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doi:10.2204/iodp.proc.308.208.2008

Methods and materials

Sediments from Brazos-Trinity Basin IV (Sites U1319 and U1320) and Ursa Basin (Sites U1322 and U1324) were sampled from cores taken during Integrated Ocean Drilling Program (IODP) Expedition 308. A total of 92 intervals were sampled for bulk chemical analysis from Brazos-Trinity Basin IV sediments, and 114 intervals were analyzed from Ursa Basin. Sediment whole rounds were sampled approximately every 1.5 m above 50 meters below seafloor and every 10 m to terminal depth. Sediments were freeze-dried and then homogenized with an agate mortar and pestle. The total organic carbon (TOC) composition of the sediment was determined from elemental analysis of total carbon (TC) and coulometric analysis of total inorganic carbon (TIC) on board the R/V JOIDES Resolution. TOC was calculated as the difference between TC and TIC (TOC = TC – TIC). Weight percent TN was also determined by shipboard elemental analysis. C/N values were calculated as the molar ratio of TOC to TN.

Carbon and nitrogen isotopic analyses were conducted at the University of Virginia Stable Isotope Laboratory. Sediment splits for organic carbon isotope analysis were directly acidified with 2N HCl to remove carbonates. The carbonate-free residues were then dried at 50°C and homogenized. Bulk sediment δ¹⁵N was reported for nonacidified sediments to preclude potential nitrogen isotope effects during the carbonate removal procedure. Bulk isotope ratios were determined on a Carlo-Erba elemental analyzer coupled under continuous flow with an OPTIMA isotope ratio mass spectrometer (GV, Manchester, UK). Carbon and nitrogen isotope values are reported according to the equation δ^xE (‰) = R_sample/R_standard – 1) × 10³, where x is the heavy isotope of the respective element (E) and R is the ratio of the heavy to light isotope (e.g., ¹³C/¹²C). Internal laboratory reference gases were calibrated against international standards for carbon (NBS-19) and nitrogen (atmospheric N₂). Analytical precision for replicate analyses of standard materials was typically δ¹³C and δ¹⁵N.

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