Estuarine Processes and Their Stratigraphic Record: Paleosalinity and Sedimentation Changes in the Hudson Estuary (North America)
Paleosalinity estimates and rates of sedimentation inferred from core samples from the Hudson estuary for the interval between 6.4 and 1.3 ka indicate a possible role for the estuarine turbidity maximum (ETM) in influencing patterns of estuarine sedimentation at centennial to millennial time scales. Currently in the estuary, sedimentation is localized via sediment trapping particularly in the vicinity of the ETM, 13–26 km upstream from Battery Park (FBP) at the southern tip of Manhattan, in water depths greater than 4 m, and on the western side of the estuary. Data presented in this paper are from cores located within the segment of the estuary 29–50 km FBP. Age constraints are provided by C-14 dating. Paleoenvironmental interpretations are based upon paleosalinity estimates, grain size variability, and sedimentary structures.
Paleosalinity was inferred on the basis of foraminiferal biofacies analysis and a new method for estimating summertime paleosalinity using oxygen isotope measurements in bivalve shell material. The isotopic analysis of a narrow size fraction (1.0–1.7 mm) representing summer growth of a single bivalve species (Gemma gemma) reduces the uncertainty related to annual changes in temperature. Data from ∼45 km FBP indicate a gradual decrease in summertime paleosalinity between 6.4 and 2.0 ka from 25–20‰ to 15–10‰ (the latter is similar to present-day values). These results are consistent with the conclusion of an earlier low-resolution study.
Sedimentation rates are generally low and are similar to the rate of sea-level rise in the Hudson River. Lowest sedimentation rates are noted in short (lower than 2 m) cores from north of the Tappan Zee Bridge (40–50 km FBP from 2.4 ka to present); in shallow water (∼2 m at mean low water, core SD-11) ∼45 km FBP; and on the eastern side of the estuary from ∼50 to 29 km FBP. Exceptions are high sedimentation rates (up to four times background) observed in cores from the western flats (SD 30, ∼45 km FBP, 4.9 to 3.4 ka) in water depths of 4 m and from the western part of the main channel (P21.7 core, ∼32 km FBP, greater than 2.3 to ∼1.3 ka).
We hypothesize that the observed pattern in sediment accumulation relates to a location for the ETM some 20 km upstream of its present position at 3 ka. Downstream migration of the ETM since 3 ka is ascribed to shoaling of the estuary, effectively squeezing the marine saltwater wedge in the same direction, and off marginal flats into the channel. Such shoaling would have enhanced the role of waves in mixing marine and fresher surface water, and reduced the effect of the ETM in focusing sediment accumulation. The results of this study are consistent with the idea that at any time, estuarine sedimentation is highly localized, suggesting a more complex depositional pattern than previously indicated in estuarine stratigraphic models.
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- Marine Geology