Changes in the C-N reservoirs in the sediments sampled at Site 1040 likely reflect changes in productivity and sediment sources with superimposed effects of diagenesis. The deformed wedge sediments at this site, thought to be older than 3 Ma (Morris et al., 2002), have relatively high concentrations of organic matter, containing up to 2400 ppm N and 2 wt% TOC. If these deformed wedge sediments are considered Pliocene or older in age, as indicated by 10Be concentrations (Morris et al., 2002), their C-N concentrations can perhaps be compared with the Pliocene part of the subducting section represented at Site 1039, just outboard of the present-day trench. Differences in the two older sediment reservoirs (Pliocene part of section at Site 1039 and deformed wedge) are consistent with the expected differing proximities of the Site 1039 section and wedge sediments to the trench and continental sediment sources during the Pliocene, with organic productivity and the terrestrial organic component increasing toward the continent. The uppermost, less deformed part of the section drilled at Site 1040 (< 130 m), corresponding to Subunit P1A and the uppermost part of Subunit P1B, shows upsection decreases in TN concentrations (from ~2000 ppm to ~1000 ppm at the top of the section) at a relatively constant TOC concentration of ~1.5 wt%, producing large changes in TOC/TN. We suggest that this increase in TOC/TN (from ~8 at 100 mbsf to >18 at the surface) could represent a somewhat more intact record of changes with time in the sources of the organic matter, with increasing or changing terrestrial/continental organic contributions in these younger, less deformed slope apron sediments interpreted as being debris flow deposits and turbidites. It is not possible, based on our data, to distinguish between origins of the deformed wedge sediments below ~80–100 mbsf as either Pliocene, or somewhat older, near-trench sediment offscraped from the incoming plate or Pliocene or somewhat older, slope sediment deformed within the wedge during slumping and formation of thrust faults resulting from proximity to the evolving décollement. The results presented here also do not bear directly on the issue of whether the Costa Rica convergent margin is at present an erosional margin, on a broader scale (see Vannucchi et al., 2003).
Small perturbations in C-N concentrations and isotopic compositions of deformed sediments in the décollement zone at Site 1040 could reflect enhanced infiltration by C-N-bearing fluids within this structurally complex zone. Although a large fraction of the organic N initially subducted in sediments is thought to be retained to great depths in subduction zones, loss of small amounts of C and N over the extremely large volumes of sediment subducting into this margin could result in C-N mobility sufficient to produce such shifts where fluid flow is highly focused.