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Alfonso Basin is a natural sedimentation environment in the La Paz Bay, southern Baja California Peninsula, located an arid subtropical region characterized by limited land runoff with a productive surface layer overlying a well-developed Oxygen Minimum Zone, very attractive for regional paleooceanographic studies because of excellent lamination of its sediments. A time-series sediment trap program was initiated in 2002 and is still in action to help understand the processes controlling variations in the biogeochemistry of this environment. INAA, ICP-MS and ICP-OES analyses, as well as flame AAS were made for major and trace elements in 105 subsamples of material collected at 350 m depth Turing 2002-2005. Principal component analysis, including data for organic and inorganic carbon, biogenic silica and the lithogenic fraction, yielded four factors. Factor 1 (25% of the total variance) showed high eigenvalues for the classical terrigenous aluminosilicate elements: Al, Fe, Mn and Sc. High loadings also occurred for the group of light to medium REEs (except for Eu). The latter are frequently associated with acid source rocks, such the granites and rhyolitic volcanics exposed on the margin of the bay. Factor 2 (18%) grouped mainly REEs, with high loading on the heavier members of the series that are more typical of basic Rocks (thick sequences of andesitic volcanics occur to the NW), as well as Ni and Rb. Factor 3 (12%) appears to represent carbonate organism production, with high loadings for inorganic carbon, Ca and moderate levels for organic carbon, while % litho is moderately negative, as is biogenic silica. The fourth factor (9%) is characterized by high negative loadings for Cd, Co, Se and moderately negative values for organic carbon. Moderate positive loadings occur only for Sr and, at lower levels, for U, Sb and % lithogenic. This suggests that this grouping is related to particles that have been partially transformed within the Oxygen Minimum Zone. The results will be examined for distinctive periods within the time-series, such as the hurricane-influenced months of August-September 2003, the generally very low fluxes Turing 2005, and unusual peaks during winter 2002.The elements listed in as controlled by last factor (mainly micronutrients and chalcophilic elements) are enriched in sinking particles of a time series, especially in some periods. The average enrichment factors (EFs), estimated using Sc as a normalizer, show high values during 2003-2005: Se (117-927), Sb (10-53), Cd (17-41), As (14-19), Pb (3-36), Sr (3-10), U (2-6), Ca (3-5) , Zn (3-5), Cu (1.3-5.4), Ba (1.8-4.3), Mo (2-2.8), Co (1.9-3.6). They were generally enriched during anomalous 2002 year especially Se (1448), Sb (470), As (239), U(61), Cd (35), Sr (31), Mo(26), Zn(15), Ca (11), Cs (9), Rb (6.5). Such an increase of EFs of some elements (As, Mo and U) occurred mainly during late spring-summer of 2002, on first stage of moderate El Niño, when the waters from subtropical and tropical Pacific Ocean presumably entered into the La Paz Bay,

Palabras clave: La Paz Bay; settling particulate matter; Alfonso Basin; Micronutrients; redox-sensitive elements; SW Gulf of California

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