IODP Proceedings    Volume contents     Search



PFT delivery

PFT was introduced into the drilling fluid (DF) stream at a rate calculated to achieve a concentration of 0.89 mgPFT/LDF. The injection rate was controlled with a single-piston high-pressure liquid chromatography (HPLC) pump, which pumps the PFT from a polypropylene carboy into the drilling mud stream. For each hole, we started pumping the tracer early enough to ensure that the tracer reached the drill bit well before drilling operations began. We kept track of drilling fluid PFT concentrations by monitoring the drilling mud pump rate (in liters per minute) and the PFT delivery rate (in milliliters PFT per minute) using shipboard instrumentation software (an example for Site U1370 is shown in Fig. F2).

Sample collection

Unconsolidated sediment was sampled on the catwalk for PFT analyses immediately after core retrieval. PFT samples were consistently sampled adjacent to the section/whole round that had been selected for microbiological analysis. Two cylindrical plugs (3 cm3 each) were taken from the bottom end of each section with 5 mL cut-off syringes. One plug was taken from the outer edge near the core liner to confirm successful delivery of the tracer (exterior samples) and the other was taken from near the center of the core to quantify drill water intrusion to the core center (interior samples). The samples were immediately extruded into 20 mL capacity headspace glass vials containing iso-octane (Sites U1365 and U1366) or water (Sites U1367–U1371) and sealed with gas-tight caps containing septa.

PFT analysis (postcruise)

The low solubility of PFT in water facilitates gas partitioning, allowing a quantitative headspace analysis via gas chromatography (GC). Due to the lack of an appropriate GC column, PFT measurements were not possible during the expedition. Therefore, we quantified the PFT concentration in the samples postcruise using a Shimadzu 17A gas chromatograph with an electron capture detector (ECD) at the Graduate School of Oceanography, University of Rhode Island. We heated the headspace vials containing the sediment samples for at least 30 min in a 70°C oven in order to release the tracer from the sediment. We directly injected 2.0 mL of headspace gas from samples, standards, and blanks manually using 3 mL plastic syringes and needles. Syringes and needles were similarly heated for 30 min at 70°C prior to injection in order to minimize absorption of PFT into the interior of the syringe and to negate sample cross-contamination. The oven temperature was set 6°C lower than the PFT boiling point (76°C) to avoid melting the plastic syringes during the heating process. Because the temperature was slightly below the PFT boiling point, PFT extraction may not have been complete. However, because PFT readily evaporates due to its inherent low viscosity and low surface tension, we believe that most of the PFT was extracted. For description of a detailed analytical protocol, we refer to Smith et al. (2000) and House et al. (2003).

Calibration, instrumental and procedural blanks, and detection limit

For standards, we diluted PFT in ultrapure methanol to 10–2, 10–4, 10–6, and 10–8 vol:vol in 20 mL capacity headspace vials (see Appendix A). We constructed a standard curve by plotting the PFT peak area versus PFT mass injected, resulting in a log-log correlation. We calculated the amount of PFT (in grams) contained within an injection by using the formula described in Appendix A. Further details on GC calibration slopes are presented in Appendix B. We analyzed both procedural and instrumental blanks to determine experimental backgrounds. Each instrumental blank consisted of a 2 mL injection of air collected in a heated gas-tight syringe from outside the laboratory. We also ran procedural blanks consisting of a 2 mL injection of ambient coring air (headspace gas sealed in an empty vial at time of sample collection), when available. Procedural blanks were collected for Holes U1367D, U1370E, U1370F, and U1371E. To calculate the detection limit, we used the average value and standard deviation of all blank runs combined. We present further details regarding detection limit analysis in Appendix C.

Estimates of maximum potential of drilling fluid intrusion and nonindigenous cells

We calculated the volume of drilling water intrusion based on the PFT content of each sample and assuming a PFT concentration of 0.89 mg/L in the drilling fluid. A detailed description of the calculation method is given in Appendix D. We then used the calculated volume of drilling water in each sample to estimate the potential number of contaminant cells. This estimate is usually based on the measured density of cells in the drilling fluid (Lever et al., 2006). However, because no drill water samples were taken for cell counts during Expedition 329 coring operations, we used the average surface seawater cell count from the 2006 site survey expedition, KNOX-02RR (1.7 × 105 cells/mL; see D’Hondt et al., 2011).