IN
PROGRESS
TWENTY YEARS OF GEOTRACES
AN INTERNATIONAL STUDY OF THE MARINE BIOGEOCHEMICAL
CYCLES OF TRACE ELEMENTS AND ISOTOPES
Oceanography
THE OFFICIAL MAGAZINE OF THE OCEANOGRAPHY SOCIETY
VOL. 37, NO. 2, JUNE 2024
June 2024 | Oceanography
SPECIAL ISSUE ON
TWENTY YEARS OF GEOTRACES
6 FROM THE GUEST EDITORS. Introduction to the Special Issue
By T.M. Conway, J.N. Fitzsimmons, R. Middag, T.L. Noble, and H. Planquette
8 PERSPECTIVE. GEOTRACES Reflections
By R.F. Anderson
13 PERSPECTIVE. A Young Scientist’s Perspective on GEOTRACES
By D.J. Halbeisen
17 PERSPECTIVE. The Value of Going To Sea on Big Ships and the Advantage
of Multiple Tracers for GEOTRACES Style Programs
By C. Jeandel
21 PERSPECTIVE. Intercalibration: A Cornerstone of the Success of the
GEOTRACES Program
By A. Aguilar-Islas, H. Planquette, M.C. Lohan, W. Geibert, and G. Cutter
25 The GEOTRACES Intermediate Data Products: Rich Resources for Research,
Education, and Outreach
By R. Schlitzer and S. Mieruch-Schnülle
34 SPOTLIGHT. Three-Minute Videos to Learn About Marine Geochemistry
By E. Masferrer Dodas, C. Jeandel, and A. Artis
35 PERSPECTIVE. GEOTRACES: Ironing Out the Details of the Oceanic
Iron Sources?
By T.M. Conway, R. Middag, and R. Schlitzer
46 The Southern Ocean Hub for Nutrients, Micronutrients, and Their Isotopes
in the Global Ocean
By G.F. de Souza and A.K. Morrison
60 Shelf-Basin Connectivity Drives Dissolved Fe and Mn Distributions in the
Western Arctic Ocean: A Synoptic View into Polar Trace Metal Cycling
By L. Jensen and M. Colombo
72 On the Variability of Equatorial Pacific Nitrate and Iron Utilization
By P.A. Rafter
85 Advances in Understanding the Marine Nitrogen Cycle in the
GEOTRACES Era
By K.L. Casciotti, T.A. Marshall, S.E. Fawcett, and A.N. Knapp
102 The “Net” Impact of Hydrothermal Venting on Oceanic Elemental
Inventories: Contributions to Plume Geochemistry from the International
GEOTRACES Program
By J.N. Fitzsimmons and J.M. Steffen
116 PERSPECTIVE. GEOTRACES: Fifteen Years of Progress in Marine Aerosol
Research
By C.S. Buck, S. Fietz, D.S. Hamilton, T.-Y. Ho, M.M.G. Perron, and R.U. Shelley
120 An Ocean of Particles: Characterization of Particulate Trace Elements by
the GEOTRACES Program
By B.S. Twining
contents VOL. 37, NO. 2, JUNE 2024
102
46
60
June 2024 | Oceanography
Oceanography | Vol. 37, No. 2
238U
234Th
234U
230Th
Dust/
Minerals
232Th
228Th
224Ra
Porewater
Release
232Th
228Ra
234Pa
228Ra
228Th
228Ac
Sinking Particle
Removal
Sinking Particle
Removal
Sinking Particle
Removal
24 days
76 kyr
1.8 yr
4.5 Gyr
250 kyr
1.2 min
14 Gyr
6.2 hrs
5.8 yr
3.7 days
1.8 yr
14 Gyr
5.8 yr
156
AAIW
NADW
AABW
131 Novel Insights into Ocean Trace Element Cycling from
Biogeochemical Models
By A. Tagliabue and T. Weber
142 New Insights into the Organic Complexation of Bioactive Trace Metals
in the Global Ocean from the GEOTRACES Era
By H. Whitby, J. Park, Y. Shaked, R.M. Boiteau, K.N. Buck, and R.M. Bundy
156 PERSPECTIVE. Timekeepers for Trace Elements in the Global Ocean:
The Thorium Stopwatches
By C.T. Hayes
162 PERSPECTIVE. The Dawn of the BioGeoSCAPES Program: Ocean
Metabolism and Nutrient Cycles on a Changing Planet
By M.A. Saito, H. Alexander, H.M. Benway, P.W. Boyd, M. Gledhill, E.B. Kujawinski,
N.M. Levine, M. Maheigan, A. Marchetti, I. Obernosterer, A.E. Santoro, D. Shi, K.Suzuki,
A. Tagliabue, B.S. Twining, and M.T. Maldonado
DEPARTMENTS
5 QUARTERDECK. A Needle in the Haystack
By C. Benitez-Nelson
167 DIY OCEANOGRAPHY. The OpenCTD: A Low-Cost, Open-Source CTD for
Collecting Baseline Oceanographic Data in Coastal Waters
By A. Thaler, S.K. Sturdivant, R.Y. Neches, and J.J. Levenson
174 DIY OCEANOGRAPHY. Design Update to “The Pressure of In-Situ Gases
Instrument (PIGI) for Autonomous Shipboard Measurement of Dissolved O2
and N2 in Surface Ocean Waters”
By B. Lowin, R. Izett, E. Taylor, C. Robertson, and S. Rivero-Calle
180 TRIBUTE. A Tribute to Richard W. Eppley
Compiled and edited by P.G. Falkowski
186 THE OCEANOGRAPHY CLASSROOM. Interviews!
By S. Boxall
188 FROM THE TOS JEDI COMMITTEE. In Pursuit of Conference Equity
By I.M. Martínez-Farrington, L. Martell Bonet, A. Alemán-Díaz, H.J. Ballenger, D. Ebanks,
M. Behl, S. Kolesar, S. Cooper, C. Garza, J.C. Lewis, M.B. Jones, and L. White
190 CAREER PROFILES. Trisha Bergmann, International Affairs Specialist, NOAA
National Ocean Service | Meredith Jennings, Senior Research Associate,
Houston Advanced Research Center
ON THE COVER
Deployment of the "trace metal clean" rosette
at 78°N from USCGC Healy as part of the 2015
US GEOTRACES GN01 cruise to the Arctic
Ocean. Water from this cast yielded data for
key biogeochemical parameters such as iron,
aluminum, radium, metal isotopes, and major
nutrients. Photo credit: Katlin Bowman Adamczyk,
US Geological Survey
SPECIAL ISSUE SPONSOR
Support for production of this special issue was
provided by the US National Science Foundation,
award OCE-2219888 to Robert Anderson.
SPECIAL ISSUE GUEST EDITORS
Tim Conway, University of South Florida
Jessica Fitzsimmons, Texas A&M University
Rob Middag, Royal NIOZ
Taryn Noble, University of Tasmania
Hélène Planquette, University of Brest
180
131
IN
PROGRESS
TWENTY YEARS OF GEOTRACES
AN INTERNATIONAL STUDY OF THE MARINE BIOGEOCHEMICAL
CYCLES OF TRACE ELEMENTS AND ISOTOPES
Oceanography
THE OFFICIAL MAGAZINE OF THE OCEANOGRAPHY SOCIETY
VOL. 37, NO. 2, JUNE 2024
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June 2024 | Oceanography
For the past 20 years, the GEOTRACES program has produced trans-
formative insights into the cycling of trace elements and isotopes (TEIs)
in the ocean. Modeled after the 1970s GEOSECS (Geochemical Ocean
Sections Study) program, the goals of this ambitious study were to con-
duct large-scale measurements of TEI concentrations and to deter-
mine the biogeochemical processes that controlled their distributions
throughout the global ocean. Perhaps the biggest hurdle to overcome
was how to accurately measure less than nanomolar concentrations of
TEIs from the deck of a 3,000 gross tonnage steel ship. Contaminants
were literally everywhere, requiring scientists and crew to become inno-
vative in their attempts to maintain clean conditions for sample collec-
tion, storage, and measurement. GEOTRACES encouraged interna-
tional partnerships while instilling a collaborative approach in the next
generation of scientists. The practices GEOTRACES developed led to
consistent high-quality measurements across diverse lab groups and
provided an extensive and open database. GEOTRACES has truly been
a labor of love among all the scientists and crews involved, and the pro-
gram continues as a lofty example of the discoveries possible when con-
ducting such wide-ranging collaborative research.
Claudia Benitez-Nelson
Associate Editor, Oceanography
D https://doi.org/10.5670/oceanog.2024.422
QUARTERDECK
A NEEDLE IN THE HAYSTACK
Physical Oceanography of
Continental Shelves
K.H. Brink
An authoritative graduate textbook and
professional reference on the physical
dynamics of the coastal ocean
“This concise, up-to-date textbook has
virtually everything a student needs for
a course on coastal ocean physics.”
—John Wilkin, Rutgers University
• Covers the physical dynamics and properties
of the coastal ocean, synthesizing theory
and global observations
• Topics include turbulent boundary layers,
wind driving, coastal-trapped waves, the
inner shelf, tides, buoyancy currents,
instabilities, and connections with the
deep open ocean
Oceanography Flipbooks
https://oceanography.publuu.com
Be sure to visit the open access Oceanography flipbook library to
page through the full GEOTRACES special issue. While there, explore
other Oceanography issues back to March 2015.
AUTHORS! Flipbooks are an exciting enhancement to Oceanography.
In these flipbooks, we can embed videos, animations, photo galleries,
and audio files in your article. For details on file sizes and formats, visit
https://tos.org/oceanography/guidelines.
Oceanography | Vol. 37, No. 2
INTRODUCTION TO THE
SPECIAL ISSUE ON
TWENTY YEARS OF GEOTRACES
AN INTERNATIONAL STUDY OF THE MARINE BIOGEOCHEMICAL
CYCLES OF TRACE ELEMENTS AND ISOTOPES
INTRODUCTION
This special issue of Oceanography cele-
brates the transformational findings of
the international GEOTRACES program
in chemical oceanography, 20 years after
drafting of the GEOTRACES Science
Plan in 2004 (GEOTRACES Planning
Group, 2006). With the section cruise
phase of the program ending soon, and a
planned pivot toward smaller- scale pro-
cess studies, this is an opportune time
to look back at the achievements of
GEOTRACES during the last two decades
and to highlight some of the advances in
our understanding of the processes that
determine the oceanic distributions of
trace elements and isotopes (TEIs).
GEOTRACES
GEOTRACES is a cooperative interna-
tional effort comprising scientists from
35 countries, supported by national fund-
ing agencies, and guided by international
steering committees. Since 2008, the pro-
gram has carried out oceanographic,
World Ocean Circulation Experiment
(WOCE)-style “section” cruises through-
out all the ocean basins. Rigorous inter-
calibration efforts stand at the foun-
dation of GEOTRACES to ensure that
basin-scale analyses made by individ-
ual nations can be compared with-
out question. This has been achieved
primarily by occupation of “crossover”
stations between cruises hosted by dif-
ferent countries—perhaps the most
evocative was a meeting of German and
US GEOTRACES Arctic cruises at the
North Pole in 2015. Moreover, the drive
for publicly available synthesized data-
sets has resulted in the production of
an online electronic atlas (https://www.
egeotraces.org/) and the release of three
intermediate data products to date, with
a fourth expected in 2025. These efforts
have led to a sea change in the quantity
of high-quality TEI data available for the
ocean, coupled with many advances in
our understanding of the biogeochemical
processes that determine these distribu-
tions (Anderson et al., 2014).
ARTICLES IN THE
SPECIAL ISSUE
This special issue begins with reflec-
tions from junior and senior scien-
tists on the GEOTRACES program,
the benefit of a multi-tracer approach
for answering biogeochemical ques-
tions, and what life is like aboard a
GEOTRACES section cruise (Anderson,
Halbeisen, and Jeandel). A key suc-
cess of GEOTRACES lies in the rigorous
intercalibration of all data incorporated
into its products, a process overseen
by the Standards and Intercalibration
Committee. This achievement is based
on crucial cooperation among scien-
tists who openly shared protocols, as
discussed in Aguilar-Islas et al., and
allows for both a holistic view of TEI
cycling within a basin (e.g., Arctic
Ocean; Jensen and Colombo) and
synthesized intermediate data prod-
ucts that are valuable for research, edu-
cation, and outreach efforts (as outlined
by Schlitzer and Mieruch-Schnülle).
In addition, from an education and out-
reach perspective, the GEOTRACES
program office has produced a series
of short educational videos, which are
highlighted by Masferrer Dodas et al.
Several articles in this issue focus
on “key parameters,” tracers that are
deemed essential to the GEOTRACES
program. For example, Anderson and
Conway et al. both focus on dissolved
iron and how GEOTRACES has elegantly
demonstrated the utility of a multi-tracer
approach and the unexpected and per-
sistent nature of iron from deep bound-
ary sources such as marine sediments
and hydrothermal venting. Fitzsimmons
and Steffen take a deeper dive into
how GEOTRACES research on hydro-
thermal plumes have transformed our
understanding of which TEIs are sup-
plied or consumed by hydrothermal
systems before influencing the wider
im M. Conway, Jessica N. Fitzsimmons, Rob Middag, Taryn L. Noble, and Hélène Planquette
FROM THE GUEST EDITORS
June 2024 | Oceanography
ocean. Hayes provides an overview of
the unique value of the thorium radio-
nuclides as timekeepers for rates in the
ocean and how they aid understand-
ing of manifold biogeochemical pro-
cesses, while Casciotti et al. describe
recent advances in our understanding
of the marine nitrogen cycle that have
come from GEOTRACES and other
efforts. Rafter then looks at how linking
together elements such as iron and nitro-
gen can enhance insights into oceanic
nutrient cycling.
The proliferation of GEOTRACES
datasets has sparked renewed appre-
ciation for the role of physical circula-
tion in influencing global TEI distri-
butions. The paper by de Souza and
Morrison takes a close look at our
increasing understanding of the role of
the Southern Ocean “hub” in driving
global nutrient and TEI distributions.
However, it is not just knowledge of oce-
anic “dissolved” TEIs and circulation that
are needed to understand marine bio-
geochemical cycles; GEOTRACES has
also provided numerous advances in
our knowledge of elemental speciation
and organic complexation, marine par-
ticles, and aerosols as featured by sev-
eral articles in this issue (Whitby et al.,
Twining, and Buck et al., respectively).
Tagliabue and Weber provide an over-
view of how a range of different ocean
biogeochemical modeling approaches
have been stimulated by—and become
invaluable in the interpretation of—
GEOTRACES datasets. Lastly, we look to
the future with Saito et al. describing the
nascent BioGeoSCAPES program, which
aims to build on advances from pro-
grams such as GEOTRACES and stim-
ulate a research program designed to
enhance understanding of the linkages
between microbes, climate change, and
biogeochemical cycles. We eagerly antic-
ipate the development of this new pro-
gram and the continuing discoveries to
come from GEOTRACES.
REFERENCES
Aguilar-Islas, A., H. Planquette, M.C. Lohan, W. Geibert,
and G. Cutter. 2024. Intercalibration: A corner-
stone of the success of the GEOTRACES program.
Oceanography 37(2):21–24, https://doi.org/10.5670/
oceanog.2024.404.
Anderson, R.F. 2024. GEOTRACES reflections.
Oceanography 37(2):8–12, https://doi.org/10.5670/
oceanog.2024.405.
Anderson, R., E. Mawji, G.A. Cutter, C.I. Measures,
and C. Jeandel. 2014. GEOTRACES: Changing
the way we explore ocean chemistry.
Oceanography 27(1):50–61, https://doi.org/
10.5670/oceanog.2014.07.
Buck, C.S., S. Fietz, D.S. Hamilton, T.-Y. Ho,
M.M.G. Perron, and R.U. Shelley. 2024.
GEOTRACES: Fifteen years of progress in marine
aerosol research. Oceanography 37(2):116–119,
https://doi.org/10.5670/oceanog.2024.409.
Casciotti, K.L., T.A. Marshall, S.E. Fawcett,
and A.N. Knapp. 2024. Advances in under-
standing the marine nitrogen cycle in the
GEOTRACES era. Oceanography 37(2):85–101,
https://doi.org/10.5670/oceanog.2024.406.
Conway, T.M., R. Middag, and R. Schlitzer. 2024.
GEOTRACES: Ironing out the details of the oce-
anic iron sources? Oceanography 37(2):35–45,
https://doi.org/10.5670/oceanog.2024.416.
de Souza, G.F., and A.K. Morrison. 2024. The
Southern Ocean hub for nutrients, micronutrients,
and their isotopes in the global ocean.
Oceanography 37(2):46–59, https://doi.org/
10.5670/oceanog.2024.414.
Fitzsimmons, J.N., and J.M. Steffen. 2024. The “net”
impact of hydrothermal venting on oceanic ele-
mental inventories: Contributions to plume geo-
chemistry from the international GEOTRACES pro-
gram. Oceanography 37(2):102–115, https://doi.org/
10.5670/oceanog.2024.421.
GEOTRACES Planning Group. 2006. GEOTRACES
Science Plan. Scientific Committee on Oceanic
Research, Baltimore, MD, https://geotracesold.
sedoo.fr/libraries/documents/Science_plan.pdf.
Halbeisen, D.J. 2024. A young scientist’s perspec-
tive on GEOTRACES. Oceanography 37(2):13–16,
https://doi.org/10.5670/oceanog.2024.403.
Hayes, C.T. 2024. Timekeepers for trace elements
in the global ocean: The thorium stopwatches.
Oceanography 37(2):156–161, https://doi.org/
10.5670/oceanog.2024.412.
Jeandel, C. 2024. The value of going to sea
on big ships and the advantage of multi-
ple tracers for GEOTRACES style programs.
Oceanography 37(2):17–20, https://doi.org/
10.5670/oceanog.2024.408.
Jensen, L., and M. Colombo. 2024. Shelf-basin
connectivity drives dissolved Fe and Mn dis-
tributions in the western Arctic Ocean: A syn-
optic view into polar trace metal cycling.
Oceanography 37(2):60–71, https://doi.org/
10.5670/oceanog.2024.410.
Masferrer Dodas, E., C. Jeandel, and A. Artis. 2024.
Three-minute videos to learn about marine geo-
chemistry. Oceanography 37(2):34, https://doi.org/
10.5670/oceanog.2024.401.
Rafter, P.A. 2024. On the variability of equa-
torial Pacific nitrate and iron utilization.
Oceanography 37(2):72–84, https://doi.org/
10.5670/oceanog.2024.411.
Saito, M.A., H. Alexander, H.M. Benway, P.W. Boyd,
M. Gledhill, E.B. Kujawinski, N.M. Levine,
M. Maheigan, A. Marchetti, I. Obernosterer, and
others. 2024. The dawn of the BioGeoSCAPES pro-
gram: Ocean metabolism and nutrient cycles on
a changing planet. Oceanography 37(2):162–166,
https://doi.org/10.5670/oceanog.2024.417.
Schlitzer, R., and S. Mieruch-Schnülle. 2024. The
GEOTRACES intermediate data products: Rich
resources for research, education, and out-
reach. Oceanography 37(2):25–33, https://doi.org/
10.5670/oceanog.2024.402.
Tagliabue, A., and T. Weber. 2024. Novel insights
into ocean trace element cycling from biogeo-
chemical models. Oceanography 37(2):131–141,
https://doi.org/ 10.5670/oceanog.2024.418.
Twining, B.S. 2024. An ocean of particles:
Characterization of particulate trace ele-
ments by the GEOTRACES program.
Oceanography 37(2):120–130, https://doi.org/
10.5670/oceanog.2024.407.
Whitby, H., J. Park, Y. Shaked, R.M. Boiteau, K.N. Buck,
and R.M. Bundy. 2024. New insights into the
organic complexation of bioactive trace met-
als in the global ocean from the GEOTRACES era.
Oceanography 37(2):142–155, https://doi.org/
10.5670/ oceanog.2024.419.
ACKNOWLEDGMENTS
We thank Robert Anderson, the US GEOTRACES
Program Office, and the US National Science
Foundation for supporting this Special Issue through
NSF award OCE-2219888. We also thank each of
the authors of the original GEOTRACES Science Plan
(GEOTRACES Planning Group, 2006), whose vision
brought us to the outstanding discoveries described
in this special issue. The international GEOTRACES
program is possible in part thanks to the support from
the US National Science Foundation (Grant OCE-
2140395) to the Scientific Committee on Oceanic
Research (SCOR).
AUTHORS
Tim M. Conway (tmconway@usf.edu) is Associate
Professor, College of Marine Sciences, University
of South Florida, St. Petersburg, FL, USA.
Jessica N. Fitzsimmons is Associate Professor,
Department of Oceanography, Texas A&M University,
College Station, TX, USA. Rob Middag is Research
Leader, Royal Netherlands Institute for Sea Research
(NIOZ), Den Burg, the Netherlands, and Honorary
Professor, Centre for Isotope Research - Oceans,
University of Groningen, Groningen, the Netherlands.
Taryn L. Noble is Senior Lecturer, Institute for Marine
and Antarctic Studies, University of Tasmania,
Hobart, Tasmania, Australia. Hélène Planquette is
Senior Researcher, CNRS, Université de Bretagne
Occidentale, IRD, Ifremer, LEMAR, Plouzané, France.
ARTICLE CITATION
Conway, T.M., J.N. Fitzsimmons, R. Middag,
T.L. Noble, and H. Planquette. 2024. Introduction to
the special issue on twenty years of GEOTRACES:
An international study of the marine biogeo-
chemical cycles of trace elements and isotopes.
Oceanography 37(2):6–7, https://doi.org/10.5670/
oceanog.2024.415.
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Oceanography | Vol. 37, No. 2
PERSPECTIVE
GEOTRACES REFLECTIONS
By Robert F. Anderson
SPECIAL ISSUE ON TWENTY YEARS OF GEOTRACES
Though geochemists have long sought to
understand biogeochemical cycles, initial
attempts to measure the extremely low
concentrations of trace elements in sea-
water were frustrated by contamination
problems. It was not until the 1970s that
contamination-free methods were devel-
oped, launching a new era of research to
characterize the biogeochemical cycles of
trace elements in the ocean.
Nevertheless, by the beginning of the
new millennium, after two decades of
improved and, generally, contamination-
free sample collection, work was pro-
ceeding so slowly that description of
the marine biogeochemical cycles of
most trace elements was beyond reach.
For example, by 2003, dissolved iron
(dFe) profiles from the surface ocean
to >2,000 m had been reported for no
more than two dozen locations world-
wide (Anderson et al., 2014). Despite reli-
able data, for the most part, they were
grossly inadequate to define biogeochem-
ical cycling of Fe.
Efforts would have to be coordinated
to characterize the global biogeochem-
ical cycle of any trace element: no sin-
gle nation, let alone an individual inves-
tigator, could hope to compile sufficient
information. This recognition led to the
creation of the GEOTRACES program
(https://www.geotraces.org/), an inter-
national study of the marine biogeo-
chemistry of trace elements and their
isotope (TEIs).
The objectives of the program were
straightforward: to determine ocean dis-
tributions of TEIs globally and to under-
stand the processes that control them
well enough to code the defining param-
eters into models. Achieving the neces-
sary global coverage required contribu-
tions from many investigators in many
nations, which, in turn, led to two further
prerequisites: intercalibration, to ensure
internal consistency of data generated by
different labs (Aguilar-Islas et al., 2024, in
this issue) and a data management sys-
tem that combined the international suite
of intercalibrated data into a single data-
base that was available in multiple for-
mats, including graphical illustration of
the results in an electronic atlas (Schlitzer
and Mieruch-Schnülle, 2024, in this
issue). These prerequisites were put into
place in advance of the global study.
International workshops held in 2007,
focusing on the Pacific, Atlantic, and
Indian Oceans, enabled investigators to
identify target locations, either where
strong sources or sinks of TEIs were
thought to exist, or where internal cycling
processes (biological uptake, regener-
ation, abiotic scavenging, transport by
ocean circulation) have a strong influ-
ence over TEI distributions. Investigation
of the Arctic and Southern Oceans began
in 2007 under the International Polar
Year (IPY). Although GEOTRACES
was not ready at that time to under-
take a full study of all TEIs of interest,
the development of new technologies
for the collection of contamination-free
samples (de Baar et al., 2008) in prepa-
ration for GEOTRACES allowed some
GEOTRACES investigators to partic-
ipate in the IPY. More complete plan-
ning for Arctic Ocean work (Jensen and
Colombo, 2024, in this issue) was orga-
nized during an international workshop
in 2009. Workshop reports, containing
recommendations for a global survey, are
available at https://www.geotraces.org/
planning-documents/.
A global survey of TEI distributions
(Figure 1) was designed using the targeted
locations noted above, enabling investi-
gators to develop, and in some cases test,
ABSTRACT. GEOTRACES is an international program that has benefited from con-
tributions by investigators in 35 nations. The program mission is to identify processes
and quantify fluxes that control the distributions of key trace elements and isotopes
in the ocean and to establish the sensitivity of these distributions to changing envi-
ronmental conditions. This perspective first summarizes the historical motivation
for the program, and then describes selected research highlights, focusing on recent
findings related to iron. The patchy distribution of iron in the ocean indicates a short
residence time, at the low end of the range of residence times estimated in models.
Iron removal from the ocean must, therefore, be rapid. Recent results from the North
Atlantic Ocean suggest that the formation of particulate authigenic iron phases may be
a factor contributing to iron removal that is faster than previously thought. This arti-
cle also identifies several areas where advancements are expected through modeling
and synthesis efforts.