9.a) Background
Although abundant in the Earth’s crust iron (Fe) is relatively insoluble in oxygenated sea water resulting in concentrations that are known to limit phytoplankton growth and nitrogen fixation rates over large areas of the ocean. For marine geochemists, one of the most important aspects of iron is that it precipitates rapidly in hydrothermal plumes, modifying the gross flux from vents to the oceans. But if only a small proportion of dissolved Fe escapes from hydrothermal plumes it could dominate the budget for deep-ocean dissolved Fe. It is therefore essential to understand the biogeochemical cycling of Fe in hydrothermal systems and their influence throughout the deep ocean. Our knowledge of Fe speciation in seawater, however, is severely limited due to a lack of measurements of Fe concentrations and its degree of organic complexation in seawater. The few existing electrochemical measurements of Fe speciation demonstrate that greater than 99% of the operationally defined “dissolved” Fe that passes through a 0.4 micron filter is strongly bound to organic ligands of presumed biological origin. Our knowledge of Fe speciation in deep waters is also severely lacking due to lack of data, while our knowledge of Fe speciation in hydrothermal plumes is virtually zero except for limited studies on size fractionation. Organic complexation has been measured on deep samples collected in the Sub-Arctic North Pacific and shown to be also highly complexed. However the origin of these ligands is completely unknown. Colloidal (0.4 micron - 0.02 microns molecular diameter) Fe and binding ligands may be the driving force in Fe hydrothermal inputs into the deep ocean keeping the Fe in solution long enough for hydrothermal inputs to play an important role in the Fe deep-ocean budget.
This study aims to investigate Fe organic complexation in deep water samples in distil and plume samples from a newly discovered vent site close to Ascension Island in the South Atlantic. Major goals of the study are to elucidate the dominant size fraction, concentration and binding strength of
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these ligands along with determining the concentration of free iron available and ultimately the organically bound fraction.
9.b) Electrochemistry equipment.
This system is used for on-board measurements of Fe-organic complexation. This data can be used with total Fe data to model the ligand concentrations, the different class of ligand (L1, L2), the conditional stability constants of these different ligand classes, and Fe(III)΄ (soluble inorganic Fe(III) hydrolysis species). The instrumentation used consists of a PAR303A hanging mercury drop electrode connected to an Ecochemie 303 Interface and an Ecochemie µAutolab 3
voltammeter; the system was run using GPES software. During D286 major problems were encountered with the system. However after servicing and air-freighting out to Ascension in time for CD169 the 303A electrode was found to be in full working order and preliminary studies were undertaken on collected samples.
9.c) Methodology.
Seawater was collected using the CTD at stations and depths in the plume and in the background (Table 9.1). Samples were pressure filtered using research grade nitrogen through an acid washed 0.4 micron polycarbonate filter and collected in Teflon bottles. Samples for immediate analysis were placed in the fridge while the majority of the samples were frozen for subsequent analysis at SOC. Previous studies have shown that immediate freezing of the samples retains the integrity of the sample for future speciation studies. Analysis will be undertaken at SOC using the technique of CLE-ACSV (Competitive ligand exchange – adsorptive cathodic stripping voltammetry) with the added ligand TAC. Complexing capacity titrations will be undertaken on the samples to determine the Fe-TAC response over a series of increasing Fe concentrations (0.1 to 20 nM). Total dissolved Fe will be measured in the laboratory at Southampton Oceanography Centre. The seawater will be subjected to UV irradiation and analysed using CSV with DHN as the added ligand. Total Fe values will also be determined using high-resolution isotope dilution inductively coupled plasma mass spectrometry after Mg(OH)2 coprecipitation or solvent extraction and determination by GFAAS (graphite furnace atomic absorption spectroscopy). After the total values have been measured, the numbers combined with the complexing capacity titrations can then be used to yield the ligand concentrations, the different class of ligand, the conditional stability constants of these different ligand classes, and Fe(III)΄ (soluble inorganic Fe(III) hydrolysis species).
Table 9.1 CTD stations sampled for Fe speciation studies during CD169. (Fridge samples in italics)
CTD # Sample # OTE Bottle #
3 1 9 24 8 5 3 10 7 3 8 8 5 14 9 2 6 10 6 6 7 7 8 9 10 12 12 14 14 16 18 20
33 11 4 5 6 8 10 12 15 3 4 5 6 11 12 13 14 16 15 16 17 18 22 15 16 17 18 22 9.d) Preliminary Results.
Only a limited number of samples were run on-board ship due to the time constraints and length required for each titration (~ 4 hours). Initial data from samples analysed on ship showed evidence for organic complexation. Figure 2 shows a complexing capacity titration for Niskin Bottle #14 from CTD cast 10. Linear analysis of this data yields a ligand concentration of 2.10 nM with a logKL of 11.9. This is consistent with deep ocean ligand concentrations and depths, comparing favourably with data from the tropical deep Pacific (2.5 nM and 11.2 log KL) and the sub-Arctic North Pacific (1.76 nM and 12.1 Log KL).
9.e) Future Work.
All frozen samples collected will be air freighted to SOC and analysed using the same equipment in the SOC clean electrochemistry laboratory. Totals will be determined and speciation calculations undertaken using the linear approach and compared with a non-linear modelling programme.
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