Modeling sediment accumulation rates

Solution of equation (10) which involves sedimentation in the presence of mixing and that of equation (11) which contains the sedimentation term only, are exponential in nature. The major conclusion which arises from this is that the logarithmic nature of the activity-depth profiles by itself is not a guarantee for undisturbed particle by particle sediment accumulation, as has often been assumed. The effects of mixing and sedimentation on the radionuclide distribution in the sediment column have to be resolved to obtain pertinent information on the sediment accumulation rates. (It is pertinent to mention here that recently Guinasso and Schink [65] have developed a detailed mathematical model to calculate the depth profiles of a non-radioactive transient tracer pulse deposited on the sediment surface. Their model is yet to be applied in detail for radionuclides. )... 

We hypothesize that the development of nonuniform distribution of both linear and sediment accumulation rates is related to the course of the rim current at the Gotland Deep. Empirical measLuements of near-bottom currents are rare (e.g., Hagen and Feistel, 2001 Hagen and Feistel, 2004). Hence, near-bottom velocities were derived from a 3D model (Fig. 14.9 Schmidt, personnel communication). 

In the paleo-oceanographic context, constant flux tracers are valuable tools because they enable the reconstruction of particulate fluxes. One of the advantages of this approach is that it allows us to establish mass accumulation rates independently from single-point age models (e g., O). These models frequently are biased and sensitive to sediment redistribution effects. For example, this holds true for paleoceanographic studies in the Quaternary, where many interpretations of the sediment record rely on potentially erroneous sediment accumulation rates derived from 5 0 stratigraphy. 

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. 

PCBs can move from local sediments into the avian food web, as judged by PCB accumulation rates of tree swallows (Tachycineta bicolor) from contaminated and reference sites (Custer et al. 1998). Patterns of relative concentrations of PCB congeners change from sediment to invertebrates, and from tree swallow eggs to nestlings (Froese et al. 1998). Dioxin-like activity (TEF) measured in tree swallow tissues could predict TEF in sediments and the reverse. Models of dioxin-like activity in the sediments of Saginaw Bay, Michigan, predicted that sediments were not harmful to tree swallows from that area (Froese et al. 1998). 

Other Applications of the Multiple-Core Approach. The bulk of this chapter has dealt with the specific application of multiple-core methodology to questions of atmospheric Hg deposition. Whole-basin Hg accumulation rates for seven lakes, calculated from multiple sediment cores, were used in a simple mass-balance model to estimate atmospheric fluxes and Hg transport from catchment soils. This approach can be used to answer other limnological questions, and the model is not restricted to Hg or atmospheric deposition. 

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. 

Figure 7 Accumulation rates of total mass, opal, and the eolian component (continental dust) of North Pacific sediments recovered at ODP Sites 885/886. Accumulation rates derived by (a) Rea et al. (1998) using an age model based on biostratigraphic and paleomagnetic age control points and (b) Higgins et al. (submitted) by...

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. 

In natural lacustrine and slowly-accumulating reservoir sediments, core dating with the isotope °Pb has been used extensively (Schell and Earner, 1986). Appleby and Oldfield (1983) found that the constant rate of °Pb supply model (CRS) provides a reasonably accurate sedimentation chronology. The basic assumption of the CRS model is that the rate of supply of excess °Pb to the lake is constant. This model, thus, assumes that the erosive processes in the catchment are steady and give rise to a constant rate of sediment accumulation (MAR) (Appleby and Oldfield, 1983). In practice, for reservoirs, this assumption is rarely met because, for example, an increase in the MAR caused by land disturbances, such as those associated with the urban development, transports additional surficial soils and sediments to the lake. This additional erosion increases the MAR and also increases the rate of supply of °Pb to the lake. In general, because excess °Pb is an atmospheric fallout radionuclide, the model works better in low sedimentation rate, atmospherically dominated lakes with undisturbed watersheds, than in high sedimentation rate, fluvially dominated urban lakes and reservoirs. 

The excess activity as a function of depth in the top 10 m of a sediment core from the Caribbean Sea. Different sedimentation rates are indicated for a model that assumes a continuous and constant sediment accumulation (see text). Redrawn from Ku eta/. (1972). 

Figure CO2 consumption rate (Q) during the hydrate accumulation for different model sediments (Wi 10%) l- sand, 2- sand with 7% of kaolinite clay, 3- sand with 7 Vo of montmorillonite clay...

Fig. 12.20 Results of a steady-state simulation with a coupled model for ocean circulation, water chemistry and sediment diagenesis. Major control parameters and forcings comprise a large-scale geostrophic flow field, primary productivity controlled by nutrient advection, export production and sediment accumulation, as well as CO input by weathering and CO -exchange with the atmosphere, a) Export production (mol m yr ), b) CaCO export production (both mol m yr ), c) wt% CaCOj, d) CaCO mass accumulation rate (g cm kyr ) (from Archer et al. 1998).

Use the simple model approach from Figure 12.12 and the general values given (5 = 85%, cp = 2,5 g cm= 500 yr) to calculate the degree of organic carbon preservation when accumulation rates are 1 and 0,1 (mg cm yr ) for NRP and, respectively, and a reactive mixed layer of 10 cm thickness. Please consider that the degradation of organic matter has an effect on the sedimentation rate (co). 

Depletion of dissolved SOq in marine sediments is displayed over widely varying depth ranges, depending mainly on the interplay of rates of microbial metabolism, sediment accumulation and diffusive sulfate replenishment. Sulfate concentration gradients can be used with sedimentation rates, diffusion coefficients and model assumptions to estimate rates or rate constants for microbial sulfate reduction in sediments (Toth and Lerman, 1977 Berner, 1978 Canfield, 1991). There is a general proportionality between rates of sediment accumulation and sulfate reduction, but this relation is only weakly predictive for specific sites because of the influence of other factors on sulfate reduction rates (e.g. organic matter composition, microbial population density, etc.). 

In water, with a viscosity approximately 50 times that of air, mineral particles similar to those above would have sedimentation rates on the order of 1.6 m day and 23 m min respectively. Such calculations (estimations, really) are important for modeling problems of sediment accumulation in dammed reservoirs, for example. 

The result of these modern soil conservation measures was to greatly curtail soil erosion. While a highly imperfect model, use of the Universal Soil Erosion Equation suggests that the erosion rates of 1975 had been reduced to about one-fourth those of 1934 (Trimble Lund, 1982). However, the measured rates of downstream sediment accumulation were only about 6% of the earlier rates (Trimble, 1999) The disparity between erosion rates from the uplands and the sedimentation rates in valleys is explained by sediment entrained from tributaries and also from errors of estimation of erosion and measurement of sediment deposits. 

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