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

Abstract:

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 (SO₄^2-) profiles, assuming steady-state consumption of SO₄^2- and CH₄ by anaerobic oxidation of methane (AOM): CH₄ + SO₄^2- → HCO₃^- + HS^- + H₂O. This reaction is one of the primary controls on CH₄ distributions in sediments. This work will present pore water SO₄^2-, sulfide (HS^-) and chloride (Cl^-) depth profiles in sediment collected from the pockmark field. Theoretical SO₄^2- distributions in the absence of AOM are compared to observed SO₄^2- 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.