Carbon and nitrogen geochemistry of wedge sediments at odp site 1040: Evidence for sediment sources, diagenetic history, and fluid mobility

We determined the C and N concentrations and isotopic compositions of sediments in the prism sampled during Ocean Drilling Program Legs 170 and 205 offshore Costa Rica, with the goals of evaluating sediment sources and extents of diagenesis and identifying any effects of infiltrating fluids on the sedimentary C and N. The sediments from Leg 170 Site 1040 contain 0.85-1.96 wt% total organic carbon (TOC) with Vienna Peedee belemnite (VPDB) δ ¹³C _VPDB from -26.3‰ to -22.5‰, and 832-2221 ppm total nitrogen (TN) with δ ¹⁵N _air from +3.5‰ to +6.6‰. Sediment TN concentrations and δ ¹⁵N values show dramatic downhole increases within the uppermost 130 m of the section and more gradual downhole decreases from 130 meters below seafloor (mbsf) to the base of the décollement at ~370 mbsf. Concentrations and isotopic compositions of TOC are relatively uniform within the entire section, showing some minor perturbation within the décollement zone. In the uppermost 100 m, upsection increases in TN concentrations at constant TOC concentrations produce significant increases in atomic TOC/TN ratios from ~8 to ~18. Carbonate (calcite) contents in the wedge sediments are generally low (<4 wt%), but the δ ¹³C and Vienna standard mean ocean water (VSMOW) δ ¹⁸O _VSMOW values vary significantly from -26.1‰ to +4.1‰ and from +30.0‰ to +35.3‰, respectively. Concentrations and isotopic compositions of TOC and TNfor sediments from Leg 205 Sites 1254 and 1255 overlap well with C-N data for sediments from the same depth intervals obtained during Leg 170 at Site 1040. At Site 1040, effects of diagenesis on the sedimentary C and N reservoirs appear to be overwhelmed by larger variations in these reservoirs related to changes in paleoproductivity and sediment sources. Previously published ¹⁰Be data for the deformed wedge sediments indicate that the sediments are at least 3-4 Ma in age, and as with some major and trace element compositions (e.g., Ba concentrations), the C-N concentration data for the wedge sediments at Site 1040 show more similarity with data for the slope apron section at Site 1041 than with the Pliocene hemipelagic part of the incoming sediment section at Site 1039 (~120-140 mbsf). The deformed wedge sediments below 130 mbsf at Site 1040 are more similar in C-N concentrations to the younger Pleistocene hemipelagic section at Site 1039, but in addition to their differing major and trace element concentrations, they have C and N isotope compositions that indicate a terrestrial organic signature larger than that represented in the hemipelagic sediment section at Site 1039. It is not possible, based on the C-N data presented here, to distinguish between the differing proposed origins of the deformed wedge sediments below ~80 mbsf at Site 1040. Sediments in the fault zones and décollement show hints of differences in C-N concentrations and isotopic compositions relative to sediments away from these structures, indicating some possible effects of infiltrating fluids on the sedimentary C and N in these zones. However, for C and N concentrations and isotopic compositions, dilution by more locally derived pore waters probably obscured signatures of the passage of more deeply sourced fluids.