Geochemistry of surficial sediments from Sebastian Vizcaino Bay, Baja California

Thirty-seven surficial sediment samples from Vizcaino Bay, Baja California, are analysed for their phosphate, organic matter, carbonate, as well as me tal contents (Al, Fe, Mn, Zn, Cu, Ni, Cr, Cd and Sr). Highest organic carbo n concentrations (up to 15.1%) are found in deeper waters of the outer bay and also in a narrow area extending NW-SE adjacent to the eastern coast. To tal carbonates are also enriched proximal to the eastern coast, with the hi ghest concentrations found on Ranger Bank (36.9%), where it is associated w ith foraminifer-rich sands. Phosphate (P2O5) content is highest (22.3%; mai nly present as francolite pellets, nodules and bone fragments) in shallow w aters (2970 meters) adjacent to Punta Santa Rosaliita to the NE of the bay, showing a sharp offshore decrease. Geochemical partition analyses indicate that, except for Sr and Cd, all the metals studied occur mainly in the detrital fraction of the sediment, with minor but varying proportions occurring in the more reactive fractions. Th e regional distribution of non-lithogenic Cu, Cr, Ni, Cd and, to a lesser e xtent Zn, is mainly controlled by their association with organic matter. Th ese elements show high concentrations in an area under influence of upwelle d waters, and in the organic-rich bathymetric deep in the outer bay. Cadmiu m is mainly associated with the most reactive fraction of the sediments (ei ther loosely adsorbed, as carbonates and/or Fe-Mn oxihydroxides), being enr iched (4.09 ppm) directly below the coastal upwelling region. Different pre cipitation processes for Cd compared to Cu, Cr and Ni occur in the area, po ssibly due to the preferential input of Cd via upwelling and/or the relativ ely shorter residence time of Cd in waters above continental margins. The d istribution of Sr is related to that of calcium carbonate, except for an ar ea proximal to the coastal lagoons on the SE coast, where relatively high S r concentrations are found (< 1725 ppm). Various metal enriched samples thr oughout the study area, suggest different lithologies and sources of the de trital components deposited. Of these, detrital Mn is associated with sedim ents derived from the peninsular mainland NE of the bay and Fe, Cu, Cr, Ni and Zn with the lithogenic detritus on Ranger Bank.