Evaluation of shallow sediment methane cycling in a pockmark field on the Chatham Rise, New Zealand

Tuesday, 16 December 2014
Richard B Coffin1, Paula S Rose1, Ingo Klaucke2, Joerg Bialas2, Ingo Andreas Pecher3 and Andrew R Gorman4, (1)Texas A & M University Corpus Christi, Department of Physical and Environmental Sciences, Corpus Christi, TX, United States, (2)GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany, (3)University of Auckland, Auckland, New Zealand, (4)University of Otago, Department of Geology, Dunedin, New Zealand
Seismic studies have identified an extensive field (>20,000 km2) of seafloor depressions, or pockmarks, on the southwestern flank of the Chatham Rise, New Zealand. It has been suggested that these pockmarks result from gas hydrate dissociation linked to sea-level changes during glacial-interglacial cycles. Gas hydrates are predominately composed of methane (CH4), a potent greenhouse gas. Surface sediment cores (~ 8 m) were collected from the pockmark field on the Chatham Rise during a research cruise in February 2013 to evaluate the association of the features with CH4 releases. A suite of geochemical parameters are interpreted to determine the methane contribution to solid phase sediment and pore water. The upward flux of CH4 in sediments is often quantified using pore water sulfate (SO42-) profiles, assuming steady-state consumption of SO42- and CH4 by anaerobic oxidation of methane (AOM): CH4 + SO42- → HCO3- + HS- + H2O. This reaction is one of the primary controls on CH4 distributions in sediments. This work will present pore water SO42-, sulfide (HS-) and chloride (Cl-) depth profiles in sediment collected from the pockmark field. Theoretical SO42- distributions in the absence of AOM are compared to observed SO42- profiles as a preliminary assessment of the influence of CH4 on sediment geochemistry in and around the seafloor depressions.

In addition isotopically-light CH4 is incorporated into sediment carbon pools via AOM and subsequent CO2 fixation. Stable carbon isotope distributions in the organic and inorganic carbon pools are presented to determine the influence of CH4 in sediments in the vicinty of the pockmarks. Collectively, the geochemical data are used to assess the role of gas hydate dissociation in pockmark formation on the Chatham Rise. Despite sesimic data interpretation in this region there is no modern day contribution of CH4 to shallow sediment carbon cycling and data are presented to assess paleogeochemical methane cycling.