Although the samples were measured using different methods, the overall patterns and the range of values were reproducible (Figs. F2, F3, F4, F5, F6, F7, F8, F9; Tables T1, T2, T3, T4, T5; only new data are listed in the tables). Dolomitic samples typically show a much lower TOC content and do not follow the general trends (Tables T2, T3, T4, T5; not shown in the figures).
TIC values measured from the eastern equatorial Pacific Sites 1225/851 and 1226/846 vary in major part between 6 and 10 wt%, reflecting the high content of calcareous nannoplankton (Mayer, Pisias, Janecek, et al., 1992) (Figs. F2, F3; Table T1). In contrast, the late Miocene carbonate crisis is well documented by a low TIC content in the diatom-rich zone between 200 and 300 mbsf at Site 1225/851 and at 262.7 and 317 mbsf at Site 1226/846 (Mayer, Pisias, Janecek, et al., 1992). At Site 1226/846, the TIC content is also relatively depleted in upper Pliocene (50–70 mbsf) and upper Pleistocene sediments (uppermost 10 mbsf).
TOC at Site 1225/851 varies between 0 and 0.4 wt% throughout the profile (Fig. F2), whereas at Site 1226/846, the TOC value reaches 1–2 wt% in the diatomaceous zones (Fig. F3; Table T1). The TOC-rich zone at Site 1226/846 between 270 and 315 mbsf correlates with the peak Mn concentration and probably reflects high Mn-reducing microbial activity (D'Hondt, Jørgensen, Miller, et al., 2003).
Peru margin sediments generally have a low inorganic carbon content (mostly <2.5 wt%), reflecting the upwelling-related siliceous and siliciclastic delivery (see "Lithostratigraphy" sections in the site chapters in D'Hondt, Jørgensen, Miller, et al., 2003) (Figs. F4, F5, F7; Tables T2, T3, T4). TIC decreases from average distal Site 1227 values of 2 wt% (Fig. F4) to <1 wt% at Site 1229 (Fig. F7), which is located at the center of the upwelling cell (Suess, von Huene, et al., 1988). Zones with ~20 wt% CaCO3 tend to occur in diatomaceous layers at Sites 1227 and 1228, whereas carbonate is almost absent in siliciclastic sediments.
TOC values at Site 1227 vary between 1 and 11 wt% (Fig. F4; Table T2). They decrease from ~6 wt% in the uppermost diatom ooze, which is late Pleistocene in age (Skilbeck and Fink, this volume), to 2 wt% in a winnowed coarse foraminiferal sand layer at 12 mbsf. Another low-TOC peak appears at the base of Pliocene siliciclastic sediments, in a winnowed glauconitic sand at 50 mbsf reported in D'Hondt, Jørgensen, Miller, et al. (2003). These two zones of low TOC in the condensed stratigraphic column correlate with high natural gamma radiation (D'Hondt, Jørgensen, Miller, et al., 2003) and are probably horizons of low deposition and erosion. Elevated oxygen indexes at these depths (400 at 12 mbsf and 200 at 50 mbsf; Suess, von Huene, et al., 1988) might be due to degradation of organic matter. Bacterial cell numbers (D'Hondt, Jørgensen, Miller, et al., 2003) match with TOC concentration at this site.
In the uppermost 1.5 mbsf at Site 1228, the highest TOC values, up to 12 wt%, were measured in Holocene siliciclastic sediments (Figs. F5, F6; Table T3). In general, the Pleistocene–Holocene sediments (0–56 mbsf) show 10-m-scale variations from 1 to 8 wt% TOC (Wefer et al., 1990). These variations correlate with variations in diatom content and color reflectance values (D'Hondt, Jørgensen, Miller, et al., 2003) (Fig. F5). They were interpreted by Wefer et al. (1990) as glacial–interglacial cycles. A relative low TOC interval is present at ~60 mbsf and is correlated with the top of Pliocene siliciclastic sediments. Rock-Eval pyrolysis data indicate immature organic matter of marine origin (kerogen type II; Suess, von Huene, et al., 1988). Below 100 mbsf, all TOC values are <1 wt% in the Pliocene siliciclastic sediments.
In the Holocene surface sediments of Site 1229, TOC values up to 7 wt% rapidly decrease to 2 wt% at 5 mbsf (Fig. F7; Table T4). Between 10 and 40 mbsf, relatively high TOC values up to 6 wt% were measured. Variations in TOC match the paleobathymetric curve based on a study of benthic foraminifers (Resig, 1990) and can thus be correlated with eustatic sea level variations. Intervals with more siliclastic input generally show increased oxygen indexes (kerogen type III), indicating degradation or a terrigenous origin of the organic matter. A middle Pleistocene siliciclastic zone at 40–100 mbsf is generally low in TOC, except in the interval from 90 to 100 mbsf, where TOC values are >3 wt%. This relatively high TOC layer correlates with the lower boundary of the methanogenic zone, where the highest bacterial cell concentrations up to 109 cell/cm3 were counted (D'Hondt, Jørgensen, Miller, et al., 2003). The occurrence of diagenetic dolomite layers also correlates with TOC (Meister et al., in prep.).
All three Peru shelf sites are characterized by a relatively thick sulfate reduction zone. An S-shaped curve of dissolved sulfate in the pore water at Site 1229 (D'Hondt, Jørgensen, Miller, et al., 2003) is reflected in the TOC profile. This can be explained by the presence of erosion surfaces and noncontinuous sedimentation at the Peru shelf sites (Sites 1227, 1228, and 1229), allowing the pore water sulfate to diffuse deeper into the sediment (see also Skilbeck and Fink, this volume; DeVries and Schrader, 1981). The non-steady-state conditions seem to be reflected in the high variation of the TOC values.
In general, TIC values are <1 wt% throughout the recovered series drilled at Site 1230 on the lower slope of the Peru margin (Fig. F8; Table T5). TOC values scatter between 2 and 3 wt% in the upper 200 mbsf but decrease to 1–2 wt% below 250 mbsf. The 1- to 2-wt% decrease at 200 mbsf correlates with a decrease in diatom content but is also associated with increased compaction and deformation related to tectonic processes in the accretionary prism (Kemp and Lindsley-Griffin, 1990). At 230 and 340 mbsf, peak concentrations of >4 wt% TOC were reached.
Despite the relatively low TOC, sulfate is consumed rapidly in the uppermost 7 mbsf (see D'Hondt, Jørgensen, Miller, et al., 2003). Below, methanogenic activity and gas hydrates are present.
A few TOC values of ~0.5 wt% were measured in the open Peru Basin during DSDP Leg 34 (Yeats, Hart, et al., 1976). They reflect the relatively low microbial activity found at this site (D'Hondt, Jørgensen, Miller, et al., 2003) within low-productivity open-ocean sediments (Fig. F9). Values decrease to near 0 wt% in the diatom-free nannofossil ooze below 55 mbsf.