Maine's pockmark fields were discovered as a result of a geological problem. Geologists Kelley and Belknap were frustrated mapping muddy estuaries by garbled signals coming below muddy embayments. Bubbles between the mud grains reflected sound back so strongly that the sound signal was obscured, or "wiped out". Thus, gassy bottoms were mapped as a consequence of the inability of scientists to see the subsurface geology. The bubbles always appeared and blocked the signal as the muddy sediment became thick. On the fathometer-like seismic reflection records, there appeared to be v-shaped channels in many muddy areas, but little else of interest. It was the side scan sonar images that displayed vast fields of deep craters in muddy bays that stunned geoscientists. The v-shaped channels of the seismic profilor were actually sections through bowl-shaped holes on the ocean bottom. The slopes on the walls of the pockmarks, up to 30?, are too steep to exist for long unless they are regularly maintained by renewed gas escape. Detailed mapping showed long, linear chains of pockmarks that initially defied explanation. Now, after years of mapping these areas and collecting deep core samples through the mud deposits, marine geologists in Maine have a hypothesis to explain what they have seen, but many more additional questions as well. The story begins about 11,000 years ago, when glaciers still occupied much of North America and world sea level was lower (Figure 4). At that time, Maine's coast was many miles seaward of the modern shore, and about 200 feet lower than today. The muddy bays of today were forested uplands, much like the coastal region today. Two common features in Maine's current landscape must have also existed in the past as well: lakes and wetlands. As the great glaciers melted, world sea level rose, and the lakes and wetlands were intersected by the sea. This process continues to occur today at the head of many estuaries. Great Salt Bay in the Damariscotta Estuary and Taunton Bay in upper Frenchman Bay both appear to be former lakes that now contain brackish to salt water. Extensive fresh water wetlands transition to salt marshes and mud flats as the rising sea drowns upper Casco Bay, for example. The defining characteristic of lake and wetland deposits forming at the land/sea boundary today is a high concentration of organic matter made up of plant and animal remains.Bacteria break down plant and animal remains to produce natural gas, or methane. Methane is a common product of septic systems and landfills for just this reason. University of Maine marine geologists, Joe Kelley, Dan Belknap and their students hypothesize that as sea level rose past lakes and wetlands long ago, some of the organic deposits were not eroded, but were buried beneath ocean mud. Over time, bacteria broke down the organic remains and generated natural gas. If the confining seafloor sediment was sandy, as off Old orchard Beach in Saco Bay, the gas simply bubbled away as it was produced. It is the muddy sediment that is more impermeable and traps the gas until its pressure builds up. In some places the gas migrated along sandy layers of sediment and erupted in linear chains. In many other locations the gas erupted out of the muddy sediment and formed large pockmarks. It apparently continues to do so. Anecdotal accounts from divers recount walls of pockmarks opening up like "the inside of Coke bottles". Kelley and colleagues have also witnessed eruptions with side scan sonar and observed muddy plumes in the water. Though some aspects of pockmarks are known, many unknowns exist. When does the gas escape? Is it during storms, or at low tides when the confining weight of the ocean water is less? How fast is the gas produced? How much is there? Could we extract natural gas from Maine's bays? Why is it that gas is observed in many other estuaries, like Chesapeake and Delaware Bays, but pockmarks are only recorded north of Casco Bay and into the Maritimes? Whatever their origin, gas and pockmark fields exist. With increasing activity offshore, we will need to reckon with them. Kelley recalls an Army Corps of Engineers proposal to dispose of dredged material at Sears Island by "filling in the holes in Belfast Bay". Kelley thinks that such an action would be disastrous, as large quantities of gas and mud would be rapidly released when tons of rock and mud were dropped onto the seafloor. He has found some evidence that large boat anchors can even release gas and form pockmarks. He also thinks that great care is required to locate utility lines away from weak gassy sediment to prevent damage when the gas is released.
weak gassy sediment to prevent damage when the gas is released.