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.
Highlights
Since at least the beginning of the Neogene, the sedimentation at the Guianese margin of the Demerara Plateau has been controlled by large contour currents (Loncke et al, 2016; Tallobre et al, 2016; Fanget et al, 2020)
This descriptive approach was already used by Tallobre et al (2019), indicating that maximum abundance of mature dark green grains in core IG-KSF-11 generally occurred during MIS 2 and in MIS 4 (∼60–70 kyr BP), in sediment layers characterised by high contents of glauconitic grains indicative of enhanced winnowing conditions (Figure 3B)
Our geochemical investigation of glauconitic grains recovered from a contourite sediment sequence at the Demerara margin suggests that authigenic glauconite can faithfully record the Nd isotopic composition of ambient bottom water masses
Summary
Since at least the beginning of the Neogene, the sedimentation at the Guianese margin of the Demerara Plateau has been controlled by large contour currents (Loncke et al, 2016; Tallobre et al, 2016; Fanget et al, 2020). At this location, slope bottom deposits are subject to recurrent winnowing that favours the appearance and mineralogical maturation of green glauconitic grains at the seawater–sediment interface (Tallobre et al, 2019). While Nd isotopes have proven helpful for reconstructing past ocean circulation patterns over various geological timescales, their application to contourite sediment accumulations at ocean margins, such as the Demerara Rise, can be skewed by sedimentary processes related to strong bottom current activity, such as winnowing and reworking
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
