Deglaciation of the Gulf of Maine

High-resolution seismic stratigraphy, sedimentology, and micropaleontology of accelerator mass spectrometer (AMS) radiocarbon-dated piston cores showed that deglaciation started very early in the Gulf of Maine. Glaciomarine deposition began as early as 18 ka in the southeastern gulf (Georges Basin) and reached the present coastline about 14 ka. The retreat of the ice margin occurred in a sequence of distinct steps. (1) Marine water entered the basin beneath the ice, creating an ice shelf. (2) This ice shelf remained stable as long as its frontal edge was buttressed against sills or ledges. (3) Once thinning and calving freed the ice edge from its support, it calved back quickly until the next sill was reached. In this fashion, the ice retreated from basin to basin until shallow water and the coastline was reached. Following the collapse of an ice shelf over a basin, the initial marine environment there was one of thick, year-round sea ice and of icebergs, prohibiting surface productivity. This year-round sea-ice phase was followed by a phase of brief summer open water during which an arctic flora developed. Further ice retreat and increasing warming led to lengthier periods of summer ice-free conditions, and with upwelling of nutrient-rich water, highly productive surface conditions prevailed. This sequence of events can be traced in the sedimentary record laterally from the southeast to the northeast, and vertically within each basin.