SAMPLE PREPARATION AND ANALYSIS STRATEGY

The samples used for this study were from several sources, and the sediments have been handled differently depending on the ODP leg on which they were recovered. All of the Leg 205 samples used in this study (Holes 1254A and 1255A) were frozen onboard after removal of pore fluid, producing squeezecakes, whereas most of the Site 1040 samples analyzed in this study were provided by R. Lutz and had been either freeze-dried and stored at room temperature or air-dried at room temperature in plastic bags.

Decomposition of organic matter after drilling, potentially affecting the C-N reservoirs in seafloor sediment samples, could be significant in these samples depending on sediment sample storage methods. The chemical pretreatments (e.g., rinsing with H₂O and dissolutions in HCl) and methods for drying samples must also be carefully considered in efforts to extract information regarding organic and inorganic C-N reservoirs. In order to evaluate these factors, we analyzed frozen Site 1254 and refrigerated Site 1040 sediment samples, which overlap in their depths of recovery with the dried samples obtained from R. Lutz, to compare the results of TOC and nitrogen concentrations and isotope compositions in samples stored (room temperature, refrigerated, and frozen), dried (frozen and low temperatures), and pretreated (H₂O or HCl rinsing) by some commonly employed methods. The results of this testing are provided in the "Appendix."

The data presented in this paper were obtained using the following methods. For analyses of carbonate (calcite), samples from Site 1040 were reacted overnight with 100% phosphoric acid at 25°C to extract CO₂ for measurement of C and O isotope compositions (McCrea, 1950). Concentrations of carbonate were determined by Hg manometry after cryogenic purification of the CO₂ (using a pentane–liquid nitrogen mixture to remove H₂O and SO₂). For analyses of TOC, dried and homogenized sediment was reacted overnight with 1-N HCl and rinsed three times with distilled H₂O before freeze-drying. Decalcified and homogenized samples were loaded into 6-mm (outside diameter) quartz tubes with Cu_xO_x-Cu reagent, sealed at high vacuum, combusted overnight at 910°C, and cracked to release CO₂, which was cryogenically purified using a pentane–liquid nitrogen trap. Concentrations of TOC were obtained by Hg manometry. For analyses of TN, bulk sediment samples without any pretreatment (except two samples from Site 1255; see Table T1) were loaded with Cu_xO_x and Cu metal reagents into 9-mm quartz tubes, which were sealed at high vacuum and combusted overnight at 910°C (Sadofsky and Bebout, 2004). The N₂ produced in these combustions was collected for isotope analysis using a molecular sieve at liquid nitrogen temperature after cryogenic purification to remove CO₂, H₂O, and SO₂. Nitrogen concentrations were determined by measuring the m/z 28 signal for calibrated inlet volumes in the mass spectrometer.

All isotope analyses were obtained using a Finnigan MAT 252 mass spectrometer in dual-inlet mode at Lehigh University (Pennsylvania, USA). ¹⁵N, ¹³C, and ¹⁸O values are reported with reference to air, Vienna PeeDee belemnite, and Vienna standard mean ocean water standards, respectively. The reproducibilities of the measured ¹⁵N, ¹³C, and ¹⁸O values were better than 0.2 (1), and uncertainties for duplicate TOC, N, and carbonate concentration measurements were <5%.