Constraints on ocean acidification associated with rapid and massive carbon injections: The early Paleogene record at ocean drilling program site 1215, equatorial Pacific Ocean

Massive amounts of 13C-depleted carbon rapidly entered the ocean more than once during the Early Paleogene, providing a geological framework for understanding future perturbations in carbon cycling, including ocean acidification. To assess the number of events and their impact on deep-sea carbonate accumulation, we investigated a 42 m thick unit of Upper Paleocene–Lower Eocene carbonate ooze, which was deposited on a subsiding flank of the East Pacific Rise. Age control was established using calcareous nannofossils and planktonic foraminifera, as well as stable carbon isotopes of bulk carbonate. Carbonate content, foraminiferal test fragmentation, and planktonic/benthic foraminiferal ratio were measured to ascertain changes in carbonate dissolution. Based on these analyses, carbonate preservation generally increased from the late Paleocene (55.4 Ma) through the early Eocene (51.4 Ma), after which it became poor to negligible. This trend was punctuated by three (and probably four) short-term intervals characterized by carbonate dissolution and negative δ13C excursions. These horizons almost assuredly correspond to the PETM (~ 55.5 Ma), H1/ETM-2 (~ 53.7 Ma), I1 (~ 53.2 Ma), and K/X (~ 52.5 Ma) events. Carbonate preservation also increased within 200 kyr after two and perhaps all four intervals. We suggest the lysocline and calcite compensation depth (CCD) generally deepened between 55.4 and 51.4 Ma but shoaled and subsequently overcompensated during and after three and likely four intervals of rapid and massive carbon injection. Oxygen isotope data further suggests these intervals were times of anomalous warmth.