Cold seep carbonates and associated cold-water corals at the Hikurangi Margin, New Zealand: New insights into fluid pathways, growth structures and geochronology

Abstract

The aim of this study is to provide combined new insights into the geochronological framework, isotope geochemical signatures and structural observations of methane related authigenic carbonate settings and associated cold-water corals from offshore New Zealand. The analysed samples are obtained from calcified sediments of three different cold seep areas at the Hikurangi Margin: Opouawe Bank, Uruti and Omakere Ridge. We focused the sub-sampling on aragonitic precipitates in vein like structures, partly still open fluid channel systems and related chemoherm structures in order to identify the timing and signature of focused marine methane emanation. The presented initial U/Th age data set indicates different generations of intensified seep activity and related carbonate precipitation between 12,400 ± 160 and 2090 ± 850 years BP. The youngest stage so far, was identified as contemporaneous cold seep activity at the southernmost (North Tower, Opouawe Bank) and northernmost (Bear's Paw, Omakere Ridge) sampling sites around 2300 years BP. Sharing the same water depth (1050 to 1100 m) these sites imply regional margin-wide tectonic or hydrological changes as controlling process. An intermediate phase of vein and channel structures within the sediment was detected for a time interval between approximately 5000 and 4000 years BP with contemporaneous settings of focused seep activity around 4300 years BP at Uruti Ridge (LM-10) and Opouawe Bank. δ 13C PDB data reflect site and carbonate type specific signatures, clustering around − 52‰ (Uruti and Omakere Ridge) and − 47‰ for the fluid pathway system and the uppermost surface at North Tower site (Opouawe Bank). Late stage precipitates in chemoherm cavities of the latter reflect significantly heavier values of about − 38‰. Porous precipitates within open fluid channel systems are characterized by decreased δ 234U ( T ) values, exceptional high Th and U concentrations and slightly lighter δ 13C PDB signatures when compared to adjacent rim-like and dense cements. This specific kind of precipitate is interpreted as indicator for phases of less vigorous fluid seepage. The observed occurrence of cold-water corals seems to be mostly depending on the abundance of authigenic carbonates as a substrate exposed to erosive bottom water currents. But, seafloor observations combined with preliminary age data indicate a significant time gap between the inferred end of cold seep activity and coral colonization. U–Th analyses of recent reef-forming coral provided an initial δ 234U (0) value of 146.3 ± 3.9‰ and 0.0013 ± 0.0002 as starting 230Th/ 234U activity ratio for coral growth in the bottom water.