Figure F2. Pore water concentration and carbon isotopic composition profiles at Leg 172 Sites 1058 and 1059, plotted as a single hole location (separated laterally by ~4 nmi). Depth is plotted as meters corrected depth (mcd) below the sediment-water interface (see Keigwin, Rio, Acton, et al., 1998, for explanation). Data above 114.31 mcd (96.15 mbsf) are from Site 1059; data below this depth are from Site 1058. A. The entire sampled sediment column including the sulfate reduction zone (SRZ) and methanogenic zone. B. The upper 30 m of the sediment column showing profiles near the methane-sulfate interface (SMI). Sulfate, methane, CO₂, and alkalinity values are reported in Table T1. Methane concentration is reported as micromolar units (µM)(Keigwin, Rio, Acton, et al., 1998); other concentration units are millimolar (mM). Carbon isotopic values are reported in per mil units (, relative to the Peedee belemnite [PDB] standard). CO₂ isotopic values are tabulated in Table T1; methane isotopic values appear in Table T2. In the shallow ¹³C_CO2 profile, the shaded portion of the profile illustrates methane carbon in the CO₂ pool likely derived from methane through anaerobic methane oxidation, because marine sedimentary organic matter (SOM) is typically more ¹³C-enriched than -30 PDB (Deines, 1980). The hatched portion of the curve shows carbon likely derived from methane because the isotopic composition of average SOM in sediments of the Carolina Rise and Blake Ridge is -21 (e.g., Borowski, 1998). The lower boundary of the curve is defined by a data point (-8.4, 33.45 mcd) that is not shown.