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

Dissolved Fe(II) concentrations were determined using an automated flow injection analysis system (FeLume II Waterville Analytical) employing a luminol chemiluminescence based detection system as described in King et al. (1995) and Heller et al. (2017).

From published and ongoing work in the Peru and Chile oxygen minimum zones an optimized methodology for Fe(II) detection by luminol has been described in Croot et al. (2019). This work details some of the challenges and limitations of using luminol for dFe(II) detection. It should be noted that as Fe(II) is a transient redox species with a short half life in oxygenated seawater that is dependent on dissolved oxygen and pH, it is not possible to archive (preserve) samples for later analysis. Intercalibrations for Fe(II) have yet to be conducted in the context of GEOTRACES studies and should be pursued where logistically possible.  

Samples for in-situ dissolved Fe(II) (dFe(II)) detection (see Supplemental Files: Table 1 and Figure 1):
157 samples were as soon as possible analysed from the Geotraces carousel from Stn 38-66;
5 ice samples from Stn43 /46 (operated by Ana Aguilar, Rob Rember);
3 small boat surface samples from Stan60,61,66 (operated by Ana Aguilar, Rob Rember);
1 Multicorer niskin sample from Stn66 (operated by Greg Cutter).

Shipboard measurements were performed by Dr. Maija I. Heller for Fe(II) following a method by King et al. The photomultiplier (PMT) for analysis was borrowed from the FS Polarstern (Germany) at the North Pole from scientist Micha Rijkenberg, NIOZ, (Netherlands) since the PMT owned by UCSC (USA) was damaged at the beginning of the cruise, likely due to a bad power supply.

Dissolved Fe(II) concentrations were determined using an automated flow injection analysis system (FeLume II Waterville Analytical) employing a luminol chemiluminescence based detection system (King et al, 1995). Luminol, prepared in 18.2 MO Milli-Q water and buffered to pH 10.3 with ultra pure ammonia (here used quartz distilled at UCSC), reacts with an Fe(II)- containing solution, resulting in luminol oxidation with concurrent chemiluminescent emission (Rose & Waite, 2001; Croot & Laan, 2002). The FeLume was fitted with a standard quartz flow cell and a Hamamatsu HC135 photon counter configured with the following settings: flow rate: 2.5 mL per minute; photon counter integration time: 200 ms; load time: 20–40 s. The mixing and reaction occur in a spiral flow cell positioned in front of a photomultiplier tube. The sample and luminol reagent were directly continually mixed in the flow cell by omitting the injection valve (Rose & Waite, 2001; Hopkinson & Barbeau, 2007; Roy et al, 2008). Once the signal was in steady-state, the mean of the last 50 data points was used to determine the signal. Fe(II) was quantified by 6-8 standard additions of Fe(II) (typically 0.025, 0.05, 0.1, 0.2, 0.4 and 0.8 nM) to samples of seawater which were stored in the dark for several days and showed low initial signals for Fe(II). A 0.01 M Fe(II) stock solution was prepared with ferrous sulfate (Sigma) in 0.2 M HCl. A working Fe(II) standard solution (100 nM) was prepared daily. Detection limits were determined for dark corrected aged seawater samples where ferrous Fe was negligible based on a standard 3r evaluation of the baseline signal (Moffett, et al. 2007; Kondo & Moffett, 2013; Heller et al., 2017).

Instrumentation:
The chemiluminescence was detected with a Hamamatsu HC135 Photon counter build into a flow injection system (FeLume, Waterville Analytical, US) borrowed from NIOZ (Netherlands) as described above (Rijkenberg et al., 2014).

Problem Report:
The required PMT for analysis was borrowed from the FS Polarstern (Germany) at the North Pole from NIOZ scientist Micha Rijkenberg, (Netherlands), since the PMT owned by UCSC (USA) was damaged at the very beginning of the cruise, most likely by a faulty power supply.

Quality Control:
See Supplemental File "Figure 2" for examples for standard additions of Fe(II) and blank calculation of the above described method.

Quality Flags:
All data have been assigned quality flags using the GTSPP convention and interpretation. Values below the detection limit were flagged as QF=6 in the GTSPP convention (also adopted by SeaDataNet and recommended by the GEOTRACES programme):
1 = good—passed lab QC and oceanographically consistent;
2 = possibly good—oceanographically consistent, but have minor sampling/instrumental issues;
3 = possibly bad—not oceanographically consistent, or have major sampling/instrumental issues;
4 = bad—failed lab QC or known issue with samples;
6 = below detection limit;
9 = data missing (including all “nd”).

BCO-DMO Processing:
Version history:
2020-05-15 - original data version;
2020-09-23 - made corrections to the following duplicate, erroneous sample and event numbers:
-- when {Event_ID} == '6303', replaced {Sample_ID} 11891 with 11591;
-- when {Start_Time_UTC} == '2015-09-10T10:19:00.00Z', replaced {Event_ID} 6249 with 6253.

Description from NSF award abstract:
In pursuit of its goal "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 environmental conditions", in 2015 the International GEOTRACES Program will embark on several years of research in the Arctic Ocean. In a region where climate warming and general environmental change are occurring at amazing speed, research such as this is important for understanding the current state of Arctic Ocean geochemistry and for developing predictive capability as the regional ecosystem continues to warm and influence global oceanic and climatic conditions. The three investigators funded on this award, will manage a large team of U.S.scientists who will compete through the regular NSF proposal process to contribute their own unique expertise in marine trace metal, isotopic, and carbon cycle geochemistry to the U.S. effort. The three managers will be responsible for arranging and overseeing at-sea technical services such as hydrographic measurements, nutrient analyses, and around-the-clock management of on-deck sampling activites upon which all participants depend, and for organizing all pre- and post-cruise technical support and scientific meetings. The management team will also lead educational outreach activities for the general public in Nome and Barrow, Alaska, to explain the significance of the study to these communities and to learn from residents' insights on observed changes in the marine system. The project itself will provide for the support and training of a number of pre-doctoral students and post-doctoral researchers. Inasmuch as the Arctic Ocean is an epicenter of global climate change, findings of this study are expected to advance present capability to forecast changes in regional and globlal ecosystem and climate system functioning.

As the United States' contribution to the International GEOTRACES Arctic Ocean initiative, this project will be part of an ongoing multi-national effort to further scientific knowledge about trace elements and isotopes in the world ocean. This U.S. expedition will focus on the western Arctic Ocean in the boreal summer of 2015. The scientific team will consist of the management team funded through this award plus a team of scientists from U.S. academic institutions who will have successfully competed for and received NSF funds for specific science projects in time to participate in the final stages of cruise planning. The cruise track segments will include the Bering Strait, Chukchi shelf, and the deep Canada Basin. Several stations will be designated as so-called super stations for intense study of atmospheric aerosols, sea ice, and sediment chemistry as well as water-column processes. In total, the set of coordinated international expeditions will involve the deployment of ice-capable research ships from 6 nations (US, Canada, Germany, Sweden, UK, and Russia) across different parts of the Arctic Ocean, and application of state-of-the-art methods to unravel the complex dynamics of trace metals and isotopes that are important as oceanographic and biogeochemical tracers in the sea.

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.