Table of Contents

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

Dissolved (0.2 micron filtered) Iron, Zinc and Cadmium concentrations and isotope ratios.

Concentration data is freely available. Isotope data (columns L-Q in dataset) was restricted until July 2014. Fe isotope data was made public 2016-11-28.

DMO NOTE:
- 2017-07-07: Changed units of error columns to match those of the concentrations.
- 2017-07-07: Changed units of Cd to pmol/kg.
- version 5: 2016-11-28. All columns were made publicly accessible.
- version 4: 2015-01-16. Fe and Fe_err changed for sample 6128.
- version 3: Revised data served on 5/21/2014. Zn_err_2s changed for samples 7765 and 7767; sample 6862 flag was changed from 2 to 1.
- version 2: 1 January 2014
- version 1: 7 January 2013

Sampling and Analytical Methodology: 
Samples were provided from the GEOTRACES trace-metal clean sampling rosette and were sampled into acid-cleaned 1L LDPE bottles by the onboard GEOTRACES clean sampling team. Upon receipt of samples on shore, they were acidified to pH 2 using clean hydrochloric acid and left at this pH for at least 3 months before processing. Every sample was simultaneously extracted and purified for the analysis of d56Fe, d66Zn, d114Cd, [Fe], [Zn], and [Cd] using published methods (Conway et al., 2013). Briefly, Fe, Zn, and Cd double-spikes were added to the acidified samples and allowed to equilibrate for > 1 hour. Metals were then extracted from seawater onto an ethylenediaminetriacetic acid resin (Nobias PA-1; Hitachi) for 2 hours at pH 2 and 2 hours at pH between 6-6.3. Metals were eluted off this resin using 3 N HNO3, and were then purified by anion exchange chromatography before being reconstituted in 0.1 N HNO3 for analysis.

Concentrations and stable isotope ratios were determined simultaneously by analysis on a Neptune multi-collector ICP-MS. Fe and Zn were analyzed in high resolution mode, while Cd was analyzed in low resolution mode. Every sample was analyzed twice, and the mean isotope ratios are presented here, expressed relative to international isotope standards. Instrumental and procedural isotopic fractionation was corrected for using a double-spike data reduction scheme. 

Concentrations were calculated using isotope-dilution to calculate ng of metal per sample, and then converted to nmol kg-1 or pmol kg-1 using the weight of each sample processed. Concentration numbers represent the mean of two ICP-MS analyses of each sample. The largest sources of error for concentration analyses are determining the weight of the sample and determining the volume of double-spike added to each sample. We approximate 2% error for each sample based on the combination of these two factors. Accuracy was checked by analysis of S, D1 and D2 SAFe reference standards (for which we determined values identical to the consensus values; see Conway et al, 2013) and inter-comparison with two other groups measuring trace metal concentrations on US GEOTRACES NAZT samples. 

Analytical accuracy of stable isotope ratios is equal to the analytical precision (i.e. the analysis is free from systematic errors; John, 2012) and is calculated separately for each sample. Uncertainty for isotope ratios is expressed as the 2 sigma uncertainty on the average isotope ratio, calculated from a combination of the internal standard error of each of two measurements and the bracketing isotope standards used, as described in Conway et al., (2013). 

Data Processing:
Data are flagged as 1 (good) or 2 (questionable). Data points flagged as questionable are considered to be potential outliers. This may be because the difference between replicate analyses is much larger than the presumed analytical error, (the separation between two replicates was > 4x the 2s error) or because the data appear oceanographically inconsistent (e.g. values which are very different from samples taken above and below), even though there is no obvious analytical reason to dismiss them.

References
Conway, T. M., and John, S. G. (2014). The biogeochemical cycling of zinc and zinc isotopes in the North Atlantic Ocean. Global Biogeochemical Cycles. 28 (10). 1111-1128. doi:10.1002/2014GB004862.

Conway, T. M., and John. S. G. (2014). Quantification of dissolved iron sources to the North Atlantic Ocean. Nature. 511. 212-215. doi:10.1038/nature13482.

Conway, T. M., and John, S. G. (2015). Biogeochemical cycling of cadmium isotopes along a high-resolution section through the North Atlantic Ocean. Geochimica et Cosmochimica Acta. 148 (1). 269-283. doi: 10.1016/j.gca.2014.09.032.

Conway, T.M., Rosenberg, A.D., Adkins, J.F., and S.G. John. (2013). A new method for precise determination of iron, zinc and cadmium stable isotope ratios in seawater by double-spike mass spectrometry.  Analytica Chimica Acta, 793: 44-52.

John, S. (2012). Optimizing sample and spike concentrations for isotopic analysis by double-spike ICP-MS. Journal of Analytical Atomic Spectrometry, 27: 2123-2131.

Much of this text appeared in an article published in OCB News, October 2008, by the OCB Project Office.

The first U.S. GEOTRACES Atlantic Section will be specifically centered around a sampling cruise to be carried out in the North Atlantic in 2010. Ed Boyle (MIT) and Bill Jenkins (WHOI) organized a three-day planning workshop that was held September 22-24, 2008 at the Woods Hole Oceanographic Institution. The main goal of the workshop, sponsored by the National Science Foundation and the U.S. GEOTRACES Scientific Steering Committee, was to design the implementation plan for the first U.S. GEOTRACES Atlantic Section. The primary cruise design motivation was to improve knowledge of the sources, sinks and internal cycling of Trace Elements and their Isotopes (TEIs) by studying their distributions along a section in the North Atlantic (Figure 1). The North Atlantic has the full suite of processes that affect TEIs, including strong meridional advection, boundary scavenging and source effects, aeolian deposition, and the salty Mediterranean Outflow. The North Atlantic is particularly important as it lies at the "origin" of the global Meridional Overturning Circulation.

It is well understood that many trace metals play important roles in biogeochemical processes and the carbon cycle, yet very little is known about their large-scale distributions and the regional scale processes that affect them. Recent advances in sampling and analytical techniques, along with advances in our understanding of their roles in enzymatic and catalytic processes in the open ocean provide a natural opportunity to make substantial advances in our understanding of these important elements. Moreover, we are motivated by the prospect of global change and the need to understand the present and future workings of the ocean's biogeochemistry. The GEOTRACES strategy is to measure a broad suite of TEIs to constrain the critical biogeochemical processes that influence their distributions. In addition to these "exotic" substances, more traditional properties, including macronutrients (at micromolar and nanomolar levels), CTD, bio-optical parameters, and carbon system characteristics will be measured. The cruise starts at Line W, a repeat hydrographic section southeast of Cape Cod, extends to Bermuda and subsequently through the North Atlantic oligotrophic subtropical gyre, then transects into the African coast in the northern limb of the coastal upwelling region. From there, the cruise goes northward into the Mediterranean outflow. The station locations shown on the map are for the "fulldepth TEI" stations, and constitute approximately half of the stations to be ultimately occupied.

Figure 1. The proposed 2010 Atlantic GEOTRACES cruise track plotted on dissolved oxygen at 400 m depth. Data from the World Ocean Atlas (Levitus et al., 2005) were plotted using Ocean Data View (courtesy Reiner Schlitzer). [click on the image to view a larger version]

Hydrography, CTD and nutrient measurements will be supported by the Ocean Data Facility (J. Swift) at Scripps Institution of Oceanography and funded through NSF Facilities. They will be providing an additional CTD rosette system along with nephelometer and LADCP. A trace metal clean Go-Flo Rosette and winch will be provided by the group at Old Dominion University (G. Cutter) along with a towed underway pumping system.

The North Atlantic Transect cruise began in 2010 with KN199 leg 4 (station sampling) and leg 5 (underway sampling only) (Figure 2).

KN199-04 Cruise Report (PDF)

Figure 2. The red line shows the cruise track for the first leg of the US Geotraces North Atlantic Transect on the R/V Knorr in October 2010.  The rest of the stations (beginning with 13) will be completed in October-December 2011 on the R/V Knorr (courtesy of Bill Jenkins, Chief Scientist, GNAT first leg). [click on the image to view a larger version]

The section completion effort resumed again in November 2011 with KN204-01A,B (Figure 3).

KN204-01A,B Cruise Report (PDF)

Figure 3. Station locations occupied on the US Geotraces North Atlantic Transect on the R/V Knorr in November 2011.  [click on the image to view a larger version]

Data from the North Atlantic Transect cruises are available under the Datasets heading below, and consensus values for the SAFe and North Atlantic GEOTRACES Reference Seawater Samples are available from the GEOTRACES Program Office: Standards and Reference Materials

ADCP data are available from the Currents ADCP group at the University of Hawaii at the links below:
KN199-04   (leg 1 of 2010 cruise; Lisbon to Cape Verde)
KN199-05   (leg 2 of 2010 cruise; Cape Verde to Charleston, NC)
KN204-01A (part 1 of 2011 cruise; Woods Hole, MA to Bermuda)
KN204-01B (part 2 of 2011 cruise; Bermuda to Cape Verde)

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.