POCKMARK FIELD GEOMORPHIC TRENDS FROM SERIAL SWATH BATHYMETRY, BELFAST BAY, MAINE, USA

Pockmarks, or seafloor craters, occur worldwide in a variety of continental shelf settings, including areas of tectonic activity, deltas and previously glaciated estuarine areas. Associated with fluid escape, the mechanisms and timescale of pockmark formation are not well constrained and several hypothesis for their formation have been proposed. One of the most studied estuarine pockmark fields in the world resides in Belfast Bay, ME, at the mouth of the region's largest river. In muddy embayments along Maine's coast, methane proliferates in the subsurface and pockmarks cover the seafloor. Sediment plumes and bubbles in the water column have been observed in the Belfast Bay, and geophysical surveys have also imaged a sediment plume emanating from a pockmark. The walls of pockmarks exhibit 30-degree slopes. These observations imply that: 1) modern venting is a possible mechanism for pockmark formation; and 2) a pockmark not actively venting should fill in overtime because of steep walls and proximity to a sediment source. Corroborating these interpretations, a comparison of two sidescan sonar surveys conducted in 1989 and 1999 found a 32% change in the field. However, this difference analysis suffered from inconsistent navigation used in the two surveys, and has an unquantifiable amount of error.

To conclusively determine the amount of seafloor change in the Belfast Bay pockmark field, serial 2-m resolution swath bathymetry surveys were collected in 1999, 2006, and 2008. All three surveys meet the International Hydrographic Organization's highest standard for bathymetric data. Difference analysis of the three datasets found no detectable change in the seafloor. All pockmarks identified in the earlier survey were present in the later surveys, and no pockmarks present in 1999 were absent in 2006 or 2008. These findings indicate that: 1) although evidence for modern gas escape exists, observed plumes and bubbles are not the fluid escape events responsible for pockmark formation; 2) pockmarks demonstrate perseverance unexplained by their morphology, and thus their maintenance may be related to geotechnical properties or local oceanographic conditions; and 3) pockmark fields may not change with processes that occur on a daily or yearly basis, but instead may be controlled by events that punctuate decades or centuries.