Table of Contents

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

Concentrations of dissolved iron and dissolved iron(II) in water-column samples from major stations occupied during first and second legs of the U.S. GEOTRACES North Atlantic zonal transect cruises.

Sample collection and processing:
Water-column samples for trace metal analysis were collected in modified 12 L Teflon-lined GO-FLO samplers (General Oceanics Inc.), deployed on the U.S. GEOTRACES clean CTD rosette system with a total of 24 samplers. All samples were filtered as soon as possible after recovery by GEOTRACES sampling personnel using 0.2 µm Supor Acropak filter cartridge (Pall Corp.) inside the U.S. GEOTRACES clean-air laboratory van. Near-surface samples (from ~2 m depth) were collected while underway on approach to each station using the Bruland-Smith trace-metal towfish, which was towed outside of the ship’s wake. Seawater from the towfish was pumped directly into a clean-air laboratory van, where it was filtered through 0.2 µm Supor Acropak filter cartridges.

Dissolved iron (dFe) analysis:
The filtered seawater samples were collected in 125 mL acid-cleaned Nalgene wide-mouth low-density polyethylene bottles, acidified to pH ~1.7 with Fisher Optima grade ultrapure hydrochloric acid, and then stored at room temperature until post-cruise analysis at Old Dominion University by Bettina Sohst. Dissolved iron was determined by flow injection analysis with colorimetric detection after in-line preconcentration on resin-immobilized 8-hydroxyquinoline [Sedwick et al., 2005, 2008], using a method modified from Measures et al. [1995]. The efficacy of our analytical method for dFe was verified through the analysis of the SAFe seawater reference materials. Over the period during which these samples were analyzed (2010-2012), we determined mean dFe values of 0.122 ± 0.021 nM (n = 7) for SAFe seawater reference material S and 1.13 ± 0.19 nM (n = 2) for SAFe seawater reference material D2, versus community consensus values of 0.095 ± 0.008 nM and 0.955 ± 0.024 nM, respectively (as of May 2013). The analytical limit of detection is estimated as the dFe concentration equivalent to a peak area that is three times the standard deviation on the "zero-loading blank" (or "manifold blank"), from which we estimate a detection limit of less than 0.04 nM [Bowie et al., 2004; Sedwick et al., 2005]. Blank contributions from the ammonium acetate sample buffer solution (added on-line during analysis) and hydrochloric acid (added after collection) are typically negligible (i.e., too low to quantify). Robust estimates of our analytical precision are derived from multiple (separate-day) determinations of the SAFe seawater reference materials, which yield analytical uncertainties (expressed as ± one relative sample standard deviation on the mean) of ±14.7% (n = 33) at the concentration level of SAFe S seawater (0.095 nM), and ±9.2% (n = 16) at the concentration level of SAFe D2 seawater (0.955 nM).

Dissolved iron(II) (dFe(II)) analysis:
The filtered seawater samples were transported from the sampling station to a shipboard clean-air laboratory van in a cooler, and were analyzed at sea by Bettina Sohst as soon as possible after sampling. Dissolved iron(II) was determined by flow injection analysis with in-line preconcentration modified after the method of Bowie et al. [2005]. The analytical limit of detection was estimated daily as the dFe(II) concentration corresponding to a signal three times the standard deviation on triplicate analyses of the blank [Bowie et al., 2004; Sarthou et al., 2011]. For the blank solution, we used filtered, aged, low-iron seawater that was stored in the dark ("low Fe(II) seawater"), for which the chemiluminescence signal was analytically indistinguishable from reagents injected without seawater [see Bowie et al., 2005]. KN199-04: The limit of detection averaged <0.06 nM for all of the daily shipboard analyses (n = 16). KN204-01: The limit of detection averaged <0.01 nM for all of the daily shipboard analyses (n = 31).

There is no standard reference material for the determination of dissolved iron(II) in seawater, thus we are unable to provide rigorous estimates of the accuracy and precision of the dFe(II) determinations. However, some indication of our analytical uncertainty is provided by repeat measurements of standards prepared in low-Fe(II) seawater, which yielded average relative standard deviations of <17% (KN199-04) or <13% (KN204-01) and <11% for triplicate injections of 0.2 nM and 0.4 nM dFe(II) standards, respectively.

References:
Bowie, A. R., P. N. Sedwick, and P. J. Worsfold (2004), Analytical intercomparison between flow injection-chemiluminescence and flow injection-spectrophotometry for the determination of picomolar concentrations of iron in seawater, Limnology & Oceanography Methods, 2, 42-54. doi: 10.4319/lom.2004.2.42.

Bowie, A. R., E. P. Achterberg, S. Ussher, and P. J. Worsfold (2005), Design of an automated flow injection-chemiluminescense instrument incorporating a miniature photomultiplier tube for monitoring picomolar concentrations of iron in seawater, Journal of Automated Methods & Management in Chemistry, 2, 37-43. doi: 10.1155/JAMMC.2005.37.

Measures, C. I., J. Yuan, and J. A. Resing (1995), Determination of iron in seawater by flow injection analysis using in-line preconcentration and spectrophotometric detection, Marine Chemistry, 50, 3-12. doi: 10.1016/0304-4203(95)00022-J.

Sarthou, G., E. Bucciarelli, F. Chever, S. P. Hansard, M. González-Dávila, J. M. Santana-Casiano, F. Planchon, and S. Speich (2011), Labile Fe(II) concentrations in the Atlantic sector of the Southern Ocean along a transect from the subtropical domain to the Weddell Sea Gyre, Biogeosciences, 8, 2461-2479, doi: 10.5194/bg-8-2461-2011.

Sedwick, P. N., T. M. Church, A. R. Bowie, C. M. Marsay, S. J. Ussher, K. M. Achilles, P. J. Lethaby, R. J. Johnson, M. M. Sarin, and D. J. McGillicuddy (2005), Iron in the Sargasso Sea (Bermuda Atlantic Time-series Study region) during summer: Eolian imprint, spatiotemporal variability, and ecological implications, Global Biogeochemical Cycles, 19, GB4006, doi: 10.1029/2004GB002445.

Sedwick, P. N., A. R. Bowie, and T. W. Trull (2008), Dissolved iron in the Australian sector of the Southern Ocean (CLIVAR-SR3 section): meridional and seasonal trends, Deep-Sea Research I, 55, 911-925, doi: 10.1016/j.dsr.2008.03.011.

Data processing:
Following subtraction of appropriate blank values, sample concentrations were calculated using least-square linear regressions fit to daily calibration curves that were obtained by additions of iron and iron(II) standard solutions to low-iron seawater.

Note on concentration units: All analyses were performed on a volumetric basis, so all concentrations are reported in units of nanomole liter-1 (nM). To convert to units of nmol kg-1, divide the nM concentrations by 1.024 kg l-1 (the approximate density of seawater at a laboratory temperature of 20 degrees C).

BCO-DMO Processing Notes:
Converted original lon values in decimal degrees west to negative values (decimal degrees east); added column for official cruise_id and for cruise_part; converted original date from dd/mm/yyyy format to date in yyyymmdd format; added 'nd' to indicate 'no data' for depths not sampled; changed parameter names; added basic GEOTRACES metadata (see below).

Additional GEOTRACES Processing:
After the data were submitted to the International Data Management Office, BODC, the office noticed that important identifying information was missing in many datasets. With the agreement of BODC and the US GEOTRACES lead PIs, BCO-DMO added standard US GEOTRACES information, such as the US GEOTRACES event number, to each submitted dataset lacking this information. To accomplish this, BCO-DMO compiled a 'master' dataset composed of the following parameters: station_GEOTRC, cast_GEOTRC (bottle and pump data only), event_GEOTRC, sample_GEOTRC, sample_bottle_GEOTRC (bottle data only), bottle_GEOTRC (bottle data only), depth_GEOTRC_CTD (bottle data only), depth_GEOTRC_CTD_rounded (bottle data only), BTL_ISO_DateTime_UTC (bottle data only), and GeoFish_id (GeoFish data only). This added information will facilitate subsequent analysis and inter comparison of the datasets.

Bottle parameters in the master file were taken from the GT-C_Bottle_GT10, GT-C_Bottle_GT11, ODF_Bottle_GT10, and ODF_Bottle_GT11 datasets. Non-bottle parameters, including those from GeoFish tows, Aerosol sampling, and McLane Pumps, were taken from the Event_Log_GT10 and Event_Log_GT11 datasets. McLane pump cast numbers missing in event logs were taken from the Particulate Th-234 dataset submitted by Ken Buesseler.

A standardized BCO-DMO method (called "join") was then used to merge the missing parameters to each US GEOTRACES dataset, most often by matching on sample_GEOTRC or on some unique combination of other parameters.

If the master parameters were included in the original data file and the values did not differ from the master file, the original data columns were retained and the names of the parameters were changed from the PI-submitted names to the standardized master names. If there were differences between the PI-supplied parameter values and those in the master file, both columns were retained. If the original data submission included all of the master parameters, no additional columns were added, but parameter names were modified to match the naming conventions of the master file.

See the dataset parameters documentation for a description of which parameters were supplied by the PI and which were added via the join method.

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.