No abstract

<p>Ultramafic (UM) soils are of particular interest due to their high content in metals for example Fe, Mn but also in Ni, Co, or Cr up to the ore grade (<strong>Butt and Cluzel, 2013</strong>). Those high metal contents combined with low contents of plants essential nutrients (Ca, K and P) imply particularly stressful conditions for the vegetation. To take advantage on these specific edaphic conditions, few plant species growing on UM soils have developed ecophysiological strategies including metal hyperaccumulation (Reeves, et al. 2018). Hyperaccumulation implies efficient metal mobilization at the soil-plant interface, i.e. roots, and the transfer to the different aerial organs of plants, which can lead to significant concentrations of metal in stems, sap, latex, and leaves. As example, for Ni, these concentrations can reach up to the percent level, while most plants contain less than 15 µg/g (dry mass) of Ni in their tissues (<strong>Brooks et al., 1977</strong>). This behaviour is expected to increase Ni-phytobioavailability by litter degradation and complexation of metal with organic ligands in the upper horizon of UM soils (<strong>Boyd and Jaffré, 2001; Zelano et al., 2020</strong>). This physiological process is also suspected to modify Ni isotope ratios due to absorption, transport and storage in the plant. However, the extent of Ni isotope fractionation in UM soils due to hyperaccumulators remains unclear and debated. While Zelano et al. (2020) suggested that the Ni sequestration by hyperaccumulators and its redistribution in the aerial organs of the plant could hinder Ni isotope fractionation in old individuals, Ratié et al. (2019) reported a preferential uptake of light isotope by roots in soils and Ni fractionation during translocation to the aerial part of the plants leading to heavier isotopic composition in soils.</p><p>The present study focuses on Ni-hyperaccumulation <em>Pycnandra acuminata</em> tree, endemic to New Caledonia. To understand the impact of Ni-hyperaccumulating plants on the Ni biogeochemical cycle, twelve soil profiles have been identified in the rainforest of Grande Terre including six profiles developed in the close vicinity of Ni-hyperaccumulating trees P. acuminata and six other profiles developed in the close vicinity of <em>Pycnandra fastuosa</em>, a non-hyperaccumulating tree also endemic in New Caledonia. Nickel concentrations found in hyperaccumulator-soil systems are higher relative to the non-hyperaccumulator-soil systems revealing the influence of <em>P. acuminata </em>and the associated leaves degradation on Ni redistribution in ultramafic soils. Ni isotope compositions and XAS spectroscopy of soil samples will help us to reveal the biogeochemical processes controlling the Ni isotopic signature in UM soils. Although focalized on New Caledonia, our study can be considered representative of the influence of hyperaccumulating trees on the biogeochemical cycle of Ni in UM soils systems worldwide.</p><p> </p><p>Boyd and Jaffré (2001), South Afr. J. Sci. 97, 535 – 538</p><p>Brooks et al. (1977), J. Geochem. Explor. 7, 49 – 57</p><p>Butt and Cluzel (2013), Elements 9(2), 123 – 128</p><p>Ratié et al. (2019), J. Geochem. Explor. 196, 182 – 191</p><p>Reeves et al. (2018), New Phytol. 218(2), 407 – 411</p><p>Zelano et al. (2020),  Plant and Soil 454(1 – 2), 225 – 243 </p>

<p><span>Lateritic soils are deep weathering profiles, developed in tectonically quiescent areas under tropical conditions and over long timescales. Laterites are key components in the regulation of element cycle in the Earth’s history but, the timing between climatic changes and lateritic weathering episodes remains unconstrained. The combination of chronometric and weathering proxies is one way to build a comprehensive story of laterite formation.</span></p><p><span>In this study, two lateritic vertical profiles were targeted on the outer part of the Guyana Shield in the Amazon Basin. This region is tectonically stable and subjected to a rainy tropical climate since the Cretaceous. The first soil profile, located in the Brownsberg Mountains, Suriname, is developed on Proterozoic Greenstone [1]. The second lateritic cover, already studied and dated using EPR technique [2], is developed over the Cretaceous sedimentary Alter do Chao formation, Brazil. Both lateritic profiles are characterized by 1/ a total depletion of soluble elements and weathering of primary minerals at the base of the profile and 2/ a desilication followed by the formation of Fe and Al duricrusts on top. Here, traditional geochemical budgets are seconded by measurements of Si isotopes in both soils (bulk and/or clay fractions) and laterite draining streams. Silicon isotopes (δ</span><sup><span>30</span></sup><span>Si) are known to be an excellent weathering proxy, fractionated during clay mineral formation [3]. </span></p><p><span>In Suriname bulk soils, heavier δ</span><sup><span>30</span></sup><span>Si is associated with lateritization due to the “buffering” quartz exerts on bulk δ</span><sup><span>30</span></sup><span>Si. However, if clay fractions are isolated, the observed strong enrichment in light Si (Δ</span><span>δ</span><sup><span>30</span></sup><span>Si<sub>clay fraction-bedrock</sub> up to -0.9‰) is in line with the weathering of primary minerals and the formation of kaolinite. The dating of this intense weathering episode is c.a. 2-9 Ma based on preliminary EPR dating of kaolinites. </span></p><p><span>Regarding the Brazilian laterite, the material forming the Alter do Chao formation already suffered weathering episodes before deposition. The combination of EPR dating [2] and δ</span><sup><span>30</span></sup><span>Si measurements on the clay fraction reveals two distinct formation phases. First, chemical weathering is limited to the 37-22 Ma period. Second, the progressive depletion of</span><span> δ</span><sup><span>30</span></sup><span>Si from the bottom to the top of the lateritic profile highlights a replacement of a first kaolinite generation by a second population through dissolution-reprecipitation around 6 Ma, as previously inferred by EPR dating [2]. </span></p><p><span>These results, in combination with elemental mass budgets, give us better constraints to estimate the intensity and the timing of element mass transfers during laterite formation.</span><span> </span></p><p><span>[1] Monsels & van Bergen (2017) <em>Journal of Geochemical Exploration </em>180, 71-90. [2] Balan et al. (2005) <em>GCA</em> 69 (9), 2193-2204. [3] Opfergelt & Delmelle (2012) <em>Comptes Rendus Geoscience</em> 334 (11), 723-738.</span></p>

<p>Laterite formations are deep regoliths, up to one hundred of meters thick, that represent about 80% of the global soil volume. Formed under tropical conditions, laterites result from successive chemical weathering reactions over long periods up to tens of millions of years. Laterites can thus be seen as both an actor of the long-term carbon cycle, through CO<sub>2</sub> consumption by silicate weathering and witness of the long-term climate evolution. Indeed, secondary minerals found nowadays in lateritic profiles may have recorded past environmental conditions that prevailed at the time of their formation. Despite the large distribution of lateritic formations around the world, their timing and processes of formations as well as their preservation over long period of time remain unclear.</p><p>Here, we investigate an entire weathering profile developed on the Guiana Shield, in Brownsberg mountains, Suriname. The sampling region has remained in equatorial position for the last 100 Myr and has seen lateritic development since early Tertiary [1]. Such latitudinal stability offers the possibility to look at links between long-term climate evolution or climatic events and long-term chemical weathering processes.</p><p>The lateritic profile shows a strong loss in both alkali and alkaline-earth elements as well as a desilication, and an enrichment in Fe, particularly in the duricrust. The study of trace elements and rare earth elements highlights various geochemical processes behind the development of a lateritic – bauxitic profile.</p><p> (U-Th-Sm)/He ages of iron oxides from the duricrust show the presence of multiple generations of Fe oxides, demonstrating that the Brownsberg profile underwent multiple dissolution and recrystallization phases since its formation, at least 19.9 ± 1.8 Ma ago. These successive weathering processes may have led to the particular enrichment in the profile such as the one observed for Fe and V in the duricrust. Measurement of d18O – dD on secondary minerals, i.e. kaolinite and Fe-oxides s.l., will help to connect mineralogical and geochemical variations with the environmental conditions that prevailed at the time of their formation [2].</p><p>[1] Theveniaut and Freyssinet, 2002. Pal. Pal. Pal., 178, 91-117</p><p>[2] Girard et al., 2000. GCA, 64 n°3, 409 – 426</p>

No abstract

Thick regoliths developed under tropical climate, namely, laterites, resulting from long-term and pronounced geochemical and mineralogical rearrangement of the parent rock in response to environmental changes. Little information is available on the timing of laterite and bauxite formations, especially on the chronology of the main weathering episodes responsible for lateritic cover formation on the Guiana shield. For this purpose, we focused on both lateritic and bauxitic duricrusts developed over the Paleoproterozoic Greenstone Belt in the Brownsberg, Suriname. The duricrust samples have a relatively simple mineralogy (i.e., goethite, gibbsite, hematite, and kaolinite) but reveal, when observed at a microscopic scale, a complex history of formation with multiple episodes of dissolution/reprecipitation. The (U-Th)/He dating of 179 Fe-oxides subsamples shows that duricrusts sampled at the top of the Brownsberg plateau have ages ranging from <0.8 Ma to ∼19 Ma. In contrast, Fe-oxides extracted from detrital duricrust boulders collected downslope indicate formation ages up to 36 Ma. This age discrepancy may indicate that a main episode of physical erosion affected this region between ca. 30 and 20 Ma. Consistently, the bauxite sampled at the mountaintop indicates a younger phase of formation, with Fe-oxides recementing fragments of a preexisting bauxitic material older than ∼15 Ma. Geochronological data also reveal a long-lasting weathering history until the present day, with multiple generations of Fe-oxides in the bauxite and the duricrusts resulting from successive cycles of dissolution and reprecipitation of Fe-oxides associated with redox cycles. This long-lasting weathering history led to geochemical remobilization and apparent enrichment in some relatively immobile elements, such as REE, aluminum, and vanadium, especially in the duricrust sampled at the mountaintop. Our geochronological, mineralogical, and geochemical study of Fe- and Al-crusts from the Brownsberg mountain provide constraints on the evolution of environmental conditions prevailing since the early Oligocene in Suriname.