HR: 1340h
AN: V13B-1471    [Abstracts]
TI: Characterizing Sedimentary Carbon and Nitrogen Subduction Fluxes in the Central America Convergent Margin (ODP Legs 170, 205)
AU: * Li, L
EM: lol2@lehigh.edu
AF: Lehigh University, 31 Williams Drive, Bethlehem, PA 18015 United States
AU: Bebout, G E
EM: geb0@lehigh.edu
AF: Lehigh University, 31 Williams Drive, Bethlehem, PA 18015 United States
AB: We present new data regarding the C and N contents and isotope ratios of the subducting calcareous sediments from the lower section (U3) of Site 1039 and the silty sediments from the accreted section sampled by Sites 1040, 1254, and 1255. Sediments in the calcareous section contain 62 to 866 ppm N with $\delta$$^{15}$N$_{Air}$ of 2.4 to 8.5 per mil and 0.09 to 0.73 wt.% TOC with $\delta$$^{13}$C$_{PDB}$ of -25.4 to -21.4 per mil. Integrating the upper diatomaceous ooze section (U1) and the silty clay section (U2) [Li et al., 2003], the subducting sediments along the 1100-km Central American convergent margin could carry inventories of 1.3\times10$^{10}$ g/yr N with average $\delta$$^{15}$N$_{Air}$ of 5.7 per mil, 1.4\times10$^{11}$ g/yr TOC with average $\delta$$^{13}$C$_{PDB}$ of -22.0 per mil and 1.5\times10$^{12}$ g/yr oxidized C with average $\delta$$^{13}$C$_{PDB}$ of 1.9 per mil. Sediments at Sites 1040, 1254 and 1255 contain 832 to 2398 ppm N with $\delta$$^{15}$N$_{Air}$ of 3.6 to 6.6 per mil, 0.85 to 3.06 wt.% TOC with $\delta$$^{13}$C$_{PDB}$ of -26.6 to -21.1 per mil, and $<$4 wt.% CaCO$_{3}$ with $\delta$$^{13}$C$_{PDB}$ of -26.1 to 4.1 per mil and $\delta$$^{18}$O$_{VSMOW}$ of 30.0 to 36.4 per mil. No obvious changes in N, TOC and carbonate contents and isotope concentrations are observed in sediments in and near the fault zones and the d\'{e}collement, indicating that fluid flow in the Central American convergent margin had a minimal effect on the TOC and N concentrations and isotopic compositions of the sediments. However, the impact of fluid processes on the sediment geochemistry could be indicated by extreme $^{13}$C-depletion in minor carbonate in the sediments; this depletion could have been produced biogenetically by incorporation of methane C. Regarding the mass-balance across the Central American subduction zone, if the C and N loss in sediments due to fluid processes and early devolatilization is not significant, the large amount of subducting C and N (even without considering the potentially comparable C and N inputs in altered oceanic crust and tectonically eroded material), relative to the smaller output fluxes in arc volcanic gases, indicates significant transfer to the deep earth, likely affecting the C and N isotopic characteristics of the mantle.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting