Reconstructing modern and past weathering regimes using boron isotopes in river sediments Christian Ercolani To cite this version: Christian Ercolani. Reconstructing modern and past weathering regimes using boron isotopes in river sediments. Geochemistry. Université de Strasbourg; University of Wollongong (Wollongong, Australie), 2018. English. NNT : 2018STRAH008. tel-02176267 HAL Id: tel-02176267 https://tel.archives-ouvertes.fr/tel-02176267 Submitted on 8 Jul 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. 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UNIVERSITÉ DE STRASBOURG École Doctorale des Sciences de la Terre et de l’Environnement Laboratoire d’Hydrologie et de Géochimie de Strasbourg (LHyGeS) THÈSE présentée par : Christian ERCOLANI soutenue le : 25 septembre 2018 pour obtenir le grade de : Docteur de l’université de Strasbourg Discipline/ Spécialité : Géochimie Reconstruction des régimes d’altération actuels et passés à partir des isotopes du bore dans les sédiments de rivière THÈSE dirigée par : LEMARCHAND Damien MCF, HDR, Université de Strasbourg DOSSETO Anthony Associate Professor, University of Wollongong RAPPORTEURS : NÄGLER Thomas Professor, Universität Bern PUCÉAT Emmanuelle MCF, HDR, Université de Bourgogne EXAMINATEURS : CHABAUX François Professeur, Université de Strasbourg VIGIER Nathalie DR., Laboratoire d’Océanographie de Villefranche INVITÉ PARIS Guillaume CR., Centre de Recherche Pétrographiques et Géochimiques, Nancy Reconstruction of modern and past weathering regimes using boron isotopes in river sediments Christian Paul Ercolani Supervisors: Dr. Anthony Dosseto Dr. Damien Lemarchand This thesis is presented as part of the requirement for the conferral of the degree: Doctor of Philosophy The University of Wollongong School of Earth and Environmental Science 2018 Christian ERCOLANI Reconstruction of modern and past weathering regimes using boron isotopes in river sediments Résumé Cette thèse a les objectifs suivants : 1. Mieux comprendre comment les isotope du bore dans les sédiments fluviaux modernes enregistrent le régime d’altération à la échelle du bassin versant. 2. Mieux comprendre comment le « signal » d’altération porté par les sédiments fluviaux est transféré des zones sources vers l’environnement de dépôt. 3. Déterminer si les isotopes du B dans les dépôts sédimentaires (paléo-canaux) peuvent être utilisés pour reconstituer les conditions paléo-climatiques et paléo-environnementales et ainsi révéler comment l’altération continentale au sens large (production et transport de sédiments) a réagi à la variabilité climatique au cours du dernier cycle glaciaire-interglaciaire (derniers 100 ka). Ces objectifs ont été examinés en étudiant les matériaux fluviaux des fleuves Gandak (Himalaya) et Murrumbidgee (NSW, Australie) et des dépôts de sédiments fluviaux de la Riverine Plain (bassin versant de Murrumbidgee, Australie). La connaissance des paramètres qui contrôlent le fractionnement isotopique du bore des sédiments fluviaux au cours de la formation et du transport a d'abord été acquise dans les systèmes modernes, puis appliquée à d'anciens dépôts de paléochenaux. Résumé en anglais This thesis has the following objectives: 1. To better understand how boron isotopes in modern fluvial sediments record the weathering regime at the catchment scale. 2. To better understand how the weathering “signal” carried by river sediments is transferred from source areas to the depositional environment. 3. To determine if boron isotopes in sediment deposits (paleochannels) can be used to reconstruct paleo-weathering and paleo-environmental conditions and reveal how continental weathering at large (production and sediment transport) responds to climatic variability over the last glacial-interglacial cycle (approximately the last 100 ka). These objectives were addressed by studying fluvial material from the Gandak (Himalayas) and Murrumbidgee (NSW, Australia) Rivers and fluvial sediment deposits from the Riverine Plain (Murrumbidgee catchment, Australia). Knowledge of the parameters that control boron isotope fractionation of river sediment during formation and transport was first gained in the modern systems and then applied to ancient paleochannel deposits. ABSTRACT Chemical weathering coupled with carbonate precipitation in the oceans is largely responsible for the sequestration of atmospheric CO 2, which balances CO 2 inputs into the atmosphere from mantle degassing and thus participates in the global climate regulation at the geological time scale. Despite the importance of chemical weathering in maintaining habitable conditions on the Earth's surface, quantification of past and present chemical weathering remains difficult. The intensity of modern chemical weathering is generally determined from the geochemistry of solutes and sediments transported by rivers. However, this approach suffers from the lithological control on the composition of the dissolved load and granulometric/mineralogical sorting during sediment transport. Alternatively, boron (B) isotopes have physicochemical properties suitable to study of water-rock interactions, including those involving a biological component. The processes responsible for B isotope fractionation are adsorption on clay and detrital particles, precipitation in secondary phases, and recycling through vegetation. While several studies have used B isotopes as a proxy to quantify chemical weathering reactions in the dissolved load of rivers, few have focused on river sediments. As a result, the parameters that control B isotope behavior during the production of secondary products and subsequent transport from source areas to the deposition environment are not fully understood. Additionally, the use of B isotopes as potential proxy for paleo-weathering and paleo- environment reconstruction is relatively unknown. In the Gandak River (Himalayas) suspended and river bank sediments were collected along the course of the river in order to better understand the parameters that control B isotope fractionation in sediments transported by rivers. Grain size fractions were analyzed for B isotopes and major and trace element ly fractionated with respect to the primary minerals, represented by the sand fraction (63 µm - 2 mm), and evolve downstream, while all other size fractions have a constant composition. The downstream evolution of B isotope compositions and concentrations in the clay fraction reflect differences in the weathering regime; dissolution reactions are dominant in the headwaters, while secondary phase precipitation reactions dominate downstream. Similarly, bank sediments and the dissolved load of the Murrumbidgee River and its tributaries (NSW, Australia) were analyzed to assess the role of lithology, climate, and geomorphology on B isotope fractionation in weathering products and their transport from source areas to the depositional environment. 3 Boron isotopes in the clay fraction of bank sediments also show an increase in isotope ratios downstream in the watershed, a behavior attributed to changes in the weathering regime in sediment production areas. The B isotope composition of clay fractions is disconnected from that of the dissolved load of the river, which reflects both an absence of isotopic exchange during transport and a disconnection between the fluid that produces the clay and the fluid that composes the dissolved phase of the river. A mass balance model reveals that the composition of the clay fraction represents a mixture of sediment sources throughout the watershed. Therefore, no chemical or isotopic evolution of the sediments was observed during transport. Paleochannel sediments in the Murrumbidgee catchment were also analyzed for boron and neodymium (Nd) isotopes, major and trace element concentrations, and sediment mineralogy. After identification and removal of samples which were affected by post-depositional alteration, distinct B isotope variations between paleochannel systems can be made. This B isotope record in the paleochannels correlates with a sea surface temperature (SST) and vegetation cover records over the last 100 ka indicating that clay formation in source areas has responded to climatic variability, particularly temperature and vegetation cover. During warmer and wetter periods (Marine Isotope Stages 1, 3, and 5), the role of vegetation on pedogenesis is greater than during colder periods (Marine Isotope Stage 2). Finally, Nd isotopes in the clay fraction show that there has been no significant change in sediment provenance for the last 100 ka. These results highlight the potential for the use of B isotopes to quantify modern and past weathering regimes, including their use in sediment deposits as a proxy to reconstruct paleo- weathering and paleo-environmental conditions. 4 ACKNOWLEDGEMENTS The opportunity to earn a doctoral degree comes but once in a life time. Success is not measured by how much you achieve, but rather how many times you attempt to achieve success. Hardships and mishaps are just part of the experience, which in the end, often lead to much better and interesting