Table of Contents

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

Sample Collection and Filtration
All seawater samples analyzed here for dissolved iron (dFe) concentration and isotope ratios (δ56Fe) were collected between 21 November and 4 December 2013 aboard the R/V Thomas G. Thompson as part of the U.S. GEOTRACES GP16 East Pacific Zonal Transect (Resing et al. 2015, Fitzsimmons et al. 2017, John et al. 2018). The EPZT encompassed 35 stations along an 8000 km transect that traversed from Peru to Tahiti (Peters et al. 2017, Moffett & German 2018). All seawater samples were collected as part of the standard U.S. GEOTRACES trace metal casts, using a trace metal clean carousel and CTD (Seabird) with a Kevlar conducting cable, fitted with 24 x 12-L Teflon-coated GO-Flo bottles (General Oceanics), following established GEOTRACES protocols (Cutter & Bruland 2012). Rosette deployments began with GO-Flo bottles open, allowing them to flush during deployment to depth, and the GO-Flo bottle was subsequently tripped on ascent at ~3 m/min, to minimize chances of contamination from rosette hardware. Upon recovery, GO-Flo bottles were transferred immediately into a clean van for subsampling and were pressurized using HEPA-filtered air. Each GO-Flo bottle was fitted with acid-cleaned Teflon tubing connected to a 0.2 um Acropak-200 capsule filter (Pall), allowing collection of filtered seawater for bulk dissolved Fe analysis into 4 L acid-clean low-density polyethylene (LDPE) bottles, after three 10% volume seawater rinses of the caps and threads (Cutter et al. 2017). Filtered seawater from Station 26 at 2650 m and 2700 m depths were combined to achieve sufficient volume for ultrafiltration processing.

Ultrafiltration and Sample Processing
Immediately after sample collection and 0.2 um filtration, 1 L of the 4 L homogenized sample was poured into a clean 1 L LDPE bottle for dissolved Fe analysis, and the remaining sample (~ 3 L) was ultrafiltered. Ultrafiltration took place using a cross flow filtration system (CFF) with a 10 kDa (~0.003 um for globular proteins; Erickson et al. 2009) regenerated cellulose membrane (Pellicon XL PLCGC) in single-pass mode (see Fitzsimmons and Boyle 2014 for detailed methods). Briefly, before collection, the CFF system was conditioned using ~500 mL of sample seawater, and the sample bottles, caps, and threads were rinsed with a minimal amount of ultrafiltered seawater. After conditioning, ~1 L of permeate (soluble fraction) and ~1 L of retentate were collected into separate 1 L LDPE bottles. All samples were then acidified to 0.012 M hydrochloric acid (HCl, Optima, Fisher) and stored for more than a year before analysis. Soluble Fe (sFe) is defined as the concentration in the permeate solution.

Dissolved Fe Concentration Analysis
Iron concentrations in the dissolved, permeate, and retentate phases were analyzed using a 15 mL aliquot of the 1 L samples. Samples were analyzed in the Sherrell lab at Rutgers University with a SeaFAST pico system (ESI, Omaha, NE, USA) and Thermo Element 1 HR-ICPMS following the SeaFAST methods of Lagerstrom et al. (2013). Samples were analyzed offline, as outlined by Jensen et al. (2020).

Dissolved Fe isotope ratio (δ56Fe) Analysis
Iron stable isotope ratios (δ56Fe) were determined using a double-spike multi-collector MC-ICPMS technique, based on Conway et al. (2013). All sample processing and Fe isotope analyses were carried out at the University of South Florida (USF). Chemical processing of samples took place in ISO-5 laminar flow hoods within the Marine Metal Isotope and Trace Element ISO-6 clean laboratory at the USF College of Marine Science. All ultrapure water used was from a Thermo Scientific Barnstead Genpure Ultrapure Water System; all plasticware was cleaned using standard procedures (Conway et al. 2013); and all reagents were Optima grade from Fisher Scientific. Briefly, the ~1 L of dissolved, permeate, and retentate seawater samples were spiked with a double spike containing 57Fe and 58Fe at a ratio of ~1:1, to provide a ~1:2 ratio of natural:spike Fe in samples. Following spiking, samples were amended to 10 mM H₂O₂ and left for 24 h after spiking. Samples were then brought to pH ~6, and Fe was extracted from seawater onto Nobias PA-1 chelating resin beads using a batch extraction technique. The Fe was then eluted from the Nobias resin using 3 M HNO₃, evaporated to dryness, refluxed with a conc. HNO₃ and conc. H₂O₂ mixture, evaporated to dryness, taken up in 1 M HCl, and purified using an AGMP-1 (BioRad) anion-exchange column chemistry technique following Sieber et al. (2019). Following column purification, samples were evaporated to dryness before being re-dissolved in 0.5 mL of 2% HNO₃ (v/v) for analysis by MC-ICPMS.

Samples were analyzed for Fe isotope ratios by Thermo Neptune Plus MC-ICPMS at USF’s Tampa Bay Plasma Facility at the USF College of Marine Science. Briefly, samples were introduced via a ~100 µL/min PFA nebulizer into an ESI Apex Ω desolvator (with Ar but no N₂ add gas), using a Ni Jet sampler cone and an Al X skimmer cone. Fe isotope ratios were measured using high resolution mode, and instrumental mass bias was corrected using the double spike technique. δ56Fe were calculated following an iterative procedure based on Siebert et al. (2001) and are expressed relative to the IRMM-014 international standard.

Problem report: Filtered seawater from Station 26 at 2650 m and 2700 m depths were combined to achieve sufficient volume for ultrafiltration processing.

Dissolved Fe Concentration Data Processing
The raw counts from the Element 1 analyses were first corrected for any instrument blank derived from a 10% nitric acid and In solution repeatedly analyzed during the instrumental session. Following this, each sample was corrected for machine drift and variation in instrumental sensitivity during analysis using the added 115In as well as instrument mass bias using an Fe elemental standard. Subsequently, all counts for isotope spiked elements were converted to concentration in nmol/kg based on the weight of the sample and known concentrations of the added isotope spike mixture. For every 40 seawater samples at least 24 "Procedure blanks" were included throughout the run. These process blanks consisted of freshly acidified ultrapure water spiked with a diluted version of the isotope spike mixture. Each blank was processed identically to a sample in an attempt to account for contamination coming from solutions (buffer, eluent) used or the SeaFAST itself. After conversion to concentration units, the most consistent procedure blank values for each metal are used to subtract an average procedure blank from all sample concentrations determined during that run. Each run, an internal standard was analyzed six times and an external standard (such as SAFe water) was analyzed two times as a "check" of our long-term precision and accuracy.

For the session in which these samples were analyzed, accuracy of the technique was assessed using analyses of SAFe consensus seawater solutions during the analytical session: SAFe S = 0.13 nmol/kg (consensus value 0.093±0.008 nmol/kg), SAFe D1 = 0.73 nmol/kg (consensus value of 0.67±0.04 nmol/kg), and SAFe D2 = 0.95 nmol/kg (compared to consensus value of 0.933±0.023 nmol/kg). An error of 5% is reported on all concentration values, based on the long-term external reproducibility of an in-house seawater solution. Dissolved Fe concentrations compared well with published dissolved Fe concentrations (Fitzsimmons et al. 2017, John et al. 2018) from the GP16 section reported in the Intermediate Data Product (IDP) 2017 (Schlitzer et al. 2018).

Dissolved Fe isotope ratio (δ56Fe) Data Processing
To account for any instrumental drift during the analytical session, each group of five samples was expressed relative to the average δ56Fe of two bracketing IRMM-014 double spike mixtures. The validity of this method for generating precise and accurate seawater δ56Fe data has been shown previously by several inter-laboratory inter-comparison exercises, as well as repeated analysis of seawater and standard reference materials (Conway et al. 2013). At USF, precision and accuracy of δ56Fe measurements are based on the long-term analysis of a 100 ng g^-1 NIST 3126a Fe standard (+0.36±0.04‰, n=190 over 2 years). Following that approach, we apply 0.04‰ as a conservative 2SD measure of uncertainty on all δ56Fe data presented here, except if the standard internal error (2SD) is larger. In those cases, which typically occur at lower sample Fe concentrations, the 2SD is taken as a more conservative estimate of error and is used instead. Dissolved δ56Fe measurements made during this study compared well with previously reported data measured on separate subsamples in another laboratory (John et al. 2018), and when direct depth comparisons were not available we compared to the mean of bracketed sampling depths. 100% of measurements taken at the same depth were within error of one another, while 83% of all δ56Fe measurements were within error across the two studies (including samples compared to bracketed depth means).

Quality Flags:
Data were flagged using the SeaDataNet quality flag scheme. For more information on SeaDataNet flags, see: https://www.seadatanet.org/Standards/Data-Quality-Control

SeaDataNet quality flag definitions:
0 = No quality control;
1 = Good value;
2 = Probably good value;
3 = Probably bad value;
4 = Bad value;
5 = Changed value;
6 = Value below detection;
7 = Value in excess;
8 = Interpolated value;
9 = Missing value;
A = Value phenomenon uncertain.

BCO-DMO Processing:
- added ISO_DateTime_UTC column.

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]

During the 2013 GEOTRACES Eastern Pacific cruise a diverse range of oceanic environments will be encountered from the high productivity/high particle flux waters off Peru to the Peru-Chile oxygen minimum zone, the hydrothermal plume of the East Pacific Rise, and finally to some of the most oligotrophic waters around Tahiti. Scientists from Rutgers University and Woods Hole Oceanographic Institution will sample suspended particulates from the same GO-Flo bottles that will be used to sample dissolved trace metals and their isotopes (TEIs) across this entire transect. The suspended matter samples will be analyzed for 42 elements, including the particle-reactive rare earth elements. In addition, core-top sediments will be collected at every water-column sampling station and analyzed for both bulk composition (i.e., relative % content of organic carbon, opal, biogenic carbonate and lithogenic components) and the same 42 elements to be analyzed in the suspended particulates. Results from this study will be used to assess the role of suspended particulates in the biogeochemical cycling of TEIs across the Eastern Pacific by addressing three key sets of questions: (1) How does uptake of TEIs into phytoplankton and non-living particles in the upper ocean drive the suspended particulate composition through the deeper water column, along the substantial gradient from the high productivity Peru margin to the oligotrophic ocean interior?; (2) How faithfully is the along-transect variability in the upper ocean transmitted to the sediment (paleo) record?; (3) What are the relative influences of vertical recycling versus lateral advection in generating the distributions of dissolved and particulate TEIs observed in the Peru-Chile OMZ?; (4) Is there a characteristic signature of OMZ activity that is preserved in core-top sediments?; (5) What dominates TEI uptake onto/into authigenic particles in hydrothermal plumes and to what extent are these processes augmented by continuing uptake in core-top sediments?; and (6) What is the net effect from submarine venting on global TEI budgets?

As regards broader impacts, the scientist from Rutgers University is collaborating with the Education Director of the Centers for Ocean Science Education Excellence Networked Ocean World (COSEE-NOW) to contribute to the MARE (Marine Activities, Resources, and Education) program by inviting teachers and high school students to workshops and presentations on climate and ocean sciences. With the help of COSEE-NOW, he also plans to create educational video clips during the Pacific cruise and the subsequent laboratory based analytical work to educate them on the use of geochemistry to understand how the ocean works. Both scientists also plan to develop a teaching module entitled "Particles, Metals, and Carbon" for an Introduction to Oceanography class taught by the Rutgers scientist. One postdoc from Rutgers University would be supported and trained as part of this project.

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.