Sedimentology and magnetic susceptibility of recent sediments from New Caledonia

The interpretation of the primary origin of the minerals carrying the magnetic susceptibility (MS) signal from ancient rocks suffers notably from the scarcity of studies on Recent sediments. To bring new data, a study of tropical coastal sediments of New Caledonia was undertaken. This island is surrounded by a nearly uninterrupted reef barrier, isolating a wide lagoon from the open ocean. The erosion of extremely varied rocks (from mantle rocks to laterites) produces different types of detrital sediments, which are mixed with the indigenous precipitated carbonates. This generates different types of coastal sediments, detrital- or carbonate-dominated or mixed. More than 300 samples were analysed for grain size, nature of sediment, MS and geochemistry (major elements). The first results show that: (a) carbonate sands and carbonate silts are characterized by lower MS than detrital sediments; (b) the MS signal of mixed sediments is mostly influenced by the proportion of detrital sediments; (c) MS is directly correlated with Mn and Fe content; (d) beachrocks are characterized by lower MS than equivalent loose sediment; (e) the MS signal of carbonate sediments is locally positively correlated with granulometry; (f) there is no MS change between surface and 20 cm deep samples; and (g) when the subsurface sediment is reducing, the MS is higher than that from surface sediment.