IMMeRSS-- Dating Past Methane Seepage

Science Center Objects

Cold seeps are locations where chemicals, including methane, hydrocarbons, sulfide and sometimes carbon dioxide, leak from sediments below the seafloor into the water above.  Much of the current knowledge on cold seeps comes from direct observations, such as the mapping of acoustic flares and the detection of elevated methane concentration in the water column, both of which are indicative of active seepage. The mapping of both seafloor and sub-seafloor structures can also provide information about the location and mechanisms of current or past methane seepage. However, direct observations of cold seeps only cover the last few decades of their activity, and imaging of structures often only provides crude estimates of when seepage was occurring.  Knowledge of the timing allows us to better understand the processes controlling the release of methane and provides a longer term (i.e., thousands of years) perspective on the question of how fast methane is being released.

Photograph of Methane-derived authigenic carbonate

Methane-derived authigenic carbonate (MDAC) features from a sample obtained at Baltimore Canyon

Evidence of past methane seepage is preserved in seafloor sediments in the form of methane-derived authigenic carbonates (MDAC). These are rocks that are specific to cold seeps and that form in situ as a result of changes in water chemistry brought on by the bacterial breakdown of methane, which in turn causes calcium carbonate to precipitate. Crucially, carbonate is a mineral that can be dated by determining the ratio between uranium (which co-precipitates with calcium from seawater when the MDAC forms) and its decay product, thorium.  Increasingly, uranium-thorium dating methods are used to study MDAC samples and infer the timing of methane emissions.  For example, these methods have recently been used to date MDAC on the Norwegian margin.

The IMMeRSS Mid-Atlantic expedition is collecting MDAC samples from seep sites using Oceaneering’s Global Explorer ROV.  These samples will be returned to the British Geological Survey’s geochronology laboratory in Keyworth, United Kingdom for analysis.  The U.S. Atlantic margin offers an exciting setting for investigations related to the chronology of past methane seepage, because cold seeps are distributed over a large geographic area, a wide range of water depths, and diverse geological settings. It will take numerous studies to collect and date enough samples to reveal the complete history of seepage along the margin.  At the moment there are only a few published dates and a handful of additional dates acquired on samples from the NSF-sponsored SeepC cruise led by Cindy Van Dover in 2015.

The USGS Gas Hydrates Project is a multidisciplinary effort that is jointly supported by the Coastal and Marine Geology Program and the Energy Research Program at the USGS. Most personnel are based in Woods Hole, Massachusetts (Coastal and Marine) and in Denver (Energy). Additional personnel are located at the Santa Cruz Coastal and Marine Science Center, the Earthquake Program in Menlo Park, and the Minerals Program in Reston.