Rare earth element and neodymium isotope tracing of sedimentary rock weathering

Abstract

Chemical weathering plays an important role in sequestering atmospheric CO 2 , 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 ( Nd ) 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  Nd difference between paired Fe-oxide and detrital fractions in river sediments (defined here as  Nd 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  Nd 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  Nd Feox-Det values agree well with previous estimates for carbonate and silicate weathering rates in large river basins.

Neodymium Isotopes in Glauconite for Palaeoceanographic Reconstructions at Continental Margins: A Preliminary Investigation From Demerara Rise

“…In this study, the evidence that leached Fe-oxyhydroxide phases significantly depart from NADW-like ε Nd values clearly point towards the presence of such pre-formed continental oxides. In fact, recent investigations have shown that river-borne Fe oxides are almost systematically characterised by more radiogenic Nd isotopic compositions relative to the associated detrital material (Hindshaw et al, 2018;Bayon et al, 2020;Jang et al, 2020). This Nd isotope decoupling between paired Fe oxide and detrital silicate fractions has been attributed to the preferential erosion and/or weathering of sedimentary rocks on continents (e.g., Bayon et al, 2020).…”

“…In fact, recent investigations have shown that river-borne Fe oxides are almost systematically characterised by more radiogenic Nd isotopic compositions relative to the associated detrital material (Hindshaw et al, 2018;Bayon et al, 2020;Jang et al, 2020). This Nd isotope decoupling between paired Fe oxide and detrital silicate fractions has been attributed to the preferential erosion and/or weathering of sedimentary rocks on continents (e.g., Bayon et al, 2020). For instance, recent sediments deposited near the mouth of the Amazon River display ε Nd values of −8 and −10.7 for leached Fe oxides and detrital fractions, respectively (Bayon et al, 2020).…”

“…This Nd isotope decoupling between paired Fe oxide and detrital silicate fractions has been attributed to the preferential erosion and/or weathering of sedimentary rocks on continents (e.g., Bayon et al, 2020). For instance, recent sediments deposited near the mouth of the Amazon River display ε Nd values of −8 and −10.7 for leached Fe oxides and detrital fractions, respectively (Bayon et al, 2020). At the Demerara Rise, the main potential sources of sediment include suspended particulate material from the large South American tropical rivers (Amazon, Orinoco, and Maroni) that are subsequently transported by ocean currents.…”

A global survey of radiogenic strontium isotopes in river sediments

“…2). Studied samples were classified into three categories based on the lithological composition of corresponding watersheds (Table S1), following the approach described in Bayon et al (2020a): 1) Rivers draining crystalline basements dominated by igneous, metamorphic and/or siliciclastic sedimentary rocks (n=19). This category includes those rivers flowing through diverse Precambrian regions in Africa (Niger, Congo, and their tributaries, Betsiboka), northern South America (Orinoco, and its right-bank tributaries), Canada (Churchill), Australia (Murchinson, Gascoyne, Fortescue, East Alligator), Fennoscandia (Kymiijoki, Ume, Lule), and a small river from the Hercynian Armorican Massif in France (Elorn); 2) Rivers draining volcanic provinces (n=3) in Northern Ireland (River Maine, Six Mile Water) and Indonesia (Brantas); 3) Rivers draining basins of mixed lithology dominated by sedimentary rocks, which include most of the major river systems investigated in this study (n=39).…”

“…In contrast with soil studies, which allow detailed reconstruction of weathering processes along soil depth profiles, but are also strongly affected by local effects related to differences in bedrock composition and primary mineral assemblages, the geochemical composition of fine-grained J o u r n a l P r e -p r o o f Journal Pre-proof sediments deposited near the mouth of rivers provides insights into weathering processes at the catchment scale. Such a global approach is ideally suited for investigating the mechanisms controlling the distribution of geochemical tracers in sediments and for ‗calibrating' weathering proxies against modern basin parameters (e.g., Bayon et al, 2016;Bayon et al, 2018;Bayon et al, 2020a;Bindeman et al, 2019). Two different grain-size fractions were investigated: 1) the finest clay-rich (<4µm) detrital material, which includes secondary clays formed in soils during silicate weathering processes; and 2) the silt-size (4-63µm) terrigenous component, corresponding to a mixture of both primary and weathered minerals transported in the suspended particulate load (Bindeman et al, 2019).…”