Sediment accumulation rate

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. )... 

Relative primary productivity, POC fluxes at 105 and 3000 m, and POC sediment accumulation rates versus latitude in the central equatorial Pacific Ocean. Data are normalized to the maximum value in each transect. Survey 1 was conducted during February-March 1992 under El Nino conditions and Survey 2 from August to September 1992 under non-El Nino conditions at longitudes ranging from 135 to 140°W. Ordinate scale is reset to 1.0 at each maximum, and the absolute magnitude (mmolCm ij-i) of each parameter is given next to its maximum. Source-. From Flernes, P. J., et al. (2001). Deep-Sea Research I 48, 1999-2023. 

Mean sediment-accumulation rates except Cedar and Mountain, for which preindustrial rates are shown. Dating parameters for Little Rock were provided by seven cores. 

For Cedar and Mountain lakes, detailed sediment cores collected from the deeper regions of each basin showed increasing sedimentation rates up-core. Thus a slightly different approach was used to provide additional temporal detail from the coarsely sectioned cores. These subsequent cores were extruded into five intervals 5-20 cm long so that dates and sediment accumulation rates could be explicitly calculated for the deeper strata. 

Dating and Sediment Accumulation. Stratigraphic Patterns Lead-210 profiles from profundal cores from each study lake show conformable declines in unsupported activity to an asymptote of supported 210Pb typically below 40-60 cm deep (Figure 3). Supported activity, attained at shallower depths in Thrush (28 cm) and Kjostad (36 cm), indicates slower linear rates of sedimentation at these sites. The activity profiles for several lakes, most notably Thrush and Kjostad, are almost perfectly exponential and thus indicate nearly constant sediment accumulation rates. Others, such as Cedar and Mountain, show flat spots and kinks that probably represent shifts in sediment flux. 

These results indicate that our use of a single average sediment-accumulation rate for the cores from Dunnigan, Meander, Thrush, Kjostad,... 

Figure 4. Plots of sediment-accumulation rate versus age for the cores in Figure 3. Error bars represent one standard deviation propagated from counting uncertainty. The apparent increase in recent sediment accumulation in Little Rock Lake is atypical of other cores from this site.

Problematic Cores. Two cores from Meander Lake exhibit sediment accumulation rates considerably higher than the basin average (cores at 4.63 and 5.77 m). The core at 4.63 m is highly inorganic (5% organic matter) and... 

Figure 7. Plots of Hg concentration versus 210Pb age for deep-water cores. Dates older than about 1800 are extrapolations based on mean dry-mass sediment accumulation rates.

In most of the study lakes Hg concentrations are spatially less variable than sediment accumulation rates, although the large range of modern Hg values for Thrush Lake is a notable exception. Although there are no obvious... 

Lakewide Hg Accumulation. Mercury accumulation rates can be calculated for individual core sites as the product of the 210Pb-based sediment accumulation rate and Hg concentration in different strata. If enough cores are analyzed, whole-lake Hg inputs can be calculated by weighting the Hg flux of each core by the portion of the depositional basin it represents. In this study we calculate Hg loading for each lake on an areal basis for two time-stratigraphic units—modem (roughly the last decade) and preindustrial (before 1850)—according to eq 2 ... 

Various workers have assembled relatively complete assessments of the annual input of the major biological nutrients (C, N, P), certain heavy metals (Mn, Cd, Pb, Cu), and petroleum hydrocarbons to Narragansett Bay. Other studies have developed inventories of the amounts of these materials in the sediments of the Bay. We have brought these data together with information on sediment accumulation rates in the Bay to determine the degree to which this one estuary serves as a sink for different types of materials in their passage between land and the coastal ocean. 

Figure 2.10 Sediments accumulation rates (cm y 1) on Amazon River shelf. (From Kuehl et al., 1986, with permission.)...

Assuming that the transport (e.g., physical mixing and bioturbation), depositional inputs (e.g., sources/quality and amounts of POM), mass sediment accumulation rate, temperature, and decomposition are in steady state, as well as lateral homogeneity of the deposit, the concentration of POM will not change over time. Although these conditions are almost never met in estuarine systems (Berner, 1980), steady-state conditions will be used here for the general purposes of this discussion. Thus, assuming steady-state conditions, the GDE for POM decay is as follows (Rice and Rhoads, 1989) ... 

The preservation of organic matter in coastal and estuarine sediments is believed to be principally controlled by productivity, sedimentation accumulation rate, bottom water and sediment redox conditions, and sorption as a function of specific surface area of sediments (see review, Hedges and Keil, 1995). In this section the focus will be primarily on factors controlling preservation or organic matter in estuarine sediments the use of chemical biomarkers for historical paleo-reconstruction of past estuarine environments is discussed in chapter 15. 

The first evidence for enhanced preservation with high net sediment accumulation rates in oceanic environments was presented in the context of organic burial efficiency—to... 

The preservation of organic matter in coastal and estuarine sediments is believed to be principally controlled by productivity, sedimentation accumulation rate, bottom water and sediment redox conditions, and sorption as a function of specific surface area of sediments. 

A summary of physical and biochemical parameters for the Amazon shelf are presented in Table 17.1. These data were used to develop the mass balance calculations described in the remainder of the paper. The rate of Amazon River sediment discharge that is cited in most studies (Meade et al. 1985) characterizes the flux at Obidos, which is hundreds of kilometers from the river mouth. The sediment fluxes that have been documented for the Amazon coastal area are the fine-grained sediment accumulation rate on the shelf (6.3 x 10 g yr, Kuehl et al. 1986, 1996) and the advection rate of particulate material northward out of the study area, primarily in nearshore environments (-1.2 X 10 g yr, Allison et al. 1995). Because some sediment may be deposited between Obidos and the river mouth, we have chosen to use the sum of the shelf... 

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