Dataset: GP16 Particulate Iron Speciation
Deployment: TN303

The method for measuring particulate iron speciation in filter-bound hydrothermal particles has been published several times (Toner et al. 2009; 2012; 2014; 2016; Breier et al. 2012). The specific methods and analyses for this data contribution are described in Hoffman et al. 2018.

From Hoffman et al. 2018 with minor modification:
In-situ Filtration and Sample Preservation - As part of the US GEOTRACES-Eastern Pacific Zonal Transect cruise (GP16), marine particles were collected by in- situ filtration along a ~8000 km transect that includes greater than 4000 km of hydrothermally influenced waters. Samples were filtered using a custom manifold attached to the deployed dual-flow McLane pumps. The overall in-situ pump program is described by Heller et al., 2017, Lam et al., 2018, Lee et al., 2018, and Ohnemus and Lam, 2015. The filter manifold was used to collect up to four 25 mm filters simultaneously on a third, un-metered flowpath of the McLane pumps: two 0.2 um polycarbonate filters for analysis by Scanning Transmission X-ray Microscopy (STXM) and two 0.2 um polyethersulfone (PES) filters for X-ray microprobe. Shipboard, the filter manifold was opened in an anaerobic chamber (Coy Labs) to prevent exposure of the sample to ambient oxygen, and all sample processing occurred under a nitrogen-hydrogen (95% N2, 5% H2) atmosphere (the anaerobic chamber was located within a self-built shipboard High Efficiency Particulate Arresting (HEPA) chamber). Filters were rinsed using N2-sparged 18.2 MΩ·cm purified double distilled water (MilliQ) via vacuum pump to remove seasalts (the pump was located outside of the HEPA bubble with a line plumbed to the anaerobic chamber). Replicate filters were: (1) placed in acid-washed petri-slides for X-ray microprobe analysis and archiving, and (2) placed in acid-washed microfuge tubes with 0.5 mL N2-sparged MilliQ for STXM preparation. Prior to leaving the anaerobic chamber, all samples were sealed in mylar bags with an oxygen-free atmosphere. Sample packs were then frozen at -20˚C to reduce the kinetics of chemical oxidation.

Sediment Core Collection – Sediment cores were collected using a Royal Netherlands Institute for Sea Research (NIOZ) mono-corer, specifically designed to collect surface sediment samples with minimal disturbance of the sediment-water interface which was suspended from the ODF (Ocean Data Facility) CTD (conductivity, temperature, depth)-rosette for the final deep cast conducted at that station. In a shipboard cold room (4°C), the overlying seawater was syphoned off, down to ~ 0.5 cm above the 1-2 cm thick visible fluff layer. Then, the fluff layer was syphoned off, being careful not to re-suspend the consolidated sediment, and solids were separated by centrifugation within hour of sediment core recovery. The pellet of fluff material was then frozen in the tip of the 50 mL polypropylene centrifuge tubes. Shore-based splitting of the fluff layer sample was accomplished by thawing the pellet and re-suspending in pH purified water (Milli-Q water with a small volume of ultraclean NH4OH), then removing representative sub-samples with a pipet while vortex mixing to maintain a homogeneous suspension. Prior to spectroscopic analysis, fluff layer materials were separated from aqueous solution by 0.2 um filtration.

Particulate Iron Speciation - The speciation of particulate Fe in filter-bound hydrothermal plume particles was measured using micro-focused X-ray fluorescence mapping (u-XRF) and micro-focused Fe 1s X-ray absorption near edge structure (uXANES) spectroscopy at beamline 10.3.2 of the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory, U.S.A, with previously developed methods for marine particles (Breier et al., 2012; Lam et al., 2012; Marcus et al., 2004; Toner et al., 2009, 2012, 2016, 2014). The data for this contribution were first reported in Hoffman et al. 2018. All particles were analyzed on original polyethersulfone (PES; trademark SUPOR) filters and all sample preparation was conducted within positive pressure (N2 or Ar) glove bags. The final step in sample preparation included application of a layer of sulfur-free mylar film (Premier Lab Supply TF-125-255) to limit exposure of the sample to ambient oxygen during analysis using the method established by Zeng et al., (2013). The monochromator was calibrated using an Fe foil XANES scan with the inflection point set to 7110.75 eV (Kraft et al., 1996). Micro-XRF and uXANES data were collected using a Canberra 7-element Ge solid-state fluorescence detector. Micro-XANES data were collected from 7012 to 7417 eV in quick-XANES mode with a single sweep of the monochromator lasting 30 s.

Data collection occurred in the following manner for each filter: (1) a large uXRF "survey" map with an area of approximately 1000 × 3000 um2, pixels of 6 × 6 um2, and incident energy of 10 keV was collected to measure the distribution of particles and elements on the filter; (2) "fine" uXRF sub-maps of approximately 100 × 100 um2, pixels of 3 × 3 um2, and incident energies spanning the Fe 1s absorption edge; and (3) Fe 1s point-XANES spectra.