SEDIMENTARY FEATURES AND ACCUMULATION RATES FROM SEDIMENT CORES IN THE NEW YORK BIGHT

The Hudson Shelf Valley (HSV), the major topographic feature in the New York Bight, was carved into predominantly sandy continental-shelf sediments by the ancestral Hudson River during the last Pleistocene low sea-level stand. The submarine valley is now a depo-center for fine-grained sediment. In the past century, several areas at the head of the valley served as dump sites for sewage sludge and dredging spoils, resulting in the addition of fine-grained sediments and contaminants to the region. We collected more than 100 cores, 20-180 cm in length, in the HSV and on the surrounding shelf on four cruises from 1993-1998 in order to study sedimentation processes and contamination levels related to the dump sites. The cores were X-radiographed and selectively analyzed for grain size distribution, radionuclides (Pb-210 and Cs-137), and major and trace elements. Models of sediment accumulation and mixing were used to quantify sedimentary processes over the last 100 years; digital analysis of X-radiographs characterized finer scale events.

The sedimentation rates in the HSV region vary with location and time. Natural shelf sediments are dominated by sand (>65-70%); anthropogenic deposits are predominantly silt with some clay (silt+clay >60-70%). Modern deposits (post 1900) in the HSV are silty with preserved sand layers, 1-50mm thick, in the upper valley that may have been deposited by high energy transport during storm events. During the 1900s, accumulation rates of the silty sediment were higher toward the valley head (~1.3 cm/yr) than down valley (~0.1 cm/yr, 35 km away). The percentage of sand increases to natural values, and the fraction of anthropogenic sediment decreases, both down valley and down core. The cessation of dumping in 1987 was followed by an apparent decrease in sedimentation rates and increase in sediment reworking. Where sediment reworking rather than deposition is the dominant sedimentary process, buried contaminants may be remobilized.