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Excess nitrogen gas (N2xs) was measured in samples collected at six locations in the eastern and western sub-basins of the Cariaco Basin, Venezuela, in September 2008 (non-upwelling conditions) and March 2009 (up-welling conditions). During both sampling periods, N2xs concentrations were below detection in surface waters, increasing to ~22 µmol N kg-1 at the oxic–anoxic interface ([O2] < ~4 µmol kg-1, ~250 m). Below the oxic–anoxic interface (300-400 m), the average concentration of N2xs was 24.7±1.9 µmol N kg-1 in September 2008 and 27.5±2.0 µmol N kg-1 in March 2009, i.e., N2xs concentrations within this depth interval were ~3 µmol N kg-1 higher (p < 0.001) during the upwelling season compared to the non-upwelling period. These results suggest that N-loss in the Cariaco Basin may vary seasonally in response to changes in the flux of sinking particulate organic matter. We attribute the increase in N2xs concentrations, or N-loss, observed during upwelling to: (1) higher availability of fixed nitrogen derived from suspended and sinking particles at the oxic-anoxic interface and/or (2) enhanced ventilation at the oxic-anoxic interface during upwelling. [ABSTRACT FROM AUTHOR]

We have analyzed the chemical compositions of 87 samples from the Cariaco Basin and 20 samples from the Orinoco Delta, Venezuela, in order to better characterize the fl uvial sources of material to the Cariaco Basin. We observe distinctive regional patterns in composition; shelf sediments found near the Tuy River, Manzanares River, the Araya-Margarita region, and in the northern portion of the Unare platform record reproducible compositional differences. However, linear mixing models using only the composition from these rivers do not satisfy the variability observed in modern sediment trap samples collected from the basin. Average trap values are best approximated by mixing upper crust (30%-50%) with local river sources (50%-70% of either the Tuy, Unare, or Neverí Rivers) and eolian dust (<2%). The chemical data suggest a glacial-interglacial switching of sources, with the Tuy-Tacarigua, Unare, and Neverí being more important during glacials, and the larger Tuy and Manzanares being more important during interglacials. The K/Al in shelf sediments (0.14-0.27) spans the range of previously documented glacial to interglacial values (0.16-0.26), and the high Ti/Al found during glacials (∼0.052) can perhaps be supplied by some of the local rivers (Ti/Al ∼0.049-0.106) and not exclusively by Saharan dust. The data and statistical models do not offer support for the presence of any signifi cant Orinoco source [ABSTRACT FROM AUTHOR]