Environmental change and oyster colonization within the Hudson River estuary linked to Holocene climate

Geophysical mapping and sampling data provide a record of changing environmental and faunal conditions within the Hudson River estuary during the midto late Holocene. On the shallow, broad marginal flats of the mesohaline Hudson, fossil oyster beds (Crassostrea virginica) are found exposed on the river bottom and buried by sediment. The shallowest beds are well imaged in chirp sub-bottom and side-scan sonar data and form discrete flow-perpendicular bands, 0.6– 1.0 km wide and up to 3 km long, which cover 30% of the river bottom. Radiocarbon-dated sediment cores indicate oysters thrived within two time periods from 500–2,400 and 5,600–6,100 cal. years B.P. Sediment and physical property data indicate a changing depositional regime consistent with the oyster chronology. Similar changes in oyster presence are found in local shell midden sites of the Lower Hudson Valley as well as elsewhere along the Atlantic coast, and may reflect climatic controls associated with warm–cool cycles during the Holocene. Oysters flourished during the mid-Holocene warm period, disappeared with the onset of cooler climate at 4,000–5,000 cal. years B.P., and returned during warmer conditions of the late Holocene. The most recent demise of oysters within the Hudson at 500– 900 cal. years B.P. may have accompanied the Little Ice Age. Introduction Humans have long settled estuary banks where food resources are abundant and accessible routes for transportation and trade are present. As estuaries emerged as favored sites for urban development and industrialization within the past two centuries, widespread degradation of estuarine ecosystems occurred. Although the modern collapse of coastal ecosystems is widely attributed to anthropogenic factors (e.g., Jackson et al. 2001), these ecosystems are also vulnerable to environmental transformation associated with sea-level rise and climate change (Walther et al. 2002). Within the Hudson River estuary, oysters were once an abundant estuarine resource, as evidenced by the presence of numerous prehistoric oyster shell middens (mounds of discarded shells) left along the shores of the Hudson by indigenous peoples. The earliest record of marine shellfishing anywhere along the western Atlantic coast is at Dogan Point on the eastern Hudson shore south of Verplanck, where oyster harvesting began 6,000 cal. years B.P. (e.g., Brennan 1974; Claassen 1995, Fig. 1). The Dogan Point shell middens contain two distinct shell layers. At the base of the middens are ‘‘giant’’ oyster (GO) shells 10–13 cm long, dating to 5,100–5,900 cal. years B.P. (Brennan 1974; Little 1995; see Radiocarbon age dating section below). Above this horizon is a younger, small oyster (SO) horizon with 7– 8 cm valve lengths. Shells from the SO horizon form two age groups (4,000–5,100 and 1,500–1,800 cal. years B.P.) separated by a distinct hiatus of roughly 2,000 years. Archeologists have debated whether the oyster hiatus revealed in the shell matrix sites of the lower Hudson reflects change in the estuarine environment or anthropogenic factors such as over-exploitation, a change in dietary preferences or migration patterns of the resident indigenous peoples (e.g., Schuldenrien 1995; Claasen,1995). In this study we present evidence from new geophysical and sampling data for fossil oyster beds within S. M. Carbotte (&) Æ R. E. Bell Æ W. B. F. Ryan Æ C. McHugh A. Slagle Æ F. Nitsche Æ J. Rubenstone Lamont-Doherty Earth Observatory of Columbia University, Box 1000, Palisades, NY, 10964, USA E-mail: [email protected] Tel.: +1-845-3658895 Fax: +1-845-3653181 C. McHugh Queens College, CUNY, Flushing, NY, 11367, USA Geo-Mar Lett (2004) 24: 212–224 DOI 10.1007/s00367-004-0179-9