Table of Contents

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

Dissolved Al, Fe and Mn concentrations (filtered by 0.45um PES filter)

Sampling method:
The samples were collected for trace metal determinations at 46 stations between 1 July and 15 August, 2006 from the RV NB Palmer using a custom-built trace metal clean rosette consisting of an epoxy painted Al rosette frame containing 12x12 L GO-FLO bottles (Measures et al., 2008).  The frame also housed an SBE 911 CTD system that included an SBE 43 dissolved oxygen sensor and a Wet labs FL1 fluorometer.  The rosette frame was lowered using a 4 conductor Kevlar cable coated with polyurethane which was passed over a Nylatron plastic block and wound on to a normal, but clean, hydraulic winch.  Sample bottles were closed while the rosette was moving upwards through clean water at ~5-10 m/minute.  Immediately after the package was recovered, the tops of the GO-FLOs were covered with plastic bags and the bottles were removed from the frame and carried into the clean van for sub-sampling.   The GO-FLO bottles were pressurized to 10 psi using 0.2 µm-filtered compressed air and samples filtered through 0.45 µm pore size acid washed, 47 mm polysulphone filters (Pall Supor 450 P/N 60173).  All sub-sampling was undertaken in the clean van using rigorous trace metal protocols.

Analytical method:
Samples obtained with this system and processed in this manner have been shown during the SAFe intercomparison cruise (Johnson et al., 2007) and GEOTRACES intercalibration cruise to produce trace metals (Al, Fe and Mn) that are, within analytical uncertainty, identical to those obtained using other currently accepted sampling methodologies for trace elements. 

1. Dissolved Al using FIA
Dissolved Al was determined using the Flow Injection Analysis (FIA) method of Resing and Measures (1994) within a few hours of sample collection on board.  Samples were drawn into pre-numbered 125 ml PMP bottles after three rinses and were stored in plastic bags in the dark at room temperature before determination which was usually within 12 -36 hours of collection. Prior to determination samples were acidified by the addition of 125 ul sub-boiling distilled 6N HCl (hereinafter 6N HCl) and were microwaved in groups of 4 for 3 minutes in a 900 W microwave oven to achieve a temperature of 60+/-10 degC.  Samples were allowed to cool for at least 1 hour prior to determination.  Samples were determined in groups of 8. 

A shipboard mixed standard (Al and Fe) was prepared in the shore-based laboratory by serial dilution of commercial Al standards into distilled water which was acidified with the equivalent of 4 ml sub-boiled 6N HCl/L.  Standards for instrument calibration were prepared daily from filtered seawater by acidifying 1 L of low Fe seawater from a previous cast with 1 ml of 6N HCl and microwaving for 5 minutes.  After 1 hour, 200 +/- 2 ml of the cooled seawater was added to each of three 250 ml PMP bottles each of which had been rinsed three times with the microwaved seawater and shaken dry.  Working standards were prepared by adding spikes of the mixed standard to these bottles, to yield a standard curve. The system blank from the addition of the acid and buffer to samples was determined by double spiking a replicate sample i.e. by adding 2 x 125 ul 6N HCl and 5 ml of sample buffer to the replicate bottle and comparing the resulting signal to the original sample. Calculation of sample concentrations was by dividing the peak height derived from sample using the A/D software by the calculated slope of the standard curve.  A 3-minute pre-concentration of sample (~9 ml) onto an 8-hydroxyquinoline (8-HQ) resin column yielded a detection limit of 0.53 nM Al and a precision of 6.5% at 2.7 nM.

2. Dissolved Fe and Mn using ICPMS
Filtered sub-samples were taken for shore-based determination of dissolved Fe and Mn by Inductively Coupled Plasma Mass Spectrometry (ICP MS) using the protocol of Milne et al. (2010), which is briefly described here.  Stored samples were acidified to 0.024 M HCl by the addition of sub-boiled 6N HCl and kept for at least one year prior to analysis.  Aliquots of the acidified samples (15 ml) were spiked with 125 μL of an ⁵⁷Fe isotope solution (⁵⁶Fe: ⁵⁷Fe = 0.073:0.971, total Fe concentration 152 nM) and then left for >24 hours to equilibrate.  Pre-concentration and extraction of the samples was performed using a flow injection manifold with an in-line micro-column containing ~200 μL of Toyopearl AF Chelate-650M resin.  Prior to extraction of the sub-samples, the system was cleaned with sub-boiled HNO₃ (1 M).  The isotope spiked samples were buffered to pH 6 using 900 μL of 2 M ammonium acetate (pH 9.1) prepared from sub-boiled acetic acid and isopiestic distilled NH₃, and then pumped at 2 mL min^-1 through the column for 6 minutes.  The column was then rinsed with 1 mL DI water to remove the seawater matrix, and the adsorbed trace elements were eluted with 1 mL 1.0 M sub-boiled HNO₃.  Extracted samples were analyzed for their ⁵⁶Fe/⁵⁷Fe ratio and Mn concentration on an ICP MS (Thermo Scientific, Element 2, Medium-Resolution) with the desolvating apparatus (Elemental Scientific, Apex-Q) with self-aspirating nebulizer (Elemental Scientific, a 400 µL min^-1 PFA). Dissolved Fe concentrations were calculated by isotope dilution.  Mn concentrations were calculated using an external standard curve.  Fe and Mn sample concentrations were corrected for the blank that was measured in the acids and buffer that were used in the preconcentration and elution process.  The acid blank was 0.034 ± 0.003 nM (Fe, n=3) and 0.013 ± 0.005 nM (Mn, n=8).  Detection limits for both Fe and Mn were calculated using 3 standard deviations of replicate measurements of the acid blank and were approximately 0.009 nM (Fe) and 0.015 nM (Mn).  Determination of Fe and Mn concentrations in the SAFe open ocean reference material and GEOTRACES shallow standards were in good agreement with the inter-laboratory averages.

Related Files and References:
Johnson, K.S. et al. 2007.  Developing Standards for Dissolved Iron in Seawater, EOS, trans American Geophysical Union, 88, 131.

Measures, C.I. et al. 2008. A commercially available rosette system for trace metal clean sampling. Limnol. Oceanogr. Methods. 6, 384-394.

Milne, A. et al. 2010. Determination of Mn, Fe, Co, Ni, Cu, Zn, Cd and Pb in seawater using high resolution magnetic sector inductively coupled mass spectrometry (HR-ICP-MS). Analytica Chimica Acta. 665, 200-207.

Resing, J. Measures, C.I., 1994.  Fluorometric determination of A1 in seawater by FIA with in-line pre concentration. Anal. Chem. 66, 4105-4111.

Quality flag:  Assignment of quality flag is by the data generator based on analytical and methodological considerations.  Scheme is based on simple flags i.e. good (0), questionable (4), bad (8).

BCO-DMO Processing/Edits
- Generated from file: "BWZ2006TM.xls" contributed by Yiwu Zhu
- Longitude converted to +/- hemisphere convention by -= 360.0 to oriignal values
- Parameter names modified to conform to BCO-DMO conventions (blanks to underscores, etc.)
- "nd" (no data) inserted in black cells

NSF Proposal Title: Collaborative Research: Plankton Community Structure and Iron Distribution in the Southern Drake Passage and Scotia Sea

The Shackleton Fracture Zone (SFZ) in Drake Passage of the Southern Ocean defines a boundary between low and high phytoplankton waters. Low chlorophyll water flowing through the southern Drake Passage emerges as high chlorophyll water to the east, and recent evidence indicates that the Southern Antarctic Circumpolar Current Front (SACCF) is steered south of the SFZ onto the Antarctic Peninsula shelf where mixing between the water types occurs. The mixed water is then advected off-shelf with elevated iron and phytoplankton biomass. The SFZ is therefore an ideal natural laboratory to improve the understanding of plankton community responses to natural iron fertilization, and how these processes influence export of organic carbon to the ocean interior. The bathymetry of the region is hypothesized to influence mesoscale circulation and transport of iron, leading to the observed patterns in phytoplankton biomass. The position of the Antarctic Circumpolar Current (ACC) is further hypothesized to influence the magnitude of the flow of ACC water onto the peninsula shelf, mediating the amount of iron transported into the Scotia Sea. To address these hypotheses, a research cruise will be conducted near the SFZ and to the east in the southern Scotia Sea. A mesoscale station grid for vertical profiles, water sampling, and bottle incubation enrichment experiments will complement rapid surface surveys of chemical, plankton, and hydrographic properties. Distributions of manganese, aluminum and radium isotopes will be determined to trace iron sources and estimate mixing rates. Phytoplankton and bacterial physiological states (including responses to iron enrichment) and the structure of the plankton communities will be studied. The primary goal is to better understand how plankton productivity, community structure and export production in the Southern Ocean are affected by the coupling between bathymetry, mesoscale circulation, and distributions of limiting nutrients. The proposed work represents an interdisciplinary approach to address the fundamental physical, chemical and biological processes that contribute to the abrupt transition in chl-a which occurs near the SFZ. Given recent indications that the Southern Ocean is warming, it is important to advance the understanding of conditions that regulate the present ecosystem structure in order to predict the effects of climate variability. This project will promote training and learning across a broad spectrum of groups. Funds are included to support postdocs, graduate students, and undergraduates. In addition, this project will contribute to the development of content for the Polar Science Station website, which has been a resource since 2001 for instructors and students in adult education, home schooling, tribal schools, corrections education, family literacy programs, and the general public.

PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH

Hewes, C. D., Reiss, C.S., .Kahru, M. , Mitchell, B.G. , and Holm-Hansen, O.. "Control of phytoplankton biomass by dilution and mixed layer depth in the western Weddell-Scotia Confluence (WSC)," Marine Ecology Progress Series, v.366, 2008, p. 15.

Hiscock, M. , Lance, V. , Apprill, A., Bidigare, R , Mitchell, B., Smith Jr. W., Barber, R.. "Photosynthetic maximum quantum yield increases are an essential component of the Southern Ocean phytoplankton response to iron," Proceedings of the National Academy of Sciences, v.105(2), 2008, p. 4775.

Holm-Hansen, O., Kahru, M., Hewes, C.. "Deep chlorophyll a maxima (DCMs) in pelagic Antarctic waters. II. Relation to bathymetric features and dissolved iron concentrations," Marine Ecology-Progress Series, v.297, 2005, p. 71.

Hopkinson, B., Mitchell, B. G., Reynolds, R. A., Wang, H., Selph, K., Measures, C., Hewes, C., Holm-Hansen, O., Barbeau, K.. "Iron limitation Across Chlorophyll Gradients in the Southern Drake Passage: Phytoplankton Responses to Iron Addition and Photosynthetic Indicators of Iron Stress," Limnology and Oceanography, 2007, p. 2540.

Hopkinson, B., Mitchell, B. G., Reynolds, R. A., Wang, H., Selph, K., Measures, C., Hewes, C., Holm-Hansen, O., Barbeau, K.. "Iron limitation Across Chlorophyll Gradients in the Southern Drake Passage: Phytoplankton Responses to Iron Addition and Photosynthetic Indicators of Iron Stress," Limnology and Oceanography, v.52, 2007, p. 2540.

Kahru, M., Mitchell, B. G., Gille, S. T., Hewes, C. D. and Holm-Hansen, O.. "Eddies enhance biological production in the Weddell-Scotia Confluence of the Southern Ocean," Geophys. Res. Let., 34,, v.24, 2007, p. L14603.