Composition of settling particulate matter and element fluxes into Alfonso Basin, La Paz Bay, Gulf of California: 2002-2008 sediment trap time series

Element fluxes between the reservoirs of the marine environment are studied in order to try to understand biogeochemical cycles of elements. To characterize the behavior of particulate elements in the southwestern Gulf of California and specifically in La Paz Bay, settling particulate matter, collected with a sediment trap during 2002-2008 period was used. Major and trace elements were determined by an instrumental neutron activation analysis. During the registered period a high variability was found as well as almost no periodicity in the total mass flux, composition and particulate element fluxes to the Alfonso Basin. The main sources for the different element contents and fluxes are influenced by terrigenous fluvial supply (tropical cyclone effect), aeolian supply (dust storms), planktonic blooms (marine biogenic supply) and authigenic sources (redox-sensitive element reduction). Introduction In order to understand the Earth system we need to study each one of its spheres, as well as establish the interactions between them. Modern geochemistry does not only study the element contents in separated parts of this system (lithosphere, biosphere, hydrosphere, cryosphere and atmosphere). It focuses on the characterization of biogeochemical cycles of elements (GEOTRACES, 2006). The creation of these cycles requires the quantification of fluxes between the ecosystem compartments. Important features to consider in the marine environment are represented by vertical fluxes of settling particulate matter (SPM) and the elements transported to the bottom sediments. These fluxes depend on the composition and amount of sedimentary particles that form in the upper layer of the aqueous reservoir, their transformations in the water column, as well as their final deposition at the bottom (Rodriguez Castaneda, 2008). The main interest of this work is to study the SPM composition and the particulate element fluxes to the La Paz Bay and try to find their possible contribution sources. The study area, La Paz Bay, is located in the southern part of the Baja California Peninsula. This embayment has a variable depth, ranging from less than 200 m beneath most of the Bay to a maximum depth of 410 m in a tectonic depression called Alfonso Basin (Figure 1). An important characteristic of the study area is the desertic arid climate with very low precipitation ( effect), aeolian supply (dust storms), planktonic blooms (marine biogenic supply) and authigenic sources (redox-sensitive element reduction). Certain flux seasonality can be seen for some elements, well correlating with periods of strong northeastern winds in fall-winter. References Boning P., Brumsack H.-J., Bottcher M.E., Schnetger B., Kriete C., Kallmeyer J. and S.L. Borchers, 2004. Geochemistry of Peruvian near-surface sediments. Geochim. Cosmochim. Acta 68: 4429-4451. GEOTRACES, 2006. An international study of the marine biogeochemical cycles of trace elements and their isotopes. Science plan. International Council for Science. Scientific Committee on Oceanic Research, Boston, USA McManus, J., Berelson, W/M., Severmann, S., Poulson, R.L., Hammond, D.E., Klinkhammer, G.P., and C. Holm, 2006. Molibdenum and uranium geochemistry in continental margin sediments: Paleoproxy potential. Geochim. Cosmochim. Acta 70: 4643-4662. Rodriguez Castaneda, A.P., 2008. Variacion de flujos de los elementos particulados en Cuenca Alfonso, Bahia de La Paz, en el periodo 2002- 2005. PhD Thesis. CICIMAR-IPN, La Paz, Mexico (in Spanish).