LATE HOLOCENE SEDIMENT ACCUMULATION IN THE LOWER HUDSON ESTUARY; SPATIAL AND TEMPORAL VARIABILITY

Seven long (230-880 cm) vibracores have been recovered from two sub-tidal shoals in the lower Hudson estuary for the purpose of examining the late Holocene record of turbidity maximum processes. In order to assess the relative roles of natural and anthropogenic factors on deposition and erosion, both temporally and spatially, sedimentological measurements and geochronology (i.e. Cs-137 and AMS C-14) are being applied. All cores are composed of silty-clay with silt laminations that are tidal in origin; significant facies changes are not observed down-core. Preliminary Cs-137 analyses indicate sedimentation rates on the order of 1.5-2 cm/yr since the 1950's, nearly six times the rate of sea-level rise at the Battery. The presence of brick fragments to 285 cm core-depth supports this rate for the post-colonial period. Recent (i.e., post-1954) shoaling appears to have been more rapid from west to east at the northern site, while CHIRP sonar and core-data suggest that the modern channel is migrating westward at the southern site, thus eroding the toe of the shoal. The dredged channel appears to be experiencing the most rapid sediment accumulation (i.e. >2 cm/yr) with higher than average porosities down-core and little in the way of biological activity. The cores exhibit multiple erosive surfaces, which account for the disparity between rapid seasonal deposition and longer-term accumulation; the frequency and nature of erosion has yet to be determined. AMS C-14 dating and trace metal analyses, both currently in progress, will provide further time-constraints for sedimentation on longer timescales. These preliminary findings suggest that anthropogenic activities, namely dredging, have had a local impact on sedimentation in the lower Hudson estuary by disrupting the equilibrium surface.