Study of the Remineralization of Organic Carbon in Nearshore Sediments Using Carbon Isotopes

摘要

Major processes affecting the carbon chemistry in the sediments of Buzzards Bay, MA were reflected by changes in the stable carbon isotope ratios of dissolved inorganic carbon sigma CO2 in sediment pore water. The delta (13)C (PDB) of Sigma CO2 varies from 1.3 o/oo in the bottom water to approximately -10 o/oo at 30 cm. The isotopic signature of sigma CO2 was used as an indicator of the processes affecting sigma CO2 in pore water. In every month, the delta (13)C of sigma CO2 added to the pore water in the surface sediments was greater than -15 o/oo, significantly greater than the delta (13)C of solid-phase organic carbon in the sediments (-20.6 o/oo). The delta (13)C of sigma CO2 added to the pore water in the sediments deeper than 7 cm was between -20 and -21 o/oo, similar to the organic carbon in the sediments. The effect of CaCO3 dissolution was not large enough to explain the observed isotopic enrichment. An additional source of (13)C-enriched carbon was bottom water carbonate ion. There was always a net flux of sigma CO2 from pore water to bottom water. However, because of pH differences, there was a flux of carbonate ion from bottom water to pore water. Because bottom water sigma CO2 was more (13)C-enriched than pore water sigma CO2, the transfer of bottom water carbonate ion to pore water was a source of (13)C-enriched carbon to the pore water. The flux of CO3(-2) from bottom water to pore water was 10-30% of the total flux of sigma CO2 from pore water to bottom water. Further, laboratory experiments strongly suggested that delta (13)C-OC(ajx) in the surface sediments (-17.8 o/oo + or - 1.9 o/oo) was greater than delta(13)C-SOC (-20.6 + or - 0.2 o/oo). Using the sigma CO2 depth profile, it was estimated that 67-85 gC/sq m are oxidized annually and 5 gC/sq m -yr are buried. The calculated remineralization rates varied seasonally with the high of 7.5 x 10 to the -9th power mol/L-sec observed in August 84 and the low .6 x 10 to the -9th power in December 83.