Nearshore sediments using carbon isotopes, A study of the remineralization of organic carbon in

摘要

A study of the remineralization of organic carbon was conducted in the organic-rich sediments of Buzzards Bay, MA. Major processes affecting the carbon chemistry in sediments are reflected by changes in the stable carbon isotope ratios of dissolved inorganic carbon (XCO2) in sediment pore water. Six cores were collected seasonally over a period of two years. The following species were measured in the pore waters: JC02, &'3 C-2CO2, P04, JH 2S, Alk, DOC, and Ca. Measurements of pore water collected seasonally show large gradients with depth, which are larger in summer than in winter. The 613C (PDB) of IC02 varies from 1.3 o/oo in the bottom water to approximately -10 o/oo at 30 cm. During all seasons, there was a trend towards more negative values with depth in the upper 8 cm due to the remineralization of organic matter. There was a trend toward more positive values below 8 cm, most likely due to biological irrigation of sediments with bottom water. Below 16-20 cm, a negative gradient was re-established which indicates a return to remineralization as the main process affecting pore water chemistry. Using the XC02 depth profile, it was estimated that 67-85 gC/m 2 are oxidized annually and 5 gC/m 2-yr are buried. The amount of carbon oxidized represented remineralization occurring within the sediments. This estimate indicated that approximately 20% of the annual primary productivity reached the sediments. The calculated remineralization rates varied seasonally with the high of 7.5 x 10-' mol/L-sec observed in August 84 and the low (0.6 x 10-) in December 83. The calculated remineralization rates were dependent on the amount of irrigation in the sediments; if the irrigation parameter is known to ±20%, then the remineralization rates are known to this certainty also. The amount of irrigation in the sediments was estimated using the results of a seasonal study of 2 2 2Rn/ 22 'Ra disequilibria at the same study site (Martin, 1985). Estimates of the annual remineralization in the sediments using solid-phase data indicated that the solid-phase profiles were not at steady-state concentrations. The isotopic signature of 2C02 was used as an indicator of the processes affecting IC02 in pore water. During every month, the oxidation of organic carbon to C02 provided over half of the carbon added to the IC02 pool. However, in every month, the 6'"C of XCO2 added to the pore water in the surface sediments was greater than -15 o/oo, significantly greater than the 613C of solid-phase organic carbon in the sediments (-20.6 o/oo). The 613C of IC02 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. Possible explanations of the 1 3C-enrichment observed in the surface sediments were: a) significant dissolution of CaC0 3 (613C = +1.7 o/oo) b) the addition of significant amounts of carbonate ion from bottom water to pore water c) an isotopic difference between the carbon oxidized in the sediments and that remaining in the sediments. The effect of CaC0 3 dissolution was quantified using measured dissolved Ca profiles and was not large enough to explain the observed isotopic enrichment. An additional source of 13C-enriched carbon was bottom water carbonate ion. In every month studied, there was a net flux of 2C0 2 from pore water to bottom water. The flux of pore water 2C02 to bottom water ranged from a minimum of 10 x 10-12 mol/cm 2 -sec in December 83 to a maximum of 50 x 10-12 mol/cm2-sec in August 84. However, because the pH of bottom water was about 8 while that of the pore water was less than or equal to 7, the relative proportion of the different species of inorganic carbon (H2CO, HCO-, C0~) was very different in bottom water and pore water. Thus, while there was a net flux of IC0 2 from pore water to bottom water, there was a flux of carbonate ion from bottom water to pore water. Because bottom water JC02 was more 13C-enriched than pore water JC0 2, the transfer of bottom water carbonate ion to pore water was a source of 13C-enriched carbon to the pore water. If the &'3C of CO2 added to the pore water from the oxidation of organic carbon was -20.6 o/oo, then the flux of C3% from bottom water to pore water must have been 10-30% of the total flux of 2C02 from pore water to bottom water. This is consistent with the amount calculated from the observed gradient in carbonate ion. Laboratory experiments were conducted to determine whether the 613C of C02 produced from the oxidation of organic carbon (613C-OCOX) was different from the 613C of organic carbon in the sediments (613C-SOC). In the laboratory experiments, mud from the sampling site was incubated at a constant temperature. Three depths were studied (0-3, 10-15, and 20-25 cm). For the first study (IEl), sediment was stirred to homogenize it before packing into centrifuge tubes for incubation. For the second study (IE2), sediment was introduced directly into glass incubation tubes by subcoring. The second procedure greatly reduced disturbance to the sediment. Rates of C02 production were calculated from the concentrations of 2C02 measured over up to 46 days. In both studies, the values of Re in the deeper intervals were about 10% of the surface values. This was consistent with the field results, although the rates decreased more rapidly in the field. In all cases, the remineralization rates during the beginning of IEl were much greater than those at the beginning of IE2. The sediment for IEl was collected in February 84. The measured value of Rc in the surface sediment of the laboratory experiment (24 x 10- mol/L-sec) was much greater than the value of Rc observed in the field in another winter month, December 83 (.62 x 10~9). The sediment for IE2 was collected in August 85. The measured values of Re in the surface sediment (6.6-12 x 10~9 mol/L-sec) were consistent with the field values from August 84 (7.5 x 10-9). The XC02 results indicated that IE2 reproduced field conditions more accurately than IEl did. The isotopic results from the experiments strongly suggested that 613C-OCOX in the surface sediments (-17.8 o/oo ± 1.9 o/oo) was greater than 6'3C-SOC (-20.6 ± 0.2 o/oo). The magnitude of the observed fractionation was small enough that the observed values of 613C-C02 in the pore waters could be explained by fractionated oxidation coupled with the diffusion of carbonate ion from bottom water to pore water. The observed fractionation was most likely due to the multiple sources of organic carbon to coastal sediments. A study of the natural levels of radiocarbon in these sediments indicated that the carbon preserved in the sediments is approximately 30% terrestrial while the rest is from phytoplankton.