The partitioning of transitional metals (Fe, Mn, Ni, Cr) in mangrove sediments downstream of a ferralitized ultramafic watershed (New Caledonia)

Abstract

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. 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.

Trace metal fractionation in the Pichavaram mangrove–estuarine sediments in southeast India after the tsunami of 2004

“…The geochemical behavior of Mn and Fe in estuarine environments certain order of redox sequence (Marchand et al 2006(Marchand et al , 2012. Fe and Mn concentrations were highest in F5 fraction followed by F3 and F4 fractions in surface sediments and cores.…”

“…Notably, these metals can accumulate in mangrove plant tissues, and pose long-term damaging effects in the food chain through biomagnification (Agoramoorthy et al 2008;Tam and Wong 2000). Hence, metal cycling in different mangrove ecosystems has been addressed by many authors (Furukawa et al 1997;Yim and Tam 1999;Tam and Wong 2000;Marchand et al 2012).…”

“…Preliminary studies carried out after the December 2004 tsunami reported high contamination of Cd and moderate to strong contamination of Cr, Pb, and Ni in coastal sediments from this area (Ranjan et al 2008a,b;Srinivasalu et al 2008;Satheeshkumar et al 2011). However, for environmental impact assessment, determination of the total metal concentration in sediments is not enough (McLaughlin et al 2000;Krupadam et al 2006;Marchand et al 2012). Instead, knowledge regarding the association of metals with different geochemical fractions (carbonate, oxide, sulfide, OM, and residual) is far more critical to understand their distribution patterns, long-term fate, and their potential uptake by the biota.…”

Concentrations and Fractionation of Carbon, Iron, Sulfur, Nitrogen and Phosphorus in Mangrove Sediments Along an Intertidal Gradient (Semi-Arid Climate, New Caledonia)

“…This mangrove (21°54′ S, 166°04′ E), covering 20 ha, does not receive any significant freshwater runoff, and was free of anthropogenic influences. This forest exhibits the typical zonation of mangroves in New Caledonia [18]: (i) at the back side of the mangrove swamp, the area is occupied by the salt-flat, a highly saline zone, only submerged during high spring tide and covered sporadically in the most downstream stretches with bushes of Sarcocornia quinqueflora; (ii) a second stand of vegetation, downstream, is occupied by Avicennia marina; (iii) finally, the seaward edge is occupied by Rhizophora stylosa, which is submerged at each tide. Tides are semidiurnal, and mangroves were fully drained at each low tide, except during neap tides where parts of the Rhizophora area remain flooded.…”

“…In New-Caledonia, mangrove forests are developed over 35,000 ha, fringing on about 80% and 15% of the western and the eastern coastline of the island, respectively [16]. Previous studies [17,18] suggested that the main factor controlling the distribution of mangrove species in New Caledonia was soil salinity, which in turn was controlled by the duration of tidal inundation and thus by the soil elevation [19]. Thus, our hypothesis are that towards the sea-side: (i) the sediment will be enriched in organic matter because of higher productivity resulting from less stressing conditions; (ii) the sediments will be more reducing because of longer waterlogging and higher organic content; (iii) and the concentrations of key nutrients will be lower because of plant uptake.…”