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Reconstructing the impact of nickel mining activities on sediment supply to the rivers and the lagoon of South Pacific Islands: lessons learnt from the Thio early mining site (New Caledonia)
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International audience. Opencast mining has exacerbated land degradation in New Caledonia, a French archipelago located in the south-west Pacific Ocean. Developed since the 1880s, mining has become the major economic activity in some catchments, which strongly disrupted sediment dynamics. Reconstructing the temporal changes of sediment source contributions is essential to understand the driving factors of soil erosion in response to i) the occurrence of cyclones, ii) the changes in mining practices during the last several decades, and iii) other soil degradation processes such as extensive soil erosion induced by fires, overgrazing and trampling of invasive species, and landslides. Accordingly, a multi-parameter analysis including gamma spectrometry, color and X-ray fluorescence measurements was conducted on a sediment core collected in a deltaic floodplain at the outlet of one of the first areas exploited for nickel mining, the Thio River catchment (397-km2). One geochemical tracer (i.e. K) has been used to quantify changes in sediment sources in the successive sediment layers deposited since the beginning of mining activity. The results showed that the contribution of mining tributaries largely dominated, with a mean contribution of 74 % (SD 13 %) of material sampled in the sediment core. This contribution notably increased after the mechanization of mining activities (i.e. from 1950s; increase of 18 %). The occurrence of Cyclone Alison in 1975 triggered the progressive transfer of mining waste accumulated on the foothills over 25 years into the river system. This tipping point could be identified in the sediment sequence, which demonstrates that over the last 41 years (i.e. 1975-2016), a ~84-cm deep sediment deposit has accumulated in the alluvial floodplain of the Thio River catchment (mean annual deposition rate of 2 cm yr-1). Currently, the progressive release and downstream transfer of this mining waste is still ongoing ~45 years after Cyclone Alison. Although environmental legislation was introduced in 1975, mining tributary contributions to sediment still dominate (80 %, SD 5 %). Overall, this multi-proxy approach to examining the cumulative effects of mining activities on downstream sediment dynamics could be implemented in other mining catchments of New Caledonia and around the world to compare the respective mining source contributions to sediment and their evolution throughout time in these contrasted areas.
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