The authigenic and detrital neodymium isotopic composition in the modern Mediterranean Sea: implications for paleocirculation proxy

Abstract

Neodymium isotopic composition (143Nd/144Nd or εNd) has been used as a tracer of water mass provenance in the modern and the past oceans. However, recent studies have questioned its reliability because some phases of detrital sediments are reactive and a partial dissolution of these phases could alter the initial authigenic signals through porewater Nd. We examined this possibility using core-top sediments from the Mediterranean Sea, an ideal region, because the water circulation and the εNd distribution of seawater and detrital fractions are relatively well constrained. Furthermore, smectite, one of the most reactive minerals, is abundant in the easternmost basin due to Nile sediment input. We obtained paired authigenic (leachates and foraminiferal tests) and detrital εNd signatures from core-top sediments along the zonal transect from 2°W to 33°E. The authigenic εNd values generally agreed with the bottom water signals, except in the easternmost Levantine and the Aegean Seas, where they were higher than the water values. These areas are characterised by high smectite abundance and enclosed nature, making local Nd source influence more visible. The authigenic εNd values in these areas were stable at about −5 to −4, while the corresponding detrital values ranged between −10 and −4. The compilation of modern seawater and detrital εNd values shows that seawater εNd covaries with detrital signals at 0–200 m water depth, and that the correlation is insignificant at depths greater than 1500 m. These results argue against a ubiquitous control of bottom water εNd by the benthic flux in the present Mediterranean Sea. Sensitivity tests suggest that the ɛNd values of Mediterranean Sea water are affected by the isotopic composition of Atlantic inflow, deep-water formation zones and local benthic sources, as well as by the size of the Atlantic inflow flux, deep-water formation and zonal water exchange. These results support the use of Nd isotopic composition as a tracer of water mass provenance in the Mediterranean Sea although a combination of isotopic data from various basins is required to distinguish the influence of these factors.