Modeling Neodymium Isotopes in the Ocean Component of the Community Earth System Model (CESM1)

Abstract

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.

Influence of Elevated Nd Fluxes on the Northern Nd Isotope End Member of the Atlantic During the Early Holocene

“…Part of the smaller mismatch found here (5.3 pmol/kg) can be attributed to the exclusion of seawater data from the uppermost 250 m in our approach. Even when considering all water depths including near‐surface water, the misfits remain smaller than listed by Rempfer et al (2011) and Gu et al (2019) (6.5 pmol/kg; Table S3). Both previous studies reported particularly large deviations in the top 2,000 m of the water column, due to too low Nd concentrations at these depths in the models compared to modern seawater.…”

“…The steady-state AMOC strength simulated in the Bern3D model under preindustrial boundary conditions is 17.7 Sv (Sv: Sverdrup = 10 6 m 3 /s), which agrees well with modern observations (e.g., Frajka-Williams et al, 2019), however, with NADW slightly shallower in the model than observed in the modern ocean ( Figure 4). As reported by previous studies (Gu et al, 2019;Rempfer et al, 2011), the MAEs of the Nd concentration and ε Nd cannot be minimized with the same set of parameters ( Figure S4). Therefore, we choose the following parameters in between the two minima as a compromise: a benthic Nd flux of 3.3 × 10 9 g/a corresponding to 5.4 pmol/(cm 2 a), a scavenging efficiency parameterized by the ratio of particulate to dissolved Nd concentration of 0.0014 (see Rempfer et al, 2011), and a riverine Nd scaling of 3.5 (corresponding to an increase of all riverine Nd fluxes by a factor of 3.5), yielding a mean Nd residence time of 690 years.…”

“…Both studies also reported MAEs, facilitating a cautious comparison of model performances. The adjustments to the Nd-module of the Bern3D model (as described in section 2.1) improved the model-data agreement for the Nd concentration compared to Rempfer et al (2011) as well as to Gu et al (2019), who reported MAEs for their control runs of 9 and 8.1 pmol/kg, respectively. Part of the smaller mismatch found here (5.3 pmol/kg) can be attributed to the exclusion of seawater data from the uppermost 250 m in our approach.…”

“…In order to improve the understanding of the Nd isotope response to AMOC variations, ε Nd has been simulated in a handful of models (Arsouze et al, 2007, 2009; Friedrich et al, 2014; Gu et al, 2019; Rempfer et al, 2011; Siddall et al, 2008). Two different approaches were used in these studies.…”

“…Two different approaches were used in these studies. First, a simplified implementation of ε Nd as a passive tracer that only changes at the continental margins (Arsouze et al, 2007; Friedrich et al, 2014) and, second, a comprehensive implementation of the Nd cycle with all its known sources and sinks (Arsouze et al, 2009; Gu et al, 2019; Rempfer et al, 2011; Siddall et al, 2008). All these studies confirmed the quasi‐conservative behavior of ε Nd on interbasin scale and thus its sensitivity to changes in water mass provenance.…”