Journal Article

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

Gao, Guohui, Colin, Christophe, Pinna‐Jamme, Rosella, Haurine, Frederic, Siani, Giuseppe, Sepulcre, Sophie, Dapoigny, Arnaud

2025

Overview

The neodymium isotope signatures (εNd) of the authigenic fraction have been extensively used to reconstruct past seawater εNd 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 εNd 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 εNd remains unaffected by the diagenesis of tephra content; (b) all three methods indicate significantly more radiogenic εNd values in tephra levels (up to 1.5 εNd 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 εNd values that are more consistent with those obtained on planktonic foraminifera; (d) the εNd 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. Plain Language Summary Neodymium isotope signatures (εNd) from the authigenic fraction of sediments are commonly used to reconstruct past ocean circulation and seawater composition. A common method involves extracting ferromanganese oxyhydroxides from bulk sediments, which is quick and straightforward but may introduce contamination from non‐seawater sources of neodymium during the process. In this study, we tested different methods for extracting εNd from a sediment core in the Adriatic Sea, known for its tephra layers, by comparing data from foraminifera and sediment leachates using three solutions: hydroxylamine hydrochloride (HH), HCl, and acetic acid (AA). We found that: (a) foraminiferal εNd is unaffected by tephra dissolution; (b) all methods showed more radiogenic εNd in tephra layers due to tephra dissolution; (c) the HH solution gave εNd values closest to foraminifera in low‐tephra samples; (d) foraminiferal εNd has remained stable over the last 20,000 years, indicating Adriatic deep water reflects local neodymium isotope composition. Key Points The HCl and AA leaching processes dissolve the tephras and bias the extracted authigenic εNd values εNd of HH leachate closely matches that of foraminifera and past seawater of the Adriatic Sea εNd of the Adriatic Deep Water stay stable during the last 20,000 years due to vertical water mixing and local lithogenic inputs

• Publisher: John Wiley & Sons, Inc,AGU and the Geochemical Society
• Subject: Acetic acid / Adriatic sea / Chemical analysis / Chemical composition / Continental interfaces, environment / Deep water / Diagenesis / Dissolving / ferromanganese oxyhydroxides / Foraminifera / Hydroxylamines / Isotope composition / Leachates / Leaching / Neodymium / Neodymium isotopes / Ocean circulation / Ocean, Atmosphere / rare earth elements / Sciences of the Universe / Seawater / Seawater composition / Sediment / Sediments / Tephra / Water analysis / Water circulation