The hydrothermal plume samples reported here were acquired aboard R/V Melville during two short cruises of opportunity conducted in 2010 (MV 1003) and 2012 (MV 1205). Surveys along the axis of the ridge-crest were conducted using the ship's Seabird 911+ CTD rosette. A combination of tow-yo, vertical casts, and "pogo" stations were employed. An ultra-short baseline (USBL) navigation beacon was attached to the CTD-rosette for all deployments to ensure that we could navigate precisely where all samples and ancillary data were collected, as well as their sample depths.
Samples collected for Helium analyses were processed at the NOAA/PMEL Helium Isotope Laboratory in Newport, OR. The gas and liquid phases of the cold-welded samples were separated using a high-vacuum extraction line. The content of each sealed Cu tube was dropped into an evacuated flask and continuously stirred with a magnetic stirrer during the extraction process. A combined charcoal-LN₂ trap was then used to pump the gas phase into aluminosilicate ampoules during the 15 minute-long gas extraction process. The ampoules were subsequently sealed with a hot flame and stored dry until analysis. Isotope ratios and concentrations of helium were determined using a dual collector, 21 cm radius, sector-type mass spectrometer specially designed for helium isotope analyses. The precision for the helium isotope ratios in seawater samples averaged 0.2% in δ³He, where δ³He is the percentage deviation of the isotopic ratio R=³He/⁴He in the water sample from the atmospheric ³He/⁴He ratio Rₐ= 1.39 x 10⁻⁶: δ³He =((R/Ra)-1)*100 [%].
Methane concentrations were determined at sea. After equilibration, the headspace gas was injected into a SRI 8610C gas chromatograph. Separation of CH₄ was done using a 15 m long Molecular Sieve 5A column and CH₄ concentrations were measured with a flame ionization detector. The measured background seawater CH₄ concentration was 0.4 nM. Sampling and analytical precision, determined through replicate draws, was <2.5% of the measured concentrations or ±0.1 nM, whichever is greater.
Analyses for total dissolvable iron (TDFe) and manganese (TDMn) were conducted on samples selected from Cruise MV1003 (2010) based on shipboard dissolved CH₄ data. Those analyses were conducted using Mg-precipitation, isotope dilution, and inductively coupled plasma mass spectrometry (ICP-MS) following standard methods described elsewhere [Saito and Schneider, 2006; Noble et al., 2008]. Briefly, unfiltered seawater samples were acidified to pH 1.7 using high purity HCl (SeaStar Inc) and then left for 4 months to allow the dissolution of labile metals in weak acid. For each analysis 13.0mL of processed sample was decanted into a centrifuge tube, spiked with ⁵⁷Fe stable isotope and precipitated using a small amount of high-purity ammonia (SeaStar Inc.) then centrifuged for 3 min at 3000 rpm (1460 x g) and decanted. The sample was resuspended in 5% high-purity nitric acid with 1ppm indium (In) and analysis was conducted on an Element 2 ICP-MS using an Aridus desolvator and platinum X-cones. Fe and Mn concentrations were calculated using the ⁵⁷Fe for isotope dilution and In as a recovery tracer [Saito and Schneider, 2006]. Precisions in the measurements were typically better than ±1.0nM for Fe and ±0.2nM for Mn.
All relevant metadata (precision, accuracy, etc.) are published in German et al., 2022 (doi: 10.1029/2021GC010317).
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