish teeth, zeolites) have been conducted. This has been combined with measurements of percent CaCO3 and observations of test ultrastructure in the scanning electron microscope. The survey of pore infilling and secondary calcite overgrowth of foraminifera indicates that preservation deteriorates at the base of the PETM, likely due to lysocline shoaling. In some sections, poor preservation is also observed in the 1-2 cm below the event. Foraminiferal preservation is best and percent sand fraction highest immediately above the base of the event. All observations support the expected shoaling of the Pacific lysocline and CCD as a result of hydrate dissociation at the PETM.