CO₂ Release From Pockmarks on the Chatham Rise‐Bounty Trough at the Glacial Termination

Seafloor pockmarks of varying size occur over an area of 50,000 km 2 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 Δ 14 C 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 δ 13 C excursion at the glacial termination that would be indicative of methane or mantle‐derived carbon at the time the Δ 14 C excursion and pockmarks were produced. An ocean general circulation model equipped with isotope tracers is unable to simulate these large Δ 14 C excursions on the Chatham Rise by transport of hydrothermal carbon released from the East Pacific Rise as previous studies suggested. Here we attribute the Δ 14 C anomalies and pockmarks to release of 14 C‐dead CO 2 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 Δ 14 C anomaly, the pockmarks may collectively represent an important source of 14 C‐dead carbon to the ocean during glacial terminations.