Spatial and temporal distribution of Fe, Ni, Cu and Pb along 140°E in the Southern Ocean during austral summer 2001/02

doi:10.1016/j.marchem.2008.05.001

Marine Chemistry

Volume 111, Issues 3–4, 16 September 2008, Pages 171-183

https://doi.org/10.1016/j.marchem.2008.05.001 Get rights and content

Abstract

The distribution of dissolved (D) and acid-dissolvable (AD) Fe, Ni, Cu and Pb in the upper water column (0–300 m depth) was determined in the Australian sector of the Southern Ocean (140°E meridian) during three cruises conducted between November 2001 and March 2002. For Ni and Cu, there was no significant difference in concentration between dissolved and acid-dissolvable species. DNi and DCu showed significant (P = 0.01) positive correlations with silicate, phosphate and nitrate, reflecting their strong nutrient-type behaviour. For Fe and Pb, the acid-dissolvable concentration mostly exceeded the dissolved concentration, reflecting the importance of labile particulate species for these elements. DPb decreased between January and February in the Polar Frontal Zone and in Antarctic continental shelf water. ADPb maxima occurred in the Antarctic Zone, resulting in a maximum AD/D ratio of 7. The mean DFe concentration in the surface mixed layer was 0.3 nM in the sub-Antarctic zone, 0.4 nM in the Polar Frontal Zone, 0.5 nM in the Antarctic Zone and increased southward beyond the Antarctic Divergence and towards the continent. DFe did not show a clear temporal change in its horizontal distribution, which was in contrast to the other nutrients and trace metals. ADFe substantially increased in Antarctic continental shelf water where the AD/D ratio reached 11. The following conclusions can be drawn from these data. (1) Ni and Cu exist exclusively as dissolved species and their distributions are mainly controlled by their biogeochemical cycling, similar to those of the major nutrients. (2) Pb is dominated by particulate species. The distribution of DPb is temporally and spatially variable due to a sporadic source and strong scavenging. (3) DFe is rather a minor fraction of total Fe in Antarctic continental shelf water where shelf sediments and Antarctic sea-ice appear to be strong sources for Fe. There is substantial temporal variation in the supply of Fe to the upper water column. DFe in the mixed layer of the open Southern Ocean is maintained at low concentrations throughout summer due to uptake by phytoplankton and scavenging.

Introduction

There is increasing interest in the biogeochemical cycling of trace metals in the ocean (Morel and Price, 2003), as reflected by the international programs GEOTRACES and SOLAS. Some trace metals are essential micronutrients for marine organisms and may limit phytoplankton growth at low concentrations. On the other hand, certain trace metals can be toxic for organisms when their stoichiometry deviates from natural conditions. Trace metals are delivered to the Southern Ocean in a number of ways, including atmospheric dust deposition, upwelling, advection from continental sediments and melting of glacial and sea-ice. Trace metals are exported out of euphotic surface waters with settling particles and remineralised at depth. The biogeochemical cycle of iron (Fe) in the Southern Ocean has attracted widespread research since the iron hypothesis was postulated by Martin et al. over two decades ago (Turner and Hunter, 2001). However, our knowledge of the distribution and cycling of Fe in the Southern Ocean remains limited, and the situation is much poorer for the other trace metals such as nickel (Ni), copper (Cu) and lead (Pb).

In the austral summer of 2001/2002, Japanese and Australian scientists carried out a collaborative multi-ship research project along the WOCE/CLIVAR SR3 meridian (approximately 140°E; Fig. 1). We investigated the spatial and temporal distribution of Fe, Ni, Cu and Pb in the upper water (300 m) column between 47 and 66°S over the period November 2001 to March 2002. Dissolved (D) and acid-dissolvable (AD) species were determined in accordance with our recent work (Ezoe et al., 2004, Kinugasa et al., 2005, Norisuye et al., 2007). The acid-dissolvable species will include labile particulate species (such as those adsorbed onto iron oxyhydroxides and clay minerals, and incorporated into organisms) which dissolve during storage. A fraction similar to acid-dissolvable is referred to as total dissolvable (TD) in some references. However, we prefer the term “acid-dissolvable”, since the solubilised percentage of the total metal is probably dependent on pretreatment conditions such as pH, duration of storage and temperature, and not yet fully evaluated (Bowie et al., 2004).

Our data come from a region of the World's oceans that is sparsely sampled. Each of the target metals has a different biogeochemical cycle in the ocean (Donat and Bruland, 1995, Chester, 2000). Although both Fe and Cu belong to a mixed nutrient-scavenging type on the basis of vertical profiles in the Pacific and Atlantic Ocean, their individual vertical profiles are largely different from each other. Ni belongs to a combined labile-refractory nutrient-type group. Pb is a scavenged-type element characterised by surface enrichment and depletion at depth. Anthropogenic Pb may be transferred to the ocean through an aeolian source (Wu and Boyle, 1997). The distribution of Fe around the study area has been reported previously (Sedwick et al., 1997, Sedwick et al., 1999, Sohrin et al., 2000, Bowie et al., 2001, Bowie et al., 2004). For the other metals, only the concentrations of Ni and Cu were determined at SOIREE site (61°S, 140°E) during a mesoscale iron enrichment experiment (Frew et al., 2001). This paper compares our results with these data and those that have been reported in other areas of the Southern Ocean, and discusses possible sources, sinks and factors controlling the biogeochemical cycling of each of the trace metals.

Section snippets

Reagents, materials and clean procedures

The reagents, materials and clean procedures used were generally identical with those established from our previous work (Sohrin et al., 2000, Ezoe et al., 2004, Norisuye et al., 2007). Deionised water (MQW) purified with a Milli-Q Gradient-A10 system (Millipore) was used for cleaning of materials and preparation of all solutions. Ultra-high purity HCl, HF, HNO₃, HOAc, NH₃, and H₂O₂ (TAMAPURE-AA-10 or 100, Tama Chemicals) were used for cleaning of materials, preparation of solutions, and

Results

All data for trace metals are given in Appendix Table 3. Vertical profiles of trace metals and nitrate + nitrite are presented in Fig. 2. During KH-01-3 (Fig. 1A), the subantarctic Front (SAF) was close to station 19, the northern branch of the Polar Front (PF-N) was to the south of station 18, the southern branch of the Polar Front (PF-S) was to the north of station 16, the southern Antarctic Circumpolar Current front (SACCF) was between stations 16 and 13, and the Antarctic Divergence (ADiv)

Nickel

DNi during the three cruises ranged from 2.2 nM to 7.9 nM, giving a mean and standard deviation of 5.8 ± 1.0 nM (n = 180). Low concentrations were found in the surface mixed layer at stations 18 and 20 of KH-01-3. The ADNi/DNi ratio (cruises CLIVAR-SR3 and JARE-43 only) was 1.00 ± 0.06 (n = 102). The good linear relationship between ADNi and DNi during JARE-43 (Fig. 4A) indicates that the speciation of Ni is dominated by dissolved species.

Frew et al. (2001) determined DNi and TDNi at 40 m

Conclusions

The distributions of Fe, Ni, Cu and Pb in dissolved and acid-dissolvable phases were investigated along 140°E in the Southern Ocean between November 2001 and March 2002. Ni and Cu were dominated by dissolved species, and their distribution was mainly controlled by nutrient-like biogeochemical cycling. The distribution of Pb showed large variations both temporally and spatially, indicating the effect of aeolian inputs and scavenging. A significant amount of Pb was present in a labile particulate

Acknowledgements

We wish to thank the crew of RSV Aurora Australis, R/V Hakuho-Maru and RV Tangaroa. Thanks to Shigenobu Takeda, Tsuneo Odate and Mitsuo Fukuchi for giving us an opportunity to participate in the multi-ship/time-series study, and to Peter Sedwick for sampling on CLIVAR-SR3. Three anonymous reviewers and two editors provided helpful comments that improved this paper. This work was supported by grants from the Ministry of Education, Science, Culture and Sports of Japan, the Research Institute for

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Spatial and temporal distribution of Fe, Ni, Cu and Pb along 140°E in the Southern Ocean during austral summer 2001/02

Abstract

Introduction

Section snippets

Reagents, materials and clean procedures

Results

Nickel

Conclusions

Acknowledgements

Deep-Sea Res. II

Mar. Chem.

Deep-Sea Res. II

Deep-Sea Res. II

Deep-Sea Res. II

Mar. Chem.

Mar. Chem.

Progr. Oceanogr.

Mar. Chem.

Mar. Chem.

Mar. Chem.

Mar. Chem.

Deep-Sea Res. II

Mar. Chem.

Deep-Sea Res. I

Anal. Chim. Acta

Deep-Sea Res. I