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 <500years 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 964.5ng/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.
Highlights
IntroductionStruve et al / Chemical Geology 453 (2017) 146–168 limited, such as the mid-depth North Atlantic (e.g., Frank et al, 2004; Robinson et al, 2007) or the mid-depth Southern Ocean (e.g., Burke et al, 2010; Thiagarajan et al, 2013; Margolin et al, 2014)
The Nd isotopic composition of seawater at all three depth profiles in the Drake Passage falls within a narrow εNd range between −7.6 ± 0.2 and − 8.6 ± 0.2 (Table 1, Fig. 5; (εNd = ((143Nd / 144Ndsample) / (143Nd / 144 NdCHUR) − 1) × 10,000; CHUR: chondritic uniform reservoir, see Jacobsen and Wasserburg, 1980), indicating a relatively homogenous water column signature across the modern Drake Passage
We presented new data that expand on the existing modern calibration of Cold-water corals (CWCs) aragonite as an archive for seawater Nd isotopes
Summary
Struve et al / Chemical Geology 453 (2017) 146–168 limited, such as the mid-depth North Atlantic (e.g., Frank et al, 2004; Robinson et al, 2007) or the mid-depth Southern Ocean (e.g., Burke et al, 2010; Thiagarajan et al, 2013; Margolin et al, 2014) Their skeletons can be dated accurately by uranium-series disequilibrium (e.g., Edwards et al, 1987; Cheng et al, 2000; Robinson et al, 2006), and growth rates from 0.5 to 2 mm/year in Desmophyllum dianthus (Risk et al, 2002; Adkins et al, 2004) and up to 26 mm/year in Lophelia pertusa (Gass and Roberts, 2006; recently synonymised to Desmophyllum pertusum, Addamo et al 2016) can provide for high resolution geochemical archives (e.g., Adkins et al, 1998; Copard et al, 2012; Montero-Serrano et al, 2013; Wilson et al, 2014; Chen et al, 2015; Hines et al, 2015; Lee et al, 2017).
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