GEOQUÍMICA DE LA MATERIA PARTICULADA EN HUNDIMIENTO Y DE LOS SEDIMENTOS RECIENTES DE CUENCA ALFONSO, BAHÍA DE LA PAZ

Abstract

ABSTRACT The trace element geochemistry in the sedimentary material of arid climatic regions with prevailing sporadic currents is still not as fully characterized as that of the template and tropical zones with higher precipitation and runoff. The drainage basin rock composition influence on the marine sedimentary material (settling particulate matter - SPM and superficial sedimentary layer) is being studied in one of these climatic areas with low precipitation, as is the case of La Paz Bay, Gulf of California, where the main contributions occur due to the the passage of tropical cyclones, sporadic rains and dust storms. Ten rocks, 15 alluvial sediments and 15 dune samples were collected in the bay drainage basin. Moreover, 64 SPM samples between 2005 and 2008 with sediment trap, as well as sediment core subsamples (110 mm, 210Pb-dated, giving the sedimentation rate of 0.61 mm/year) were obtained in the deepest part of this water body, called Alfonso Basin. It can be observed from the alluvial sediments that As, U and P elements are being supplied as erosion products of marine rocks, which present elevated phosphorous content, such as phosphorites and phosphatic sandstones of El Cien Formation. This fact is due to these rocks being formed in upwelling conditions with high primary productivity, as well as in suboxic basins, which is in the present time occurring in Alfonso Basin. The latter is registered by As (47.5 mg/kg) and U (42.8 mg/kg) enrichment events in SPM composition, sometimes relating to bloom-associated elevated biogenic Ca values. On the other hand, the historical registry (sediment core) indicates some of these element variation events, especially U that presents a high value peak (19 mg/kg) in the core depth of 35 mm that corresponds to the year 1950. Calcium, instead, seems to be supplied to the basin by planktonic blooms and primary productivity, which makes it possible to correlate with events already reported by other authors during the trap time series. In the sediment core case, this element presents a gradual increase after 1950 and its behavior is almost identical to the sea surface temperature reconstruction (NOAA’s ERSST.v3b). This is probably due to the preference of calcareous organisms for warm conditions. On the other hand, the terrigenous supply, estimated by Sc, shows in the particulate matter a relation with tropical cyclones, as well as with strong wind and gust events. In the sediment core x some changes in the fluvial regime are observed, where a high content of this element period from 1890 to 1980 stands out.