Rare earth element and neodymium isotope tracing of sedimentary rock weathering

Type Article
Date 2020-10
Language English
Author(s) Bayon Germain1, Lambert Thibault2, Vigier Nathalie3, de Deckker Patrick4, Freslon Nicolas1, 5, Jang Kwangchul6, Larkin Christina S.7, Piotrowski Alexander M.7, Tachikawa Kazuyo8, Thollon Maude1, Tipper Edward T.7
Affiliation(s) 1 : IFREMER, Marine Geosciences Unit, Brest, France
2 : Institute of Earth Surface Dynamics, University of Lausanne, Switzerland
3 : Laboratoire Océanographique de Villefranche sur Mer (LOV, OOV), CNRS, UPMC University of Paris VI, Villefranche-sur-Mer, France
4 : Research School of Earth Sciences, The Australian National University, Canberra, Australia
5 : Université de Bourgogne, UMR CNRS 6282 Biogéosciences, Dijon, France
6 : Division of Polar Paleoenvironment, Korea Polar Research Institute, Incheon, South Korea
7 : Department of Earth Sciences, University of Cambridge, UK
8 : Aix Marseille University, CNRS, IRD, INRAE, Collège de France, CEREGE, Aix-en-Provence, France
Source Chemical Geology (0009-2541) (Elsevier BV), 2020-10 , Vol. 553 , P. 119794 (15p.)
DOI 10.1016/j.chemgeo.2020.119794
WOS© Times Cited 14
Keyword(s) World rivers, Iron oxides, Neodymium isotopes, Mid-REE enrichment, Concavity index, Silicate weathering, Carbonate weathering, Sulphide weathering

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

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Bayon Germain, Lambert Thibault, Vigier Nathalie, de Deckker Patrick, Freslon Nicolas, Jang Kwangchul, Larkin Christina S., Piotrowski Alexander M., Tachikawa Kazuyo, Thollon Maude, Tipper Edward T. (2020). Rare earth element and neodymium isotope tracing of sedimentary rock weathering. Chemical Geology, 553, 119794 (15p.). Publisher's official version : https://doi.org/10.1016/j.chemgeo.2020.119794 , Open Access version : https://archimer.ifremer.fr/doc/00641/75351/