Total organic and inorganic carbon were measured onboard the drill ship and in shore-based laboratories using six different methods. Shipboard coulometry was used for analysis of total carbon (TC) and TIC during Leg 112. For analytical procedures, we refer to the "Explanatory Notes" chapter in the Leg 112 Initial Reports volume (Shipboard Scientific Party, 1988) and Emeis et al. (1990). During Legs 138 and 201, a carbon-nitrogen-sulfur elemental analyzer was available for TC measurements (Mayer, Pisias, Janecek, et al., 1992; D'Hondt, Jørgensen, Miller, et al., 2003).
Additional TOC data are available from Rock-Eval pyrolysis, from both shipboard (Suess, von Huene, et al., 1988; Mayer, Pisias, Janecek, et al., 1992; D'Hondt, Jørgensen, Miller, et al., 2003) and Leg 112 shore-based measurements (Emeis et al., 1990; Emeis and Morse, 1990).
A high-resolution profile of carbonate and organic carbon for Leg 112 Site 680 was analyzed on shore by Wefer et al. (1990) using the method of Weliky et al. (1983). Using a combined wet-oxidation acidification method, carbonate and organic carbon were discriminated and CO2 was analyzed using a thermal conductivity detector.
During Leg 201 shore-based investigation, samples from Sites 1227, 1229, and 1230 were analyzed for d13C of TC and weight percent of TC at University of Southern California (Los Angeles, USA) using an Isoprime GV Instruments mass spectrometer interfaced with Carlo Erba 2500 NC elemental analyzer in the continuous flow mode. Precision (2s) for daily runs for d13C is ± 0.1‰ or better, based on internal standards. Precision for TC (weight percent) is 0.1 wt% or better (based on duplicate runs on different days). Weight percent TOC is reported based on the isotopic mass balance, where it was assumed that d13C of TOC is –21‰ and d13C TIC is 0‰. Mass balance calculations using d13C values of –20‰ or –22‰ for organic matter produced variations of estimated TOC content of ±0.15 wt%. Compared to the high variations of ±5 to 10 wt% measured at the Peru margin shelf sites, a possible contribution of terrestrial organic matter would only slightly affect TOC values (< ±1 wt%) calculated by this method.
High-resolution profiles of total organic matter (TOM) in the uppermost 5–10 meters below seafloor (mbsf) at the Peru margin sites were also measured during Leg 201 shore-based investigation at the Department of Environmental Sciences (University of Technology at Sydney, Australia), using the method of Dean (1974). Between 0.4 and 1.2 g of dried sediment was combusted for 1 hr at 550°C in a Moloney furnace. The difference in weight at room temperature before and after the combustion (TOM in weight percent) was converted to TOC using the formula of Redfield et al. (1963) for the average composition of marine organic matter. As mixing with terrestrial organic matter would increase the carbon concentration of the organic matter, using the Redfield ratio gives minimum values for the TOC. But in any case, a contribution of terrestrial material at the Peru margin sites, would not strongly affect TOC compared to the high variations observed at these sites. For measurement of inorganic carbon, the residual TOM sample was combusted for an additional hour at 1000°C.
A few samples from Legs 201 and 112 (including the dolomitic samples; 10 mg each) were analyzed on shore using a UIC, Inc. coulometer system at the Geological Institute, ETH Zurich (Switzerland).