HOLOCENE BENTHIC FORAMINIFERAL BIOFACIES AND MUDDY LITHOFACIES FROM THE CONTINENTAL SHELF SOUTH OF MASSACHUSETTS, USA: RAPID GRADED DEPOSITION AND SEA-LEVEL RISE

The middle to outer continental shelf (60-150 meter water depth) south of Cape Cod, Massachusetts is unique because it is the only location on the eastern U.S. continental shelf south of the Gulf of Maine that accumulates muddy sediments in the “New England mud patch” (NEMP) at high rates (~30 cm/kyr), providing high-resolution records of climate and sea-level change. We examined benthic foraminiferal biofacies, percent planktonic of total foraminifers, and lithofacies changes from two generations of NEMP Vibracores and piston cores taken in the region by the USGS. In contrast to other areas of the US Atlantic shelf dominated by sand, the mud patch shows a graded profile, with increasing mud across the shelf. Our benthic foraminiferal studies show that despite strong shelf currents focusing mud in this area, deposition is in situ, reflecting typical middle neritic biofacies. Lower Holocene (11.7-8.1 ka) sediments are sandier and are dominated by a relatively uniform Bulimina marginata biofacies despite rapid relative sea-level rise from ~35 to ~15 m below present and shallowing from 85 to 65 m water depth. An Early Holocene peak in percent planktonic foraminifera is associated with warm-water taxa; we suggest the invasion of warm taxa was from warm core rings associated with a modeled increased tidal energy due to sea-level rise. The Middle Holocene (8-4 ka; sea level 15-7 m below present) saw a transition to a Elphidium-Eponides-Bulimina fauna at ca. 7-6 ka, associated with a slowing of sea-level rise. Globuliminaincreased in abundance during the last 3,000 years as relative sea-level rise slowed to 1 mm/yr. The dominance of Globulimina noted in the greater than 250 um size fraction is less obvious in older cores due to post-coring dissolution. Single specimen δ¹⁸O analyses of Globulimina over the past 3000 yrs show a ~1‰ variation, consistent with a shelf bottom water seasonal cycle of ~4-5°C and 0.3-0.4 psu. There is no temperature trend in the Common Era though the uppermost 10 cm (since ~1750 CE) shows a warming trend reflecting recovery from the Little Ice Age and anthropogenic warming.