Down-core sediment profiles

Figure 13.16 Down-core sediment profiles of (a) total organic carbon (TOC) and (b) atomic C N ratios at two locations in the York River estuary. (Modified from Arzayus...

Down-core porewater profiles of Mn2+, Fe2+, and H2S in sediments from Aarhus Bay estuary (Denmark) show sharp gradients in the subsurface peaks of Mn2+ and Fe2+, indicating the reduction of Mn and Fe oxides in the upper 2 and 4 cm, respectively... 

Figure 7.13 Theoretical down-core profiles of radon activity illustrating that 222Rn is typically not in (a) secular equilibrium with its parent 226Ra at the sediment-water and water-atmosphere interfaces because of inert characteristics and loss by (b) diffusion and (c) gas bubble ebullitive stripping from sediments to overlying waters and then to the atmosphere. (Modified from Martens and Chanton, 1989.)...

The depth profile of the TOC content (Fig. 3) in the piston core shows the highest TOC content (1.07%) at the surface and an abrupt decrease to 0.6% at 7 cm. Between 7 and 50 cm, the TOC content fluctuates between 0.47 and 0.75%. Below 50 cm, TOC content shows little change. The down-core profile of sulfate in porewater (Fig. 3) shows a typical depletion trend with depth. The sulfate depletion rate is very rapid in the upper 40 cm, with a mean gradient of — 0.18mMcm whereas the depletion rate becomes much lower (—0.02 mM cm ) below 50 cm. TS contents measured in this piston core (Fig. 3) range from 0.02 to 0.10% with no apparent down-core trend. These values are also significantly lower than the mean value (0.22%) of modem fine-grained sediments (Berner, 1982). 

In detail, most of the concentration profiles reveal increasing values from the bottom of the core up to a depth of 100 cm, reaching a first maximum there. Subsequently, the values decrease to a fairly constant level at a depth between 80 cm and 30 cm. The averaged values in this zone are Cd 6 pg/g, Ni 50 pg/g, Cr 85 pg/g, Cu 180 pg/g, Pb 240 pg/g and Zn 1100 pg/g. A second concentration maximum was observed at a depth of approx. 25 cm. In the top layers of the sediment core the concentrations decrease down to values similar to those in the 30-80 cm zone. These trends are best developed in the concentration profiles of lead, copper, zinc and cadmium (Fig. 4). 

Correspondence between the vertical distribution of total organic carbon (TOC) at station 3C (approximately 6-8 km downcurrent from the outfall system) and the mass emissions of suspended solids from the outfall system during the period 1946-1981 is illustrated in Fig. 6. Following World War 11 and up until 1971, the monotonic increase in emissions of suspended solids from the LACSD paralleled the population trend in Los Angeles. Thereafter, solids emissions declined in response to improved source control and advances in waste treatment (Stull et al, 1996). The vertical concentration profile of TOC in the 3C (1981) core records the historical trend in effluent solids emissions and indicates that, for this period, the outfalls dominated sedimentation of organic carbon on the shelf. The dechne in emissions of suspended solids from the outfalls after 1971 became a matter of concern because of the potential for remobilization of heavily contaminated sediments that had been laid down in earlier years. 

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