The effects of anaerobic methane oxidation on benthic foraminiferal assemblages and stable isotopes on the Hikurangi Margin of eastern New Zealand

Abstract

A study of benthic foraminiferal assemblages from cold hydrocarbon seeps of the Hikurangi Margin, New Zealand, was undertaken during RV SONNE cruise SO191-3 to establish what effects, if any, methane-influenced pore waters had on the foraminiferal assemblages and the carbon isotopes of their tests. The results of this study indicate that Hikurangi Margin foraminiferal assemblages are consistent with modern and fossil seeps worldwide Foraminiferal distribution, species richness, density and diversity were little different between seep and non-seep sites, though there were noteworthy exceptions The non-seep reference core samples were greatly enriched in agglutinate species, which constituted 45% of the total assemblage of that core By contrast, seep sites contained a maximum of 11% agglutinate foraminifera Seep and non-seep sites were not separated by density or diversity, except for the single site beneath a bacterial mat that exhibited the lowest density and diversity of all sites Assemblages at all sites were dominated by Uvigerina peregrina, a species known to inhabit fossil and modern seeps elsewhere, but that also is well documented from normal marine environments Carbon isotope data significantly differentiates seep and non-seep sites through the greater C-13 depletion and heterogeneity of delta C-13 values in seep foraminifera delta C-13 values for U peregrina were as low as -15 2 parts per thousand PDB. In addition, specimens of Pyrgo depressa and Hoeglundina exhibited minimum delta C-13 values of -29.8 parts per thousand and -35.7 parts per thousand PDB respectively, showing the influence of carbon that was derived from sulphate dependent anaerobic oxidation of methane. Authigenic carbonates and vesicomyid bivalve shells from seep Station 198 also recorded depleted delta C-13 values, with the lightest values (-55 4 parts per thousand PDB) clearly within the range of biogenic methane sources.