Rhodolith-forming coralline red algae from New Caledonia (SW Pacific) record half a century of sea-surface temperature variations and mining history

We present a continuous, high-resolution tropical record of Mg/Ca and trace elements back to the 1960s, using laser ablation ICP-MS on rhodoliths (i.e. free living forms of coralline red algae) of the species Sporolithon durum. The analysed rhodoliths are composed entirely of branched system forming mostly spherical specimens of 8-10 cm in diameter. Chronology was constrained by 20 radiocarbon dates calibrated with the 1960s-70s atomic bomb-spike curve, in conjunction with commonly used growth band counting and Mg/Ca minima-maxima peaks. Mg/Ca variations are reproducible between different branches of a single specimen as well as between three distinct rhodoliths from the same site in the SW lagoon of New Caledonia. All records present a significant correlation with the local sea-surface temperatures at a monthly to inter-annual resolution (respectively, 0.59< r <0.70 and 0.62< r <0.72; p<0.0001), thus confirming the global potential of coralline red algae for palaeo-temperature reconstructions, as suggested by recent studies from the North Atlantic and North Pacific Oceans (e.g. Halfar et al. 2000, Kamenos et al. 2008, Hetzinger et al., 2009). Compared to the youngest part of the record, Mn/Ca, Fe/Ca and Ni/Ca ratios present significantly higher values for the period prior to 1980, which corresponds to the high production period of an open-air, Ni-extraction mine located in the water catchment area of the Coulée River, ~10km from the studied rhodolith bed. The mining production at the site ceased in 1981. After that date, Mn, Fe and Ni concentrations in the rhodoliths show a steady decline. Co/Ca, however, shows no such trend, indicating different behaviour and availability for these metals, all related to the weathering of exposed laterite formations. Average metal records corrected from the anthropogenic mining effect (except for Co/Ca) present significant correlations with the inter-annual local rainfall signal (r=0.62, r=0.60, r=0.48; p<0.0001 for Mn/Ca, Fe/Ca and Ni/Ca, respectively) for most of the studied period, in accordance with a freshwater-derived origin of the metals at the study site. This also suggests a potential use of metal concentrations in rhodoliths as an indirect tracer for local rainfall patterns.