A record of mining and industrial activities in New Caledonia based on trace elements in rhodolith-forming coralline red algae

We investigate the ability of coralline red algae to record historical nickel mining activity that occurred between the early 1960s and 1981 in the Coulée River watershed, southwest New Caledonia. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) was used to determine high-resolution (sub-annual) variations of trace metal ratios (Mn/Ca, Fe/Ca, Ni/Ca and Co/Ca) in three ~45 years old, free-living forms of the rhodolith coralline red alga Sporolithon durum from Ricaudy Reef in the inner lagoon of New Caledonia, 15 km from the Coulée River mouth. We find increased Mn/Ca and Fe/Ca ratio values track the intensity of mining in the Coulée River catchment whereas Ni/Ca ratio values appear more sensitive to the ore type (saprolite versus laterite) that were targeted during two separate stages of mining activity. Mn/Ca, Fe/Ca and Ni/Ca decrease following the cessation of mining in 1981, but did not reach current values until >10 years after that date. Surprisingly, Co/Ca variations do not correlate with mining activity but rather increase steadily through the record period. We argue this relates to nickel refining and smelting activity over the past half century. Our results further suggest that the local, inter-annual rainfall variability may influence the metal concentrations recorded in the rhodoliths during the period of intense mining activity, but the link between metal uptake and rainfall is not entirely clear for the entire growth period of the rhodoliths.