Contribution of black carbon in recent sediments of the Gulf of Cadiz: Applicability of different quantification methodologies

In the Southwestern Iberian Peninsula, vegetation fires are rather common, and produce a considerable amount of charred material that, due to erosion, is fluvially transported to the continental margins. The present study constitutes the first comparative assessment of the contribution of this charred material (generally black carbon, BC) to marine sediments of the inner continental shelf of the Gulf of Cadiz (GoC). Given the ambiguity of BC definition and the heterogeneity of quantification techniques, four of the most common and well established methodologies were applied to obtain a representative range of the BC content on four surface sediments: chemical-thermal oxidation (GBC), molecular markers (benzenopolycarboxylic acids, BPCA), thermogravimetry coupled to mass spectrometry (TG-DSC-QMS), and chemical oxidation with sodium chlorite. The BC results obtained for the GoC sediments are compared, and the applicability of each technique on marine sediments is discussed. The mean BC values of the four methods (4.4-14.4% total organic carbon) were within ranges previously reported for marine sediments from diverse origins. However, a large variability was observed within individual samples when comparing the BC results obtained with the four methods (relative standard deviation from 31% to 75%). The BPCA approach produced the highest BC values, whereas the GBC method the lowest. This reflects the analytical variability derived from the different windows composing the BC continuum. The results derived from the TG-DSC-QMS application generated certain uncertainties, despite the correction factor introduced to reduce the BC overestimation previously reported for this method. Considerable differences in the BC content of spatially close samples evidenced the complexity of coastal currents and sediment fluxes in the area, suggesting the existence of other factors affecting the BC distribution. The lack of correlation between the BC and TOC, as well as between the BC and lignin contents in all samples, ruled out the occurrence of charring artifacts. Preferential degradation and/or erosion of BC could explain the BC distribution in the area. The use of ancillary terrigenous biomarkers (lignin and inorganic silica) pointed to a preferential fluvial transport of the terrestrial organic matter and indicated that the longer distance from the source, the greater degradation of the OM. In addition, the northern dominant winds in the region may have favored the additional introduction of airborne transported BC in the marine system. (C) 2011 Elsevier Ltd and INQUA. All rights reserved.