Erosion of an ultramafic massif: impact of geochemistry and sedimentary deposits in tropical lagoons in New Caledonia
摘要
New Caledonia is a hotspot of biodiversity but holds 25% of the world’s nickel resources making mining the main sector of its economic development. Ultramafic massifs are composed of metal-rich laterites and peridotites, such as Co, Fe, Mg, and Ni. Natural weathering exacerbated by open-pit mining contributes significantly to sediment transport in rivers and lagoons. In the early 2000s, a new mine was opened in the Koniambo ultramafic massif, a catchment area of the Voh lagoon. The main objectives of this study were (i) to examine the distribution of metals (Al, Ca, Co, Cr, Cu, Fe, Mg, Mn, Ni, Pb, Sb, Sr, Ti, V, Zn) in marine sediment and (ii) to assess the historical deposition fluxes resulting from the weathering of the Koniambo massif. Our results confirm that sediments are strongly influenced by continental weathering, with a contribution from volcano-sedimentary rocks (Al, Cu, Pb, Sb, Ti, V, and Zn) and ultramafic laterites (Co, Cr, Fe, Mn, and Ni). Sediment accumulation rates have been estimated at about 2.7 cm year−1 near the shore and 4.5 cm year−1 near the coral reef, which is up to 18 times larger than in the natural ecosystem. Ultramafic laterites account for more than 91% of Ni inputs into the lagoon (10.4 g m−2 year−1), followed by Cr with about 75% of inputs and a sediment deposition flux of 5.04 g m−2 year−1. Given the environmental context, this study highlights the importance of examining ecotoxicological risks in order to evaluate the impact of metals on the lagoon’s sustainability.