Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
• Data Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Program Information
• Funding

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.

Data processing: Data processing for XRF maps included deadtime correction and plotting with beamline 10.3.2 software (Marcus et al., 2004). Data processing for the XANES spectra included deadtime correction, energy calibration, pre-edge subtraction, post-edge normalization, and linear least-squares fitting with a reference database using beamline 10.3.2., Athena, SixPack, and mriftty softwares (Marcus et al., 2008, 2004; Ravel and Newville, 2005; Webb, 2005; Nicholas et al. 2017). 

A table was generated showing the best fits to each experimental spectrum in terms of the exact reference spectra used in the best fits Table 1 "mrfitty". These reference spectra were then binned according to the type of particulate Fe they represent using the Table 2 "key" worksheet to create a "binned" (Table 3) version of the data. The final version of the data provided to BCO-DMO is the "annotated" data in Table 4. The original Excel file containing Tables 1-3 is available for reference: BCO-DMO-Toner-TN303-particulate-iron-speciation.xlsx (51 kb)

Problem report: Appearance of "native Fe" in two samples (GT# 8564 and 10160) are considered contaminants.

BCO-DMO Processing:
- modified parameter names (replaced spaces with underscores; removed parentheses; applied GEOTRACES naming conventions where applicable);
- added links to spectral files from the data file names;
- joined to EPZT master events file for location, date, and cast information.

From the NSF Award Abstract
The mission of the International GEOTRACES Program (https://www.geotraces.org/), of which the U.S. chemical oceanography research community is a founding member, is "to identify processes and quantify fluxes that control the distributions of key trace elements and isotopes in the ocean, and to establish the sensitivity of these distributions to changing environmental conditions" (GEOTRACES Science Plan, 2006). In the United States, ocean chemists are currently in the process of organizing a zonal transect in the eastern tropical South Pacific (ETSP) from Peru to Tahiti as the second cruise of the U.S.GEOTRACES Program. This Pacific section includes a large area characterized by high rates of primary production and particle export in the eastern boundary associated with the Peru Upwelling, a large oxygen minimum zone that is a major global sink for fixed nitrogen, and a large hydrothermal plume arising from the East Pacific Rise. This particular section was selected as a result of open planning workshops in 2007 and 2008, with a final recommendation made by the U.S.GEOTRACES Steering Committee in 2009. It is the first part of a two-stage plan that will include a meridional section of the Pacific from Tahiti to Alaska as a subsequent expedition.

This award provides funding for management of the U.S.GEOTRACES Pacific campaign to a team of scientists from the University of Southern California, Old Dominion University, and the Woods Hole Oceanographic Institution. The three co-leaders will provide mission leadership, essential support services, and management structure for acquiring the trace elements and isotopes samples listed as core parameters in the International GEOTRACES Science Plan, plus hydrographic and nutrient data needed by participating investigators. With this support from NSF, the management team will (1) plan and coordinate the 52-day Pacific research cruise described above; (2) obtain representative samples for a wide variety of trace metals of interest using conventional CTD/rosette and GEOTRACES Sampling Systems; (3) acquire conventional JGOFS/WOCE-quality hydrographic data (CTD, transmissometer, fluorometer, oxygen sensor, etc) along with discrete samples for salinity, dissolved oxygen (to 1 uM detection limits), plant pigments, redox tracers such as ammonium and nitrite, and dissolved nutrients at micro- and nanomolar levels; (4) ensure that proper QA/QC protocols are followed and reported, as well as fulfilling all GEOTRACES Intercalibration protocols; (5) prepare and deliver all hydrographic-type data to the GEOTRACES Data Center (and US data centers); and (6) coordinate cruise communications between all participating investigators, including preparation of a hydrographic report/publication.

Broader Impacts: The project is part of an international collaborative program that has forged strong partnerships in the intercalibration and implementation phases that are unprecedented in chemical oceanography. The science product of these collective missions will enhance our ability to understand how to interpret the chemical composition of the ocean, and interpret how climate change will affect ocean chemistry. Partnerships include contributions to the infrastructure of developing nations with overlapping interests in the study area, in this case Peru. There is a strong educational component to the program, with many Ph.D. students carrying out thesis research within the program.

Figure 1. The 2013 GEOTRACES EPZT Cruise Track. [click on the image to view a larger version]

NSF Award Abstract:
Particles play essential roles in the cycling and distribution of trace elements and isotopes (TEI). For instance scavenging onto particles and removal by particle export is a major sink for many TEIs. However the driving factors affecting the spatial extent and temporal variability of TEI scavenging remain largely unknown. The U.S. GEOTRACES East Pacific Zonal Transect between Peru and Tahiti will cross three biogeochemically important areas, setting the stage for researchers to constrain unknowns surrounding particle scavenging.

Scientists from Woods Hole Oceanographic Institution and University of Minnesota-Twin Cities will address fundamental questions on the location of particle formation, particle-dissolved species interactions, dominant particle phases, and the evolution of particle phases during transport. As regards particle phases, the researchers will measure particulate organic carbon (POC), calcium carbonate, opal, lithogenics, and oxyhydroxides, as well as the trace element compositions of size fractionated particles covering three distinct zones: large lateral productivity gradients, a major oxygen deficient zone off the coast of Peru, and a world renowned hydrothermal plume emanating from the southern East Pacific Rise. Furthermore, since previous results suggest particle type affects scavenging, investigators will specifically measure acid-leachable trace metals, particle concentration, and major particle composition. These combined measurements will allow them to disentangle the relative importance of hydrothermal particles, benthic nepheloid layers, and high productivity margins for scavenging of TEIs. The cruise transect will also offer unique opportunities to examine the speciation of Fe and POC in hydrothermal particles away from the vent field and the effects of aggregation and POC coatings on TEI partitioning.

As regards broader impacts, the researchers plan to create an educational module on particulate trace metals in the ocean which would become part of a five part GEOTRACES webinar series developed by Dr. Benjamin Twining at the Bigelow Laboratory for Ocean Sciences in collaboration with the Center for Ocean Sciences Education Excellence-Ocean Systems at the University of Maine. One graduate student from Woods Hole Oceanographic Institution and one graduate student from the University of Minnesota, Twin Cities, would be supported and trained as part of this project. In addition, an undergraduate student would participate in the research during the summer month as part of the Woods Hole Diversity Initiative Partnership in Education program.

GEOTRACES is a SCOR sponsored program; and funding for program infrastructure development is provided by the U.S. National Science Foundation.

GEOTRACES gained momentum following a special symposium, S02: Biogeochemical cycling of trace elements and isotopes in the ocean and applications to constrain contemporary marine processes (GEOSECS II), at a 2003 Goldschmidt meeting convened in Japan. The GEOSECS II acronym referred to the Geochemical Ocean Section Studies To determine full water column distributions of selected trace elements and isotopes, including their concentration, chemical speciation, and physical form, along a sufficient number of sections in each ocean basin to establish the principal relationships between these distributions and with more traditional hydrographic parameters;

* To evaluate the sources, sinks, and internal cycling of these species and thereby characterize more completely the physical, chemical and biological processes regulating their distributions, and the sensitivity of these processes to global change; and

* To understand the processes that control the concentrations of geochemical species used for proxies of the past environment, both in the water column and in the substrates that reflect the water column.

GEOTRACES will be global in scope, consisting of ocean sections complemented by regional process studies. Sections and process studies will combine fieldwork, laboratory experiments and modelling. Beyond realizing the scientific objectives identified above, a natural outcome of this work will be to build a community of marine scientists who understand the processes regulating trace element cycles sufficiently well to exploit this knowledge reliably in future interdisciplinary studies.

Expand "Projects" below for information about and data resulting from individual US GEOTRACES research projects.