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.