Seep-bubble characteristics and gas flow rates from a shallow-water, high-density seep field on the shelf-to-slope transition of the Hikurangi subduction margin

We analyse an area of high density submarine methane gas seeps situated on the shelf to slope transition (130–420 m water depth) on the northern region of New Zealand's Hikurangi margin, off Poverty Bay. Multibeam and singlebeam echo sounder data collected in 2014 and 2015 revealed >600 seeps, at much greater density than any previously mapped areas of seepage on the Hikurangi margin. To broadly constrain the output of methane from these seeps, we have estimated the flow of methane at individual seeps, utilising perspective-measurements applied to still frames from a deep towed camera system to measure the dimensions of rising bubbles. We combine bubble size and rise-rate distributions with singlebeam acoustic data to estimate gas flow rates at six selected seeps sites. Our results predict a wide range (3.0–2249 mL/min) of methane release into the water column. If we assume that the six seeps we analysed are representative of the entire seep population, and that gas flow is constant, we can extrapolate across the seep field and infer a gas release of 30 to 2415 t of methane per year into the ocean.