Neodymium isotopes and concentrations in aragonitic scleractinian cold-water coral skeletons - Modern calibration and evaluation of palaeo-applications

Abstract

Cold-water corals (CWCs) are unique archives of mid-depth ocean chemistry and have been used successfully to reconstruct the neodymium (Nd) isotopic composition of seawater from a number of species. High and variable Nd concentrations in fossil corals however pose the question as to how Nd is incorporated into their skeletons. We here present new results on modern specimens of Desmophyllum dianthus, Balanophyllia malouinensis, and Flabellum curvatum, collected from the Drake Passage, and Madrepora oculata, collected from the North Atlantic. All modern individuals were either collected alive or uranium-series dated to be < 500 years old for comparison with local surface sediments and seawater profiles. Modern coral Nd isotopic compositions generally agree with ambient seawater values, which in turn are consistent with previously published seawater analyses, supporting small vertical and lateral Nd isotope gradients in modern Drake Passage waters. Two Balanophyllia malouinensis specimens collected live however deviate by up to 0.6 epsilon units from ambient seawater. We therefore recommend that this species should be treated with caution for the reconstruction of past seawater Nd isotopic compositions. Seventy fossil Drake Passage CWCs were furthermore analysed for their Nd concentrations, revealing a large range from 7.3 to 964.5 ng/g. Samples of the species D. dianthus and Caryophyllia spp. show minor covariation of Nd with 232Th content, utilised to monitor contaminant phases in cleaned coral aragonite. Strong covariations between Nd and Th concentrations are however observed in the species B. malouinensis and G. antarctica. In order to better constrain the source and nature of Nd in the cleaned aragonitic skeletons, a subset of sixteen corals was investigated for its rare earth element (REE) content, as well as major and trace element geochemistry. Our new data provide supporting evidence that the applied cleaning protocol efficiently removes contaminant lithogenic and ferromanganese oxyhydroxide phases. Mass balance calculations and seawater-like REE patterns rule out lithogenic and ferromanganese oxyhydroxide phases as a major contributor to elevated Nd concentrations in coral aragonite. Based on mass balance considerations, geochemical evidence, and previously published independent work by solid-state nuclear magnetic resonance (NMR) spectroscopy, we suggest authigenic phosphate phases as a significant carrier of skeletal Nd. Such a carrier phase could explain sporadic appearance of high Nd concentrations in corals and would be coupled with seawater-derived Nd isotopic compositions, lending further confidence to the application of Nd isotopes as a water mass proxy in CWCs

Sea-ice control on deglacial lower cell circulation changes recorded by Drake Passage deep-sea corals

“…Both D. dianthus and Caryophyllia spp. reliably record dissolved Nd isotopic compositions of ambient seawater in the Drake Passage (van de Flierdt et al, 2010;Struve et al, 2017), providing strong confidence in coral-based reconstructions in this setting.…”

“…(n = 1) and Paraconotrochus antarcticus (n = 3). In the present day, UCDW and LCDW have fairly uniform Nd isotopic compositions at the coral sampling locations (εNd = -8.2 ± 0.5, 2SD) (Struve et al, 2017) that reflect the balance of Atlantic (εNd ~-13) and Pacific (εNd ~-4) waters in the Southern Ocean (van de Flierdt et al, 2016). The homogeneity of Nd isotopes in the modern Southern Ocean reflects the smaller Nd isotope gradient between these water masses where they enter the Southern Ocean, as well as strong diapycnal mixing within the ACC (Watson et al, 2013), although this situation may have differed in past climate states.…”

“…However, a comprehensive interpretation of such changes in deep water chemistry requires independent constraints on water mass sourcing. Neodymium (Nd) isotopes have the potential to provide insights into water mass sources to the Southern Ocean because Atlantic and Pacific-derived waters have contrasting Nd isotopic compositions (Carter et al, 2012;van de Flierdt et al, 2016;Struve et al, 2017) and past compositions can be reliably recovered from deep-sea corals (van de Flierdt et al, 2010;Struve et al, 2017). To date, direct evidence for deglacial Nd isotopic compositions in the Drake Passage comes from only a single coral at UCDW depths (Robinson and van de Flierdt, 2009), with no data constraining the composition of the glacial lower cell within the ACC.…”

Expedition 382 Preliminary Report: Iceberg Alley and Subantarctic Ice and Ocean Dynamics

“…Where possible, past seawater Nd isotope compositions will also be extracted from discrete ferromanganese nodules or carbonate constituents to ensure their seawater origin. The Nd isotope composition in the modern water column is well resolved in the Drake Passage (Stichel et al, 2012;Struve et al, 2017) and the Pacific Rickli et al, 2014) and Atlantic (Stichel et al, 2012) sectors of the Southern Ocean.…”

Efficient Extraction of Past Seawater Pb and Nd Isotope Signatures From Southern Ocean Sediments

“…Cold-water corals represent a good alternative because their age can be well constrained. Nevertheless, only recently cold-water corals were shown to be a robust archive both for extracting seawater derived Pb (Lee et al, 2014(Lee et al, , 2017Wilson et al, 2017) and Nd (Colin et al, 2010;Struve et al, 2017;van de Flierdt et al, 2006;Wilson et al, 2014). However, fossil cold-water corals are usually not found in situ in abyssal water depths below the aragonite or calcite compensation depths.…”