The Concentration and Isotopic Composition of Carbon in Marine Sediments Affected by a Sewage Discharge
<p>The impact of the discharge of primary treated effluent of a high suspended solids concentration on the sedimentary environment of the Palos Verdes Shelf was investigated. Analyses of the carbon content and the isotopic composition of sedimentary organic material with depth in the sediments...
| Summary: | <p>The impact of the discharge of primary treated effluent of a high suspended solids concentration on the sedimentary environment of the Palos Verdes Shelf was investigated. Analyses of the carbon content and the isotopic composition of sedimentary organic material with depth in the sediments and the areal distribution of these properties relative to the location of the Los Angeles County Sanitation Districts' ocean outfalls indicate that the outfalls are extremely important in determining the chemical and physical properties of sediments on the Palos Verdes Shelf. The analyses of various sedimentary cores specifically indicate that sludge-like matter has accumulated to depths of as great as twenty centimeters during the past 38 years.</p>
<p>Calculation of the accumulation rates of organic carbon suggests that the present outfall-induced sedimentation rate for organic carbon in the immediate vicinity of the discharge is as much as about 260 times that of the natural sedimentation of organic carbon for the Palos Verdes Shelf. One result of this rapid input of organic carbon is the occurrence of a relatively large area of surface anaerobic sediments. The relationship between the mass accumulation rates of organic carbon and the areal extent of surface anaerobic conditions is further discussed as related to the suspended solids concentration of discharged effluent.</p>
<p>The uppermost outfall-influenced sediments are shown to be of high water content and to have a total organic carbon concentration as high as 12 percent which has a δC<sup>13</sup> value of about -24.5 o/oo. Particulate organic carbon discharged from the outfall has narrow-ranged δC<sup>13</sup> values of about -23.5 o/oo, thus indicating that the sedimentary carbon has incurred a diagenetic change resulting in the preferential decay of a higher δC<sup>13</sup> fraction, or that there has been a preferential deposition near the outfall of particulates with a more negative δC<sup>13</sup> value. Other possibilities are also discussed.</p>
<p>The lower pre-outfall natural sediments on the Palos Verdes Shelf have organic carbon concentrations of about one percent. The δC<sup>13</sup> values for these natural sediments are about -22.5 to -23.0 ‰ which is typical of other data reported in the literature for sediments of marine basins offshore of southern California.</p> |
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