Oxidants, estuarine sediments organic matter

Dankers and Laane [43] compared two methods, based on wet oxidation with potassium peroxydisulphate and loss on ignition at 560°C, for the determination of particulate organic carbon in estuarine suspended matter. They found that in estuarine sediments with a high clay content, loss on... 

The rate of organic matter input (i.e., sedimentation) and the availability of SOl control the rate of SO4 reduction in sediments. Sulfate is rarely limiting in marine systems except in brackish estuarine waters and with depth in sediments where SOl has been depleted. Sulfate reduction rates in sediments can span several orders of magnitude, but typical near-shore rates in the upper 5-10 cm of marine sediments are often 50-500 nmol cm d (Skyring, 1987). Rates in sediment with unusually rapid sedimentation rates can be reach 2,000 nmol ml d (Grill and Martens, 1987). Sulfate reduction in salt marshes and microbial mats can reach rates as high as 4,000 nmol ml d and 14,000 nmol ml -d respectively (Canheld and Des Marais, 1991 Hines et al., 1999). Anaerobic CH4 oxidation supports a large fraction of the SO4 reduction in some marine sediments (Table 6). 

In these examples as well as for most aquatic sediments, the principal diagenetic reactions that occur in these sediments are aerobic respiration and the reduction of Mn and Fe oxides. Under the slower sedimentation conditions in natural lakes and estuaries, there is sufficient time (years) for particulate organic matter to decompose and create a diagenetic environment where metal oxides may not be stable. When faster sedimentation prevails, such as in reservoirs, there is less time (months) for bacteria to perform their metabohc functions due to the fact that the organisms do not occupy a sediment layer for any length of time before a new sediment is added (Callender, 2000). Also, sedimentary organic matter in reservoir sediments is considerably more recalcitrant than that in natural lacustrine and estuarine sediments as reservoirs receive more terrestrial organic matter (Callender, 2000). 

The sediments are also important sites for ther removal of fixed nitrogen from coastal waters. Because there is relatively little NOs in the shallow waters overlying coastal and estuarine sediments, diffusion of NOs from bottom water is not a major source of N for denitrification. However, rapid organic matter oxidation results in the release of NH4+ to the pore waters. NH4+ can be converted to Ni by a nitrification/denitrification cycle or by NH4+ oxidation coupled to the reduction of NOs ... 

Thus, pore water studies provide evidence that some trace elements (As, Co, Cr) become more soluble on redox dissolution of Mn-Fe oxides in marine and sediments. However, these studies have failed to detect a parallel release of trace elements (other than Co) with Fe and Mn during redox events (Sakata, 1985 Morfett et al., 1988 Achterberg et al., 1997). It appears that in spite of the loss of labile organic matter and oxides of Fe and Mn, the trace elements remain relatively immobile. In oceanic or estuarine sediments it has been proposed that sulphides have a role to play in fixing the trace elements under reducing conditions. This has also been demonstrated in sulphate-rich freshwater systems (Huerta-Diaz et al., 1998). However, Cu and Pb, at least, clearly remain firmly attached to freshwater sediment even in the absence of measurable sulphides, and where Mn and Fe are being actively released via reduction (Sakata, 1985). 

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