Optimisation of the marine Nd isotope scheme in the ocean component of the FAMOUS general circulation model
Abstract. The neodymium (Nd) isotope composition (εNd) of seawater can be used to trace large-scale ocean circulation features. Yet, due to the elusive nature of marine Nd cycling, particularly in discerning non-conservative particle-seawater interactions, there remains considerable uncertainty surrounding a complete description of marine Nd budgets. Here, we present an optimisation of the Nd isotope scheme within the fast coupled atmosphere-ocean general circulation model (FAMOUS), using a statistical emulator to explore the parametric uncertainty and optimal combinations of three key model inputs relating to: (1) the efficiency of reversible scavenging, (2) the magnitude of the seafloor benthic flux, and (3) a riverine source scaling, accounting for release of Nd from river sourced particulate material. Furthermore, a suite of sensitivity tests provide insight on the regional mobilisation and spatial extent (i.e., testing a margin-constrained versus a seafloor-wide benthic flux) of certain reactive sediment components. In the calibrated scheme, the global marine Nd inventory totals 4.27 × 1012 g and has a mean residence time of 727 years. Atlantic Nd isotope distributions are represented well, and the weak sensitivity of North Atlantic Deep Water to highly unradiogenic sedimentary sources implies an abyssal benthic flux is of secondary importance in determining the water mass εNd properties under the modern vigorous circulation condition. On the other hand, Nd isotope distributions in the North Pacific are 3 to 4 εNd-units too unradiogenic compared to water measurements, and our simulations indicate that a spatially uniform flux of bulk sediment εNd does not sufficiently capture the mobile sediment components interacting with seawater. Our results of sensitivity tests suggest that there are distinct regional differences in how modern seawater acquires its εNd signal, in part relating to the complex interplay of Nd addition and water advection.
Suzanne Robinson et al.
Status: final response (author comments only)
- RC1: 'Comment on egusphere-2022-937', Anonymous Referee #1, 21 Dec 2022
- RC2: 'Comment on egusphere-2022-937', Jianghui Du, 17 Feb 2023
- RC3: 'Comment on egusphere-2022-937', Anonymous Referee #3, 17 Feb 2023
Suzanne Robinson et al.
Suzanne Robinson et al.
Viewed (geographical distribution)
Review Robinson et al, Optimisation of the marine Nd isotope scheme in the ocean component of the FAMOUS general circulation model This paper present an optimization procedure of the Neodymium scheme implemented in the FAMOUS ocean general circulation model. It provides very interesting insights on Nd cycle and sensitivity to scavenging and sedimentary and river sources. The approach is performed with a statistical emulator that is convenient for the scientific goal attempted, and derives analysis that are in accordance with the procedure. However the approach has two drawbacks which limit the scope of the article:First it does not include the Aeolian dust input in the optimization procedure. This source is also subject to large uncertainties in its parameterization, in particular in the value of the solubility in dust where the values âârange between 2 and 50%. This source has an impact on a much wider spatial extension than the rivers. It also substantially modifies the surface concentration and isotopic Nd values ââwhich have an important weight in the optimization procedure. This arbitrary choice in the method is prejudicial and not justified in the document, and potentially tends to overestimate the role of rivers (Lines 477- 490: improvement in surface Nd concentration and isotopic composition is obtained with enhanced river inputs. However, dust deposition, as it strongly impacts surface modelling results, could moderate this conclusion if it was also included in the optimization procedure)
Secondly North Atlantic simulated isotopic composition is too radiogenic compared to the observation (Lines 545-550 ; 565-568; 690-707; 840-845). Radiogenic values simulated in the North Atlantic is more probably a consequence of a too strong exchange around Island which is highly radiogenic and influences (too much) the whole north Atlantic ocean Nd isotopic composition distribution.
Sensitivity tests in the north Atlantic are a good idea (section 4), but the strategy is not satisfying. It has an interest to test the flux around the Labrador Sea, but it generates little sensitivity. Tests around Island should have generated a higher sensitivity; the signal is highly radiogenic and advected through the subpolar circulation. This strategy lead to erroneous conclusions upon the sensitivity of north Atlantic water to the local sources, that are repeated all along the paper.
For these reasons, I recommend a major revision of this paper before publication.
Comparison of modelled inventory with inventory derived from observation (line 499): What is the uncertainty on the inventory derived from the observations, in order to justify that modeled inventory are underestimating or overestimating it (especially REF-CONC2)