Nd isotopes as a paleocirculation tracer: consideration of sedimentary context and assessment through a north-south Atlantic transect

Abstract

Authigenic neodymium (Nd) isotope ratios have been extensively used over the past two decades to reconstruct past changes in the global ocean circulation. Nevertheless, their effectiveness as a water mass tracer is currently strongly disputed based on the potential for modification by non-conservative processes. This study compares authigenic Nd isotope records along a meridional transect in the Atlantic from sediment core sites chosen with careful consideration of sedimentary conditions that are conducive to preserving the original seawater signal, including high carbonate/low lithogenic detritus content, oxic conditions, and exclusion of sites near continental margins and regions impacted by local effects. Records over the last 1.5 Myr shows north-to-south patterns such that, with few exceptions, at each point in time, at nearly every location, Nd isotope ratios are lower toward the north, and higher toward the south. The same relationship is observed in seawater studies of the present-day Atlantic Meridional Overturning Circulation (AMOC) . Moreover, each site shows interglacial-glacial cyclicity, indicating systemic changes through time impacting the entire Atlantic basin. In addition, a comparison of Nd isotope records over the last glacial cycle reaffirms these same meridional trends at high temporal resolution. The coherence of the present-day circulation dynamics and past meridional Nd isotope patterns provide strong evidence that such transects can be used to track temporal changes in the water mass structure and circulation in the Atlantic, if the deep-sea cores are carefully chosen for favorable conditions to preserve the original seawater Nd isotope signature.