Seafloor pockmarks on the Chatham Rise, New Zealand: Possible causes and links to glacial cycles.

An area of the seafloor of >50,000 km² on the Chatham Rise and Bounty Trough east of New Zealand’s South Island is covered by seafloor depressions.  Distribution and type of these depressions seem to be bathymetrically controlled, with smaller depressions occurring between ~500-700 m water depth and larger ones in water depths of >800 m.  Formation of these features is enigmatic.  The smaller features display typical features of pockmarks caused by sudden escape of fluids and gas.  Echosounder and seismic data furthermore reveal wide-spread buried pockmarks that appear to have been formed repeatedly near glacial-stage maxima.  Some of the buried pockmarks appear to be stacked, often at a slight offset, underlain by positive-polarity reflections, and aligned with structures that promote fluid escape.  These patterns are compatible with repeated release of fluids from deep sources and precipitation of authigenic material.  Some of the larger seafloor depressions appear to involve interaction with the Southland Current.  These depressions have been interpreted as contouritic mounds although alternative hypotheses have been proposed and they may be linked to deeply rooted fluid migration.

Pronounced Δ¹⁴C anomalies during the last glacial termination, around the time of formation of the most recent pockmarks, indicate release of significant amounts of geologic carbon.  The pockmark fields coincide with the extent of the flat-subducted Hikurangi Plateau.  We hypothesize formation of the pockmarks is linked to repeated release of CO₂ that originates from carbonates on top of the Hikurangi Plateau.  We will discuss this hypothesis, open questions in particular related to the “valve” mechanism controlling repeated release and pockmark formation, as well as alternative mechanisms for possible formation of seafloor depressions in the study area.