Table of Contents

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

Seawater samples were collected using the GEOTRACES trace-metal clean sampling system (rosette with 24 12-liter GO-FLO bottles) or a towfish for surface samples (Cutter and Bruland, 2012). Samples were filtered through acid-cleaned AcroPak capsule filters (0.2 micrometers (μm)) into acid-cleaned LDPE bottles. For concentration and isotope analysis at the University of South Florida (USF), 2 liters of filtered seawater were collected, acidified ashore at USF by addition of 2.4 milliliters (mL) 10 molar (M) Teflon-distilled HCl, and stored for at least 12 months at pH ∼2 before processing.

Seawater samples were processed for isotope analysis following Sieber et al. (2019). Briefly, a double-spike for cadmium (Cd), iron (Fe), and zinc (Zn) was added prior to batch extraction using Nobias PA-1 chelating resin, followed by purification by anion-exchange chromatography using AG-MP1 resin. Isotope analyses were performed on a Thermo Neptune Plus MC-ICPMS in the Tampa Bay Plasma Facility at the University of South Florida using an Apex-Q (Cd) or Apex Ω (Fe, Zn) introduction system.

We express Cd stable isotope ratios in delta notation (δ¹¹⁴Cd) relative to the NIST SRM-3108 Cd standard. A secondary standard, BAM-I012, was analyzed over 8 sessions on the same timescale as the samples to provide an estimate of long-term instrumental precision. We obtain a value of −1.32 ± 0.06‰ (2SD, n = 172), in agreement with consensus values (Abouchami et al., 2013). Using the 2SD of offsets from the mean of full replicate measurements based on 26 pairs of replicate analysis on separate seawater samples collected at the same depth (the GP15 sampling strategy collected overlapping samples between casts), we obtain a second estimate of external precision (0.05‰), which is similar to analytical precision. Therefore, we consider a 2SD uncertainty of 0.06‰ as a conservative estimate of analytical precision, and have applied it to all samples, except for low-concentration samples where the larger internal error is considered a more conservative estimate of uncertainty. Concentrations were calculated using the isotope dilution technique based on on-peak blank, interference and mass-bias corrected ¹¹⁴Cd/¹¹¹Cd ratios measured simultaneously with isotope analyses (Sieber et al., 2019). We express uncertainty (1SD) on Cd concentrations as 2%, based on replicate analysis on separate seawater samples collected at the same depth (n = 26).

We express Fe stable isotope ratios in delta notation (δ⁵⁶Fe) relative to the IRMM-014 standard. A secondary Fe standard, NIST-3126, was analyzed over 44 sessions to provide an estimate of long-term instrumental precision. We obtain a value of +0.36 ± 0.05‰ (2SD, n = 604), in agreement with consensus values (Conway et al., 2013). As a second estimate of external precision, we use the 2SD of offsets from the mean of full replicate measurements based on 26 pairs of replicate analysis using separate seawater samples collected at the same depth (0.08‰), which is similar to the analytical precision. Therefore, we consider a 2SD uncertainty of 0.05‰ as an estimate of analytical precision, and have applied it to all samples, except for low-concentration samples where the larger internal error is considered a more conservative estimate of uncertainty. Concentrations were calculated using the isotope dilution technique based on on-peak blank, interference and mass-bias corrected ⁵⁷Fe/⁵⁶Fe ratios measured simultaneously with isotope analysis. We express uncertainty (1SD) on Fe concentrations as 2%, based on replicate analysis on separate seawater samples collected at the same depth (n = 26).

We express Zn stable isotope ratios in delta notation (δ⁶⁶Zn) relative to the JMC-Lyon standard. A secondary Zn standard, AA-ETH, was analyzed on the same timescale as the samples (over 10 sessions) to provide an estimate of long-term instrumental precision. We obtain a value of +0.28 ± 0.03‰ (2SD, n = 147), in agreement with consensus values (Archer et al., 2017). As a second estimate of external precision, we use the 2SD of offsets from the mean of full replicate measurements based on 26 pairs of replicate analysis using separate seawater samples collected at the same depth (0.03‰), which is similar to the analytical precision. Therefore, we consider a 2SD uncertainty of 0.03‰ as an estimate of analytical precision, and have applied it to all samples, except for low-concentration samples where the larger internal error is considered a more conservative estimate of uncertainty. Concentrations were calculated using the isotope dilution technique based on on-peak blank, interference and mass-bias corrected ⁶⁷Zn/⁶⁶Zn ratios measured simultaneously with isotope analysis. We express uncertainty (1SD) on Zn concentrations as 5%, based on replicate analysis on separate seawater samples collected at the same depth (n = 26).

Data Processing:
Microsoft Excel was used for managing data.

Concentrations are calculated using isotope dilution. Data reduction of isotope data is based on the iterative approach of Siebert et al. (2001).

Data are assigned quality flags according to GEOTRACES Flag Policy  (https://www.geotraces.org/geotraces-quality-flag-policy/); obvious outliers are marked as 4, missing data as 9, and good data as 2.

BCO-DMO Processing:
- moved data for station 18.6 from RR1814 to RR1815;
- changed incorrect years of 2013 and 2016 to 2018;
- for station 18.6, changed end date from 2018-10-21 to 2018-10-25;
- renamed fields to comply with BCO-DMO naming conventions;
- added start and end date-time columns in ISO 8601 format.

A 60-day research cruise took place in 2018 along a transect form Alaska to Tahiti at 152° W. A description of the project titled "Collaborative Research: Management and implementation of the US GEOTRACES Pacific Meridional Transect", funded by NSF, is below. Further project information is available on the US GEOTRACES website and on the cruise blog. A detailed cruise report is also available as a PDF.

Description from NSF award abstract:
GEOTRACES is a global effort in the field of Chemical Oceanography in which the United States plays a major role. The goal of the GEOTRACES program is to understand the distributions of many elements and their isotopes in the ocean. Until quite recently, these elements could not be measured at a global scale. Understanding the distributions of these elements and isotopes will increase the understanding of processes that shape their distributions and also the processes that depend on these elements. For example, many "trace elements" (elements that are present in very low amounts) are also important for life, and their presence or absence can play a vital role in the population of marine ecosystems. This project will launch the next major U.S. GEOTRACES expedition in the Pacific Ocean between Alaska and Tahiti. The award made here would support all of the major infrastructure for this expedition, including the research vessel, the sampling equipment, and some of the core oceanographic measurements. This project will also support the personnel needed to lead the expedition and collect the samples.

This project would support the essential sampling operations and infrastructure for the U.S. GEOTRACES Pacific Meridional Transect along 152° W to support a large variety of individual science projects on trace element and isotope (TEI) biogeochemistry that will follow. Thus, the major objectives of this management proposal are: (1) plan and coordinate a 60 day research cruise in 2018; (2) obtain representative samples for a wide variety of TEIs using a conventional CTD/rosette, GEOTRACES Trace Element Sampling Systems, and in situ pumps; (3) acquire conventional CTD hydrographic data along with discrete samples for salinity, dissolved oxygen, algal pigments, 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 data to the GEOTRACES Data Assembly Centre (via the US BCO-DMO data center); and (6) coordinate all cruise communications between investigators, including preparation of a hydrographic report/publication. This project would also provide baseline measurements of TEIs in the Clarion-Clipperton fracture zone (~7.5°N-17°N, ~155°W-115°W) where large-scale deep sea mining is planned. Environmental impact assessments are underway in partnership with the mining industry, but the effect of mining activities on TEIs in the water column is one that could be uniquely assessed by the GEOTRACES community. In support of efforts to communicate the science to a wide audience the investigators will recruit an early career freelance science journalist with interests in marine science and oceanography to participate on the cruise and do public outreach, photography and/or videography, and social media from the ship, as well as to submit articles about the research to national media. The project would also support several graduate students.

NSF Award Abstract:
The goal of the international GEOTRACES program is to understand the distributions of many chemical elements and their isotopes in the oceans. The National Science Foundation is supporting a U.S. GEOTRACES sampling expedition in the Pacific Ocean 2018, from Alaska to Tahiti. This award will focus on measurement of the stable isotopes of iron (Fe), nickel (Ni), zinc (Zn), copper (Cu), and cadmium (Cd) in seawater, filtered particles, and atmospheric aerosol particles on this expedition. The trace metals Fe, Ni, Zn, Cu, and Cd strongly influence marine biogeochemistry and carbon cycling. Measurements of isotope ratios -- the relative abundance of different forms of the same chemical element -- provide insights not possible from concentration measurements alone. The investigators will use isotope data to learn more about the inputs and outputs of these elements to the ocean as well as the biological processes that influence their distributions within the oceans, leading to greater understanding of the role these metals play in oceanic carbon cycling. The award will also host an international inter-lab comparison exercise for Cd isotope ratios at low concentrations in seawater. The project will support an early career investigator, a postdoctoral researcher, and undergraduate and graduate students.

The investigators will measure delta-56Fe, delta-60Ni, delta-65Cu, delta-66Zn, and delta-114Cd at high spatial resolution along the U.S. GEOTRACES Pacific Meridional Transect (PMT). The resulting oceanic sections of all five isotope systems will enable them to distinguish between competing ideas about the controls on trace metal distributions. A wide variety of hypotheses about marine biogeochemical trace metal cycling, addressing topics of global importance will be explored and tested, including: 1) How do different sources, productivity and export regimes, coupled with overturning circulation, control the distribution of Fe, Ni, Zn, Cu and Cd and their isotopes in the Pacific? 2) Do oxygen minimum zones act as sinks for Cd, Zn, Cu and Ni, while acting as sources for Fe? and 3) What sources are most important for supplying Fe to the North Pacific Ocean? The PMT section crosses two high nutrient-low chlorophyll (HNLC) regions and two oligotrophic gyres, as well as transecting the oldest waters in the ocean, allowing the investigators to use high-resolution isotope datasets to investigate competing hypotheses about the effect of vertical, horizontal and in situ biogeochemical processes on the distribution of all five metals and their isotopes. Aerosol dust, volcanogenic and reducing margin sediments, and hydrothermal vents such as the East Pacific Rise have all been hypothesized as major contributors to the dissolved Fe cycle in the Pacific. Measurement of iron isotope signatures in aerosols and near to oceanic sources will enable the identification and quantification the importance of these different iron sources and processes in supplying iron to the iron-limited surface ocean, especially important for the two HNLC regions along the PMT section.

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.