1. Rare earth element and neodymium isotope tracing of sedimentary rock weathering
- Author
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Alexander M Piotrowski, Edward T. Tipper, Patrick De Deckker, Germain Bayon, Kazuyo Tachikawa, Nicolas Freslon, Maude Thollon, Kwangchul Jang, Nathalie Vigier, Christina S. Larkin, Thibault Lambert, Géosciences Marines (GM), Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER), Institute of Earth Surface Dynamics, University of Lausanne, Université de Lausanne = University of Lausanne (UNIL), Laboratoire d'océanographie de Villefranche (LOV), Institut national des sciences de l'Univers (INSU - CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Institut de la Mer de Villefranche (IMEV), Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), Research School of Earth Sciences [Canberra] (RSES), Australian National University (ANU), Biogéosciences [UMR 6282] (BGS), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), Division of Polar Paleoenvironment, Korea Polar Research Institute (KOPRI), Department of Earth Sciences [Cambridge, UK], University of Cambridge [UK] (CAM), Centre européen de recherche et d'enseignement des géosciences de l'environnement (CEREGE), Institut de Recherche pour le Développement (IRD)-Aix Marseille Université (AMU)-Collège de France (CdF (institution))-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE), Unité de recherche Géosciences Marines (Ifremer) (GM), Institute of Earth Surface Dynamics, Université de Lausanne (UNIL), Biogéosciences [UMR 6282] [Dijon] (BGS), Centre National de la Recherche Scientifique (CNRS)-Université de Bourgogne (UB)-AgroSup Dijon - Institut National Supérieur des Sciences Agronomiques, de l'Alimentation et de l'Environnement, Aix Marseille Université (AMU)-Collège de France (CdF (institution))-Institut National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement (INRAE)-Institut de Recherche pour le Développement (IRD)-Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS), Work funded through an IEF Marie Curie fellowship (SI-PALEO, Grant No. FP7-PEOPLE-2012-IEF 327778), by NERC Grant (NE/P011659/1) for research into sediment compositions of large rivers, by an Australian Research Council DP grant (DP0772180) for the collection of Australian river samples., University of Lausanne (UNIL), Centre de Recherches de Climatologie [UMR Biogéosciences] (CRC), Université de Bourgogne (UB)-AgroSup Dijon - Institut National Supérieur des Sciences Agronomiques, de l'Alimentation et de l'Environnement-Centre National de la Recherche Scientifique (CNRS)-Université de Bourgogne (UB)-AgroSup Dijon - Institut National Supérieur des Sciences Agronomiques, de l'Alimentation et de l'Environnement-Centre National de la Recherche Scientifique (CNRS), Bayon, G [0000-0002-6791-4953], De Deckker, P [0000-0003-3003-5143], Jang, K [0000-0003-3777-2728], Tachikawa, K [0000-0002-9522-8600], and Apollo - University of Cambridge Repository
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010504 meteorology & atmospheric sciences ,sub-01 ,Mid-REE enrichment ,Geochemistry ,Silicate weathering ,Weathering ,Concavity index ,engineering.material ,Structural basin ,010502 geochemistry & geophysics ,Geologic record ,01 natural sciences ,Iron oxides ,chemistry.chemical_compound ,Geochemistry and Petrology ,[SDU.STU.GC]Sciences of the Universe [physics]/Earth Sciences/Geochemistry ,[CHIM]Chemical Sciences ,0105 earth and related environmental sciences ,geography ,geography.geographical_feature_category ,Sulphide weathering ,Neodymium isotopes ,Geology ,15. Life on land ,World rivers ,Silicate ,Craton ,chemistry ,13. Climate action ,Carbonate weathering ,engineering ,Carbonate ,Sedimentary rock ,Pyrite - Abstract
Chemical weathering plays an important role in sequestering atmospheric CO2, but its potential influence on global climate over geological timescales remains debated. To some extent, this uncertainty arises from the difficulty in separating the respective contribution of sedimentary and crystalline silicate rocks to past weathering rates in the geological record; two types of rocks having presumably different impact on the long-term carbon cycle. In this study, we investigate the use of rare earth element (REE) and neodymium isotopes (εNd) in leached iron oxide fractions of river sediments for tracing the origin of weathered rocks on continents. A new index, called ‘concavity index’ (CI), is defined for measuring the degree of mid-REE enrichment in geological samples, which enables the determination of the source of iron oxides in sediments, such as seawater-derived Fe-oxyhydroxide phases, ancient marine Fe oxides derived from the erosion of sedimentary rocks, and recent secondary oxides formed in soils via alteration of crystalline silicate rocks or pyrite oxidation. Using this index, we demonstrate that the εNd difference between paired Fe-oxide and detrital fractions in river sediments (defined here as ∆εNd Feox-Det) directly reflects the relative contribution of sedimentary versus crystalline silicate rocks during weathering. While rivers draining old cratons and volcanic provinces display near-zero ∆εNd Feox-Det values indicative of dominant silicate weathering (0.5 ± 1.1; n = 30), multi-lithological catchments hosting sedimentary formations yield systematically higher values (2.7 ± 1.2; n = 44), showing that sedimentary rock weathering can be traced by the occurrence of riverine Fe oxides having more radiogenic Nd isotope signatures compared to detrital fractions. This assumption is reinforced by the evidence that calculated ∆εNd Feox-Det values agree well with previous estimates for carbonate and silicate weathering rates in large river basins. Examining the influence of climate and tectonics on measured Nd isotopic compositions, we find that ∆εNd Feox-Det is strongly dependent on temperature in lowlands, following an Arrhenius-like relationship that reflects enhanced alteration of silicate rocks and formation of secondary Fe oxides in warmer climates. In contrast, in high-elevation catchments, ∆εNd Feox-Det defines striking correlation with maximum basin elevation, which we also interpret as reflecting the intensification of silicate weathering and associated Fe oxide formation as elevation decreases, due to the combined effects of thicker soils and warmer temperature. Overall, our new findings are consistent with previous assertions that the alteration of sedimentary rocks prevails in high-elevation environments, while silicate weathering dominates in floodplains. This novel approach combining REE and Nd isotopes opens new perspectives for disentangling the weathering signals of sedimentary and crystalline silicate rocks in the geologic record, which could be used in future studies to reassess the causal relationships between mountain uplift, erosion and climate throughout Earth's history.
- Published
- 2020
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