El papel del hierro y el azufre en la biogeoquímica del sedimento en los ecosistemas acuáticos

Resumen

The biogeochemical role of sedimentary iron and sulfur was studied in sediments from an acidic reservoir, El Sancho (Huelva), polluted with acid mine drainage, and in marine coastal sediments from the inner Cadiz Bay. The experimental approach included laboratory manipulative experiments with sediment slurries and intact cores and field studies with the careful analysis and interpretation of a number of biogeochemical variables in sediment records.The laboratory experiment with sediment and water collected from El Sancho, demonstrates clearly the strong effect of seasonal changes in the redox conditions at the sediment-water interface on the biogeochemical cycling of S, Fe, C, N. Oxic conditions stimulated the aerobic oxidation of organic matter and of reduced inorganic compounds like FeS, Fe2+ and H2S, but also enhanced anoxic biotic and abiotic processes like sulfate reduction and the oxidation of H2S by Fe(III) hydroxides. Net H+ production increased along the oxic period in water over the sediment and within the sediment, whereas the situation is reversed under hypoxic conditions. This mechanism of acidity transfer between the sediment and the water column probably involved dissolution-precipitation reactions between Fe and Al mineral phases that were capable of buffering the pH in the water phase. The quick responses of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) pools to changes in O2 availability suggest that in addition to changes in microbial degradation rates of OM, reversible reactions of adsorptiondesorption of OM from sediment particles and of OM co-precipitation with Fe(III) hydroxides or as Fe(III)-organic carbon complexes play an important role in controlling the net exchange of DOC and DON at the sediment-water interface during redox transitions conditions. Cádiz Bay sediments, sulfate reduction and TCO2 production rates increased with the addition of Ulva sp. detritus to sandy and muddy sediment slurries in the presence of different amount of reactive iron. The content of reactive iron controlled very effectively the concentration of H2S in the incubations. The H2S was quickly removed from the aqueous phase by reaction with Fe(III) to form insoluble iron sulfides (FeS and FeS2), decreasing H2S accumulation with the amount of reactive Fe. The study of OM burial and sources that carried out in different habitats of inner Cadiz bay showed C burial rates decreased landward along the tidal range. An inverse trend was observed for N burial rate, suggesting a relatively high deposition of low C:N ratio organic matter in the high marsh. On the contrary, S burial rates did not show any accumulation pattern along the tidal range. The sedimentary organic matter seem to be derived from a well-mixed combination sources of subtidal and high-marsh sources, with no clear evidence of a higher contribution from the particular species or community inhabiting the sediments. The environmental reconstruction of the inner Cadiz Bay for the last 300 y suggested that the major changes took place during the 20th century, when the population around Cadiz Bay tripled. Thus, increases in sediment accumulation rates, organic matter inputs and heavy metal contents are interpreted as direct effects of this increase in human population and its associated environmental impacts. On the other hand, the variations in the sedimentary Corg/N, δ13C, and δ15N seem to indicate an increase in marine productivity and a higher contribution of benthic macroalgae due to higher N inputs from wastewater discharges and fertilizer runoff over this period. Moreover, this study revealed post depositional alteration, probably induced by human activities, in the sedimentary records of the inner Cadiz bay. These and other diagenetic overprints were differentiated from variability induced by environmental changes, via the interpretation of sedimentary profiles using a multi-proxy approach.