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Neodymium (Nd) isotopic composition (eNd) is an important tracer for water mass mixing and the reconstruction of past ocean circulation. To allow for a direct model‐data comparison, we have implemented Nd isotopes in the ocean component of the Community Earth System Model (CESM1.3). The model is able to capture the major features of the observed modern distribution of both eNd and Nd concentrations. Our model provides a useful tool for the interpretation of eNd reconstructions. For example, we show that in an idealized North Atlantic freshwater hosing experiment, eNd changes in the Atlantic are documenting primarily the changes in water mass mixing and are hardly affected by the concomitant and large changes in the marine biological productivity and organic matter fluxes. However, the hosing experiment also shows that the end‐member changes due to the change of ocean circulation can influence the interpretation of eNd in the Atlantic, depending on the location. The implementation of Nd, together with other existing tracers, such as δ¹⁸O, ²³¹Pa/²³⁰Th, δ¹³C, and radiocarbon in the same model, can improve our understanding of past ocean circulation significantly.

Neodymium (Nd) isotopic composition (εNd) is an important tracer for water mass mixing and the reconstruction of past ocean circulation. To allow for a direct model‐data comparison, we have implemented Nd isotopes in the ocean component of the Community Earth System Model (CESM1.3). The model is able to capture the major features of the observed modern distribution of both εNd and Nd concentrations. Our model provides a useful tool for the interpretation of εNd reconstructions. For example, we show that in an idealized North Atlantic freshwater hosing experiment, εNd changes in the Atlantic are documenting primarily the changes in water mass mixing and are hardly affected by the concomitant and large changes in the marine biological productivity and organic matter fluxes. However, the hosing experiment also shows that the end‐member changes due to the change of ocean circulation can influence the interpretation of εNd in the Atlantic, depending on the location. The implementation of Nd, together with other existing tracers, such as δ¹⁸O, ²³¹Pa/²³⁰Th, δ¹³C, and radiocarbon in the same model, can improve our understanding of past ocean circulation significantly.

Neodymium (Nd) isotopes are an important geochemical tool to trace the present and past water mass mixing as well as continental inputs. The distribution of Nd concentrations in open ocean surface waters (0�100 m) is generally assumed to be controlled by lateral mixing of Nd from coastal surface currents and by removal through reversible particle scavenging. However, using 228Ra activity as an indicator of coastal water mass influence, surface water Nd concentration data available on key oceanic transects as a whole do not support the above scenario. From a global compilation of available data, we find that more stratified regions are generally associated with low surface Nd concentrations. This implies that upper ocean vertical supply may be an as yet neglected primary factor in determining the basin-scale variations of surface water Nd concentrations. Similar to the mechanism of nutrients supply, it is likely that stratification inhibits vertical supply of Nd from the subsurface thermocline waters and thus the magnitude of Nd flux to the surface layer. Consistently, the estimated required input flux of Nd to the surface layer to maintain the observed concentrations could be nearly two orders of magnitudes larger than riverine/dust flux, and also larger than the model-based estimation on shelf-derived coastal flux. In addition, preliminary results from modeling experiments reveal that the input from shallow boundary sources, riverine input, and release from dust are actually not the primary factors controlling Nd concentrations most notably in the Pacific and Southern Ocean surface waters.

International audience; Using a cost-efficient climate model, the effect of changes in overturning circulation on neodymium isotopic composition, Nd , is systematically examined for the first time. Idealized sequences of abrupt climate changes are induced by the application of periodic freshwater fluxes to the North Atlantic (NA) and the Southern Ocean (SO), thus mainly affecting either the formation of North Atlantic Deep Water (NADW) or Antarctic Bottom Water (AABW). Variations in Nd reflect weakening and strengthening of the formation of NADW and AABW, changes in Nd of end-members are relatively small. Relationships between Nd and the strength of NADW or AABW are more pronounced for AABW than for NADW. Atlantic patterns of variations in Nd systematically differ between NA and SO experiments. Additionally, the signature of changes in Nd in the Atlantic and the Pacific is alike in NA but opposite in SO experiments. Discrimination between NA and SO experiments is therefore possible based on the Atlantic pattern of variations in Nd and the contrariwise behavior of Nd in the Atlantic and the Pacific. In further experiments we examined the effect of variations in magnitudes of particle export fluxes. Within the examined range, and although settling particles represent the only sink of Nd, their effects on Nd are relatively small. Our results confirm the large potential of Nd as a paleocirculation tracer but also indicate its limitations of quantitative reconstructions of changes in the Atlantic Meridional Ocean Circulation.

Antarctic Intermediate Water (AAIW) plays important roles in the global climate system and the global ocean nutrient and carbon cycles. However, it is unclear how AAIW responds to global climate changes. In particular, neodymium isotopic composition (εNd) reconstructions from different locations from the tropical Atlantic have led to a debate on the relationship between northward penetration of AAIW into the tropical Atlantic and the Atlantic meridional overturning circulation (AMOC) variability during the last deglaciation. We resolve this controversy by studying the transient oceanic evolution during the last deglaciation using a neodymium-enabled ocean model. Our results suggest a coherent response of AAIW and AMOC: when AMOC weakens, the northward penetration and transport of AAIW decrease while its depth and thickness increase. Our study highlights that as part of the return flow of the North Atlantic Deep Water,the northward penetration of AAIW in the Atlantic is determined predominately by AMOC intensity. Moreover, the inconsistency among different tropical Atlantic εNd reconstructions is reconciled by considering their corresponding core locations and depths, which were in fluenced by different water masses in the past. The very radiogenic water from the bottom of the Gulf of Mexico and the Caribbean Sea, which was previously overlooked in the interpretations of deglacial εNd variability, can be transported to shallow layers during active AMOC and modulates εNd in the tropical Atlantic. Changes in the AAIW core depth must also be considered. Thus, interpretation of εNd reconstructions from the tropical Atlantic is more complicated than suggested in previous studies.

We use the Bern3D model of intermediate complexity to examine the marine cycle of isotopes 231Pa and 230Th and the relationship between the particle-bound ratio Pap/Thpand changes in the formation of the North Atlantic Deep Water (NADW). Model parameters describing reversible scavenging of isotopes by organic particles, opal, calcite and resuspended sediments were systematically varied and alternative sink parametrisations explored. It proves difficult to simultaneously achieve a good agreement with observations of dissolved and particle-associated concentrations of 231Pa and 230Th (Pad, Thd, Pap, Thp) as well as the particle-bound ratio Pap/Thpwithin the classical concept of reversible scavenging alone. Agreement between simulated and observed Pad, Thdand estimates of mean ocean residence times is improved by taking into account simplified representations of additional sinks at the sea floor (bottom scavenging) and at continental boundaries (boundary scavenging). We also find improved agreement between model and data by increasing lateral advection, in particular for Pad. These results point to the importance of sink processes that act in addition to reversible scavenging to shape the steady state distribution of 231Pa and, to a lesser degree, of 230Th. In transient experiments in which the strength of the Atlantic meridional overturning circulation (AMOC) is periodically turned on and off, we find a strong statistical relationship between variations in AMOC strength and Pap/Thpat great depths in the Northwest Atlantic region. These conclusions are robust across the range of sink parametrisations, that are consistent with estimates in the mean ocean residence time of 231Pa and 230Th. Our results indicate that the relationship between Pap/Thpand AMOC-strength may not be fundamentally affected by uncertainties in sink processes, at least on the large spatial and temporal scale considered here, and support the idea that changes in Pap/Thpin sediments of the Northwest Atlantic are indicative of changes in AMOC strength. Taking into account our simplified approach, our results indicate that the relationship between Pap/Thpand AMOC-strength in the deep Northwest Atlantic is not affected by boundary scavenging or bottom scavenging. Our results thus support the idea that changes in Pap/Thpin sediments of the Northwest Atlantic are indicative of changes in AMOC strength.