Extraction of seawater-derived neodymium from different phases of deep sea sediments by selective leaching

Abstract:

In order to deduce reliable information about the interaction of the oceans with the climate system as a whole in the past, the reconstruction of water mass circulation is crucial. The analysis of seawater-derived neodymium isotopes (¹⁴³Nd/¹⁴⁴Nd, expressed as ε_Nd) in marine sediments provides a unique proxy for deep water provenance in particular in the Atlantic [1]. The ε_Nd signature and thus the mixing proportion of the local bottom water masses is archived in authigenic phases in the sediment. Obtaining seawater ε_Nd from authigenic accretions bound to foraminiferal tests has lately become the preferred since most reliable method [2]. Attempts have also been made to extract the Nd-rich authigenic metal fraction by leaching it off the bulk sediment and thereby use this proxy with less effort, in the highest possible resolution and in sediments where foraminifera are not sufficiently present. However, often other sedimentary components are also leached in the process and contaminate the extracted Nd [3,4].

In this project several core-top and older sediments across the Atlantic have been leached in ten consecutive steps with either dilute buffered acetic acid or an acid-reductive solution. The leachates were analysed on their elemental and Nd isotope compositions, as well as rare earth element (REE) distributions. By graduating the total leaching procedure into smaller stages the results display which processes take place in the course of sediment leaching in the laboratory and which components of the sediment are most reactive. Thus, they help to better evaluate the quality of sediment leaches for ε_Nd analysis. Clearly, organic calcite acts as a fast reacting buffer and at the point where its amount is sufficiently reduced the leaching of other components commences and the Nd concentration peaks. Corruption of the extracted ε_Nd signal by non-authigenic sources in many cases occured early in the leaching sequence, indicating that only very cautious leaching can reliably extract an authigenic ε_Nd signal from diverse sedimentary environments.

References:

[1] Crocket, K. C. et al. (2011), Geology 39, 515-518

[2] Tachikawa, K. et al. (2014), Quat. Sci. Rev. 88, 1-13

[3] Elmore, A. C. et al. (2011), G³ Vol. 12/9

[4] Wilson, D. J. et al. (2013), GCA 109, 197-221