Ice-rafted debris as a source of non-conservative behaviour for the εNd palaeotracer: insights from a simple model

Abstract

Neodymium isotopic composition (εNd) has enjoyed widespread use as a palaeotracer, principally because it behaves quasiconservatively in the modern ocean. However, recent bottom water εNd reconstructions from the eastern North Atlantic are difficult to interpret under assumptions of conservative behaviour. The observation that this apparent departure from conservative behaviour increases with enhanced ice-rafted debris (IRD) fluxes has resulted in the suggestion that IRD leads to the overprinting of bottom water εNd through reversible scavenging. In this study, a simple water column model successfully reproduces εNd reconstructions from the eastern North Atlantic at the Last Glacial Maximum and Heinrich Stadial 1, and demonstrates that the changes in scavenging intensity required for good model-data fit is in good agreement with changes in the observed IRD flux. Although uncertainties in model parameters preclude a more definitive conclusion, the results indicate that the suggestion of IRD as a source of non-conservative behaviour in the εNd tracer is reasonable and that further research into the fundamental chemistry underlying the marine neodymium cycle is necessary to increase confidence in assumptions of conservative εNd behaviour in the past.

Large-scale Pacific-sourced intrusion into the glacial South Atlantic

“…Nevertheless, contamination of the record of ODP 1094 evoked from volcanic material is possible due to the vicinity to the volcanically active Bouvet region 39 . Another possible explanation for the different behavior of ODP 1094 is local relabeling of the water column and bottom water through reversible scavenging by ice-rafted debris, Patagonia-sourced dust or volcanic ash particles 36,40,41 . This mechanism should, however, impact all three studied sediment cores similarly, which is not the case.…”

Active Nordic Seas deep-water formation during the last glacial maximum

“…However, since a similar signature to the Nordic Seas is not seen in the NE Atlantic (Figs. 2 and 3), non-conservative processes (changing seawater εNd via input of Nd sourced from local sediment) may mask the εNd record of export to the NE Atlantic 24,34 . This dense water could also have exited the Nordic Seas via the Denmark Strait, as has been previously suggested 18,35 ; however, no suitable εNd records have yet been obtained from the Denmark Strait.…”