Arsenic and uranium in the settling particulate matter and sediments of Alfonso Basin, la Paz Bay

Arsenic and uranium contents in surface sediments of La Paz Bay, in a sediment core and settling particulate matter collected in Alfonso Basin are presented. Arsenic is most enriched in sediments of the southwestern bay, probably associated with terrigenous supply. Uranium accumulates in sediments of the deeper part of the embayment. The profile of uranium in the core is characterized by higher concentrations near the surface and lower contents below 40 mm. Uranium concentration maximum at 35 mm is probably related to low oxygen in the water column at the deposition time. The contents of As and U in settling particles from 2002-2008 were greatest in late spring-early summer (May-July 2002, 2007 and 2008). The accumulation timing corresponds with the intrusion of oxygen-depleted waters from the Equatorial Pacific Ocean into the Gulf of California. Maximum uranium and arsenic contents in the settling particles coincide with moderate "El Niño" of 2002-2003.