Table of Contents

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

Total aerosol trace metals (Li, Na, Mg, Al, P, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, As, Se, Rb, Sr, Zr, Cd, Sn, Sb, Cs, Ba, La, Nd, Pb, Th, U) from the 2010 and 2011 U.S. GEOTRACES North Atlantic Transect cruises.

Please note that some US GEOTRACES data may not be final, pending intercalibration results and further analysis. If you are interested in following changes to US GEOTRACES NAT data, there is an RSS feed available via the BCO-DMO US GEOTRACES project page (scroll down and expand the "Datasets" section).

Sample Collection
Aerosol samples were collected on acid-washed Whatman 41 (W41) cellulose ester filter papers deployed in one of two high-volume aerosol samplers (model 5170-VBL, Tisch Environmental) deployed simultaneously operating at approximately 1.2 cubic meters air per minute. The aerosol samplers were deployed on the ship’s flying bridge (14 m above sea level) as high off the water as possible. Contamination from the ship’s stack exhaust was avoided by controlling aerosol sampling with respect to wind sector and wind speed using an anemometer interfaced with a Campbell Scientific CR800  datalogger and PC. The samplers were allowed to run when the wind was +/- 60 degrees from the bow and > 0.5 m/s. When the wind failed to meet these two criteria, the motors were shut off automatically and not allowed to restart until the wind met both the speed and direction criteria for 5 continuous minutes. The anemometer was deployed nearby on a separate pole in 'free air' where turbulence from the wind crossing the bow did not cause the wind vane to wobble excessively.

Trace Metal Determination
For determinations of total aerosol trace metal composition, the W41 filter discs were digested in tightly capped Teflon vials using sequential additions of concentrated acid solutions [nitric acid (Optima) and hydrofluoric acid (Optima)] at 150 degrees C (Morton et al., 2013, accepted). Any solution remaining after each digestion step was heated to dryness. After the final digestion and dry down, the samples were re-dissolved with 0.32 M nitric acid for analysis using inductively-coupled plasma mass spectrometry (ICP-MS; Agilent 7500CS and Thermo Element 2). All filter digestions were performed under a Class 100 laminar flow hood at the National High Magnetic Field Laboratory at Florida State University (FSU).

Filters for both the total and soluble aerosol concentration measurements were stored frozen prior to processing.

Blank solutions for the acid digestions were prepared by digesting W41 discs that had been deployed in the aerosol samplers for 1 h while not in operation. For example, blanks accounted for 65.6 ± 0.52 ng m-3 air Fe (representing an average of 0.09 ± 0.001% of the Fe sample concentrations). Trace metal concentrations in the blanks were subtracted from all acid-digested filter samples. Leachate blanks were prepared by passing 100 mL of deionized water through W41 filters that had also been deployed in the same way as the acid digestion blanks. Blank leachates averaged 0.004 ± 0.001 ng m-3 air dissolved Fe (representing an average of 1.6 ± 0.38% of the Fe sample concentrations) and were subsequently subtracted from all leachate sample concentrations. Relative standard deviations of replicate analyses averaged 7% for total Fe concentrations. Replicate analyses of soluble Fe produced relative standard deviations of 10%.

References:
Buck, C.S., et al. 2010. The solubility and deposition of aerosol Fe and other trace elements in the North Atlantic Ocean: observations from the A16N CLIVAR/CO2 repeat hydrography section. Marine Chemistry 120, 57-70. doi: 10.1016/j.marchem.2008.08.003.
Buck, C. S., Landing, W.M.,  Resing,  J. A.,  Lebon, G. T. 2006. Aerosol iron and aluminum solubility in the northwest Pacific Ocean: Results from the 2002 IOC cruise. Geochemistry, Geophysics, Geosystems. 7. doi: 10.1029/2005GC000977.
Morton, P.L. et al. 2013. Methods for the sampling and analysis of marine aerosols: results from the 2008 GEOTRACES aerosol intercalibration experiment. Limnology and Oceanography Methods 11:62-78. doi: 10.4319/lom.2013.11.62.

Concentrations below the detection limit are flagged as "below_DL" (originally " <_DL")  in the data columns. Representative detection limits were calculated using an air volume of 1200 cubic meters. See PDF of detection limits for this dataset.

Quality flag definitions:
BDL = below detection limit.
0 = No QC performed.
1 = Good data.
2 = Probably good data.
3 = Probably bad data that is potentially correctable.
4 = Bad data.
5 = Value changed.
6 = Sample < blank.
8 = Interpolated value.
9 = Missing value.

BCO-DMO made the following modifications:
- Replaced blanks with 'nd'.
- Replaced ' < DL'  with 'below_DL'. Replaced '< blank' with 'lt_blank'.
- Changed PI-supplied cruise IDs of KN204-2 and KN204-5 to KN204-01A and KN204-01B. Added 'cruise_part' column to distingush between parts A and B of cruise KN204-01.
- Copied averages and variances to all  rows containing data for that sample (where applicable).

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 name 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.