Abstract
Understanding fluid expulsion is key to estimating gas exchange volumes between the seafloor, ocean, and atmosphere; for locating key ecosystems; and geohazard modelling. Locating active seafloor fluid expulsion typically requires acoustic backscatter data. Areas of very-high seafloor backscatter, or “hardgrounds,” are often used as first-pass indicators of potential fluid expulsion. However, varying and inconsistent spatial relationships between active fluid expulsion and hardgrounds means a direct link remains unclear. Here, we investigate the links between water-column acoustic flares to seafloor backscatter and bathymetric metrics generated from two calibrated multibeam echosounders. Our site, the Calypso hydrothermal vent field (HVF) in the Bay of Plenty, Aotearoa/New Zealand, has an extensive catalogue of vents and seeps in <250 m water depth. We demonstrate a method to quantitatively link active fluid expulsion (flares) with seafloor characteristics. This allows us to develop predictive spatial models of active fluid expulsion. We explore whether data from a low (30 kHz), high (200 kHz), or combined frequency model increases predictive accuracy of expulsion locations. This research investigates the role of hardgrounds or surrounding sediment cover on the accuracy of predictive models. Our models link active fluid expulsion to specific seafloor characteristics. A combined model using both the 30 and 200 kHz mosaics produced the best results (predictive accuracy: 0.75; Kappa: 0.65). This model performed better than the same model using individual frequency mosaics as input. Model results reveal active fluid expulsion is not typically associated with the extensive, embedded hardgrounds of the Calypso HVF, with minimal fluid expulsion. Unconsolidated sediment around the perimeter of and between hardgrounds were more active fluid expulsion sites. Fluids exploit permeable pathways up to the seafloor, modifying and refashioning the seafloor. Once a conduit self-seals, fluid will migrate to a more permeable pathway, thus reducing a one-to-one link between activity and hardgrounds. Being able to remotely predict active and inactive regions of fluid expulsion will prove a useful tool in rapidly identifying seeps in legacy datasets, as well as textural metrics that will aid in locating nascent, senescent, or extinct seeps when a survey is underway.
Highlights
Seafloor fluid expulsion is a key mechanism for large-scale gas exchange between the seafloor, ocean (Mcginnis et al, 2006; Reeburgh, 2007), and potentially the atmosphere (Gentz et al, 2014; Sahling et al, 2014; Shakhova et al, 2014; Römer et al, 2017)
NCHVF04, with the highest Kappa value of 0.65, performed significantly better than the individual mosaic models with equivalent scale parameters (SPs) (Figure 10)
In the North in particular, the 30 kHz mosaic better highlights areas of veryhigh to high backscatter than the 200 kHz, as there is difficulty differentiating between sandy substrate and hardgrounds in the higher frequency mosaic using backscatter alone (Figure 4C)
Summary
Seafloor fluid expulsion is a key mechanism for large-scale gas exchange between the seafloor, ocean (Mcginnis et al, 2006; Reeburgh, 2007), and potentially the atmosphere (Gentz et al, 2014; Sahling et al, 2014; Shakhova et al, 2014; Römer et al, 2017). Varying and inconsistent spatial relationships between observable fluid expulsion, hardgrounds, and the surrounding seafloor have been noted previously (Naudts et al, 2008; Naudts et al, 2010; Thorsnes et al, 2019). This unclear relationship reflects the complexity of hardground genesis, gas migration, fluid expulsion processes, and spatio-temporal differences in expulsion and embedment. Once the conduit closes or ceases activity, hardgrounds will remain at the surface or in the subsurface, with surface hardgrounds thenceforth observable in seafloor acoustic backscatter (Judd and Hovland, 2009; Levin et al, 2016). Formalizing the link between gas flares and hardgrounds has remained elusive due to spatiotemporal differences between active expulsion and hardground formation, which can post-date expulsion activity
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