Table of Contents

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

Methodology:Quantitative vacuum extraction and mass spectrometry.

Sampling and Analytical Procedures:
Water samples were transferred from Niskin bottles and stored in crimped copper tubing. Gases were extracted from the water samples in a shore-based vacuum system (see references for description) and stored in aluminosilicate glass ampoules. The extracted gases were chemically purified and cryogenically separated, then analyzed for argon, krypton, and xenon concentrations using ion counting in a triple filter quadrupole mass spectrometer. Argon was measured using ion current manometry corrected for small linearity effects. Krypton and xenon were determined using ratiometric isotope dilution. All measurements were standardized to air abundances. Details are given in Jenkins et al (2019) and Stanley et al (2007).

Samples were acquired at sea in replicate. During the generation of this dataset 53 pairs of replicates were analyzed to assess reproducibility. The results are summarized in the attached Supplemental File named "GP15_ArKrXe_Replicates.csv" (note the results listed in this Supplemental File are reported as molar quantities).

Dissolved helium, neon, and helium isotopes were also measured on the same samples and are available at BCO-DMO (see "Related Datasets").

Data Processing:
Raw data were acquired and reduced using custom software written in VB.NET. Data were processed using MATLAB (2020b) and stored in a PostgreSQL V9.3 database.

Quality Flags:
Quality flags follow the ODF (WOCE) scheme. Defined as:
1 = Sample for this measurement was drawn from water bottle but analysis not received.
2 = Acceptable measurement.
3 = Questionable measurement.
4 = Bad measurement.
5 = Not reported.
6 = Mean of replicate measurements
7 = Manual chromatographic peak measurement.
8 = Irregular digital chromatographic peak integration.
9 = Sample not drawn for this measurement from this bottle.

BCO-DMO Processing:
- renamed fields to comply with BCO-DMO naming conventions.

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 trace chemical elements and their isotopes in the oceans. An essential part of this effort is determining the sources of trace elements to the oceans. One important such source comes from seafloor hydrothermal venting. This project will support the measurement of helium isotopes on the 2018 U.S. GEOTRACES expedition in the Pacific Ocean, running south from Alaska to Tahiti. Helium isotopes provide important information on the presence and dispersion of hydrothermal plumes. On a larger scale, they can also be used to calculate global-scale inputs of hydrothermally-sourced trace elements and isotopes. Thus, the proposed work is closely tied to many other projects associated with this expedition. The measurement of helium isotopes will also allow enable the investigation of physical mixing rates within the upper ocean to help interpret the fate of other trace elements. At the northern end of the section, this approach will also be used to investigate the rate at which other trace elements and isotopes from seafloor hydrothermal venting are delivered to the more biologically productive surface ocean. In addition to supporting the GEOTRACES effort and participating in community-scale outreach efforts associated with the expedition, the investigators will also work with a visual artist to communicate their science to the public in new ways.

The U.S. GEOTRACES Pacific Meridional Transect (56°N to 20°S, along 152°W) in late 2018 will intercept: strong margin fluxes, sub-Arctic high-nutrient, low-chlorophyll waters, the oldest deep waters in the world's oceans, the distal ends of multiple hydrothermal plumes, oxygen minimum zones, subpolar and equatorial upwelling, and, in the South Pacific near 20°S, some of the most oligotrophic waters known. This section, together with the US GEOTRACES East Pacific Zonal Transect (EPZT) conducted in 2013, will also close off a large volume of the Pacific Ocean. Consequently, this work will facilitate new, improved evaluations of regional-scale fluxes of hydrothermally-sourced trace elements and isotopes (TEIs) to the oceans. Because the 2018 expedition will intercept multiple hydrothermal plumes, this project will also allow the investigation of three key questions emerging from the larger U.S. and international GEOTRACES programs: 1) Is there a significant source of hydrothermally-sourced Fe and other TEIs to the euphotic zone in the North Pacific, through upwelling, as was demonstrated for the Southern Ocean from the 2013 EPZT results? 2) How do TEI:He-3 ratios in hydrothermal plumes vary with different geologic settings of their vent-sources? 3) How do the TEI:He-3 ratios imparted to hydrothermal plumes vary along the thermohaline conveyor as a function of varying water column chemistry? Throughout the section the investigators will also combine upper-water column helium-3 measurements with water column tritium concentrations (extrapolated from data collected on past expeditions) to estimate water column ventilation time scales that can be used to quantify rates of TEI transformation across the wide range of biogeochemical regimes to be intercepted (tropical, subtropical, subpolar). In regions characterized by upwelling, precise measurements of helium isotopes can be combined with canonical gas-exchange rate estimates to make flux-gauge determinations of upwelling rates for some key TEIs.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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.