Comparison of Th-234, Th-228, and Pb-210 fluxes with fluxes of major sediment components in the Guaymas Basin, Gulf of California

Sediment trap samples were collected from the Guaymas Basins to examine the factors that control the flux of Th-234 Th-228 and Pb-210. Total mass, org anic carbon, biogenic opal, carbonate and lithogenic fluxes were measured t o compare with the radionuclide fluxes. The fluxes of the three radionuclid es are strongly correlated with each other. However, a comparison of the ra dionuclide fluxes with total mass and major sediment component fluxes revea led two separate correlations with distinctly different slopes. Therefore, the flux of these particle-reactive radionuclides is not controlled by the total mass or the major sediment component fluxes. The correlation showing the highest radionuclide flux relative to the total mass and major sediment components (highest slope) occurs in the winter of the El Nino years, sugg esting more efficient scavenging during this period. During the non-El Nino winter and the summers, thr flux of the three radionuclides increases much less with the increase in the total mass and major sediment components (lo wer slope). Mechanisms typically used to explain highly efficient scavengin g were examined, These mechanisms include an increase in residence time of particles in the water column, iron and manganese scavenging, finer particl e size, and higher productivity. Evidence exists to discount all of these t ypical mechanisms. El Nino causes a change in the winter hydrographic condi tions in the Guaymas Basin and allows the migration of tropical Pacific pla nkton species into the central Gulf of California. The evidence is consiste nt with the hypothesis that the plankton species produced under the hydrogr aphic conditions of the El Nino winter are responsible for the more efficie nt scavenging of the particle-reactive radionuclides in the Guaymas Basin. The possibility also exists that a factor (or factors), associated with the El Nino winter that has not been examined, is responsible for the highly e fficient scavenging. (C) 1999 Elsevier Science B.V. All rights reserved.