Element Accumulation Rates

On the basis of the bulk sediment accumulation rates and the measured C, N, and P contents in the sediment cores, the accumulation rates for the single elements organic carbon, total nitrogen, and total phosphorus were calculated. 

The basinwide mean values for organic carbon, total nitrogen, and total phosphorus (expressed in g/(m year)) are 14.8 13.7, 1.60 1.58, and 0.20 0.18, respectively. Because the element concentrations (e.g., TOC, Fig. 14.10 top left) are much less variable than the bulk sediment accumulation rates (Fig. 14.8), the spatial distribution pattern of element accumulation rates mainly reflects the total mass accumulation rates. 

FIGURE 14.9 Maximum bottom velocities of the Eastern Gotland Basin as derived from a 3D model for the model year 1980-1981 (M. Schmidt, lOW, personnel communication). 

The general spatial pattern of mean element accumulation rates is dominated by the very low rates in the southern part. 

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The insensitivity of the sediment record of deep lakes to changes in trace element supply, does not mean that the trace element concentration profile in the sediment cannot show sharp changes. A change in the particle deposition rate will cause an instantaneous change in sediment composition through dilution. However, for deep lakes, particularly for low Kd values, this dilution effect is reduced by the high water column trace element inventory (Fig, 8b). This means that in deep lakes it is important not to infer external trace element supply simply using sediment trace element accumulation rates. 

If neither convection nor chemical reaction occurs within the element, the rate of accumulation of diffusing molecules is then equal to the net contribution by diffusion ... 

Net accumulation rate in element due to flow in axial direction ... 

In their studies of metals in Chesapeake Bay, however, Bieri et al. (1982) claim that more than 60 % of both the Pb and Mn input is retained in the bed sediments. In their recent studies of heavy metals in Delaware Bay (USA), Church, Tramontano and Murray (1984 and later personal communication) calculated retention of 92 % of the Mn, 37% of the Cu and 32 % of the Cd input to that estuary. However, losses from the estuary in that analysis were based on calculations of the probable flux out of the mouth of the Bay using a layered flow model. When sediment concentrations and accumulation rates were used, only small amounts of Mn and Cd appeared to be retained in the system (Church, personal communication). At this point we are not aware of any convincing evidence that clearly contradicts the findings regarding the behavior of Pb, Cu,Mn or Cd in Narragansett Bay. Unfortunately, the number of mass balances for these elements is so small that this is not a particularly reassuring claim. 

Each radionuclide of eth type is characterized by its half-life r , the rate H of input flow to water area Q, the accumulation rate Ha in living organisms a(pA,BA,Z,F,L), and the removal rate HeD with dead elements of the ecosystem. As a result, the concentration Qe of radionuclide e in is described by the... 

Assuming that particle-reactive radionuclides and many other trace elements are scavenged by particles settling through the water column, and that these fluxes of sediment and the associated radionuclide have remained constant over time, an accumulation rate (A) can be calculated using the following equation ... 

There are characteristic chemical properties of cosmic dust that have been involved in the study of sediment accumulation rates. The platinum group elements, such as iridium and osmium, offer good examples. Attempts to use iridium in this way have had the important result of indicating a giant meteorite impact at the Cretaceous-Tertiary boundary (Alvarez et ai, 1980) but it has not been proven important in determining chronometry. 

There have been two attempts to use elemental measurements to determine accumulation rates of marine deposits independent of the cosmic dust He approach or the tacitly assumed constant-flux model used in the h and Pa approaches. One depends on a trace metal that tracks the finegrained (clay) fraction of deep-sea sediments, the other on the addition of a hydrogenous element to the accumulating marine deposit. 

Elemental ratios versus accumulation rates— In most sections of the geological record, timescales are insufficiently resolved to calculate sedimentation rates at a scale capable of yielding meaningful accumulation rate data. In recent years, however, orbital timescales have been developed for this purpose in some ancient stratigraphic sequences (Park and Herbert, 1987 Meyers et al., 2001). In this paper, the presentation of accumulation rates with the... 

Zwolsman, J.J.G., Berger, G.W. Van Eck, G.T.M. (1993) Sediment accumulation rates, historical input, postdepositioned mobility and retention of major elements and trace metals in salt marsh sediments of the Scheldt estuary, SW Netherlands. Marine Chemistry 44, 73-94. 

The role of the benthic interface is not to be defined exclusively by one transport direction. On the one hand, deposition and burial of marine sediments remove elements from the marine cycles over geological time periods. Yet, as the example of the carbon cycle shown above already demonstrated, an immense proportion of accumulated particles are subject to dissolution or microbial decomposition in the course of early diagenesis (cf. Chapter 9). Marine sediments therefore also act as a secondary source of remineralized dissolved components. The coexistence of these two fundamental, but oppositely directed mass movements constitutes one of the most essential phenomena at the seafloor. Next to studies on particle fluxes through the water column and element-specific accumulation rates, the quantification of benthic flux rates across the sediment/ water boundary represents the third pre-condition for obtaining a complete balance of the marine material cycles. 

In every equation of change, one of the terms on the left side represents the accumulation rate process based on a stationary volume element. Hence, one seeks an expression for the accumulation of kinetic energy per unit volume of fluid in a stationary control volume ... 

There is, however, an additional dimension to the problem of accumulation rates. The interpretation of two specific types of component is very sensitive to mass accumulation rate. First, elements that are transported to the lake in a soluble form, and only partially captured by the lake, have concentrations which can be highly sensitive to the sediment accumulation rate. Second, any component for which the supply rate is completely independent of catchment particle supply rates, is sensitive to variable dilution. For many atmospherically supplied trace elements both of these situations apply. The model described below can be used to evaluate these effects. 

If an element that is independent of the dominant panicle supply (i.e., in solution from catchment, or from the atmosphere) is captured efficiently by the lake system, then variation in the particle mass accumulation rate will cause an inverse variation in its sediment... 

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