The PFT concentrations measured for all sediment samples are listed in Table T1, and a summary of the data is also shown in Table T2, sorted by sampling location (center of the core vs. core periphery), drilling technique (advanced piston corer [APC] vs. extended core barrel [XCB]), and site (deepwater sites vs. continental margin and shelf sites). This comparison reveals three factors that influence the potential for seawater contamination of sediment cores.
The potential seawater contamination for Leg 201 sites is in a similar range as previous data for ODP Site 1149 southeast of Japan on the seaward side of the Izu-Bonin Trench at a water depth of 5830 m (Table T3) (Smith et al., 2000b).
For six cores from deepwater Holes 1226E and 1230B, PFT samples were obtained with 1-mL syringes diagonally across freshly cut core sections on the catwalk. After sampling, the sediment samples were placed into 2-mL GC vials, closed in the open air to avoid contamination with ubiquitous laboratory PFT, and weighed before GC analysis. The PFT concentrations in core cross sections do not always follow a predictable gradient, with maxima at the core liner and minima in the center of the core (Table T4). On average, the six transects show lower PFT concentrations in the samples from the inner core (samples 2-6; average = 0.012-0.044 ng PFT/g sediment) and elevated PFT counts in the outer samples at the core liner (samples 1 and 7; average = 0.797 and 0.031 ng PFT/g sediment).
Contamination tests are especially significant in evaluating contamination risks for samples that are used for microbial cultivations. Sediment samples are mixed anaerobically (under nitrogen) with sterile saline solution on a 1:4 volume basis. These slurries are then used for a variety of microbial enrichments and quantifications, including most probable number dilutions on defined media or extracts of natural substrates (see "Microbiology" and media recipes in Tables T4, T5, and T6, all in the "Explanatory Notes" chapter).
Table T5 shows the data for potential seawater contamination and microsphere counts for slurry samples and allows an assessment as to whether detectable concentrations of the easily diffusible PFT tracer coincide with visible microspheres. Whereas the chemical tracer indicates the extent of seawater contamination and influx of dissolved compounds, the prokaryote-sized microspheres demonstrate that prokaryotes could actually penetrate the sediment. In other words, they outline the prokaryote contamination potential. Three slurry samples with high potential seawater contamination, in the range of 0.05-0.4 µL seawater/mL slurry (Sections 201-1226B-2H-3 [center]; 201-1229A-11H-5 [center]; and 201-1230A-38X-1 [center]), also contained microspheres, bacterial mimics. These slurry samples are most likely contaminated. In slurry samples with potential seawater contamination levels below ~0.05 µL/mL, microspheres were not found at all or were observed a single time. A possible source of experimental error was found in Section 201-1226B-47X-2, which had high microsphere counts but no detectable PFT. Prolonged gassing of the anaerobic slurry with nitrogen could have removed the PFT tracer, while retaining the microspheres.