PERVASIVE SHALLOW SEAWATER CIRCULATION AT METHANE SEEPS – IMPLICATIONS FOR SEEP ECOSYSTEMS AND MARINE BIOGEOCHEMICAL CYCLES

Cold seeps contain some of the richest ecosystems on the seafloor, and are important regions for element cycling between the lithosphere and hydrosphere. Fluid flow at seeps drives the interaction between anaerobic oxidation of methane (AOM), communities of micro- and macro-biota, and the precipitation of authigenic carbonates and sulfides. Estimates of fluid flow rates based on solute and temperature profiles typically indicate negligible to low upward flow rates over large portions of seep sites, and strong upward flow often exceeding 1 m/yr at discrete locations. Here we summarize direct measurements of fluid flow rates at seeps made over the past two decades with benthic fluid flow meters, and a newly collected continuous 3-year record of flow rates and composition from a bacterial mat site at Hydrate Ridge. Collectively, the fluid flow meter data show that downward fluid flow is chronic at seeps and that shallow seawater circulation is ubiquitous with implications for our understanding of seep ecosystems and marine biogeochemical cycles.

The occurrence of widespread shallow seawater circulation at seeps reconciles the extreme rates of upward fluid flow observed based on solute and temperature profiles that cannot be supported by internal sources such as sediment compaction and mineral dehydration. We suggest the shallow subseafloor flow patterns are a surface manifestation of subsurface gas dynamics, and highlight the importance of 2-phase flow in enhancing biogeochemical reaction rates and sustaining benthic communities. We propose that water is driven out of the sediments by ascending gas bubbles and is replaced by the drawdown of bottom water rich in electron acceptors. In our conceptual model, it is not advection of CH₄-rich pore water that sustains seafloor bacterial mat communities, but rather the dissolution of gas bubbles providing methane immediately below the seafloor coupled to a strong downward flux of SO₄-rich water. This greatly enhances HS^- production through AOM directly below the bacterial mats and the diffusive HS^- flux to the benthic communities. This coupled gas transport-seawater circulation process significantly increases solute transport leading to greatly enhanced biogeochemical turnovers and sustaining bacterial mat communities in a persistent downward aqueous flow regime.