The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferraltilized ultramafic watershed (New Caledonia)
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C. Marchand, J.-M. Fernandez, B. Moreton, L. Landi, E. Lallier-Vergès & F. Baltzer


In New Caledonia, one third of the Island is composed of ultramafic rocks, and lateritic soils enriched in Fe, Ni and Cr. Open-cast mining occurs all around the Island, and processes of erosion and sedimentation, which occur naturally along the coastline, are strongly amplified by mining activities (coastal development). Due to their position, at the interface between land and sea, mangroves receive extensive amounts of particles emanating from rivers through estuaries. The purpose of this study is to understand the distribution and partitioning of some transitional metals (Fe, Mn, Ni, Cr) in sediments and pore-waters in a mangrove swamp, which is situated downstream of a catchment characterized by lateritic soils that were exploited a century ago. Quantitative analyses on bulk and after selective extraction, were carried out on cores collected along a transect within the intertidal zone, i.e. beneath a Rhizophora stylosa stand, an Avicennia marina stand, and within a salt flat. Mean metal concentrations were (μmol g - 1): Fe (1997) > Ni (44.2) > Cr (31.9) > Mn (8.8). Thus, Ni, Cr and Fe concentrations in this study are substantially higher than mangrove world average. In addition, Ni concentrations are 10 to 100 times higher than in other New Caledonian mangrove developing downstream of a catchment not composed of ultramafic rocks. The studied mangrove is characterized by gradients of water and organic contents with depth and along the intertidal zone, which induced different redox conditions, and thus different metals partitioning. Transitional metals are deposited in the mangrove mainly as oxides and/or oxy-hydroxides, that are subsequently dissolved by bacteria for the decomposition of organic matter, and which leads to a strong increase of metals in the dissolved phase. Then, dissolved metals were precipitated with organic and sulfide compounds. To conclude, organic diagenesis in mangrove sediments leads to the transfer of transitional metals from oxide form to organic and sulfide forms.

Main Results and Conclusions:
  • Background of New Caldonian Mangroves:
    • “In New Caledonia, extensive mangroves are fringing 80% of the western coastline of the Island and 20% of the eastern side.
    • “These mangroves act as a buffer between a lagoon of more than 20,000 km2, delimitated by an almost continuous coral barrier reef of over 1500 km in total length and recently registered as an UNESCO World Heritage site, and a land that is characterized by mining activities (Ni-ore) exploiting mainly lateritic soils. (Coastal Development).
    • “New Caledonia is currently the third largest nickel producing country in the world.”
    • “In a previous study (Marchand et al., 2011a), we suggested that in New Caledonia the main factor controlling the distribution of mangrove species is the soil salinity, which in turn is controlled by the duration of tidal immersion and thus by soil elevation (Baltzer, 1982).”
  • Accumulation of transitional metals in sediments:
    • Mangrove sediment can trap metals: “Due to of the high rate of organic accumulation and the fact that fine particles can be trapped in mangroves, this ecosystem can act as a sink for trace metals flowing down rivers from catchments (Harbison, 1986).”
    • Averages of metal in mangrove and salt flat sediment: “Mean trace metal concentrations were (μmol g− 1): Fe (1997) > Ni (44.2) > Cr (31.9) > Mn (8.8). Mn concentrations are in the range of those measured in other mangroves all around the world. On the contrary, Ni, Cr and Fe concentrations in this study are substantially higher than world average.”
    • “Ni concentrations are 10 to 100 times higher than in a mangrove developing downstream of a catchment not composed of ultramafic rocks.”
  • Summary—Mangroves help decrease the presence of toxic metals in the environment:
    • “This combined study of pore water parameters, sedimentary organic content and transitional metal partitioning in a mangrove developing downstream an ultramafic watershed evidenced the fact that mangroves act as a natural reactor, transforming the forms of metals during their transfer from watershed to coastal waters.”
    • “Concentrations of Fe and Ni associated with the organic and the oxide fractions increased with depth and from the salt flat to the Rhizophora stand.
    • “Consequently the organic enrichment of mangrove sediments from the salt flat to the Rhizophora forest induced the transfer of trace metals from oxide form to organic and sulfide forms.
    • “Within anoxic conditions, Fe and Mn also precipitated as carbonates.
    • “Finally, taking into account, the richness of the upper mangrove sediment in Mn associated with the oxide bound and residual phases, oxidation of Cr (III) to Cr (VI) is thought to occur. Nonetheless, within the anoxic and organic-rich layers, Cr (VI) can be reduced by organic matter and sulfides, inducing a lower bioavailability and toxicity of Cr.”
Works Cited:

Baltzer, 1982 F. Baltzer Géodynamique de la sédimintation et diagenèse précoce en domaine ultrabasique – Nouvelle Calédonie Travaux et Documents de l'ORSTOM, 152 (1982) (283 pp.).

Harbison, 1986 P. Harbison Mangrove muds: a sink and a source for trace metals Marine Pollution Bulletin, 17 (1986), pp. 246–250.

Marchand et al., 2011a C. Marchand, M. Allenbach, E. Lallier-Vergès Relationships between heavy metals distribution and organic matter cycling in mangrove sediments of Conception Bay, New Caledonia Geoderma, 160 (2011), pp. 444–456.