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

Abstract

Contourite sediment accumulations at continental margins are related to strong bottom water circulation, where intense winnowing can result in neoformation of authigenic grains of glauconite at the seafloor. In this study, we investigated whether such glauconite grains could faithfully record ambient bottom-water neodymium (Nd) isotopic compositions, and hence be used as paleoceanographic archives. To this purpose, we measured Nd isotopic compositions (εNd) in a series of glauconitic grains, foraminiferal assemblages, leached Fe-Mn oxyhydroxide phases, and detrital clays separated from a contourite sediment record at the Demerara slope off French Guiana (IG-KSF-11; 2370 m water depth), at a location where the present-day εNd distribution along the water column is well characterised. We show that the εNd composition of core-top glauconite grains (−12.0 ± 0.5) agrees with the expected NADW-like seawater signature at the same location and water depth (−11.6 ± 0.3), while departing from measured εNd values for corresponding detrital clays (−11.3 ± 0.2), foraminiferal (−10.9 ± 0.2), and Fe-Mn oxyhydroxide fractions (−9.2 ± 0.2). This finding indicates that glauconitic grains at this particular location are probably best suited for paleoceanographic reconstructions than foraminifera and leached Fe-oxyhydroxide fractions, which appear to be influenced by sediment redistribution and the presence of terrestrial continental Fe-oxides, respectively. Using rare earth elements (REE), we tentatively propose that the acquisition of seawater Nd isotopic signatures by glauconite is controlled by the presence of authigenic REE-bearing phosphate-rich phases intertwined within clay mineral sheets, while confirming previous findings that the process of glauconitisation results in the progressive loss of REE within glauconitic grains. Preliminary paleoceanographic implications suggest strengthened bottom-water circulation of the glacial analogue of NADW at this particular location and water depth, with a εNd signature (between −10.8 and −11.5) similar to that of modern NADW.

The contrasting origins of glauconite in the shallow marine environment highlight this mineral as a marker of paleoenvironmental conditions

“…Previous work [Amorosi andCentineo, 2000, Tribovillard et al, 2021] showed that the glauconite of the Boulonnais chalk responded to the typical logic of formation: very slow sedimentation rate in the "Chalk Sea" in connection with the great transgression of the Upper Cretaceous, prolonged exchanges with seawater at the sediment-water interface, formation of a synsedimentary glauconite rich in iron and potassium, showing relatively large grains and an absence of particle size sorting. The long duration of the exchanges between the seawater and the sediment-water interface could have been caused by the reduced flux of particles or by winnowing induced by currents affecting the seabed [Giresse, 1985, Giresse et al, 2021, see the works of and references therein]. In contrast, the glauconite of the Aptian-Albian sandstones and that of most of the Jurassic deposits studied here show smaller, well sorted grains, which are therefore not synsedimentary but reworked from their source zones.…”

Quaternary Glauconitization on Gulf of Guinea, Glauconite Factory: Overview of and New Data on Tropical Atlantic Continental Shelves and Deep Slopes

“…The temperatures reported for glauconitization from low-latitude sea bottoms of the continental shelf are generally about or slightly below 15 • C. For the first time, glauconitization proceedings were documented near 3 • C at water depths beyond 2100 m on the Ivory Coast-Ghana Ridge and Guyana margins [7,8].…”

“…However, these winnowing conditions can sometimes induce more or less marked stratigraphic gaps, which disrupt stratigraphic analysis. Despite their sometimes incomplete character on the scale of the last isotopic stages, the contouritic accumulations offer a glauconitic archive, often allowing the geochemical analysis of paleooceanic changes [7]. The vertical fluctuations in green-grain concentrations correspond to microsequences that are linked to the pulses of energy of the bottom water.…”

Exploring Icehouse Cyclicity Pattern and Seawater Dynamics on an Ancient Carbonate Platform With Nd Isotopes (Carboniferous, Southern Kazakhstan)

“…Another important property of Nd, which is particularly relevant to paleoceanographic studies, is that Nd hosted in solid phases is remarkably immobile during diagenesis (Banner, 2004; Luo et al., 2021; McLennan, 1989). Authigenic Nd in sediments is primarily stored in biogenic phosphates, banded‐iron formations, carbonates, Fe‐Mn nodules, and glauconites (e.g., Alexander et al., 2009; Bayon et al., 2004; Giresse et al., 2021; Zheng et al., 2013). While biogenic phosphates (conodonts, fish teeth) were initially regarded as the most suitable material for extraction of Nd signatures of seawater (e.g., Dopieralska et al., 2006, 2012; Grandjean et al., 1987; Holmden et al., 1996; Martin & Scher, 2004), in more recent years, carbonate fossils (benthic and planktonic foraminifera, aragonitic deep‐sea corals) and calcite micrite were used as archives of the past Nd isotope composition (e.g., Belka et al., 2021; Elmore et al., 2011; Haley et al., 2005; L. F. Robinson et al., 2014; Theiling et al., 2017; van de Flierdt et al., 2010; Wei et al., 2019).…”