Evidence for lysocline shoaling at the Paleocene/Eocene Thermal Maximum on Shatsky Rise, northwest Pacific

The Paleocene/Eocene Thermal Maximum (PETM) was a transient interval of global warming ∼55 m.y. ago associated with transformation of ecosystems and changes in carbon cycling. The event was caused by the input of massive amounts of CO ₂ or CH ₄ to the ocean-atmosphere system. Rapid shoaling of the lysocline and calcite compensation depth (CCD) is a predicted response of CO ₂ or CH ₄ input; however, the extent of this shoaling is poorly constrained. Investigation of Ocean Drilling Program (ODP) Sites 1209-1212 at Shatsky Rise, which lies along a depth transect, suggests a minimum lysocline shoaling of ∼500 m in the tropical Pacific Ocean during the PETM. The sites also show evidence of CaCO ₃ dissolution within the sediment column, carbonate "burn-down" below the level of the carbon isotope excursion, and a predicted response to a rapid change in deepwater carbonate saturation. Close examination of several foraminiferal preservation proxies (i.e., fragmentation, benthic/planktonic foraminiferal ratios, coarse fraction, and CaCO ₃ content) and observations of foraminifers reveal that increased fragmentation levels most reliably predict intervals with visually impoverished foraminiferal preservation as a result of dissolution. Low CaCO ₃ content and high benthic/planktonic ratios also mirror intervals of poorest preservation.