气候变化领域集成服务门户

The neodymium isotope proxy has become a valuable tool for the reconstruction of past ocean water mass provenance and mixing. For its accurate application, knowledge about the origin and preservation of Nd in sedimentary archives is crucial. Recently, concerns have emerged regarding the applicability of neodymium isotopes as a conservative palaeo water mass tracer, given potential Nd fluxes from sediments into bottom waters (Abbott et al., 2015a) and inferred relabelling of ocean waters by settling detrital material (Roberts and Piotrowski, 2015). Consequently, a decoupling of water mass provenance and proxy variations may arise. We investigate the mobility of Nd around extreme detrital sedimentation events such as glacial ice rafting pulses and turbidite deposition in the Northeast Atlantic. The constructed records from sediment leachates span extreme Nd isotope variations including volcanic (epsilon Nd similar to 0) and Laurentian (epsilon Nd similar to -27) sources. We find that Nd was released into pore waters from reactive detritus inside some detrital layers during early diagenesis, thereby overprinting any archived bottom water Nd signature and precluding the reconstruction of past water mass provenance during the affected time intervals. However, we do not observe any definite indication of diffusive vertical migration of Nd into adjacent layers. Furthermore, bottom water Nd isotope signatures were not modified to a measurable degree by any potential benthic flux of Nd during the deposition of these detrital sediment layers. Consequently, the Nd isotope composition of the pelagic glacial Northeast Atlantic water masses were resilient to such episodic large detrital fluxes.

Apart from extreme local sedimentation events, we confirm the presence of detritally overprinted deep waters north of 47 degrees N during the peak glacial from comparison of Northeast Atlantic depth transects. We furthermore suggest that the sensitivity of deep waters to this overprinting effect increased during periods of reduced Atlantic Meridional Overturning Circulation and elevated ice rafting.

Overall, our study demonstrates that a thorough evaluation of the proportion of Nd originating from physical water mass advection versus in situ chemical inputs is crucial for the reliable application of Nd isotopes as a water mass tracer. (C) 2018 Elsevier Ltd. All rights reserved.