SCALES OF INTRA-ANNUAL SEDIMENTATION IN THE HUDSON RIVER ESTUARY AS REVEALED BY SHORT-LIVED RADIOISOTOPES

A one-year sediment-coring time series was conducted in the turbidity maximum region of the Hudson River Estuary to document intra-annual deposition and erosion, part of an interdisciplinary project to examine relationships between estuarine hydrography and sedimentation. Undisturbed sediment cores were collected near instrumented bottom tripods at three-month intervals from October 2000 to October 2001 and analyzed for physical and geochemical properties. Deposition timescales were constrained by downcore profiles of short-lived radioisotopes Th-234 and Be-7 measured shortly after collection. To identify temporal changes in deposit thickness and accumulated mass, radioisotope inventories were computed and sediment properties measured (i.e., x-ray density, porosity structure, and grain size).

New mud deposits, 10-30 cm thick, were observed in October 2000 and June 2001 within a 10-15 sq km area of the estuary, a consequence of hydrographic trapping of suspended sediments derived from spring runoff. These "seasonal" deposits were composed of 2-20 mm thick beds of alternating homogenous clayey silt and laminated sandy silt bound by microbioturbated erosional and hiatal surfaces, altogether indicative of net accretion via depositional pulses and minor non-depositional/erosional episodes. Seasonal deposits exhibit near-uniform activity profiles of Th-234 and Be-7, suggestive of very rapid, if not episodic, deposition. Anomalously high activities of excess Th-234 reveal that suspended sediments are transported to the lower estuary during periods of high river flow and bathed in relatively saline waters, only to be advected up-estuary and sequestered in the turbidity maximum zone later in the year. Hydrographic data and continuous instrumentation observations of bed evolution made during the study period are in remarkable agreement with the sedimentology; acoustic altimetric time series show net accretion on decimeter length scales and fortnightly timescales forced by spring-tidal flows. Based on the sedimentary and instrumentation records, we hypothesize that the laminated beds are created through spring-tidal shear sorting and resedimentation of bed materials deposited locally during previous spring tides, not by spring-ebb cyclicity as suggested by some conceptual models of tidalite formation.