Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • BCO-DMO Processing Description
  • Problem Description
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Program Information
• Funding

Sampling methods at sea followed the GEOTRACES cookbook (Cutter et al., 2017). Water samples were collected in 10-liter Niskin bottles with nylon-coated closure springs and Viton O-rings. The seawater collected was filtered with Pall Acropak 500 filters (0.8- and 0.45-micrometer (μm) pore size) into LDPE cubitainers. Shallow casts of the ODF rosette collected single 10-liter samples for Th isotopes, 231Pa, and Nd isotopes. Otherwise, 4 to 5 liters were collected for each sample. After filtering, samples were adjusted to a pH of ~2 with 20 milliliters (mL) 6 M HCl (redistilled Fisher Scientific Trace Metal grade HCl). Samples were stored and shipped to labs double-bagged at ambient temperatures.

All Neodymium (Nd) isotope samples were preconcentrated using C18cartridges (Waters Corp., Sep-Pak classic, 360 mg, 55-105 μm) loaded with complexing agent of 2-ethylhexyl hydrogen phosphate (HDEHP) and 2-ethylhexyl dihydrogen phosphate (H2MEHP) mixture (Sigma Aldrich), following Pahnke etal., 2012. Specifically, C18 cartridges were first cleaned in a 0.5 N HCl bath overnight, passed through 10 mL of 6 N HCl and then flushed with >500 mL of Milli-Q water. Prior to sample introduction, 300 μL of complexing agent HDEHP/H2MEHP was loaded on a clean cartridge. Seawater samples were adjusted to pH ≈ 3.5 by adding Optima® ammonium hydroxide, then were pumped through the cartridges at 20 mL per minute by a peristaltic pump. Cartridges were first eluted with 10 mL of 0.01 N HCl to remove barium. The cartridges were then eluted with 35 mL of 6 N HCl at 10 mL per minute by a peristaltic pump to collect rare earth elements (REEs). REEs were dried and further purified using column chemistry (Biorad® AG-50 followed by Eichrom® Ln-Spec) to separate Nd from the other REEs (Pahnke etal., 2012). All Nd isotopes were measured at Oregon State University (OSU) with a Nu3 multicollector-inductively coupled plasma-mass spectrometer (MC-ICP-MS). The instrument was coupled with an Apex-2 desolvating nebulizer. All measured Nd isotopic compositions were corrected for mass fractionation using an exponential law with 146Nd/144Nd = 0.7219. The standard JNdi-1 was measured between every sample as sample-standard bracketing. JNdi-1 standards of 50 parts per billion (ppb), yielded internal reproducibility (2σ) of ±0.30 εNd units. External reproducibility was determined by multiple (n= 55) runs of an in-house Specpure Nd standard (2σ error = 0.5 εNd units; See Table 1). The total procedural blank was measured by quadrupole ICP-MS at ~3.5 picomoles (pM) Nd. Our Limit Of Detection (LOD) for Nd isotopic analyses on the Nu3 was set to <2 V total Nd; generally equivalent to a solution of ~3 ppb Nd, or 3 nanograms (ng) Nd. This LOD is somewhat arbitrary, but was established in light of unacceptable error on the 143Nd/144Nd ratios produced below this level.

Currently being processed.

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. Neodymium (Nd) isotopes and rare earth elements (REE) are widely recognized for their utility as tracers of water transport and as indicators of the sources of trace elements to the oceans. Neodymium isotopes have been designated as "key parameters" to be measured on GEOTRACES expeditions. This project would measure Nd isotopes on a 2018 GEOTRACES expedition in the Pacific Ocean. The North Pacific is a particularly interesting place to measure Nd isotopes because North Pacific and North Atlantic deep waters represent the two end-members of deep ocean composition, and the few data available so far in the North Pacific differ from expectations based on mixing of known water types.

The US GEOTRACES Pacific Meridional Transect (PMT) from Tahiti to Alaska will play an important role in solving the "mystery" of North Pacific neodymium isotope ratios. The PMT is designed to extensively sample the oldest waters in the ocean, along with Alaskan coastal margins, volcanogenic sediments, the meridional dust gradient originating from volcanic arcs and the Asian and North American continents, and large gradients in biological productivity. The primary purpose of this project is to characterize the Nd isotope ratio of seawater along the transect. In order to address fundamental questions on the Nd and REE cycle in the Pacific, the investigators will also quantify boundary exchange effects, atmospheric inputs, provenance of particles, impacts of biological productivity, and fluxes into and out of seawater, in order to assess Nd and REE sources and sinks, and to calibrate Nd isotopes as a paleocirculation proxy in the Pacific. Specific focus areas include: (1) Nd isotopic characterization of the shallow and deep waters, (2) margin processes associated with the Alaska volcanic arc shelf, to study chemical exchanges between particulates, sediments, and seawater Nd isotopes and REE, (3) changing meridional dust sources and its impacts, (4) relationships between bottom sediment types, compositions, nepheloid layers, and their influence on the water column, (5) the effects of pore water flow on the water column; (6) the signal transfer between the deep water and sedimentary archives (e.g., fossil fish teeth, Fe-Mn oxide coatings). The project will advance the career of a postdoctoral research scientist, and support numerous undergraduate students.

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.