Evaluating methane outputs from an area of submarine seeps along the northern Hikurangi Margin, New Zealand
Collated global marine surveys have documented large volumes of gaseous methane able to escape from deeply-buried deposits into global oceans as seeps. Seeps are evident where permeable faults and fracture networks allow for the upward transportation of methane from buried deposits into the water column as plumes of rising bubbles. Seep bubbles dissolve the majority of their constitutive methane into the surrounding water column as they rise; however there is evidence of more-prominent seeps transferring undissolved methane through the water column and into the atmosphere. Due to the biologic origins of methane, the global distribution of buried methane de-posits is highly varied and diﬃcult to predict. High uncertainties in seep locations have resulted in all previous estimations of the global proportion of atmospheric methane attributed to seeps to have very large associated errors. These are mainly due to large extrapolations over global oceans based on ﬁndings from surveyed seep ﬁelds. A 2014 NIWA research voyage saw the discovery of an abundant seep ﬁeld situated at uncharacteristically shallow water depths (150–300 m below sea level) along the raised continental shelf of the Hikurangi Margin, New Zealand. In comparison to other globally documented seep ﬁelds, the Hikurangi Margin seeps are numerous (estimated between 585 and 660 surveyed seeps) and cover a large area (∼ 840 km²). Prior to the discovery of this seep ﬁeld, there was only evidence of 36 seeps along the entire Hikurangi Margin. Acoustically surveyed bubble-rise paths of newly discovered seeps also show evidence of seeps extending the entire height of the water column. The large number of shallow ﬂares present in the abundant seep ﬁeld represent the potential for considerable amounts of gaseous methane outputs. To further investigate these seeps, NIWA voyages TAN1505 and TAN1508 that took place in June and July of 2015 employed a range of scientiﬁc equipment to analyse features of the rising seep bubbles. Part of these investigations involved the video recordings of rising seep bubbles from the seaﬂoor as well as acoustically surveying rising bubbles using a singlebeam and multibeam echsounder. We have used video and acoustic data sets to create multiple tools and computational techniques for better assessing features of seeps. We have developed photogrammetric tools that can be used in Matlab to compute bubble-size distributions and bubble-rise rates from still frames of underwater video footage. These bubble parameters have then been combined with singlebeam recorded ﬂare proﬁles to calculate the ﬂux of emitted methane at the seaﬂoor. These calculations were carried out using the FlareFlow Matlab module, devised by Mario Veloso. To assess the number of seeps in a multibeam surveyed region, we have created vertically-summed intensity maps of the obtained water column data. Summed-intensity maps display localised high-amplitude features, indicative of seeps. Seep indicators have been used to (1) map the distribution of seeps of the surveyed Hikurangi Margin, (2) assess the total surveyed seep count, and (3) identify regions where seep concentrations are particularly high. We have combined methane ﬂuxes from analysed seeps with regional seep-distribution maps to approximate the rate at which gaseous methane is escaping from the seaﬂoor across the seep ﬁeld. Extrapolating seep emissions over the surveyed area approximates 0.99×10⁵ ±0.64×10⁵ m³/yr of undissolved methane is being released across the seep ﬁeld. Using models of methane preservation, combined with staggered depth models of ﬂares, we have approximated that ∼ 0.2% of the methane emitted at the seaﬂoor is able to reach the atmosphere.