Early Paleogene variations in the calcite compensation depth: new constraints using old boreholes across Ninetyeast Ridge in the Indian Ocean

摘要

Major variations in global carbon cycling occurred between 62 and 48 Ma. To better constrain the cause and magnitude of these changes, the community needs early Paleogene carbon isotope and carbonate accumulation records from widely separated deep-sea sediment sections, especially including the Indian Ocean. With the potential for renewed scientific drilling in the Indian Ocean, we examine lithologic, nannofossil assemblage, carbon isotope, and carbonate content records for late Paleocene – early Eocene sediment recovered at three existing sites spanning Ninetyeast Ridge: Deep Sea Drilling Project (DSDP) Sites 213 (deep, east), 214 (shallow, central), and 215 (deep, west). The sediment sections are not ideal, because they were recovered in single holes using rotary coring methods. Site 214 was very shallow during the late Paleocene, when it received significant amounts of neritic carbonate. The δ13C records at Sites 213 and 215 are similar to those generated at several locations in the Atlantic and Pacific. The prominent high in δ13C across the Paleocene carbon isotope maximum (PCIM) occurs at Site 215, and the prominent low in δ13C across the early Eocene Climatic Optimum (EECO) occurs at both Site 213 and Site 215. The Paleocene–Eocene thermal maximum (PETM) and the K/X event are found at Site 213 but not at Site 215, presumably because of coring gaps. Carbonate content at both Sites 213 and 215 drops to 13C (~ 52 Ma). This reflects a rapid shoaling of the calcite compensation depth (CCD), and likely a major decrease in the net flux of 13C-depleted carbon to the ocean. Our work further constrains knowledge of the early Paleogene CCD, but more importantly suggests that excellent early Paleogene carbonate accumulation records might be recovered from the central Indian Ocean with future scientific drilling.