THE EFFECTS OF METHANE SEEPAGE ON BENTHIC FORAMINIFERA FROM THE HIKURANGI MARGIN OF NEW ZEALAND

Cold methane seeps are common features of continental margins both passive and active, and where associated with gas hydrates, are reservoirs for vast quantities (~600 Tg CH4/yr) of carbon. In addition, they support unique, chemosynthesis-based ecosystems. As part of the COMET program funded by the German government investigating methane in the global geochemical cycle, a study of benthic foraminiferal assemblages from cold hydrocarbon seeps of the Hikurangi Margin, New Zealand, was undertaken during RV SONNE cruise SO191-3. The object of this study was to establish what effects, if any, methane-influenced porewaters had on the foraminiferal assemblages and the stable isotopes of their calcite shells. Results showed that foraminiferal assemblages differed little in density, diversity and species richness between seep and non-seep sites. A definite distinction existed in the number of agglutinate species, with non-seep sites containing up to 45% agglutinates and seep sites a maximum of 11%. Isotope data, on the other hand, significantly distinguish seep and non-seep sites, with the seep sites displaying considerably more 13C depletion and isotopic heterogeneity. For example, δ13C values for Uvigerina peregrina were as low as –15.2‰ PDB, and Pyrgo depressa and Hoeglundina elegans specimens exhibited minimum δ13C values of –29.8‰ and –35.7‰ PDB respectively, showing the influence of carbon derived from sulfate-dependent anaerobic oxidation of methane. Authigenic carbonates and vesicomyid bivalve shells from seep sites also recorded depleted δ13C values, with the lightest values (–55.4‰ PDB) clearly within the range of biogenic methane sources. The use of integrated data from authigenic carbonates and foraminifera proves to be a useful technique in the elucidation of fluid-flow characteristics in methane seep provinces.