New Perspectives on the 143Nd/144Nd Palaeoceanographic Tracer on Foraminifera: The State-of-the-Art Frontiers of Analytical Methods

Abstract

The 143Nd/144Nd isotopic composition of foraminifera represents one of the most prominent palaeoceanographic proxies enabling tracing of particular water masses due to the short residence time of Nd in seawater and large-scale provinciality resulting from its lithogenic origin. Despite its wide use, the applicability of foraminiferal Nd isotopic systematics as a palaeoceanographic tracer is largely hampered by the amount of material needed for precise analyses. Here, we analyzed for the first time foraminiferal samples of a combined weight of only ∼300–500 μg (ca. 300 pg to 1 ng Nd loaded). We followed a methodology that utilizes Nd separation from the matrix and provides precise 143Nd/144Nd determinations by thermal ionization mass spectrometry with sensitive 1013 Ω amplifiers. Two different techniques analyzing NdO+ and Nd+ (single/double filament configurations) were employed and tested to assess the limits both in terms of in-run precision and accuracy concluding that the former method yielded an external reproducibility on the determined 143Nd/144Nd better than ∼90 ppm (2RSD; relative standard deviation) for 100 pg Nd loads. The overall veracity was tested by multiple analyses of Holocene and Miocene foraminifera from the Mediterranean region. Pooled data were obtained in agreement with previous studies attesting that multiple analyses of even very low Nd loads (≤1 ng) can lead to sufficient precision. As revealed by our newly obtained data, individual foraminiferal species show a significant naturally induced 143Nd/144Nd variability that needs to be considered when pursuing Nd isotopic analysis.