Short box cores (to 30 cm bsf) and seafloor carbonate grab samples were acquired at mapped hydrocarbon seep sites (600–1200 m water depths) during the 2007 RV SONNE SO191 cruise on the Hikurangi Margin offshore eastern North Island, New Zealand, to evaluate the influence of methane seepage on sedimentologic, biotic, mineralogic and stable isotopic attributes of seabed sediments. Sedimentary horizons in the box cores consist of siliciclastic silts and sands, shell beds and nodular, microcrystalline aragonite bands up to 15 cm thick. The megafauna is dominated by infaunal to semi-infaunal chemosymbiotic bivalves (Calyptogena, Lucinoma, and Acharax), as well as associated worms and carnivorous and grazing gastropods. Burrows in silts, some occupied by worms or juvenile Acharax, mainly have simple morphologies more typical of high-energy, nearshore settings than deep-sea environments, while a few are large and sparsely branched with wall scratch marks inferred to be of decapod crustacean origin.
The box core silts and nodular carbonate samples vary in TOC content from 0.2 to 0.9 wt.%, carbonate content from 4 to 78%, and δ13C and δ18O values from − 50.3 to − 0.6‰ PDB and + 0.77 to + 3.2‰ PDB, respectively. Low carbonate content silt samples have the most enriched δ13C values, implying a seawater source for their pore water bicarbonate. Negative δ13C and positive δ18O values typify the nodular, microcrystalline aragonite bands, indicating formation during microbially mediated, sulphate-dependent anaerobic oxidation of methane (AOM) in a cold, near-seafloor environment, as is also supported by lipid biomarker data. A clear isotopic mixing trend of decreasing δ13C and increasing δ18O and carbonate content in the fine (< 100 µm) carbonate fraction of the host silts also has been reported from other methane seep provinces, and suggests a heterogeneous influx of methane-rich seep fluids through the shallow seabed, displacing pore waters of seawater origin.
Seafloor grab carbonate samples show greater textural variability and are divided into two types. Fresh-appearing, grey to white blocks yield similar mineralogic, isotopic and biomarker signatures to the nodular carbonate bands in the cores. A largely seep-related epifauna affiliated with these grab samples include chemosymbiotic bathymodiolin mussels, siboglinid tube worms, methanotrophic suberitid sponges and grazing limpets which, in places, are entombed within the carbonate. By contrast, some dark reddish to brown, iron-stained, microcrystalline dolomitic slabs and tubular concretions support abundant non-seep epifaunal encrusters from the surrounding deep sea, indicative of long exposure on the seafloor. Distinct stable isotopic signatures of these exhumed dolostones (δ13C c. − 20‰ PDB, δ18O up to + 7‰ PDB) suggest derivation from an oxidised methane pool, and pore fluid freshening by gas hydrate dissociation or from mobilized burial fluids that have undergone clay mineral dehydration.
