Impuestos Sobre los que Aplican Beneficios Ambientales

The initial aim of this study was to improve our understanding of the marine iron cycle by investigating the processes influencing the dissolved iron distribution in two different environments where samples were collected as part of this project: the Celtic Sea shelf edge, and the open Atlantic Ocean. However some of the open ocean samples from an Atlantic Meridional Transect (AMT-12) cruise were found to be contaminated

for iron (see Chapter IV). The focus of this project was therefore limited to the study of processes (i.e. sources, removal and transport) influencing dissolved iron distribution at the Celtic Sea shelf edge with the aim of giving a conceptual framework for future studies in highly dynamic environments of this sort.

An appropriate analytical tool was needed that would overcome problems due to the ultra-low Fe concentrations expected, risks of contamination, and high reactivity of iron. The first objective was to develop a working and compact analyser which would have an appropriate limit of detection (pico-molar), requiring very little sample handling to minimise risks of contamination, and allowing close to real-time measurements. The chosen technique was a flow-injection analyser with chemiluminescence detection (FIA-CL) which currently exists in two versions to determine: i) Fe(II), and Fe(II+III) by reducing Fe(III) to Fe(II) (Fe(II) technique); and ii) both Fe(II) and Fe(III) directly (Fe(II)+(III) technique), in seawater.

In Chapter II, a literature review is presented to give the principles of both versions of the FIA-CL. The chosen version, the Fe(II) technique, was based on an existing method (Bowie et al., 1998) to allow the determination of dissolved iron(II) in seawater or dissolved Fe(II+III) after a reduction step. This technique was relatively easy to mechanically set up with the collaboration of Dr. Matt Mowlem from the Ocean Engineering Division (OED, NOCS), but its operation was found more difficult than at first thought. Full descriptions of the analyser are given in Appendices 1 and 3 to 5. The developmental stages are explained and an overview of the analytical problems and experiments carried out to solve them is presented. However, given the difficulty of obtaining a reliable calibration curve, it was decided to move on to the alternative method.

The FIA-CL analyser was modified to the Fe(II)+(III) technique since only a few modifications in instrumentation were required and descriptions are presented in Appendices 6 and 7. Despite some difficulties, this version of the Fe(II)+(III) based on the design of Obata et al. (1993) and de Jong et al. (1998), was successfully developed. The development of the technique and the solutions found for the problems experienced as well as a full description of the working analyser in its final stage are described in Chapter III.

The analyser was then used to determine dissolved iron in the samples collected. A rigorous data quality check was carried out on both the analysis and the integrity of the samples, to ensure the quality of these data. The quality of the analysis was checked based on its accuracy, precision, blank level, and limit of detection. At this stage, some samples were discarded from the data set due to suspicion of contamination. Investigations were carried out in order to determine its source(s). Criteria are given for the evaluation of the quality of the analysis and samples. A full description of this procedure is presented in Chapter IV.

The second objective was to examine the dissolved iron data in order to investigate processes influencing its distribution, using associated data obtained simultaneously. This study was carried out on samples collected at the Celtic Sea shelf edge during the summer of 2003 and is described in Chapter V. Oceanographic data at each station are presented in Appendix 9. Several processes were examined: i) sources of dissolved iron in near-seafloor waters; ii) removal and stabilisation of dissolved iron near the seafloor;

iii) transport of dissolved iron both horizontally and vertically; and iv) the influence of primary production on the dissolved iron distribution in the euphotic zone, and the possibility for iron limitation at the Celtic Sea shelf edge was considered. Part of this work (i.e. sources and transport of dissolved iron) has been submitted to Marine Chemistry, and a copy of the first draft of the manuscript is included as Appendix 10.

An additional sample set collected during a transect along the North Scotia Ridge between the Falkland Islands and South Georgia not carried out within this project, was analysed for total dissolvable iron (leachable at pH ~ 2). These iron data were used to investigate primary production limitation in these polar waters (see Chapter V). A paper has already been published using the results presented here. Claire Holeton et al. (2005) examined variations in physiological state of phytoplankton communities in the Southwest Atlantic sector of the Southern Ocean using fast repetition rate fluorometry. The article had already been submitted once when the samples collected for iron were analysed, using the newly developed technique presented here. These iron data were used to support the data already presented in the paper. As Claire Holeton carried out the majority of the work towards this paper, a copy was not included in the main body of the thesis.

CHAPTER

II.

IMPLEMENTING A METHOD TO DETERMINE

VERY LOW CONCENTRATIONS OF

II.1. Analytical challenges

It was as early as the 1930s that the potential role of iron as a limitation to marine primary production was first suggested (Gran, 1931). This idea was however not investigated further until the 1980s owing to the low data quality when attempting to measure nanomolar seawater concentrations of iron, as a result of sample contamination and not sufficiently low analytical limits of detection. Since then, new analytical techniques have been developed with a better appreciation of the sources of contamination. Ultra-clean sampling procedures (e.g. (Bruland et al., 1979)) are now used, including careful washing of the sampling bottles; working in clean rooms; using high purity reagents (Moody and Lindstrom, 1977). Such procedures now permits the measurement of picomolar concentrations of iron in open ocean waters (Moody, 1982; Achterberg et al., 2001). The methods used to determine iron concentrations in natural waters can be divided into two groups (Table II.1).

Iron measured Technique used Detection

limit (pM) Reference

LAND-BASED TECHNIQUES

Chelex-100 + GF-AAS 50 (2s) (Landing and Bruland, 1987)

8-HQ + ICP-MS 640 (Sohrin et al., 1998)

Fe(II+III)

Isotope dilution ICP-MS 50 (Wu and Boyle, 1998)

SHIPBOARD TECHNIQUES

Fe(II) FIA + Ferrozine +

spectrophotometry 100 (Blain and Treguer, 1995)

Fe(II), or Fe(II+III)

FIA + phenanthroline +

spectrophotometry 42 (Adams and Powell, 2001)

Fe(II+III) _{spectrophotometry} FIA + DPD + 16 (Weeks and Bruland, 2002)

80 (Gledhill and van den Berg, 1995)

~ 10 (Rue and Bruland, 1995)

100 (Croot and Johansson, 2000)

Total Fe, Fe(III), or organically complexed Fe

AdCSV

13 (Obata and van den Berg, 2001)

Fe(III), or

Fe(II+III) FIA-CL luminol + H2O2 50 ₁₀ (Obata_(Obata et al., 1993)

et al., 1997)

Fe(II) or

Fe(II+III) FIA-CL luminol 8-12 (Bowie et al., 2002a)

Table II.1: Figures of merit of some of the most recent techniques used to determine iron in

seawater. Analytical limit of detection = 3 times the standard deviation of the blank (3s), unless

specified otherwise.

1) Land-based techniques, i.e. graphite furnace atomic absorption spectrometry (GF-

AAS), or inductively coupled plasma mass spectrometry (ICP-MS). These methods are not used at sea because of the size, weight, and fragility of the instruments, in addition to the costs involved. Low detection limits are obtained using solvent extraction as a

preconcentration step, but resins (e.g. Chelex-100 or 8-hydroxyquinoline (8-HQ)) are nowadays generally preferred as sample handling and pre-treatment are minimised (Table II.1). However, these techniques do not allow measurement of redox or organically complexed iron (Achterberg et al., 2001).

2) Shipboard techniques commonly require compact, portable, robust, and relatively

low-cost instrumentation. The adsorptive cathodic stripping voltammetry (AdCSV) method has a relatively good sensitivity, and allows inorganic and organic iron speciation determination (Table II.1). However, its limit of detection is not always sufficient for measurements in iron limited regions, and analysis requires a long deposition time (up to 10 minutes) to achieve a sufficiently high sensitivity. Such long deposition could be disrupted by the ship’s vibrations (Achterberg et al., 2001). A recent development of the AdCSV method has significantly lowered its limit of detection and shortened the analysis time using 2,3-dihydroxynaphthalene (DHN) as ligand and the calatytic effect of the Fe(II)/Fe(III) redox couple on the reduction of bromate (Obata and van den Berg, 2001), resulting in a method adapted to work in iron- poor waters.

Most of the current shipboard techniques involve the use of flow-injection analysis (FIA) with in-line preconcentration. These methods consume small amounts of reagents and simplify sample handling (thus reducing contamination risks) and increase throughput. Different types of detectors can be used including spectrophotometric methods using ferrozine to determine Fe(II) (King et al., 1991; Blain and Treguer, 1995); or 1,10-phenanthroline (Adams and Powell, 2001); or N,N-dimethyl-p- phenylenediamine (DPD) to determine Fe(II+III) (Measures et al., 1995; Weeks and Bruland, 2002)) (Table II.1). However, ferrozine may shift the iron redox speciation reducing Fe(III) to Fe(II) (Hong and Kester, 1986), and may not be sensitive enough for open ocean surface waters in iron-depleted regions, whereas the DPD method is sensitive enough but does not allow measurements of the iron redox speciation (Achterberg et al., 2001).

The most commonly used technique to determine iron in HNLC regions is flow- injection analysis with chemiluminescence detection (FIA-CL) using luminol (5-amino- 2,3-dihydro-1,4-phthalazinedione) (Bowie et al., 1998). This method has a flow- injection system coupled to a photo-multiplier tube (PMT) to detect the light produced

by the chemiluminescence reaction of luminol induced by iron (see Chapter II.2.3). This technique has been chosen for the current work because it potentially allows close to real-time measurements (3-10 minutes), it requires relatively low-cost, compact and portable instrumentation, it has very good sensitivity (pico-molar), and potentially allows direct Fe(II) determination.