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doi:10.2204/iodp.proc.344.209.2018

Methods

Pore fluid sampling

Whole-round cores recovered, cut, and capped on the catwalk were taken to the shipboard laboratory. Samples collected between the seafloor and 50 meters below seafloor (mbsf) were processed in a nitrogen-filled glove bag to avoid oxidation of redox-sensitive elements. All other samples were processed under ambient atmospheric conditions. Cores were generally processed within 24 h of recovery and were kept capped in a nitrogen-filled environment at 4°C until processing. For pore fluid collection, after extrusion from the core liner the surface of each whole-round sample was carefully scraped with a spatula to remove potential contamination from seawater and sediment smearing. The sediment was then loaded into a titanium squeezer, modeled after the stainless steel squeezer of Manheim and Sayles (1974), and subjected to pressures as high as 30 MPa but generally <20 MPa using a hydraulic press. As the sediment is pressurized and squeezed, pore fluid passes through a prewashed Whatman Number 1 filter and titanium screen into an acid-washed 60 mL plastic syringe attached to the bottom of the squeezer assembly. The pore fluid is then passed through an additional 0.2 µm Gelman polysulfone disposable filter and subsampled into acid-washed high-density polyethylene bottles. For trace-metal and REE analysis, the pore fluid samples were preserved with the addition of ultraclean HNO₃ to pH < 2 and stored at 4°C until analysis onshore.

REE concentration measurement

We measured pore fluid REE concentrations using the seaFAST2 system from Elemental Scientific, Inc., attached as the sample introduction unit to a Thermo X-Series II ICP-MS. The seaFAST2 system uses an ion exchange column with ethylenediaminetriacetic acid/iminodiacetic acid functional groups to selectively preconcentrate transition metals and REEs. An ammonium acetate buffer prepared as 14.5 M ammonium hydroxide and 17.4 M glacial acetic acid washes out the alkali and alkaline earth-matrix elements from the column. A solution of 2 M HNO₃ then elutes the REEs from the column and onto the ICP-MS for quantification. Of the alkali and alkaline earth elements, Ba²⁺ is particularly problematic for the analysis of REEs because Ba²⁺ can form oxides in the plasma that interfere with the signal of both ¹⁵³Eu and ¹⁵¹Eu, Eu’s two main isotopes. For the pore fluid samples in this study in particular, dissolved Ba²⁺ averaged ~1.5 µM, although values could reach as high as 18 µM in some cases (see the Expedition 344 summary chapter [Harris et al., 2013a]). Before the REEs can be accurately measured, the signal from Ba must be removed or corrected for in the measured Eu signals. The seaFAST2 system is well-suited to separating the Ba²⁺ signals from REEs in seawater, thereby minimizing the potential of isobaric interferences from the formation of oxides (e.g., Hathorne et al., 2012; Yang and Haley, 2016). The timing of the Ba²⁺ peak during the elution step was closely monitored to ascertain that there was no significant contribution from BaO formation on the Eu peak. The ICP-MS instrument was tuned to minimize oxide formation in the plasma by monitoring the formation of CeO, which was measured and minimized to levels of <2% formation.

Analytical precision was monitored through replicate measurements (N = 19) of an in-house seawater standard (NBP1097) and was quantified with a relative standard deviation of <8% for all REEs except for Sm at 11%, Ce at 20%, and Gd at 22% (Table T1). External reproducibility was monitored through replicate analyses (N = 10) of the PPREE1 acid mine drainage reference material diluted 10,000-fold to match the REE concentrations with that of seawater and pore fluid and found to fall within one standard deviation of the most probable values reported by Verplanck et al. (2001) (Table T1). Detection limits were determined as three times the standard deviation of the blank signals and are listed in Table T1. Average method blank levels were below these detection limits. Sample aliquots and instrument operating conditions for each analytical run are given in the “Appendix.”

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