High metal inputs to closed seas: The New Caledonian lagoon

The islands of New Caledonia are largely composed of ultrabasic rocks (peridotites), severily weathered, rich in Fe, Mn and Co, and where several ore deposits of Ni and Cr are extensively mined. Sediment cores from the bay of Dumbea, in the south of the main island, and from the northern part of the lagoon (Belep Islands), less affected by the mining activities, were analyzed for their mineral composition and metal concentrations. In the surficial sediments, oxides and silicates, including Fe serpentine and smectites, undergo rapid transformation or neoformation in a short time, in particular in the confined bay of Dumbea. Fe is largely present as goethite, and in deeper layers (60-100 cm) as hematite and magnetite. Chromite can be identified at each horizon. The metal concentrations decrease from the near shore areas, in particular the vicinity of the Dumbea river mouth to the open part of the lagoon. This trend is more important for Ni than for Fe or Cr. Fe ranges from 3.5 to 9% (dry weight), Ni from 200 to 2000 mu g/g, Cr from 700 to 2000 mu g/g, Co from 20 to 150 mu g/g and Mn from 130 to 900 mu g/g; yet the concentrations are lower than concentrations found in the ultrabasic rocks or laterites of the watershed. To try to understand the behavior of metals during the sedimentation-diagenesis events, we evaluated the sediment accumulation rate, and used different sequential leaching procedures. Fe, Mn, Ni, Cr and Co are mainly present as, or bound to, oxides or oxyhydroxides, even in the deeper layers (> 100 cm) where the organic content is relatively high (about 6% of organic C). Metals are mainly transported from the land to the lagoon as oxides and dispersed in the lagoon sediments, where they are diluted with a large amount of carbonaceous sediment. During diagenesis, a significant part of Mn, Co and Ni are dissolved; but, unlike Mn and Co, which seem to coprecipitate with carbonate, most of the Ni is released into the waters of the lagoon. Apparently no horizon of the sediment has undergone significant in-situ metal enrichment.