Ni cycling in mangrove sediments from New Caledonia
Introduction
Mangrove intertidal forests cover more than 70% of tropical and subtropical coastlines and play a major role in the ecological balance of these areas (Valiela et al., 2001, Duke et al., 2007, Nagelkerken et al., 2008). They receive inputs from natural or anthropogenically impacted watersheds and may act as filters between land and sea for various metallic contaminants (Harbison, 1986, Lacerda et al., 1993, Tam and Wong, 1995, Tam and Wong, 2000). As a result, and due to their potential toxicity to living organisms (Di Toro et al., 1992), the distribution of trace metals in mangrove ecosystems has received significant attention during the last decade (Otero et al., 2009, Lewis et al., 2011, Bayen, 2012, Nath et al., 2013). This is especially true in New Caledonia where mangroves act as a buffer zone between massive lateritic deposits enriched in transition metals such as Fe, Cr, Ni, Co, and Mn (Becquer et al., 2001, Quantin et al., 2001, Dublet et al., 2012), and a lagoon of over 20,000 km2, recently registered as an UNESCO World Heritage site.
Mangrove sediments are known to have a large capacity to accumulate metallic elements (Tam and Wong, 2000, Qiu et al., 2011, Marchand et al., 2012), which is partly attributed to their large content of organic matter that can act as a trace metal complexing agent (Nissenbaum and Swaine, 1976). In addition, mangrove sediments are subjected to sulfate-reduction processes (Howarth and Merkel, 1984, King, 1988), which lead to precipitation of sulfide minerals that are able to trap chalcophile elements as well as Co and Ni in sulfidic coastal or marine sediments (Huerta-Diaz and Morse, 1992, Morse and Luther, 1999, Burton et al., 2006, Burton et al., 2008). In mangrove sediments, trace metal trapping processes are, however, expected to depend on biogeochemical gradients in salinity and in redox and organic matter contents that are driven by the distance to the seafront, the magnitude of the tide, and the types of mangrove tree species (Otero et al., 2009, Marchand et al., 2011, Marchand et al., 2012). In addition, oxic and anoxic diagenetic reactions can operate in coastal sediments as a function of time, due to tide cycles (Otero et al., 2009), seawater flooding (Johnston et al., 2010, Wong et al., 2010, Wong et al., 2013), as well as freshwater inputs (Burton et al., 2008). Complex redox cycling in mangrove sediments may thus significantly impact the speciation of metallic elements across the intertidal zone (Clark et al., 1998, Tam and Wong, 2000, Marchand et al., 2006a, Marchand et al., 2012, Otero et al., 2009). Such biogeochemical cycles were studied in the mangrove sediments of the Island of Pai Matos (Cananeia — SP, Brazil), where Fe speciation, determined from sequential chemical extactions, changes significantly as a function of the redox boundaries delineating intertidal and depth zonations (Otero et al., 2009). Similarly, Noël et al. (2014) determined iron speciation in mangrove sediments from New Caledonia using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy and confirmed that the major variations in the distribution of Fe-oxyhydroxides, pyrite and iron-bearing clays, rely on the redox gradients generated by both the topographic position across the intertidal zone and the depth within the sediments. Compared to that of iron, the speciation of nickel in mangrove sediments has been less studied. Chemical extraction studies have however reported the potential role of sulfides, oxyhydroxides and carbonate minerals as well as of organic matter in nickel trapping (Clark et al., 1998, Marchand et al., 2012).
The objective of the present study was to improve our understanding of the biogeochemistry of nickel in mangrove sediments as a function of chemical gradients and of vegetal cover changes across the intertidal zone. For this purpose, we chose mangrove sediments that exhibit exceptional nickel content downstream from lateritized ultramafic ore deposits from New Caledonia. Complemented by X-ray Diffraction (XRD) and Scanning Electron Microscopy – Energy Dispersive X-ray Spectroscopy (SEM-EDXS) analyses, EXAFS spectroscopy was used for this purpose because it has been shown to be suitable for quantifying the variety of Ni-species in lateritic materials that are the source of nickel in the mangrove sediments in New Caledonia (Dublet et al., 2012, Dublet et al., 2014). The spatial distribution of nickel species was quantified in three cores drilled beneath typical mangrove tree covers (Rhizophora spp., Avicennia marina, and salt flat) in order to evaluate the long-term influence of the redox gradients on the speciation and mobility of Ni in sediments of these fragile ecosystems.
Section snippets
Sampling site and sampling procedures
The site we chose for study is typical of the topographic, vegetal and redox gradients of New Caledonian mangroves. It is located in Vavouto Bay (20°59′S; 164°49′E) at the mouth of the Talea-Coco River that drains the ultramafic Koniambo regolith (Noël et al., 2014). It stands on the west coast of the main New Caledonia island (20°S–23°S), which is characterized by a semi-arid climate (∼750 mm of rain/year; Maitrepierre, 2004). As in many mangrove ecosystems, the intertidal zone in New Caledonia
Mineralogy of the mangrove sediments
Located upstream of the mangrove forest studied, the Koniambo regolith consists of deeply weathered ultramafic rocks and thus hosts high concentrations of Fe, and to a lesser extent, Ni, Co, Cr, and Mn (Becquer et al., 2001, Quantin et al., 2001, Fandeur et al., 2009, Dublet et al., 2012, Dublet et al., 2015). It is composed of an upper laterite unit consisting of goethite with minor amounts of hematite and Mn-oxides, and of a deeper saprolite unit consisting of serpentine and talc, with minor
Evolution of the Ni-speciation along the major redox gradients
The present study shows that nickel in the salt flat sediments as well as in the upper horizon beneath the Avicennia and Rhizophora stands is hosted primarily by phyllosilicate minerals and goethite, which have been shown by Dublet et al. (2012) to be major Ni species in the Koniambo regolith, located upstream of the mangrove forest. This result confirms that a major Ni input to the mangrove sediments is related to erosion of the lateritized ultramafic outcrops. Beyond these
Acknowledgments
This study was performed in the framework of a UPMC Paris 6/CNRS (IMPMC)/IRD – Koniambo Nickel SAS (KNS) joint project. Field sampling campaigns were greatly facilitated by Sylvain Capo (KNS-GLENCORE), Andy Wright (KNS), Gregory Marakovic (KNS), and Jacques Loquet (Koniambo Environmental Committee). The authors thank Imene Esteve (IMPMC) for her help during Scanning Electron Microscope observations and analyses and Ludovic Delbes (IMPMC) for carrying out the X-ray diffraction experiments under
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