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). Each contourite sequence is characterised by high abundance of glauconitic grains (especially in the moat area), but also in a moderately to strongly discontinuous sediment accumulation associated with relatively low sedimentation rates. In contrast with nearby shallower settings along the French Guiana margin that have experienced continuous fine-grained hemipelagic sedimentation during the Late Quaternary (Häggi et al., 2017; Zhang et al., 2017; Crivellari et al., 2018), the Demerara contourite system, as other contourite deposits (e.g., Giresse and Wiewióra, 2001; Giresse, 2008), is characterised by deposition of discontinuous sediment records, which prevents their use for paleoceanographic reconstructions. Additionally, intense bottom current activity at Demerara and other contourite settings at continental margins can also result in substantial sediment redistribution, with potential effect on the reliability of commonly used paleoceanographic archives in marine sediment records.

The purpose of this study is to show the potential interest of using glauconitic grains in such discontinuous sedimentary records for paleoceanographic reconstructions based on radiogenic neodymium isotopes. Neodymium (Nd) isotopic ratios are commonly measured in biogenic material, such as fish teeth or foraminifer shells, or in the associated Fe-Mn oxyhydroxide fractions of the sediment, as tracers of bottom water circulation (e.g., Frank, 2002; Goldstein and Hemming, 2003). In the ocean, each water mass is tagged with a distinctive Nd isotopic composition (typically expressed using the epsilon notation ε_Nd), which reflects, to a large extent, that of the continental masses surrounding their source region (e.g., Tachikawa et al., 2017; Robinson et al., 2021). While Nd isotopes have proven particularly 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.

In oceanic environments, glauconitic grains are the result of transformation and neoformation processes taking place at the water–sediment interface. Various granular supports (mudclasts, faecal pellets, and foraminifera infillings) include inherited 1:1 interstratified clays (or Te-Oc; i.e., clay minerals consisting of one tetrahedral sheet and one octahedral sheet, such as kaolinite) that become gradually replaced by 2:1 clays (Te-Oc-Te) dominated first by smectite, and then glauconite. These processes are accompanied by aluminium and silicium depletion in inherited minerals, together with significant enrichment of iron and potassium in neoformed minerals (e.g., Odin and Matter, 1981). While K in neoformed clays is supplied by ambient seawater, the source of Fe is most likely derived from reactive phases of the terrigenous sediment, such as Fe-oxide and oxyhydroxide phases (Odin and Matter, 1981; Odin and Fullagar, 1988; Giresse and Wiewióra, 2001; Wiewióra et al., 2001; Giresse, 2008; Banerjee et al., 2016). In all cases, these thermodynamic reactions operate at slow kinetic rates, especially regarding the acquisition of seawater-derived K, thereby requiring prolonged residence time at the water–sediment interface, presumably over thousands of years (e.g., Giresse, 1975), and thus reduced sedimentation rates. Earlier studies suggested that such conditions were mostly met at continental platforms, in particular at their outer edges, far from any continental source of terrigenous material. More recently, however, several investigations of contouritic deposits along continental margins revealed important glauconitic accumulations at water depths between ∼2000 and 3000 m (Giresse et al., 1998; Giresse and Wiewióra, 2001; Giresse, 2008; Stow and Faugères, 2008; Faugères and Mulder, 2011). Similar to continental shelf analogues, the occurrence of abundant glauconitic grains in contouritic systems indicates prolonged exposure at the water–sediment interface, reflecting here recurring sediment reworking related to the winnowing action of contouritic currents. Due to Fe incorporation, glauconite grains typically display higher specific gravity (between ∼2.4 and 2.9 g/cm³; Yadav and Sharma, 1992) than the average surrounding sediment (∼1.7 g/cm³; Tenzer and Gladkikh, 2014). As a consequence, authigenic grains of glauconite are less likely to be remobilised and dispersed by currents, compared to empty foraminiferal tests. Additionally, because glauconite formation occurs over relatively long timescales at the seawater–sediment interface, unlike other archives of bottom water chemistry (e.g., fish teeth and sedimentary Fe-Mn oxyhydroxide phases), glauconite grains could possibly integrate the signature of bottom water masses over prolonged periods of time (Giresse and Wiewióra, 2001; Giresse, 2008), which, while preventing their use in high-resolution studies, would provide an effective means for yielding reliable average estimates on past ε_Nd signatures of bottom water masses. To date, however, this hypothesis has never been tested.

To address this issue, we have investigated the Nd isotopic composition of glauconite grains recovered from contourite sediment deposits at Demerara Rise; at a location where ε_Nd signatures of modern water masses are already well characterised (Huang et al., 2014). This ε_Nd comparison extends to more conventional paleoceanographic archives (uncleaned foraminifers and leached sedimentary Fe-oxyhydroxide phases), being also complemented by the additional use of rare earth elements (REE) and major element ratios, such as K₂O/Al₂O₃ and Fe₂O/Al₂O₃. Since the different phases of glauconitisation include the transformation and neoformation of phyllosilicate minerals, a careful investigation is also required to assess the degree to which Nd isotopes in glauconite grains may reflect the detrital ε_Nd signature of inherited clay minerals within the sediment. Following proxy calibration using core-top sediments, preliminary interpretations will be made of a set of Nd isotopic data for buried glauconitic grains, in the light of the changing paleoceanographic context since the last glacial period.

This study focuses on an area of contourite accumulation located at Demerara Rise, along a regional slope failure headscarp, where the bottom current influence is the strongest, with flow velocity up to ∼30 cm/s (Tallobre et al., 2016; Figure 1). At this location (∼2400 m water depth), sedimentary dynamics is controlled by uneven accelerations of contour currents that define condensed intervals of deposits, or even lead to significant stratigraphic gaps in the sediment record (Loncke et al., 2016; Tallobre et al., 2016, 2019). Under such conditions, the application of both oxygen isotopes and radiocarbon chronostratigraphic methods has proven generally unreliable (Tallobre et al., 2019), yielding δ¹⁸O depth profiles that cannot be directly tuned to nearby continuous records of hemipelagic sediments (Huang et al., 2014; Häggi et al., 2017).

The present-day distribution of radiogenic Nd isotopes along the water column at Demerara Rise identifies the presence of distinct water masses (Figure 2), including the West North Atlantic Central waters (WNACW; ε_Nd ∼−13), from surface to ∼500 m water depth, the Antarctic Intermediate Water (AAIW; ε_Nd ∼−10.5), between ∼ 500 and 1200 m depth, the North Atlantic Deep Water (NADW; ε_Nd from ∼−12 to −11.5), from ∼ 1200 and 4000 m depth, and finally, the Antarctic Bottom Water (AABW; ε_Nd ∼−10.5) below 4000 m depth (Huang et al., 2014). In previous investigations, the application of Nd isotope measurements to various paleoceanographic archives retrieved from Demerara Rise (Huang et al., 2014) and other locations in the Atlantic have proven particularly useful to reconstruct past ocean circulation patterns, indicating prominent changes in the Atlantic Meridional Overturning Circulation (AMOC) over glacial–interglacial timescales, with impact on heat transport and carbon storage in the surface and deep ocean, respectively (e.g., Rutberg et al., 2000; Piotrowski et al., 2004, 2005; Pahnke et al., 2008; Böhm et al., 2015; Howe et al., 2016; Lippold et al., 2016; Pöppelmeier et al., 2020). A survey of the spatial distribution of Nd isotopes in the Atlantic during both the recent Holocene and the last glacial periods indicated relatively unradiogenic NADW-like ε_Nd values (∼−13) below 1500 m water depth at Demerara Rise (Howe et al., 2016). Further south in the Atlantic Ocean, the Nd isotopic composition of NADW becomes progressively more radiogenic, reflecting gradual dilution of northern-sourced waters with overlying and underlying Antarctic water masses (Howe et al., 2016). As a consequence, the unradiogenic “tongue” of NADW can be traced as the water mass flows southward in the South Atlantic at water depths between ∼2000 and 4000 m (e.g., von Blanckenburg, 1999; Howe et al., 2016). During the Last Glacial Maximum (LGM), the deep Atlantic water column displayed a greater influence of southern-sourced waters consistent with a reduced flux of NADW to the Southern Atlantic (Rutberg et al., 2000; Howe et al., 2016; Pöppelmeier et al., 2020). At the Demerara margin, Nd isotopic measurements were previously acquired on uncleaned foraminifera from a nearby sediment core collected at 947 m depth, bathed at present-day by AAIW (Huang et al., 2014; see core 46CDH on Figure 1). The obtained ε_Nd record for the past 25,000 years indicated similar Nd isotopic compositions during the Holocene and the LGM (ε_Nd ∼−10), while revealing pronounced ε_Nd excursions towards unradiogenic values (between −11 and −12) during the short-lived North Atlantic cold periods of the Heinrich Stadial 1 (HS1) and the Younger Dryas (YD), interpreted as reflecting strong reduction of the AMOC leading to reduced influence of northward flowing AAIW in the equatorial Atlantic (Huang et al., 2014).

This study was conducted on sediment core IG-KSF-11 (07°51.85N, 052°29.25W; 2370 m water depth), recovered off French Guiana during the IGUANES cruise (R/V L’Atalante; Loncke L., 2013¹). The sedimentary records retrieved from this dynamic sedimentary environment are characterised by the occurrence of abundant glauconite grains (Tallobre et al., 2019). The sediment is dominated by homogeneous grey-green silty mud with alternation of discrete sand layers (Tallobre et al., 2016, 2019). Core IG-KSF-11 was specifically chosen for this study because foraminifera δ¹⁸O measurements suggest a continuous and coherent stratigraphy for the upper 0–80 cm sediment interval that is considered in this study (Figure 3A), which covers approximately the last 60,000 years BP (Tallobre et al., 2019). However, similar to the other sediment records retrieved from the Demerara contourite system, IG-KSF-11 suffers from inherent chronostratigraphic uncertainties due to the presence of recurrent winnowing and other erosional processes, resulting in hiatuses and frequent radiocarbon age inversions in the lower part of the core (from ∼2.5 to 6 m depth; Tallobre et al., 2019). As a consequence, in this study, the analysed sediment layers were only assigned a period of deposition (i.e., marine isotope stage 1, MIS 2…), rather than a precise stratigraphic age (Tables 1, 2).

For this study, a series of glauconite grains and planktonic foraminifers were collected from six distinct sediment layers (0, 8, 20, 29, 40, and 70 cm) representative of specific time intervals (Holocene, MIS 2, MIS 3) in core IG-KSF-11. Evidence for continuous stratigraphy in this section of core IG-KSF-11 suggests that collected glauconite grains were formed in situ. There is no evidence for gravity-induced sediment deposition in this part of the margin. At the Demerara margin, glauconite grains mostly occur as internal fillings of foraminifera. The grains were isolated under the microscope and were then classified into three main colour categories: (1) beige/grey, (2) light green/green, and (3) dark green (Figure 4). Representative samples of each category were selected for SEM observation, using a SEM HITACHI S-4500 (University of Perpignan). Microprobe chemical microanalyses were performed on hemispherical sections obtained after grains were broken. Scanning was performed on both 100 and 2 μm² areas in order to obtain representative semi-quantitative analyses of bulk grains and newly formed nano-crystallites.

For geochemical and Nd isotope analyses of glauconite grains, a few grains of ∼300–400 μm size were used. Between 30 and 90 mg of powdered sample were digested on hotplate (120°C; 1 day) with distilled 6 M HCl, following a procedure adapted from Yadav and Sharma (1992). This method achieves efficient preferential dissolution of glauconite, yielding almost quantitative recovery of K, while leaving behind more resistant Al- and Si-rich silicate minerals. This implies that major element abundances and ratios (e.g., K₂O/Al₂O₃ and Fe₂O₃/Al₂O₃; see section “Discussion”) reported here for glauconitic grains may slightly differ from values that would otherwise be obtained for bulk glauconite digested using HF-based mineralisation methods. For the analyses of foraminiferal tests, about 10–15 mg of mixed planktonic foraminifera assemblages was prepared using the methodology described in Tachikawa et al. (2014), being dissolved using dropwise addition of ultrapure 1 M acetic acid in order to reduce any potential leaching of associated silicate detritus.

In addition, a total of nine bulk sediment samples from the same core interval were processed for isolating Fe-Mn oxyhydroxide phases and detrital clays. Bulk sediments were first treated using a sequential leaching procedure (Bayon et al., 2002) that successively removes biogenic carbonates (with 5% v/v acetic acid), Fe-Mn oxyhydroxide phases (with a mixed 15% acetic acid—0.5 M hydroxylamine hydrochloride solution) and organic matter (with 15% v/v hydrogen peroxide). For Fe-Mn oxyhydroxide phases, the resulting leachates were filtered through 0.45 μm PTFE Nalgene filters, evaporated to dryness, and finally redissolved using ultrapure HNO₃ prior to subsequent elemental analyses. Clay-rich detrital fractions (<4 μm) were separated from the residual detrital material using low-speed centrifugation (Bayon et al., 2015) and further digested by alkaline fusion (Bayon et al., 2009).

Major and trace element analyses for 6 M HCl digests of glauconite grains were determined at the Pôle Spectrométrie Océan (PSO, Brest) with a Thermo Scientific Element XR sector field ICP-MS, using the following masses (²⁴Mg, ²⁷Al, ³¹P, ³⁹K, ⁴⁴Ca, ⁵⁵Mn, and ⁵⁷Fe) acquired in medium mass resolution. Yttrium and REE concentrations were also measured in low resolution mode. Elemental abundances were calculated using the Tm addition method, following the procedures described in Barrat et al. (1996, 2012). The internal precision for all measurements was better than 2%. The precision and accuracy of our data were assessed by analysing a series of silicate rock certified reference materials (CRM) having various chemical compositions (AN-G, AGV-1, BCR-1, DNC-1, DR-N, G-2, and WS-E), digested using conventional HF-HCl-HNO₃ method. The results obtained for these reference materials were in full agreement with reference values from the literature (typically < 8%), with precisions generally better than 10% (RSD; n = 3 for all of the CRMs). Neodymium was isolated using conventional ion chromatography, and isotopic measurements were performed at the PSO-Brest, using a Thermo Scientific Neptune multi-collector ICPMS. Nd isotopic compositions were determined using sample-standard bracketing, by analysing an in-house standard solution (SPEX-Nd) every two samples, yielding a mean value of 0.511687 ± 0.000007 (2 SD, n = 14). Mass bias corrections were made with the exponential law, using ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219. Analyses of the JNdi-1 standard solution during the course of this study gave ¹⁴³Nd/¹⁴⁴Nd of 0.512114 ± 0.000010 (2 SD, n = 8), in full agreement with the reference value of 0.512115 (Tanaka et al., 2000), and corresponding to an external reproducibility of ∼ ± 0.2 ε (2 SD). Note that epsilon Nd values [ε_Nd = (¹⁴³Nd/¹⁴⁴Nd_SAMPLE/¹⁴³Nd/¹⁴⁴Nd_CHUR − 1) × 10⁴] were calculated using the chondritic (CHUR) ¹⁴³Nd/¹⁴⁴Nd value of 0.512638 (Jacobsen and Wasserburg, 1980).

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. The acquisition of ε_Nd seawater signatures in glauconitic grains appears to be controlled by the presence of LREE-enriched phases, possibly corresponding to dispersed authigenic phosphate phases intertwined within the neoformed clay mineral sheets. The absence of a strong detrital control on measured Nd isotopic compositions in glauconite grains is further inferred from the evidence for a progressive loss of REE with increasing degrees of glauconitisation; a process that also results in gradual enrichments in Fe and K. Overall, our new results suggest that the application of Nd isotopes to glauconite grains could serve as useful proxies for paleoceanographic reconstructions at continental margins, wherever intense winnowing and/or erosional processes may prevent the use of other more conventional archives of past seawater ε_Nd compositions, such as uncleaned foraminifera or leached Fe-Mn oxyhydroxide fractions. In particular, the combined use of Nd isotopes (as tracers of water mass) and various elemental ratios, such as Fe₂O₃/Al₂O₃ and K₂O/Al₂O₃ (as tracers for the degree of glauconitisation), could provide complementary information of both the source and strength of past bottom water circulation. An important requisite prior to establishing Nd isotopes in glauconite grains as robust paleoceanographic archives will be to further demonstrate the preliminary findings reported in this study, by investigating additional sites from the Demerara Rise or from any other contourite systems where bottom water masses and local detrital sediment may display contrasted Nd isotopic compositions.

The original contributions presented in the study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author/s.

PG and GB contributed to the conception and design of the study and wrote the manuscript with contributions from all authors. CT contributed to the first visual analysis and counting of glauconitic grains in core IKSF11. PG and CT prepared the glauconitic grains and conducted SEM observations and analyses. GB performed all REE and Nd isotopic compositions. LL was the head scientist of the IGUANES cruise allowing the acquisition of IKSF11 core and contributed to the regional understanding of the study area. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.