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

This dataset reports dissolved elemental mercury (Hg) concentration measured continuously while the ship is moving, but not while on station, with samples taken from the ship's underway sampling system.

Samples are collected with 10 minute integration and the data submitted is the average concentration and standard deviation for each day while the ship is moving. The water is supplied by the ship's underway sampling system and the analytical methods rely on the equilibration of Hg-free air bubbled through the water, which is collected and analyzed using an automated Tekran 2357X air sampling system. The water concentration is calculated based on the air concentration assuming equilibrium and using Henry's Law (Mason et al., 2017; DiMento et al., 2019). The automated analyzer that detects the elemental Hg is automatically calibrated with an in situ permeation tube but was also calibrated manually each week during the cruise with a manual injection.

This parameter, because of the nature of its collection, was not assigned GEOTRACES sample numbers. The sampling depth is that of the ship's inlet system, which varies somewhat but is around 5 m on average so these are surface water samples.

Dissolved elemental Hg is reported in units of fmol/L
Detection limit = 0.4
Spike recovery (%) = 98.2±4.3

BCO-DMO Processing:
- added date-time fields in ISO8601 format;
- applied GEOTRACES DOoR barcoded names;
- renamed fields to conform 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:
Human activity has greatly increased the amount of mercury (Hg) in the environment, and particularly in the surface ocean. Most of the Hg enters the ocean from the atmosphere as a gas, on particles, or in precipitation. Complex physical and chemical processes at the interface between the ocean and atmosphere control the amount of Hg that is retained and therefore that can ultimately accumulate in seafood. Methylmercury (MeHg) is a chemical form of Hg that is commonly retained in organisms and impacts the health and development of humans and wildlife. This research will assess concentrations of Hg together with its "methyl" forms in the atmosphere and surface ocean at sea from Alaska to Tahiti. The spatial extent of the cruise will allow comparison of the air-sea exchange and concentrations of mercury in both the North Pacific where human emissions are large and in remote regions with minimal human impact. The researchers will use established techniques and develop new methods to examine the fate and transport of mercury within the surface ocean. These findings will contribute key Hg data to the GEOTRACES program and thus enhance its overall impact as part of an extensive marine trace element study. Findings will have potential to inform public policy and global environmental treaties related to Hg, thus providing data to evaluate human risk from Hg in present and future climate scenarios. Educational impact will include support for a graduate student and their dissertation using the field data, as well as several undergraduates that will gain high level, hands-on research experience.

The research will take advantage of recent analytical advances that enable high resolution determination of the concentrations and forms of inorganic Hg in the surface ocean and atmosphere. The analytical approach will also be expanded to include measurements of methylated Hg compounds, including MeHg and dimethylmercury. These measurements, and ancillary data collected during the GEOTRACES Pacific Meridional Transect cruise, will allow assessment of both atmospheric input and in situ oceanic loss for the dominant forms of inorganic and methylated Hg. Exchange will also be evaluated in the context of the suite of environmental variables collected by collaborators during the cruise. The resulting data will help assess the long-term impact of anthropogenic inputs of Hg to the atmosphere and ocean, and the factors that influence the loss of Hg from the ocean by gas evasion. The studies will build on previous results obtained as part of the GEOTRACES program and other NSF-funded studies, adding novel measurements and building an enhanced understanding of the sources and sinks of Hg to the open ocean.

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.