PROVENANCE AND AGE OF LOESS ON LONG ISLAND, NY, USA

Loess ubiquitously overlies Late Wisconsinan glacial sediments on Long Island and is a major reason Long Island’s soils have been so productive. The provenance of the loess was studied using single grain ⁴⁰Ar/³⁹Ar biotite and muscovite ages from two localities along the North Shore of Long Island: Caumsett State Park and the Stony Brook University campus (Zhong, 2002). Muscovite from Caumsett State Park loess has Ar/Ar ages ranging from 200-400 Ma consistent with its source being basement rocks to the north in Connecticut. Biotite in loess from Caumsett State Park did not give reliable ages.

Muscovite Ar/Ar ages from loess on the Stony Brook University campus are similar to those at Caumsett State Park. Biotite Ar/Ar ages of loess on Stony Brook campus also have a mode between 200 and 400 Ma consistent with their source being the basement rocks to the north in Connecticut. However, there are a small number of Ar/Ar ages for biotite that are up to 1800 Ma suggesting that a small proportion of the loess has sources to the west in the mid continent.

Optically Stimulated Luminescence (OSL) ages for a 2.7 m thick loess deposit at Wildwood State Park on the North Shore of Long Island are 13,780 ± 1,100 years for the bottom of the deposit, 13, 400 ± 1,250 years for the middle and 7,730 ± 690 years for just below the soil line. These data are consistent with the hypothesis that the immediate source of Long Island loess is glacial sediment deposited in glacial Lake Connecticut in Long Island Sound basement. The deposition of loess would have started when the proglacial Lake Connecticut drained and its bottom was exposed at about 15 ka (Lewis and Stone, 1991). The deposition ceased when the sea entered the Long island Sound basin at about 12 ka (Lewis and Stone, 1991) and its bottom was no longer exposed. The ca 8 ka age corresponds well to the Holocene cooling episode known as the 8.2 ka event (Dean et al., 2002; Yu and Wright, 2001). At that time a drier and colder climate might have been responsible for loess deposition on Long Island.