Chesapeake pore waters

Burdige, D.J. (2001) Dissolved organic matter in Chesapeake Bay sediment pore waters. Qrg. Geochem. 32, 487-505. 

The solubility products and reactions used here as a guideline to saturation states are given in Table VI. The results of the calculations for phosphate compounds are plotted as -log lAP (lAP is the ion activity product) as a function of depth at each station (Figs. 49 and 50). Only data from box cores collected during 1975-1976 and some selected horizons from the gravity cores are shown. Hydroxyapatite was supersaturated by a factor of lO -lO at all stations and is not plotted precipitation of this phase is known to be kinetically hindered in seawater (Martens and Harriss, 1970). Bray (1973) and Norvell (1974) inferred likely equilibrium of pore waters with whitlockite [Ca3(P04)2] in Chesapeake Bay and anoxic lake sediments, respectively. Long Island Sound pore waters also tend to have activity products close to those predicted for saturation with respect to whitlockite, although distinct undersaturation is found for most sediment intervals at NWC. 

The forms of the pore-water and solid-phase profiles of Fe and Mn found in Long Island Sound (LIS) are similar in many basic features to those reported from other sedimentary basins, particularly the southern Chesapeake Bay (Bricker and Troup, 1975 Robbins and Callender, 1975 Holdren et al., 1975 Elderfield, 1976 Froelich et ai, 1979). The exact profile shape and depth distribution of metal concentrations from a given depositional environment reflect the depth-dependent transport-reaction processes occurring there. It was shown in Part I that in LIS, biogenic transport of particles and solutes differs from one area to another and that these differences cause significant variation in the way products of decomposition build up in the upper meter of sediment. These same transport processes can be expected to directly or indirectly influence the sedimentary distribution of Fe and Mn, depending on the type and rate... 

The objective of this section is to describe the major features of lanthanides in the water column of anoxic basins and briefly discuss the mechanisms responsible for spatial and temporal variations. Two examples will be used, the Black Sea and Chesapeake Bay. The next subsection on lanthanides in pore waters follows naturally in that many of the processes operating in anoxic water columns occur in marine sediments. 

A time-series study of Chesapeake Bay demonstrated that the dissolved lanthanides have a large seasonal cycle in both the water column and pore water in response to the development of anoxia in the spring and reoxygenation in the fall (Sholkovitz et al. 1992). Short-term variations in fractionation accompany the release and removal of lanthanides from the water column under seasonally-varying redox conditions. 

Large scale fractionation accompanies the seasonal cycle of the dissolved lanthanide concentrations in Chesapeake Bay. This is illustrated in fig. 37 which presents the time series of the Nd/Yb ratio for bottom waters (and upper pore waters). This light to heavy lanthanide ratio increases in the spring and is followed by an equally large decrease in the fall. At its maximum in the summer, the Nd/Yb ratio is three times greater than its winter baseline ratio. Hence, the lighter lanthanides are preferentially released in the spring and summer and then preferentially removed in the fall. 

As described in sect. 3, small sample volumes (usually 10-50 ml) and low concentrations make it extremely difficult to measure the lanthanide concentrations of pore waters. Just as lanthanide concentrations in the water column increase several-fold under conditions of low or no oxygen, lanthanides in pore waters respond to redox conditions. There are a small number of studies of lanthanides in pore waters and they focus on sub-oxic and anoxic environments of estuaries and coastal regions. These include Chesapeake Bay (Sholkovitz and Elderfield 1988, Sholkovitz et al. 1992) Buzzards Bay (Elderfield and Sholkovitz 1987, Sholkovitz et al. 1989) and Saanich Inlet (German and Elderfield 1989). The first published paper on lanthanides in pore waters is less than a decade old (Elderfield and Sholkovitz 1987). All the above studies used ID-TIMS as the method of analysis. Pore water data are compiled in table A13. 

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