ACCUMULATION RATES AND ORGANIC CARBON SOURCES IN SHELF SEDIMENTS FROM THE GULF OF CÁDIZ (SW SPAIN)

Sediment cores (up to 15 cm long) from water depths of 13-25 m were investigated for excess 210Pb (210Pbxs) activity, total organic carbon and total nitrogen content (OC and TN, respectively), and C/N-ratios. The distribution of excess 210Pb in sediment cores was used to determine modern (last 100 yr) mass accumulation rates on the north-western shelf of the Gulf of Cádiz. Mass accumulation rates range from 0.11 to 0.19 g cm-2 y-1 in the Bay of Cádiz, and from 0.26 to 0.51 g cm-2 y-1 in the Guadalquivir prodelta area. These rates fall within ranges reported for several continental shelves and margins, like the Barcelona continental margin, the Bay of Biscay and the Gulf of Lion. Overall organic carbon content of the sediments is relatively low (< 1.0 %) by comparison with riverine and offshore sediments from the Gulf of Cádiz. The mean organic carbon content values were higher in sediment from Guadalquivir prodelta area (0.76 %; n = 27) than in the Bay of Cádiz (0.34 %; n = 32). The C/N molar ratios were lowest at the sediment surface (9-10) and highest in the deeper parts (11-15). Freshly produced plankton organic matter (OM), has C/N-ratios of 5-7; preferential degradation of N-containing compounds during early diagenesis results in values up to 12, and terrestrial OM dominated by cellulose and lignin has significantly higher C/N-ratios of 20-500. Thus, the C/N values found here, mostly < 12, and increasing C/N-ratios with increasing sediment depth indicate that in the investigated sediments, C/N-ratios were primarily controlled by ongoing degradation of marine OM.