Depth profiles of 210Pb, 226Ra, 228Th, 7Be, 137Cs and 40K in coastal sediments off the Thio River mouth (New Caledonia)

New Caledonia, an archipelago located in the south-west Pacific Ocean, is a leading producer of nickel. Mining activities began in 1875 and intensified after World War II, when extraction processes were mechanised. This has had a significant impact on sediment runoff to coastal areas. Since 1975, environmental regulations have been introduced to mitigate the industry's environmental impact. A coastal sediment archive was used to assess the long-term effects of terrestrial mining activities on the coastal environment, as well as the effectiveness of these measures. On 22 April 2022, the MGE-KS17 sediment core was sampled at a depth of 15 metres, one kilometre from the mouth of the Thio River (21°35.832' S; 166°14.055' E), using a Kullenberg corer aboard the research vessel Alis. The upper 150 cm of the core was dated using 210Pb and 137Cs on a pluridecadal time scale. The core was carefully extruded in 1 cm increments, with approximately 5 g of dry sediment from selected layers taken to determine the activities of the radioisotopes of interest (²¹⁰Pb, ²²⁶Ra, ²²⁸Th, ⁷Be, ¹³⁷Cs and ⁴⁰K). Excess 210Pb was then used to establish the chronological framework of core MGE-KS17. The dating was based on a combination of CIC (constant initial concentration) and CF-CS (constant flux: constant sedimentation) models.