Biogeochemical Process of Organic Carbon in Sediments

The vertical distribution of OC in Bohai sea sediments is shown in Fig. 2.17. The core sediments can be divided into three parts surfacial/subsurfacial 

Because the three stations in C area are located at one section in an NE-SW direction, studying the section distribution (Fig. 2.19) could help to understand the source and mineralization of OC. OC in 0 20 cm varied between 

55% and 0.75% and was present in laminated distribution, which indicated that OC originated from the sedimentation in seawater. Below 25 cm, OC was present in regular gradient horizontal distribution and decreased clearly from Cl to C3. It is possible that a flood happened in a corresponding deposition time. Freshwater with a great quantity of OC flowed into this area and led to the accumulation of OC in this direction. 

From the measured results (Table 2.6), it can be seen that pH shows an increasing trend with deposition depth. The process of mineralization is shown the degradation of OC makes the sedimentation environment more reductive so that acidity decreases and pH increases. However, the distribution of OC in surfacial/subsurfacial sediments is complicated, and pH shows an obviously increasing trend. Therefore, OC is related to not only pH but also the biomass and distribution of the microorganisms in surfacial/subsurfacial sediments. In addition, due to the shallow water column and active dynamics of the seawater, organic-rich sediment can be easily re-suspended into the water column and be utilized by organisms. The middle sediments are the mineralization area and OC decreases with an increase in pH. 

Commonly, values of Fe +/Fe +, Eh, and Es in surfacial sediments are high and the OC contents are high too. With the increase in deposition depth. 

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