HR: 1340h
AN: OS33A-0574    [Abstracts]
TI: Characterization of phosphorus in sinking particles in Cariaco Basin, Venezuela
AU: * Ranhofer, M L
EM: mranhofer@geol.sc.edu
AF: University of South Carolina, Department of Geological Sciences, Columbia, SC 29208 United States
AU: Benitez-Nelson, C
EM: cbnelson@geol.sc.edu
AF: University of South Carolina, Department of Geological Sciences, Columbia, SC 29208 United States
AU: Thunell, R
EM: Thunell@geol.sc.edu
AF: University of South Carolina, Department of Geological Sciences, Columbia, SC 29208 United States
AB: Phosphorus (P) is an essential nutrient utilized by all organisms for biological productivity, yet little is known about its cycling within the marine realm. In this study, we used a five-step sequential sediment extraction method (SEDEX) to examine the composition of P in sinking particles obtained from one of the world's largest anoxic basins, Cariaco Basin, Venezuela. This method, which is usually applied to sediments, chemically separates particulate P into 5 phases: loosely bound or exchangeable (P$_{Ex}$), iron-bound (P$_{Fe}$), authigenic (P$_{Aut}$), detrital (P$_{Det}$), and organic (P$_{Org}$). Samples were collected from November 2000 to April 2002 using four automated sediment traps moored at depths of 275, 455, 930 and 1250 m, with the 275 m trap located just above the oxic/anoxic interface. Our results indicate that the composition of particulate P changes dramatically as the material sinks through the water column. The 275 m trap was comprised of a mixture of P$_{Ex}$ (57 %), P$_{Org}$ (23 %) and P$_{Fe}$ (15 %), whereas the deepest trap was dominated by P$_{Org}$ (55 %) and P$_{Ex}$ (26 %). Total P fluxes decreased by a factor of four between 275 and 1250 m (85.7 versus 18.5 \mu mol m$^{-2}$ d$^{-1}$), with much of this decrease due to the $>$ 75 % loss of P$_{Ex}$ (49.2 to 4.9 \mu mol m$^{-2}$ d$^{-1}$) and P$_{Fe}$ (12.6 to 1.8 \mu mol m$^{-2}$ d$^{-1}$). P$_{Org}$ decreased by only 50 % between these two depths (19.9 versus 10.2 \mu mol m$^{-2}$ d$^{-1}$). Our results are consistent with the expectation that P$_{Ex}$ is the most labile fraction of P and P$_{Fe}$ is rapidly reduced in the anoxic bottom waters of Cariaco Basin. P$_{Ex}$ correlates well with organic C (C$_{Org}$) only in samples from the shallowest trap (r$^{2}$ = 0.6), while in the deeper traps there is a strong correlation with terrigenous material (r$^{2}$ = 0.6). This implies a change in the major source, and hence lability of this material as it sinks to the sea floor. P$_{Org}$ was very well correlated to C$_{Org}$ at all depths (r$^{2}$ $>$ 0.9), implying little to no preferential remineralization below 275 m. Additional strong relationships between Porg and opal and CaCO$_{3}$ became apparent when upwelling versus non-upwelling time periods were separated, implying different mechanisms of Porg production throughout the year. Our results imply that water column P transformations are complex and need to be considered when studying the cycling of P in marine systems.
DE: 4807 Chemical speciation and complexation
DE: 4825 Geochemistry
DE: 4845 Nutrients and nutrient cycling
DE: 4800 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting