ASSOCIATED TERRESTRIAL AND MARINE FOSSILS IN THE LATE-GLACIAL PRESUMPSCOT FORMATION, SOUTHERN COASTAL MAINE, AND THE MARINE RESERVOIR EFFECT ON RADIOCARBON AGES

Recent excavations for the Mercy Hospital complex on the Fore River in Portland, Maine exposed well-preserved white spruce (Picea glauca) trees in marine clay of the Presumpscot Formation. The exposures were ~ 150 m SW of the site where spruce logs were discovered in the Presumpscot clay in 1976 and showed similar stratigraphy. The new site revealed broken trunk segments of at least five trees lying on their sides in association with other fragments of arboreal and non-arboreal plants, including an ecologically diverse assemblage of mosses, indicating transport and deposition of clasts of a peaty-humus terrestrial surface soil layer. Buds of balsam poplar (Populus balsamifera) occur with twigs and needles of white spruce in the peat clasts. The plant remains occur in clay and a basal fine-sand layer in the lowest 1 m of the Presumpscot Formation. These fossiliferous marine sediments abruptly overlie the sloping surface of a deposit consisting of poorly-sorted gravel and gravelly sand, interpreted as a glaciomarine subaqueous fan. All the trees died at the same time during winter dormancy. The oldest contained 200 rings. The new exposures also yielded marine shells including Mytilus edulis, Macoma balthica, Serripes groenlandicus, Cerastoderma pinnulatum, and barnacles. Barnacle fragments and complete Mytilus valves were found within 30 cm of the tree remains. This juxtaposition enables comparison of marine and terrestrial radiocarbon ages to evaluate the local marine reservoir correction. Radiocarbon ages of nine wood samples ranged from 11,907 +/- 31 to 11,721 +/- 40 ¹⁴C BP. Ages of closely associated shell samples were 12,850 +/- 65 ¹⁴C BP (Mytilus) and 12,800 +/- 55 ¹⁴C BP (barnacle). If this marine-terrestrial age difference of ~ 1000 years is applicable to other shell ages in SW Maine, it will reduce or eliminate the discrepancy between the marine deglaciation chronology in this part of the state and the ice-retreat history based on the New England varve chronology to the west. The local sea-level curve suggests the tree remains were deposited in water depths of ~ 30-37 m. The terrestrial fossil assemblage most likely washed out to sea from one of the nearby islands or peninsulas within a few kilometers. It was buried quickly, with no evidence of sorting, as marine clay accumulated on the sea floor.