Freeze-dried samples were analyzed for calcium carbonate using the carbonate bomb technique of Müller and Gastner (1971). The carbonate bomb procedure differs from the coulometrics procedure (Engleman et al., 1985) used on board the JR to determine carbonate carbon concentrations but yields comparable results (P. Meyers, unpubl. data). Weighed samples were reacted with 3N HCl, and the volume of CO2 released from each sample was measured and compared to the volumes released from known amounts of pure CaCO3 to determine the percentage in the sample. The carbonate-free residue remaining after acid treatment was collected, rinsed, and dried for use in CHNS determinations.
Amounts of organic carbon and nitrogen in the carbonate-free residues were directly measured with a Carlo Erba 1108 CHNS analyzer. This procedure involves heating the sample at 1020ºC and measuring the combustion products by gas chromatography (Verardo et al., 1990). Known amounts of sulfanilamide (C6H8N2O2S) are used to calibrate the instrument and to calculate the quantities of C, N, and S released from each sample. Total organic carbon (TOC) concentrations were calculated on a whole-sediment basis, adjusting for the carbonate concentrations determined from the bomb technique. C/N ratios were calculated on an atomic basis.
Meyers and Silliman (1996) compared the direct determination procedure of the TOC concentration of carbonate-free sediment samples used here with the by-difference analysis that is routinely used on board the JR and that was employed to measure TOC concentrations in sediments from the other 12 Leg 175 sites (Wefer, Berger, Richter, et al., 1998). The direct procedure is more reliable at TOC concentrations of <0.1 wt%, but agreement between the two procedures is good above this value.