Vertical profiles sedimentation rate

Figure 10 Vertical profiles of methylation rates (al, a2) and sulfide/oxygen concentrations (bl, hi) in the sediments of sandy (1) and muddy (2) sites in a salt marsh (Barn Island, Connecticut, USA). Maximum methylation occurs in the top 2 cm of these sediments and is coincident with the redox transition zone. Note also that the rates are an order of magnitude faster in the sandy sediments (source Langer et al., 2001).

To illustrate the behavior of the cylinder model and also to demonstrate how irrigated burrows can be expected to influence Mn " profiles, a representative vertical profile predicted by Eqs. (6.14) and (6.15) for Mn " has been plotted for the case A i = 0 (Fig. 19). The production rate for Mn ", r, rz, and L are those for core NWC-4 based on the solid-phase dissolution rate of Section 6.4.1 (Table V divided by average porosity 0.750) and the cylinder-model values of Table V in Part I. The value of D is estimated from the molecular diffusion coefficient at infinite dilution T = I9°C (Li and Gregory, 1974), multiplied by a correction factor for sediment structure of 0.56. This factor was approximated by

[Pg.392]

There are strong nitrification and denitrification in Zhujiang River Estuary sediments and the average nitrification, denitrification, and nitrate reduction rates ranged from 0.32 to 2.43 imnol/(m h), 0.03 to 0.84 mmol/(m h), and 4.17 to 13.06 mmol/(m -h), respectively. The vertical profiles of the sediments showed that the nitrification and denitrification processes mainly took place in the depth from 0 to 4 cm and there were differences at different sampling sites. The rates of nitrification, denitrification, and nitrate reduction were dominated by Eh, nitrate, and ammonium concentrations in sediment and DO in overlying water (Xu et ah, 2005). 

We evaluated the ages of every layer by virtue of the depth and sediment rate. From bottom to surface, the core sample was deposited from 1914 to 2004 with about 6 years interval at every layer. Fig. 3.74 showed the vertical profile of EPAHs varying with depth and time. In general, EPAHs decreased with depth... 

Reaction rates of nonconservative chemicals in marine sediments can be estimated from porewater concentration profiles using a mathematical model similar to the onedimensional advection-diffusion model for the water column presented in Section 4.3.4. As with the water column, horizontal concentration gradients are assumed to be negligible as compared to the vertical gradients. In contrast to the water column, solute transport in the pore waters is controlled by molecular diffusion and advection, with the effects of turbulent mixing being negligible. 

Even if rate measurements in sediments are made using whole core incubations, e.g., when the inhibitor is a gas, it is still difficult to obtain a depth distribution of the rate (usually, an areal rate is obtained). A sophisticated measurement and model based system that avoids direct rate measurements has been used to overcome this problem. Microelectrodes, which have very high vertical resolution, are used to measure the fine scale distribution of oxygen and NOs" in freshwater sediments. By assuming that the observed vertical gradients represent a steady state condition, reaction-diffusion models can then be used to estimate the rates of nitrification, denitrification and aerobic respiration and to compute the location of the rate processes in relation to the chemical profiles (e.g., Binnerup et ai, 1992 Jensen et ai, 1994 Meyer et ai, 2001 Rysgaard et ai, 1994). Recent advances and details of the microelectrode approach can be found in the Chapter by Joye and Anderson (this volume). 

In comparing the biomarker signatures of the Upper Marsh and Lower Marsh, the most distinguishing feature is the different rate of decline of total viable microbial biomass with depth in the upper part of the profiles. In the Upper Marsh an order of magnitude decrease in the viable biomass occurred in the upper 20cm while in the Lower Marsh profile a similar order of magnitude decrease occurred over a vertical distance of c. 40 cm. This difference reflects the varying rates of sediment accretion on the two marshes. 

During the winter, Mn concentrations continue to decrease in the top few centimeters compared to fall profiles. Maximum concentrations occur deeper in the sediment column and the vertical extent of concentration peaks can broaden or Mn increase well below the interface (Figs. 5-7). These changes are consistent with a continued seasonal decrease in the production rates of Mn, as well as a decrease in consumption rates by precipitation with anaerobic metabolites or biogenic transport at depth in the sediment. Both changes are caused by the low winter temperatures (2-4°C) and the associated decrease in biological activity. 

Fig. 19. Comparison of the one- and two-dimensional models for Mn distribution in the top 0-18 cm of sediment at NWC. The production rate in both cases is that found for core NWC-4. The anoxic precipitation rate is assumed to be zero. The effective cylinder geometry used in the two-dimensional model is that determined for NH4 in Part I r, = 0.14 cm, rj = 4.5 cm. The basal gradient is constrained to be zero. The diffusion geometry created by irrigated burrows results in a vertical pore-water solute profile exhibiting apparent precipitation.

Fig. 12.4 Effects of the depth resolution in pore water concentration profiles on calculating the rates of diffusive transport. Three samples drawn from surface sediments are shown to possess different resolutions (intervals 0.5 cm - dots, 1.0 cm diamonds, 2.0 cm - squares). All values are sufficient to plot the idealized concentration profile within the hounds of analytical error, yet very different flux rates are calculated in dependence on the depth resolution values. In the demonstrated example, the smallest sample distance indicates the highest diffusion (2.98 mmol cmA f ). As soon as the vertical distance between single values increases, or, when the sediment segments under study grows in thickness, the calculated export across the sediment-water boundary diminishes (2.34-t.64mmol cm yr ). In our example, this error which is due to the coarse depth resolution can be reduced by applying a mathematical Fit-function. A truncation of 0.05 cm yields a flux rate of 2.84 mmol cm yr. (The indicated values were calculated under the assumption of the presented porosity profile according to Pick s first law of diffusion - see Chapter 3. A diffusion coefficient of 1 cmA f was assumed. Adaptation to the resolution interval of 2.0 cm was accomplished by using a simple exponential equation).

Sediment accumulation determined by various profile analyses is calculated using sedimentation and bulk density of the wetland soil profile. The flux rate is calculated from the vertical accretion rate (R) and the bulk concentration of the material (Q) using the expression... 

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