Black carbon and other refractory forms in recent sediments from the Gulf of Cadiz, Spain

Black carbon (BC) produced by incomplete combustion of fossil fuels and vegetation is a ubiquitous form of refractory organic matter (ROM) widely distributed in the environment. The stable non-reactive, mainly aromatic, BC has received special interest in recent years as a possible carbon sink in soils and sediments. At present, several methods have been used, with applications for soil and sediments, although no single method has been accepted to detect BC forms, and therefore providing the analytical data required to estimate BC contribution to terrestrial and sedimentary carbon fluxes and the global carbon cycle. This study considers the potential of the combined use of analytical pyrolysis (Py-GC/MS) and solid state 13C NMR to determine the presence of BC and to characterize the refractory organic matter in a set of recent marine sediments from the Southwest Atlantic coast of Spain (Gulf of Cadiz). The content of BC-like material found in sediments (3.0 - 16.5 % of total organic carbon) was in the range previously reported for this kind of samples and decreases from coast to offshore. Solid state CP-MAS 13C NMR and Py-GC/MS analysis of the isolated ROM revealed a conspicuous aliphatic domain remained in the samples after chlorite oxidation. Our results suggests that, in recent sediments from fluvial and marine environments, not only BC and refractory carbon forms with aromatic nature may be sequestered in a stable form; it also seems probable that the ROM pool contains a significant amount of C immobilized in the form of alkylic compounds.