METHODS

Inorganic carbon was determined on board the JOIDES Resolution and details of methods are published elsewhere (Sacks, Suyehiro, Acton, et al., 2000). Briefly, ~10 mg of freeze-dried, powdered sediment was reacted with 2-N HCl. The evolved CO₂ was colormetrically titrated using monoethanolamine. Quantification of the evolved CO₂ was based on pure calcite standards. Carbonate content (in weight percent) was calculated from the inorganic carbon (IC) content, assuming that all the carbonate was present as calcite and using the following equation:

CaCO₃ (wt%) = IC (wt%) x 8.332.

Total carbon (TC), hydrogen, nitrogen, and sulfur were analyzed using a Carlo Erba 1500 CNS analyzer. About 10 mg of freeze-dried, powdered sediment was combusted with V₂O₅ at 1000°C in a stream of oxygen. Nitrogen oxides were reduced to N₂ gas, and the mixture of evolved gases was separated by gas chromatography. Detection of separated gases was performed on a thermal conductivity detector (TCD) using sulfanilamide as a calibration standard. The amount of organic carbon (C_org) was calculated as the difference between total carbon and IC. It is possible, however, that not all carbonates reacted with dilute HCl, resulting in an overestimation of C_org values. To verify C_org content, freeze-dried, powdered samples of known weight were reacted with vapors of concentrated HCl overnight. The residue was freeze-dried and combusted in a LECO C/S elemental analyzer. Differences in C_org content obtained from these two protocols were <5%. For isotopic analyses, aliquots of freeze-dried, decalcified samples were placed in quartz tubes with ~1 g of CuO. The tubes were sealed under vacuum and placed in an oven at 850°C overnight. The evolved CO₂ was cryogenically purified and collected in a pyrex tube. The purified CO₂ was isotopically analyzed on a Finnigan MAT 252, using triphenylamide as an internal standard. Isotopic values are reported in per mil, according to the following standard equation:

¹³C = (R_sample/R_standard - 1) x 1000,

where R = ¹³C/¹²C.

Accuracy (<0.05) and precision (<0.15) for the isotopic data were established on the basis of repeated analyses of the studied samples and standards of known isotopic composition.