Chronic shallow seawater circulation driven by subsurface gas dynamics, at the Southern Hydrate Ridge seep system, Cascadia

Abstract

[author abstract] Cold seeps, like Southern Hydrate Ridge (SHR) on the Cascadia margin, are ubiquitous features along continental margins where fluids discharge from the seafloor into the water column. Previous short-term fluid flow studies at SHR and other seep systems have observed upward flow of altered fluids concentrated in regions of bacterial mats and downward flow of seawater-like fluids in regions of vesicomyid clams. To investigate longer-term variability in magnitude and direction of fluid flow at SHR, three continuous-measurement flowmeter/chemical samplers (Mosquitos) were deployed in bacterial mats to measure fluid flow rates and solute fluxes. This two-year continuous record of fluid flow shows that the flow rate at SHR is temporally and spatially variable, however there is an annual trend of ~24-67 cm/yr of downward fluid flow in regions of bacterial mats. This net downward transport of seawater circulation is more constant than previously recognized and the results indicate that shallow seawater circulation is the key in reconciling the discrepancy between bottom up and top down measurements of fluid flow at seep sites. Mechanisms driving this shallow seawater circulation are explored in addition to the connection between fluid flow and the ecology of associated benthic communities.