Evaluación geoquímica de elementos mayoritarios y oligoelementos en los sedimentos de Bahía Concepción (B.C.S., México)

The geochemistry of major components and trace elements of the sediments in Concepción Bay is of great interest. This highly productive semi-closed marine ecosystem of the western Gulf of California has alternating oxic-suboxic-anoxic conditions in the water column. Being in an arid semidesertic zone, it receives only episodic water and terrigenous sediment input associated with the summer-early autumn tropical cyclones or meterological fronts in the winter and is affected by a subaerial or shallow hydrothermal water discharges caused by active tectonics. A set of 158 surface sediment samples, 24 samples of the bed sediments from the arroyos discharging to this bay and one sample from the crust of the wall of the playa Santispac hot water spring were collected in October 1997. The following analyses were done: grain size, calcium carbonate content, organic carbon, major elements (Ca and Fe) and 34 trace elements. A high abundance of calcium carbonate (41% on average) occurs in the coarse sediments of the shallow western part of bay, especially in front of the Mulegé estuary, playa Santispac, playa Santa Barbara and El Remate. The finer particles, enriched in organic carbon (2% on average), prevail in the deeper central part of the bay along its longitudinal axis. Various associations of the major components, major and trace elements can be distinguished from the results of Principal Component Analysis (PCA): (i) association I constituted by Ba, Cs, Cu, Fe, Hf, lanthanides, Rb, Th, U and Zn; (ii) association II, which includes Ca, CaCO₃, Cd and Pb; (iii) association III presented by Br, Cd, C_org, Cu, Hg, Hf, Ni and Zn; (iv) association IV, which joins Se, Ta and Zr and (v) association V constituted by Co and Cr.