HR: 1330h
AN: T32A-0908    [PDF]
TI: Carbon and Nitrogen Input Fluxes in Subducting Sediments at the Izu-Bonin and Central America Convergent Margins
AU: * Li, L
EM: lol2@lehigh.edu
AF: Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015 United States
AU: Sadofsky, S J
EM:
AF: Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015 United States
AU: Sadofsky, S J
EM:
AF: Present address, SFB 574, University of Kiel, Kiel, 24148 Germany
AU: Bebout, G E
EM:
AF: Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015 United States
AB: We are investigating the N and C contents and isotope ratios in subducting sediment sections in the two MARGINS Subduction Factory focus sites, the Izu-Bonin (IB) and Central America (CA) convergent margins (using samples obtained on DSDP/ODP Legs 125, 170, 185, and 205). The sediments from IB (Site 1149, and Leg 129) contain 5 to 661 ppm N with $\delta^{15}$N$_{Air}$ values of +2.5 to +8.2 per mil (weighted average +4.6 per mil). Reduced-C concentrations range from 0.02 to 0.35% with $\delta^{13}$C$_{PDB}$ values from -28.1 to -21.7 per mil. Calcite in carbonate-rich layers has $\delta^{13}$C of +1.7 to +2.8 per mil and $\delta^{18}$O$_{VSMOW}$ of +28.5 to +29.7 per mil. In comparison with IB, the CA sediment section (Site 1039) has far higher N content (663 to 2380 ppm N with $\delta^{15}$N of +3.9 to +7.1 per mil; weighted average +5.6 per mil). Calcite in Site 1039 carbonate-rich layers has$\delta^{13}$C of +0.1 to +3.0 per mil and $\delta^{18}$O of +29.9 to +32.1 per mil. At Site 1149, down-section decrease in N content, accompanied by decrease in $\delta^{15}$N and C/N, is thought to reflect diagenesis, whereas at Site 1039, down-section decrease in N content (near 2000 ppm at surface, near 1000 ppm at 150 km) occurs without an obvious shift in $\delta^{15}$N (reduced C data not yet available). Based on the C-N concentration data we've obtained, sediment C-N input fluxes are estimated at, for the IB margin, 2.5$\times$10$^{6}$ g/km.year for N, 1.7$\times$10$^{7}$ g/km.year for reduced C, and 9.2$\times$10$^{8}$ g/km.year for oxidized C. For the CA margin, we estimate input fluxes of 8.9$\times$10$^{6}$ g/km.year for N, and 1.3$\times$10$^{9}$ g/km.year for oxidized C (work on reduced C is underway). Our input fluxes for C and N differ significantly from previously published input fluxes for the two margins based on estimated subducting sediment C-N concentrations, but errors are large. For the CA margin, the sediment-only N input flux of 9.8$\times$10$^{9}$ g/year (for the entire 1100 km trench length), based on our results for Site 1039, can be compared with the arc volcanic output flux of 8.1$\times$10$^{9}$ g/year of Fischer et al. (2002, Science; N input flux of 6.4$\times$10$^{9}$ g/year estimated by those authors), indicating the incomplete return of subducted N to the surface in arcs. The magnitude of the N input flux in altered oceanic crust remains unknown (work on AOC N-$\delta^{15}$N is underway) but should also be considered in such comparisons. All estimates of this type assume uniformity in the incoming sediment section along-strike in active trenches (known not to be the case). Shifts from $\delta^{15}$N values measured for the two sediment sections to values near +7 per mil for the deeply subducted sediment component as suggested by studies of volcanic gases (e.g., Fischer et al., 2002; Hilton et al., 2002) could be accomplished by moderate loss of isotopically light N during metamorphic devolatilization across forearcs.
DE: 0312 Air/sea constituent fluxes (3339, 4504)
DE: 0330 Geochemical cycles
SC: Tectonophysics [T]
MN: 2003 Fall Meeting