Sediment depth profiles

Anthropogenic inputs to intertidal environments are often direct, through point-source waste disposal, but they are also indirect, from riverine, marine and/or atmospheric sources. Trace metals are partitioned between each component of the intertidal sediment-water system they are found in solution ( bulk water or interstitial water) and associated with suspended and deposited sediments. This chapter is concerned with the biogeochemistry of trace metals in deposited intertidal sediments. Two main sections follow in the first, an overview of surface sediments and sediment depth profiles is presented, and in the second, a case study is given of the historic record of Zn from saltmarsh sediments in the Severn Estuary, UK. 

Sediment depth profiles as records of intertidal pollution history... 

Finally, a recent approach to the study of trace metal distribution in sediment depth profiles deserves mention. This is a factor-analysis technique which is used to determine the main environmental condition prevailing at the place and time when the sediment was deposited, or the main process responsible for modification of the sediment after deposition (Buckley et al, 1995). The study in Halifax Harbour, Nova Scotia, (Buckley et al, 1995), established the following groups ... 

Trace metal concentrations in intertidal sediment depth profiles will no doubt continue to provide useful historic records of pollution in the future. In particular, dated profiles that take into account sediment characteristics and diagenetic processes (where appropriate) are of value. Future studies will perhaps be more process-oriented, e.g. by combining pore-water analysis with solid phase data and through a detailed investigation of the nature and significance of the organic matter present. 

The record of zinc pollution in a sediment depth profile at Tites Point, Severn Estuary, UK a case study... 

Early work on trace metals in sediment depth profiles in the Severn Estuary provided an overview of trace metal concentrations in marsh sediments, in order to establish a chemostratigraphy (Allen, 1987c Allen Rae, 1987 Allen, 1988). Later work (Allen et al, 1990) provided a preliminary investigation of the post-depositional behaviour of Cu, Zn and Pb at one location (Tites Point). Subsequent intensive sampling of an adjacent profile has allowed a more detailed appreciation of trace metal concentrations and behaviour. 

Figure 8-2 shows the depth profiles of the saturation index omegadel), the solution rate, and the respiration rate. At the shallowest depths, the saturation index changes rapidly from its supersaturated value at the sediment-water interface, corresponding to seawater values of total dissolved carbon and alkalinity, to undersaturation in the top layer of sediment. Corresponding to this change in the saturation index is a rapid and unresolved variation in the dissolution rate. Calcium carbonate is precipitating... 

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. )... 

A quantitative and fairly easy method to obtain particle reworking rates in deep sea sediments became possible after the elegant work of Nozaki et al., [68] based on 210Pb distribution in them. The radioactive half-life of 210Pb is too short (22.6 yrs) to produce measurable depth profiles in deep sea sediments based on sedimentation alone since its activity would be limited to the top 1 mm layer. In such a case its depth profile predominantly records the effects of particle reworking and its distribution can be approximated as ... 

For stations B and C, where the LAS concentrations were higher than for A, the variation in total LAS concentration with sediment depth was determined by the homologues of 12 and 13 carbon atoms (Fig. 6.5.4). These homologues present a strong tendency to sorption and are readily biodegradable. In interstitial water, the vertical profile of the LAS concentration is similar to that observed for the sediment, particularly at stations B and C. The homologue-specific partition coefficient did not vary much with depth, because there is no appreciable variation in the composition of the sediment with depth [34]. 

As shown in Table 11.3, the concentrations of trace elements in the water column is - despite anthropogenic pollution - extremely small (10 11 - 10 7 M) illustrating the remarkable efficiency of the continuous "conveyor belt" of the settling adsorbing and scavenging particles. The sedimentary record reflects the accumulation of trace elements in sediments and a profile of concentration vs sediment-depth (or age) gives a "memory record" on the loading in the past (Fig. 11.9). 

Figure 16. Depth profiles from three ODP Sites, showing Li isotopic composition variations in pore waters (open symbols) and associated sediments (filled symbols), (a) Site 918, Irminger Basin, north Atlantic (Zhang et al. 1998) (b) Site 1038, Escanaba Trough, northeastern Pacific (James et al. 1999) (c) site 1039, Middle American Trench off of Costa Rica (Chan and Kastner 2000). The average composition of seawater is noted on each profile with dashed line (note different scales). Whereas sediments have relatively monotonous compositions, pore waters have compositions reflecting different origins and processes in each site. Interpretations of the data are summarized in the text under, Marine pore fluid-mineral processes. ...

The results of concentration measurements are presented as vertical profiles similar to those for the water column, with the vertical axis representing increasing depth below the sediment-water interfece. Depth profiles of concentrations can be used to illustrate downcore variations in the chemical composition of pore waters or in the solid particles. Dissolved concentrations are typically reported in units of moles of solute per liter of pore water. Solid concentrations are reported in mass/mass units, such as grams of carbon per 100 grams of dry sediment (%C) or mg of manganese per kg of dry sediment (ppm Mn). 

In the surface sediments, the physical processes of compaction, bioirrigation, and bioturbation also influence downcore concentration profiles. The concentration minima and maxima tend to be broadened by the action of benthic animals, whereas compaction has a sharpening effect on the depth profiles of the solids. 

Idealized depth profiles of redox species in (a) open-ocean sediments (water depth >1000m) and (b) coastal sediments (water depth <1000 m). 

By coupling flow field-flow fractionation (flow FFF) to ICP-MS it is possible to investigate trace metals bound to various size fractions of colloidal and particulate materials.55 This technique is employed for environmental applications,55-57 for example to study trace metals associated with sediments. FFF-ICP-MS is an ideal technique for obtaining information on particle size distribution and depth profiles in sediment cores in addition to the metal concentrations (e.g., of Cu, Fe, Mn, Pb, Sr, Ti and Zn with core depths ranging from 0-40 cm).55 Contaminated river sediments at various depths have been investigated by a combination of selective extraction and FFF-ICP-MS as described by Siripinyanond et al,55... 

Figure 28-1 Depth profile of nitrate in sediment from freshwater Lake Spbygard in Denmark. A similar profile was observed in saltwater sediment. Measurements were made with a biosensor containing live bacteria that convert N03 into N20, which was then measured amperometrically by reduction at a silver cathode. [From l h. Larsen, i Kjosr. and N. P. Revsbech. A Microscale NO Biosensor for Environmental Applications," Anal- Chem. 1997, <59.3527.]... 

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