This project is part of the international GEOTRACES program, which was created to allow a comprehensive, coordinated study of trace elements and isotopes (TEIs) in the oceans. This project uses the radionuclide pair Lead-210 (210Pb) and its grand-daughter Polonium-210 (210Po) to provide important biogeochemical rate information pertinent to the TEIs that were measured during the US GEOTRACES Meridional Transect in the Pacific from Alaska to Tahiti in late 2018. Many processes in the ocean cannot be directly observed and, as such, tracers such as 210Po and 210Pb can be used to provide important constraints on their rates and pathways. 210Po (half-life = 138 days) and 210Pb (half-life = 22.3 years) decay on timescales that are useful to study 1) atmospheric deposition of trace elements, 2) scavenging of particle-reactive trace elements, 3) export of particulate organic carbon (POC) from the photic zone, and 4) the fate of hydrothermal plumes dispersed from the mid-ocean ridge system. Data reported here are activities of 210Po and 210Pb on particles in two size classes obtained by in situ pumping. Sampling was conducted in 2018 at various depths through the water column at eleven stations along the GEOTRACES Pacific Meridional Transect (GP15). Particulate samples were collected by Phoebe Lam's group using McLane in-situ pumps (Lam et al., 2015). Two particle sizes were collected by pumping: a large particle fraction collected on a 142-millimeter (mm) 51-micrometer (μm) polyester mesh prefilters (Sefar 07-51/33) and a small particle fraction collected on paired 0.8-μm polyethersulfone Pall Supor 800 membrane filters. The filters were dried on board in a laminar flow hood, split, and placed in a polyethylene bag. Aliquots sent to Stony brook comprised 28-280 liters (L) (average ~100L) filtered for the small particle fraction and 60-330L (average ~250L) for the large particle fraction.

Table of Contents

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

Sampling was conducted in 2018 at various depths through the water column at eleven stations along the GEOTRACES Pacific Meridional Transect (GP15), carried out on R/V Roger Revelle (cruises RR1814 and RR1815). Particulate samples were collected by Phoebe Lam's group using McLane in-situ pumps (Lam et al., 2015). Two particle sizes were collected by pumping: a large particle fraction collected on a 142-millimeter (mm) 51-micrometer (μm) polyester mesh prefilters (Sefar 07-51/33) and a small particle fraction collected on paired 0.8-μm polyethersulfone Pall Supor 800 membrane filters. The filters were dried on board in a laminar flow hood, split, and placed in a polyethylene bag. Aliquots sent to Stony brook comprised 28-280 liters (L) (average ~100L) filtered for the small particle fraction and 60-330L (average ~250L) for the large particle fraction. More details on pump operation can be found in the GP15 Cruise Report (see Supplemental Files section).

The particulate filters were dried on board the ship in a laminar flow hood, split, and placed in a polyethylene bag. Aliquots sent to Stony Brook comprised 28-280 liters (L) (average ~100L) filtered for the small particle fraction and 60-330L (average ~250L) for the large particle fraction. Due to the equipment limitations and potential hazards when doing acid digestions at sea, on-board treatment was not possible for these samples. Instead, filter aliquots from Leg 1 were sent to the lab during the port stop in Hilo, Hawaii on 21 October. Leg 2 filters were sent back from Papeete, Tahiti at the conclusion of the cruise on 24 November.

In the laboratory, filters were placed in microwave digestion vessels and spiked with Polonium-209 (209Po) tracer (1.76 disintegrations per minute (dpm)) and 10 milligrams (mg) stable lead (Pb). 5 milliliters (mL) each of concentrated HCl, HNO3, and HF were then added to the vessel. The mixture was microwave-digested for 1 hour at 180° Celsius (C). After digestion, the resulting solution was decanted into a 25 mL Falcon tube, and the digestion vessels were rinsed with small aliquots of deionized (DI) water. The Falcon tubes were centrifuged for 10 minutes at 2000 rotations per minute (rpm) and the supernatant was pipetted into 50 mL Teflon beakers. The beakers were heated to almost complete dryness before 10 mL of concentrated HCl was added. This process was repeated to ensure that the HCl was the only acid present. 20 mL of 6 molar (M) HCl was added, the solution was decanted into a glass beaker and the Teflon beaker was rinsed with two aliquots of 30 mL DI water. The beakers were labeled, and ascorbic acid was added to the solution to reduce any Fe(III) to Fe(II) and prevent its plating. A silver planchet embedded in a Teflon stirring magnet was added to each sample, which was then plated at 80°C for 3 hours (Flynn, 1968; Lee et al., 2014), rinsed with DI water and acetone and counted in a Canberra/Mirion Quad Alpha spectrometer to determine initial 210Po. Residual Po was removed from solution by adding scrap silver for ~5 days and then transferring the samples to 125 ml polycarbonate bottles for storage and ingrowth of additional 210Po. The second plating was carried out after 6 to 8 months of storage. Data reduction was carried out using the methods reported in Rigaud et al. (2013).

Known Problems/Issues:
Calculation of the particulate 210Pb activity requires measurement of the concentration of stable Pb added during sample dissolution to determine the recovery of 210Pb from dissolution through plating. The sample solutions were stored for 6  to 8 months after the initial Po plating and removal of residual Po to permit ingrowth of additional 210Po from 210Pb decay. At the time of the second plating, aliquots were taken to measure the stable Pb. A discrete number of samples were completed in late 2019 to early 2020 and Pb recoveries were determined. However, Stony Brook University closed for an extended period starting in March 2020 due to the Covid-19 pandemic. Upon reopening, measurement of stable Pb in the stored aliquots showed losses in storage. As well, analytical instrumentation used to measure Pb (Element 2 ICP-MS) was no longer functioning and required extensive maintenance and repair. Because of these problems, it was decided to use average Pb recoveries for these samples, based on the Pb recoveries of samples analyzed before the shutdown.

Data Processing:
Calculation of radioactivities (disintegrations per minute (dpm) or Becquerels (Bq)) from count rates (counts per minute) measured in the alpha spectrometers were made using the procedure outlined in Rigaud et al. (2013).

Quality flags were applied following the GEOTRACES flag definitions where 1 = "good data". The Quality Flag Policy is available at https://www.geotraces.org/geotraces-quality-flag-policy/ or via the attached Supplemental File.

The following notation is used in the dataset: NS = not sampled; NC = not calculated.

BCO-DMO Processing:
- moved rows for station 18.3 from the RR1815 file to the RR1814 file;
- renamed fields to comply with BCO-DMO naming conventions;
- converted dates and times to ISO 8601 format (UTC);
- added the following columns/values from the RR1815 Bottle File (v6): Cast_number, Event_ID. Data were joined to the bottle data using the GEOTRACES sample number (Sample_ID).

Website: http://www.geotraces.org/

Coverage: Pacific Meridional Transect along 152W (GP15)

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. These chemical species play important roles in the ocean as nutrients, tracers of current and past oceanographic processes, and as contaminants from human activity. Their biogeochemical cycling has direct implications for research in such diverse areas as the carbon cycle, climate change, and ocean ecosystems. This project will use measurement of two natural radionuclides -- lead-210 and polonium-210 -- to provide important information about the rates of processes that affect trace elements and isotopes (TEIs) that will be measured during a U.S. GEOTRACES expedition in the Pacific Ocean in 2018. The research proposed here will address key tasks formulated within the GEOTRACES Science Plan.

Many processes in the ocean cannot be observed directly but tracers such as polonium-210 (half-life = 138 days) and lead-210 (half-life = 22.3 years) that have unique chemical properties and relevant decay timescales can be used to provide important constraints on their rates and pathways. The goals of this research are to: 1) use Pb-210, along with another project measuring Be-7, in aerosols and precipitation to characterize aerosol and TEI sources, 2) determine scavenging rates of particle-reactive TEIs through the water column using Po-210 and Pb-210, 3) use Po-210 / Pb-210 disequilibrium in the upper water column as a proxy for the sinking flux of particulate organic carbon (POC), and 4) use Pb-210as a tracer of the influence of hydrothermal processes on water column distributions of TEIs. This work will build on a database of Po/Pb distributions in the world ocean (and the Pacific Ocean, in particular) obtained through programs such as GEOSECS, GEOTRACES, and independent studies. A graduate student will be trained as part of this project. The lead investigator, Cochran, plans to incorporate information about GEOTRACES sampling strategies in the planning for a travelling exhibition on "The Oceans" through his adjunct appointment at the American Museum of Natural History (New York). Project partner Kadko plans to incorporate GEOTRACES work in an international graduate course through the Nippon Foundation, Partnership for Observation of the Global Oceans Center of Excellence.

Website: http://www.geotraces.org/

Coverage: Global

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.