Factors controlling distribution of Rare Earth Elements in Aragonitic Skeletons of Deep-Sea Scleractinian Corals explored with Laser Ablation Inductively Coupled Plasma Mass Spectrometry

The systematic variability in the properties of the rare earth elements (REEs) imparts distinct signatures on their abundance in different water masses, making this group of elements a potential tracer of ocean circulation. A record of seawater REE abundances in the ocean interior may exist in the aragonitic skeletons of deep-sea scleractinian corals (DSC), which incorporate REE in trace quantities. However, factors controlling the incorporation and distribution of REEs in DSC remain poorly understood. Using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), we characterized and quantified spatially resolved distribution of REEs in modern and fossil DSC skeletons from several oceanic provinces. REEs concentrations vary between 0.1 and 100 ppb, with greater abundances for lighter REE and were in agreement with published data obtained by dissolution of DSC aragonite. Spatial distribution of REEs within DSC septa appears to be similar to that previously reported for uranium – lower [REE] in the vicinity of the coral centers of calcification, increasing through the septa outwards. Seawater-like shale-normalized REE patterns are found in most of the studied modern corals. One notable exception was found in DSC specimens from an active segment of a mid-ocean ridge (the Reykjanes Ridge, south of Iceland). The REE concentrations in these specimens were an order of magnitude higher than in most examined DSCs and showed characteristic basaltic patterns, likely capturing the ephemeral signature of hydrothermal input from the ridge. Fossil corals specimens exhibit greater variability in REE abundance and the shapes of their shale normalized patterns than modern corals. Factors driving the observed variability, including micro-Fe-Mn oxides and diagenetic alternation (e.g. borings) on micro-scale have been addressed, to better constrain the criteria for potential usage of DSC corals as paleo-archives of the ocean REE distribution in the past.