Assessment of Seawater Nd Isotope Signatures Extracted From Foraminiferal Shells and Authigenic Phases From Volcanogenic Sediments of the Adriatic Sea

Abstract The neodymium isotope signatures ( ε Nd) of the authigenic fraction have been extensively used to reconstruct past seawater ε Nd and hydrological circulation. Among the various methods, sequential extraction of hydrogenic ferromanganese oxyhydroxides from bulk sediments represents a rapid and straightforward approach that may potentially induce artifacts due to the potential release of non‐seawater‐derived Nd during the extraction procedure. Here we investigated different methods for extracting past seawater Nd isotope compositions from a core collected in the Adriatic Sea whose tephra layers have been previously well documented. We analyzed ε Nd in planktonic foraminifera samples and in non‐decarbonated sediment leachates obtained with three solutions commonly used in the context of the Mediterranean Sea: (a) 0.02 M hydroxylamine hydrochloride (HH) solution, (b) 1N HCl, and (b) a 25% (v/v) acetic acid (AA). Our results show that (a) the foraminiferal ε Nd remains unaffected by the diagenesis of tephra content; (b) all three methods indicate significantly more radiogenic ε Nd values in tephra levels (up to 1.5 ε Nd unit), which is attributed to tephra dissolution accounting for 2.7% of extracted Nd; (c) of the three leaching methods applied to samples with low tephra content, hydroxylamine hydrochloride (HH) yields ε Nd values that are more consistent with those obtained on planktonic foraminifera; (d) the ε Nd values of planktonic foraminifera in core MD90‐917 remained constant indicating that the Adriatic deep water primarily reflects the local Nd isotope composition over the last 20 kyr.