NOAA/WDS Paleoclimatology - Global Ocean Glacial Termination Radiocarbon Modeling Results

Seafloor pockmarks of varying size occur over an area of 50,000 km2 on the Chatham Rise, Canterbury Shelf and Inner Bounty Trough, New Zealand. The pockmarks are concentrated above the flat-subducted Hikurangi Plateau. Echosounder data identify recurrent episodes of pockmark formation at ~100,000-year frequency coinciding with Pleistocene glacial terminations. Here we show that there are structural conduits beneath the larger pockmarks through which fluids flowed upward toward the seafloor. Large negative D14C excursions are documented in marine sediments deposited next to these subseafloor conduits and pockmarks at the last glacial termination. Modern pore waters contain no methane, and there is no negative d13C excursion at the glacial termination that would be indicative of methane or mantle-derived carbon at the time the D14C excursion and pockmarks were produced. An ocean general circulation model equipped with isotope tracers is unable to simulate these large D14C excursions on the Chatham Rise by transport of hydrothermal carbon released from the East Pacific Rise as previous studies suggested. Here we attribute the D14C anomalies and pockmarks to release of 14C-dead CO2 and carbon-rich fluids from subsurface reservoirs, the most likely being dissociated Mesozoic carbonates that subducted beneath the Rise during the Late Cretaceous. Because of the large number of pockmarks and duration of the D14C anomaly, the pockmarks may collectively represent an important source of 14C-dead carbon to the ocean during glacial terminations.