Accumulation rates and geochemistry of sediments in the northeastern shelf of the Gulf of Càdiz (SW Iberian Peninsula)

In June 2006, surveys were carried out northeastern shelf of the Gulf of Cádiz, within the multidisciplinary project ‘Littoral areas of green-house gases emission: influence of coastal input and benthic production’. Two areas have been selected in order to assess emissions of CO2 to the atmosphere. These emissions can be related to continental inputs of inorganic carbon (Guadalquivir estuary) or to anthropogenic inputs of organic carbon (Bay of Cádiz). In addition, the geochemistry of bottom sediments has been assessed by means X-ray fluorescence analysis, and accumulation rates by Pb-210 and Cs-210 activity profiles. Sedimentation rates and mass accumulation rates found in this study were in the ranges detected in the inner Bay of Cádiz (0.16-0.27 cm yr-1 and 0.09-0.2 g cm-2 yr-1). Particle reworking (bioturbation) is a critical processes that may be responsible for the mixing of the surface sediments, as observed from the excess Pb-210 profile. The thickness of the surface mixed layer is about 3-4 cm, and bioturbation partially disturbed the downcore profile. The origin of the anomalous metal concentrations is mostly due to persistent and widespread mining activity in both catchments. In addition to this continuous input, a total of 5 million cubic meters of acidic metal-rich wastes were released into the Guadalquivir delta zone after the collapse of a tailing dam in the pyrite mine of Aznacollar, in 1998. Finally, the present data on Pb-210 and Cs-210 activity profiles do not allow to distinguish pulse sedimentation events, and this is suggested to be an important subject for further research in the area of study