Dynamics of continental surfaces: weathering and erosion processes

Dating of lateritic soils (TA, EB, EF, SL, MG). Within the framework of the CAPES COFECUB (Brazil), PICS / CNRS (Brazil) and SYSTER / INSU (India) programs, we have sampled and analyzed lateritic profiles and ferruginous cuirasses to apply the kaolinite dating method developed in our group. Preliminary data obtained in the above-mentioned projects or during Maximilien Mathian's thesis already show soil profiles deep seated in the tertiary era, which will have to be compared to paleoclimatic data and geodynamic events at regional or global scale.

Mineralogical and isotopic tracing of erosion sources (TA, EF, GM, SL, MG). Thanks to a sampling campaign in the Amazon basin during an EC2CO program, we have investigated whether iron complexed to colloidal organic matter, a marker of soil erosion, was stable during physicochemical shocks occurring at some confluences in the Amazon basin. We have shown that the mixture of the Rio Negro and the Solimoes, characterized by a pH deviation of 2 units and traced by hydrogen isotopes, destabilizes the organometallic complexes and leads to the formation of iron oxide colloids [Mulholland et al., 2015]. The mixture is therefore non-conservative. In addition, in collaboration with F. Poitrasson (GET) we have determined that the Rio Negro colloids are enriched in heavy iron isotopes, a signature acquired upstream during oxidation and complexation processes in the podzolic environment, while the chemical signature of the Amazon is compatible with that of the continental crust.

Finally, within the framework of the PIREN-Seine program, we have highlighted with the LSCE and the UVSQ the influence of the anthropization level and the seasonal hydrological conditions on the speciation and export of zinc in the Orge River [Le Pape et al. 2014]. With the LGE-IPGP we have identified the crystallochemical and isotopic signatures of zinc sources in the Seine basin [Bonnot et al. 2016].

Crystal-chemistry of transition metals under tropical climate and lateritic ores formation (FJ, GO, EF, GM, CM, JB). Lateritic ores forming after several million years of rock weathering under tropical climates represent major mineral resources. For instance, New Caledonia's lateritic deposits account for 20 to 30% of the world's currently known nickel reserves. However, despite this importance, the geochemical mechanisms responsible for the genesis of these deposits are still questioned. These questions concern both the richest clay-type ores (garnierite) that have been mined for more than 150 years and the oxide type ores at the top of the profiles that are being more and more mined. Thanks to a 4-years project funded by CNRT Nickel and Environment (www.cnrt.nc/), we were able to gain further insights into the origin of these deposits and we have proposed an alternative model to explain the formation of the garnieritic ores [Fritsch et al., 2016]. We also clearly identified the mineralogical mechanisms that explain nickel accumulation and depletion along lateritic ores of New Caledonia [Dublet et al., 2012], thus setting the basis for evaluating the factors that control nickel distribution in these lateritic ores [Dublet et al., 2014; Dublet et al., 2015]. In parallel our work on cobalt and manganese revealed that these two elements behave similarly from a crystal-chemistry point of view during the differentiation of the lateritic profile [Dublet et al., accepted]. This work has also revealed the complex crystal-chemistry of the various cobalt-bearing manganese oxides and allowed us to propose a sequence of crystallization of these mineral species in the lateritic ores of New Caledonia [Ploquin et al., submitted].

Carbon dynamics under tropical climate: the role of mangroves (CM). Mangroves occupy 75% of the tropical and subtropical coasts and constitute a sink for atmospheric CO₂ and a source of nutrients for coastal waters (Lee et al., 2014), a function that could evolve with climate change. For the past 4 years, we have developed a mangrove observatory following a climatic latitudinal gradient between New Zealand, New Caledonia and Vietnam. We have shown that carbon storage in mangrove sediments depends on the age of the mangrove and its intertidal position [Marchand 2017], seasonal variations of microphytobenthos on the surface of sediments [Leopold et al ., 2013; 2015], as well as CO₂ emissions at the water-atmosphere interface [Leopold et al., 2017]. Net CO₂ exchanges at the ecosystem level are followed by eddy-covariance stations that have been installed in each country of the observatory. The first results on the famous mangroves of the Heart of Voh in New Caledonia show the importance of water stress on the productivity of the ecosystem [Leopold et al., 2016].

The urbanization of tropical coastlines and the massive increase in wastewater discharge into the mangrove forest could significantly alter the dynamics of carbon in the ecosystem. We have thus shown that part of the aquaculture effluents could be recycled by soil and mangroves, increasing the productivity of the latter, but that the mangrove was only a partial filter that can not handle a too large volume [Molnar et al. Al., 2013; 2014 ; Aschenbroich et al., 2015]. Different environmental quality monitoring indicators have been identified [Debenay et al., 2015: Della Patrona et al., 2016].