**Pockmark formation and sediment redistribution by gas escape in Maine
estuaries**

submitted by Walter Barnhardt

PROBLEM: The escape of natural gas and interstitial water from the
seafloor creates crater-like depressions in muddy, unconsolidated
sediment that are known as pockmarks (PMs). Natural gas and associated
PMs are recognized in many parts of the world but have received
relatively little attention in shallow estuarine environments. In
particular, the localized venting of fluids to produce PMs is poorly
understood. This proposed research will apply the broad range of USGS
expertise to study the influence of geochemistry, sedimentology, and
physical oceanography on the origin and evolution of shallow-water PMs
in the western Gulf of Maine. The project addresses several science
directions in the new USGS Science Strategy:

-   *National Hazards, Risk and Resilience* - The formation of PMs poses
    significant hazards for offshore structures, involves massive
    redistribution of nearshore sediment (often contaminated), and might
    record past earthquake activity.

-   *Climate Variability and Change* - Although quantities released are
    poorly quantified, the natural gas (methane?) involved in pockmark
    formation worldwide makes significant contributions to atmospheric
    warming and the marine carbon inventory.

-   *Understanding Ecosystems and Predicting Change* - Methane release
    supports locally enhanced biological activity through food webs
    based on methanogenic bacteria. Eruption of PMs disturbs benthic
    habitat and recycles contaminants/nutrients stored in estuarine
    sediments.

OBJECTIVES:

> 1\) Evaluate the full extent, stratigraphy, and geotechnical
> properties of gas-charged areas
>
> *Size and spatial distribution of PMs related to geologic framework?
> Physical properties of sediment?*
>
> 2\) Determine the age and origin of gas
>
> *What is the depositional environment of source beds? Rate of natural
> gas production?*
>
> 3\) Test hypotheses of occurrence and evolution of PM fields
>
> *Controls on gas migration and escape? Processes involved in
> excavation, sediment dispersal? Time scales?*

APPROACH: Examine and compare 3-4 estuaries with varying bedrock
structure, sediment cover, exposure to wave and tidal energies, degree
of fluvial influence and proximity to neotectonic activity. All contain
thick deposits of gas-charged mud but some with similar settings do not
have PMs. Evaluate geologic framework with high-resolution mapping based
on data from swath bathymetry, sidescan sonar, subbottom profiling,
bottom photography, sediment sampling and geotechnical tools. Determine
sediment accumulation rates with ^210^Pb and other techniques. Determine
age and origin of the gas by composition (C~1~:C~2+~) and isotopic
analyses (C, H). Measure sediment-transport processes with advanced
instrumentation. Analyse fluid migration and sediment dispersal with
numerical modeling.

RELEVANCE AND IMPACT: The formation of PMs probably is the single most
important process driving the geologic evolution of many estuaries in
formerly glaciated regions. This research deals directly with important
aspects of sedimentology and oceanography in nearshore marine
environments. It has implications for sedimentary petrology (rock
analogues, oxidation/reduction of marine shales), global climate change
(methane release, carbon budgets), hydrogeology (fluid migration and
trapping), and benthic ecology (disturbance, exotic communities). The
results will likely spur additional biological research on the trophic
role of escaping gas and chemical research on the rates of reactions
producing the gas. It will also support a court-ordered cleanup of Hg
contamination in the Penobscot River and Bay, and provide fundamental
data for habitat researchers concerned with dam removal.

PARTNERSHIPS: The CMGP-WHSC has long-established relationships with the
Maine Water Science Center (WRD), Univ. of Maine, Maine Geological
Survey, and the Penobscot River Restoration Project.