EVIDENCE FOR CHANGES IN THE BELFAST BAY POCKMARK FIELD, MAINE

Pockmarks were first recognized in Belfast Bay in 1985, and three side-scan sonar surveys of sections of Belfast Bay were completed in 1989, 1998 and 2000. The surveys from 1989 and 1998 imaged the entire pockmark field, while the 2000 survey was conducted with a high-resolution, digital side-scan sonar over an area where large ships frequently anchor. The 1998 survey confirmed that pockmarks increase in size towards deeper water and sediment thickness and till outcrops control location of some linear pockmark chains. Hypotheses tested were: 1) the Belfast Bay field is active; and 2) human activities are initiating pockmark formation. Despite differences in navigation (Loran C vs. DGPS) and data gaps in the mosaics, the 1989 and 1998 surveys were compared to determine the level of activity in the pockmark field. Features were matched on GIS maps of the field with the aid of landmarks, such as groups of pockmarks, pockmark chains, and till outcrops. Only the presence or absence of individual pockmarks was discernable; changes in pockmark size were not resolvable due to differences in the methods of GIS coverage creation. Comparison of the 2000 survey with the 1998 survey revealed significant changes in the seafloor over the 2-year period. Creation and destruction of pockmarks and fishing drag marks was clearly evident. The 2000 data set also points to anchor and fishing drags as possible mechanisms of initiation for pockmarks. Comparison of the three surveys shows that new pockmarks are forming and older, less active pockmarks were seen to fill in Belfast Bay. Possible seasonal fluctuations in pockmark activity were recognized.