Fine-scale strata formation in biologically and physically dominated estuarine systems within the lower Chesapeake and York River subestuary.

摘要

To investigate the relationship between biological and physical mixing in forming strata, the lower mainstem of Chesapeake Bay has been contrasted with the York River Subestuary. By using radioisotope profiles from sediment cores, comparisons are made in terms of depth and rate of sediment mixing, deposition and accretion. Within the lower Chesapeake Bay two sites were selected as biologically dominated, both are located within the bay stem plains and are characterized by muddy sand and an abundance of large, deep-dwelling organisms. X-radiographs indicate complete biological reworking of sediments. 210Pb profiles reveal low sediment accretion rates within the mainstem sites (0.1 cm y−1), but significant differences in biological mixing depths (25 vs 40 cm) and biodiffusivity (>80 vs 6–30 cm3 y−1).; Within the upper York River, transient, longitudinal erosional furrows regularly form within a broad flat secondary channel. Varying furrow morphologies were observed depending on tidal flow, ranging from: (1) no bedforms during the higher flow conditions such as spring tide; to (2) large patches of meandering furrows as the mean flow decreases; to (3) large, variably spaced (5–7 m) linear furrows during the lowest mean current conditions of neap tide. A 35 month time series using kasten cores reveals that along with $\sim$25 cm differences in mixing depths due to the fortnightly time formation and destruction of furrows, a $\sim$100 cm depth scale signal of mixing exists annual to interannual time frame which is unrelated to the formation of erosional furrows.; Throughout much of the energetic microtidal York River, the seabed is characterized by deep physical mixing (25–200 cm). A strong cross-estuary gradient is observed with one side, including channel, flank and shoal, dominated by frequent deep erosion and re-deposition (physical mixing), while physical mixing is reduced on the other side resulting in a greater preservation of biological mixing. Within the physically dominated side of the river, the mixed layer is characterized by 210Pb profiles with one or more segments ($\sim$25–100 cm thick) of nearly uniform excess activity. X-radiographs reveal that the mixed layer consists of centimeter to decimeter scale units of finely to coarsely laminated strata bounded by hiatal surfaces, indicative of physical mixing. The physical mixing results in an impoverishment benthic community which is composed primarily of small, opportunistic species. Mixing in the biologically dominated side of the river is generally shallow (40 cm), with low 210Pb biodiffusion rates (0.43–3.35 cm 2 y−1).; 210Pb based particle residence time within the mixed layer are on the order of centuries. Estimates of the sediment mass in the physically mixed layer is equivalent to $\sim$70 years of river sediment yield, this is consistent with century-scale residence times.; Although sediment mixing within the Lower Chesapeake Bay is controlled by biological processes and sediment mixing in much of the York River is controlled by physical processes, in both places particle residence times in the seabed are generally on the century time-frame. However, when considering the cycling of pore-water nutrients, organic matter and particle bound contaminants, the type of seabed mixing is as important as the particle residence time in determining the ultimate fate and fluxes of these constituents.