Trends in Basalt and Sediment Core Contamination During IODP Expedition 301

Abstract

Perfluorocarbon tracers (PFTs) are used during cruises of the Ocean Drilling Program (ODP) and Integrated Ocean Program (IODP) to measure sample contamination with drilling fluid. Drilling fluid is supplied with a constant PFT concentration that can then be detected and quantified in sediment and basalt core samples. During IODP Expedition 301, we used washing (2×) and flaming to effectively remove PFT from the exterior of basalt rocks. Near-complete removal from the exterior allowed us to demonstrate that the interior of basalts was only minutely, if at all, contaminated with drilling fluid. We examined horizontal and vertical trends in sediment core contamination. Contamination decreased greatly between the core exterior to halfway along the core radius, and slightly from halfway to the center of cores, and was generally very low in halfway and center portions. Clay cores were, on average, more contaminated than cores with fine sand. Contamination was typically highest in the two uppermost sections (sections 1 and 2) and lower below (sections 3–5). There was no relationship between depth of core origin and contamination. To determine mechanisms of contamination in halfway and interior parts of cores, we estimated the diffusive flux of PFT from the core liner towards the core center. Based on conservative estimates, we concluded that diffusion did not account for any of the PFT measured in halfway and interior parts of cores in this study. Any measurable PFT concentrations in halfway and center parts of cores were caused by advection.

Keywords:

• perfluorocarbon
• tracer
• chemical
• basalt
• sediment
• subsurface
• marine
• ODP
• IODP

Acknowledgments

We would like to thank Dennis Graham (IODP shipboard technical staff) for his invaluable technical support in operating the gas chromatograph, as well as Andrew T. Fisher (Expedition 301 co-chief scientist), Marc Gasser-Rubinat and Casey Miller (both UNC Chapel Hill) for their helpful advice in the interpretation of PFT data. Moreover, we would like to gratefully acknowledge the NASA Astrobiology Institute “Subsurface Biospheres,” the Joint Ocean Alliances, and the Integrated Ocean Drilling Program for financial and logistical support.

Notes

^a All values except extreme outliers.

^b All values measured in all 4 studies.

^c All values measured in all 4 studies except extreme outliers.