ON HOW THE EROSION OF THE COLORADO RIVER DELTA AFFECTS THE SEDIMENT GEOCHEMISTRY OF THE NORTHERN GULF OF CALIFORNIA

Water and sediment fluxes from the Colorado River to the Northern Gulf of California (NGC) were interrupted by river damming during the last century. It has been estimated that this human intervention has led to a 99.5 % reduction of the original sediment discharge to the Colorado River Delta (CRD); the deltaic basin having developed from an estuarine setting to a hypersaline, anti-estuarine and erosive one. Current hydrographic conditions in the NGC are causing the dispersion of deltaic, mainly clay-sized sediments, along the Baja California Coast, reaching at least as far south as the Great Island region (Tiburón Basin). Main transport mechanisms are: a seasonal gyre in the central NGC, a gravity current flowing south of the CRD and probably intense bottom currents in Tiburón Basin. Additional sediment sources to the NGC are identified, mainly as aerosols and fluvial inputs from the Mexican mainland. The understanding of the fate of dissolved and particulate metal species previously trapped in the CRD is important, especially since estuarine conditions do not longer prevail in this area. Delta derived clayey sediments in the NGC are relatively enriched in metals, with metal concentrations varying significantly in sediments from the NGC (e.g., Fe: 0.35-2.83 % and Mn: 97-922 mg g^-1). Normalisation of Fe and Mn against the relative abundance of <4 mm sediments indicates that the main source of these metals is the CRD. Partition analyses indicate that Fe is mainly associated with the less soluble lithogenic component throughout the NGC, while Mn is mainly partitioned in the exchangeable (ca. 15%) and carbonate (ca. 32%) sediment phases of the clayey sediments. Using the relative abundance of clays and the partitioning of Mn as tracers, it can be seen that the influence of CRD sediments extents at least as far as the central Tiburón Basin, where < 45 mg g^-1 Mn is found to be potentially exchangeable with the marine environment.