HOLOCENE SLIDE SCARS AS THE DOMINANT LOCATION FOR CHEMOSYNTHETIC BIOLOGICAL COMMUNITIES IN MONTEREY BAY

Numerous seafloor sites have been discovered along continental margins where a supply of reduced inorganic compounds, such as hydrogen sulfide or methane, support chemosynthetic biological communities (CBC), authigenic mineralization, and early diagenesis. While CBC sites were initially viewed as rare, isolated occurrences, they now have been encountered along all continental margin segments where much of the seafloor has been visually imaged. The question is increasingly whether these are actually common features along continental margins and what controls their distribution. Here, we report the first quantitative evaluation of the distribution of seafloor fluid seepage indicators (e.g., living CBC and authigenic mineralization) along the California continental margin off Monterey Bay. A systematic analysis of dive videos and navigation from the 1,192 benthic ROV dives conducted by MBARI provides data on the occurrence and distribution of CBC. These seepage indicators are common, occurring within 2% of the 25 m square grid cells within which there have been bottom observations within 45 km of the head of Monterey Bay, and within 9% of the visited cells that are below 550 m water depth. A surprising result is that the frequency of seepage indicators does not increase in the known fault zones. Instead, these seepage indicators are most commonly observed on steep and recently eroded seafloor, particularly on recent slide scars. Examination of the surfaces of slide scars of various ages shows progressive stages of post slope failure evolution. The existence of CBC on the fresher surfaces can be explained simply as a result of erosion. Fluid flow may not be required for these "seep" communities to exist. CBC undergo a succession based on their ability to maintain their access to the redox boundaries on which they depend on as an energy source. Bioerosion allows them to track the progressively retreating redox boundaries as the geochemical re-equilibration on sole of the slide occurs. Caves on the steep face of the deep–sea slide scars are carved as organisms dig into the substrate.