ORCID Profile
0000-0002-3838-5976
Current Organisation
UNSW Sydney
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Publisher: Copernicus GmbH
Date: 12-04-2017
DOI: 10.5194/BG-2017-122
Abstract: Abstract. The ocean's nutrient cycles are important for the carbon balance of the climate system and for shaping the ocean's distribution of dissolved elements. Dissolved iron (dFe) is a key limiting micronutrient, but iron scavenging is observationally poorly constrained leading to large uncertainties in the external sources of iron and hence in the state of the marine iron cycle. Here we build a model of the ocean's coupled phosphorus, silicon, and iron cycles embedded in a data-assimilated steady-state global ocean circulation. The model includes the redissolution of scavenged iron, parameterization of subgrid topography, and small, large, and diatom phytoplankton functional classes. Phytoplankton concentrations are implicitly represented in the parameterization of biological nutrient utilization through an equilibrium logistic model. Our coupled nutrient model thus carries only three nutrient tracers whose three-dimensional steady-state distributions can be found efficiently using a Newton solver. The very efficient numerics allow us to use the model in inverse mode to objectively constrain many biogeochemical parameters by minimizing the mismatch between modelled and observed nutrient and phytoplankton concentrations. We consider a family of possible solutions corresponding to a wide range of external iron source strengths. Iron source and sink parameters cannot jointly be optimized because of local compensation between regeneration, recycling, and scavenging. All optimized solutions have a similar mismatch with the observed nutrient concentrations and very similar large-scale dFe distributions. However, the relative contributions of aeolian, sedimentary, and hydrothermal iron to the total dFe concentration differ widely depending on the sources. Both the magnitude and pattern of carbon and opal export are well constrained with global values of (10.3 ± 0.4) Pg C yr−1 and (171. ± 3.) Tmol Si yr−1. We diagnose the carbon and opal export supported by aeolian, sedimentary, and hydrothermal iron. The geographic patterns of the export supported by each iron type are well constrained across the family of solutions. Sedimentary-iron supported export is important in shelf and large-scale upwelling regions, while hydrothermal iron contributes to export mostly in the Southern Ocean. The globally integrated export supported by a given iron type varies systematically with the fractional contribution of its source to the total iron source. Aeolian iron is most efficient in supporting export in the sense that its fractional contribution to export exceeds its fractional contribution to the total source by as much as ~ 30 % for carbon and ~ 20 % for opal export. Conversely, sedimentary and hydrothermal iron are less efficient with a fractional export that is less than their fractional sources. For the same fractional contribution to the total source, hydrothermal iron is less efficient than sedimentary iron for supporting carbon export but about equally efficient for supporting opal export.
Publisher: American Geophysical Union (AGU)
Date: 08-2016
DOI: 10.1002/2016JC011821
Publisher: Copernicus GmbH
Date: 21-09-2017
Abstract: Abstract. The ocean's nutrient cycles are important for the carbon balance of the climate system and for shaping the ocean's distribution of dissolved elements. Dissolved iron (dFe) is a key limiting micronutrient, but iron scavenging is observationally poorly constrained, leading to large uncertainties in the external sources of iron and hence in the state of the marine iron cycle. Here we build a steady-state model of the ocean's coupled phosphorus, silicon, and iron cycles embedded in a data-assimilated steady-state global ocean circulation. The model includes the redissolution of scavenged iron, parameterization of subgrid topography, and small, large, and diatom phytoplankton functional classes. Phytoplankton concentrations are implicitly represented in the parameterization of biological nutrient utilization through an equilibrium logistic model. Our formulation thus has only three coupled nutrient tracers, the three-dimensional distributions of which are found using a Newton solver. The very efficient numerics allow us to use the model in inverse mode to objectively constrain many biogeochemical parameters by minimizing the mismatch between modeled and observed nutrient and phytoplankton concentrations. Iron source and sink parameters cannot jointly be optimized because of local compensation between regeneration, recycling, and scavenging. We therefore consider a family of possible state estimates corresponding to a wide range of external iron source strengths. All state estimates have a similar mismatch with the observed nutrient concentrations and very similar large-scale dFe distributions. However, the relative contributions of aeolian, sedimentary, and hydrothermal iron to the total dFe concentration differ widely depending on the sources. Both the magnitude and pattern of the phosphorus and opal exports are well constrained, with global values of 8. 1 ± 0. 3 Tmol P yr−1 (or, in carbon units, 10. 3 ± 0. 4 Pg C yr−1) and 171. ± 3. Tmol Si yr−1. We diagnose the phosphorus and opal exports supported by aeolian, sedimentary, and hydrothermal iron. The geographic patterns of the export supported by each iron type are well constrained across the family of state estimates. Sedimentary-iron-supported export is important in shelf and large-scale upwelling regions, while hydrothermal iron contributes to export mostly in the Southern Ocean. The fraction of the global export supported by a given iron type varies systematically with its fractional contribution to the total iron source. Aeolian iron is most efficient in supporting export in the sense that its fractional contribution to export exceeds its fractional contribution to the total source. Per source-injected molecule, aeolian iron supports 3. 1 ± 0. 8 times more phosphorus export and 2. 0 ± 0. 5 times more opal export than the other iron types. Conversely, per injected molecule, sedimentary and hydrothermal iron support 2. 3 ± 0. 6 and 4. ± 2. times less phosphorus export, and 1. 9 ± 0. 5 and 2. ± 1. times less opal export than the other iron types.
Publisher: Copernicus GmbH
Date: 17-09-2018
Publisher: Copernicus GmbH
Date: 16-10-2018
Publisher: Copernicus GmbH
Date: 06-03-2023
DOI: 10.5194/EGUSPHERE-2023-363
Abstract: Abstract. Accurate predictive modelling of the ocean's global carbon and oxygen cycles is challenging because of uncertainties in both biogeochemistry and ocean circulation. Advances over the last decade have made parameter optimization feasible, allowing models to better match observed biogeochemical fields. However, does fitting a biogeochemical model to observed tracers using a circulation with known biases robustly capture the inner workings of the biological pump? Here we embed a mechanistic model of the ocean's coupled nutrient, carbon, and oxygen cycles into two circulations for the current climate. To assess the effects of biases, one circulation (ACCESS-M) is derived from a climate model and the other from data assimilation of observations (OCIM2). We find that parameter optimization compensates for circulation biases at the expense of altering how the biological pump operates. Tracer observations constrain pump strength and regenerated inventories for both circulations, but ACCESS-M export production optimizes to twice that of OCIM2 to compensate for ACCESS-M having lower sequestration efficiencies driven by less efficient particle transfer and shorter residence times. Idealized simulations forcing complete Southern Ocean nutrient utilization show that the response of the optimized system is sensitive to the embedding circulation. In ACCESS-M, Southern Ocean nutrient and DIC trapping is partially short-circuited by unrealistically deep mixed layers. For both circulations, intense Southern Ocean production deoxygenates Southern-Ocean-sourced deep waters, muting the imprint of circulation biases on oxygen. Our findings highlight that the biological pump's plumbing needs careful assessment to predict the biogeochemical response to environmental changes, even when optimally matching observations.
Publisher: Copernicus GmbH
Date: 24-11-2021
DOI: 10.5194/GMD-2021-338
Abstract: Abstract. Spatially distant sources of neodymium (Nd) to the ocean that carry different isotopic signatures (εNd) have been shown to trace out major water masses, and have thus been extensively used to study large-scale features of the ocean circulation both past and current. While the global marine Nd cycle is qualitatively well understood, a complete quantitative determination of all its components and mechanisms, such as the magnitude of its sources and the paradoxical conservative behavior of εNd, remains elusive. To make sense of the increasing collection of observational Nd and εNd data, we develop the global neodymium ocean model (GNOM) v1, the first inverse model of the global marine biogeochemical cycle of Nd. The GNOM is embedded in a data-constrained steady-state circulation that affords spectacular computational efficiency, which we leverage to estimate biogeochemical parameters via systematic objective optimization. Owing to its matrix representation, the GNOM model is additionally amenable to novel diagnostics that allow us to investigate open questions about the Nd cycle with unprecedented accuracy. The GNOM is open-source and freely accessible, is written in Julia, and its code is easily understandable and modifiable for further developments and experiments.
Publisher: American Geophysical Union (AGU)
Date: 10-2016
DOI: 10.1002/2016GB005418
Publisher: The Open Journal
Date: 05-01-2022
DOI: 10.21105/JOSS.03814
Publisher: American Geophysical Union (AGU)
Date: 06-2021
DOI: 10.1029/2020GB006863
Abstract: We develop novel locally defined diagnostics for the efficiency of the ocean's biological pump by tracing carbon throughout its lifetime in the ocean from gas injection to outgassing and counting the number of passages through the soft‐tissue and carbonate pumps. These diagnostics reveal that the biological pump's key controls on atmospheric pCO 2 are the mean number of lifetime pump passages per dissolved inorganic carbon (DIC) molecule at the surface and the mean aphotic sequestration time of regenerated DIC. We apply our diagnostics to an observationally constrained carbon‐cycle model that features spatially varying stoichiometric ratios and is embedded in a data‐assimilated global ocean circulation. We find that for the present‐day ocean an average of 44 ± 4% of DIC in a given water parcel makes at least one lifetime passage through the soft tissue pump, and about 4% makes at least one passage through the carbonate pump. The global mean number of lifetime pump passages per molecule, including the fraction with zero passages, is and for the soft‐tissue and carbonate pumps. Using idealized perturbations to sweep out a sequence of states ranging from zero biological activity ( ppmv) to complete surface nutrient depletion ( ppmv), we find that fractional changes in are dominated by fractional changes in the number of soft‐tissue pump passages. At complete surface nutrient depletion, the mean fraction of DIC that has at least one lifetime passage through the soft‐tissue pump increases to 69 ± 5% with .
Publisher: Copernicus GmbH
Date: 26-07-2023
Abstract: Abstract. Accurate predictive modeling of the ocean's global carbon and oxygen cycles is challenging because of uncertainties in both biogeochemistry and ocean circulation. Advances over the last decade have made parameter optimization feasible, allowing models to better match observed biogeochemical fields. However, does fitting a biogeochemical model to observed tracers using a circulation with known biases robustly capture the inner workings of the biological pump? Here we embed a mechanistic model of the ocean's coupled nutrient, carbon, and oxygen cycles into two circulations for the current climate. To assess the effects of biases, one circulation (ACCESS-M) is derived from a climate model and the other from data assimilation of observations (OCIM2). We find that parameter optimization compensates for circulation biases at the expense of altering how the biological pump operates. Tracer observations constrain pump strength and regenerated inventories for both circulations, but ACCESS-M export production optimizes to twice that of OCIM2 to compensate for ACCESS-M having lower sequestration efficiencies driven by less efficient particle transfer and shorter residence times. Idealized simulations forcing complete Southern Ocean nutrient utilization show that the response of the optimized system is sensitive to the embedding circulation. In ACCESS-M, Southern Ocean nutrient and dissolved inorganic carbon (DIC) trapping is partially short circuited by unrealistically deep mixed layers. For both circulations, intense Southern Ocean production deoxygenates Southern-Ocean-sourced deep waters, muting the imprint of circulation biases on oxygen. Our findings highlight that the biological pump's plumbing needs careful assessment to predict the biogeochemical response to ecological changes, even when optimally matching observations.
Publisher: Cambridge University Press (CUP)
Date: 28-03-2018
Publisher: Copernicus GmbH
Date: 03-12-2018
Abstract: Abstract. Iron fertilization is explored by tracking dissolved iron (DFe) through its life cycle from injection by aeolian, sedimentary, and hydrothermal sources (birth) to burial in the sediments (death). We develop new diagnostic equations that count iron and phosphate regenerations with each passage through the biological pump and partition the ocean's DFe concentration according to the number of its past or future regenerations. We apply these diagnostics to a family of data-constrained estimates of the iron cycle with sources σtot in the range 1.9–41 Gmol yr−1. We find that for states with σtot 7 Gmol yr−1, 50 % or more of the DFe inventory has not been regenerated in the past and 85 % or more will not be regenerated in the future. The globally averaged mean number of past or future regenerations scales with the bulk iron lifetime τ∼σtot-1 and has a range of 0.05–2.2 for past and 0.01–1.4 for future regenerations. Memory of birth location fades rapidly with each regeneration, and DFe regenerated more than approximately five times is found in a pattern shaped by Southern Ocean nutrient trapping. We quantify the intrinsic fertilization efficiency of the unperturbed system at any point r in the ocean as the global export production resulting from the DFe at r per iron molecule. We show that this efficiency is closely related to the mean number of future regenerations that the iron will experience. At the surface, the intrinsic fertilization efficiency has a global mean in the range 0.7–7 mol P (mmol Fe)−1 across our family of state estimates and is largest in the central tropical Pacific, with the Southern Ocean having comparable importance only for high-iron-source scenarios.
Publisher: Zenodo
Date: 2022
Publisher: Wiley
Date: 13-11-2022
Publisher: Copernicus GmbH
Date: 26-05-2017
Publisher: Copernicus GmbH
Date: 16-08-2018
DOI: 10.5194/BG-2018-379
Abstract: Abstract. Iron fertilization is explored by tracking dissolved iron (DFe) through its life cycle from injection by external sources (birth) to burial in the sediments (death). We develop new diagnostic equations that count iron and phosphate regenerations with each passage through the biological pump and partition the ocean's DFe concentration according to the number of its past and future regenerations. We apply these diagnostics to a family of data-constrained estimates of the iron cycle with sources σ in the range 1.9–41 Gmol yr−1. We find that for states with σ 7 Gmol yr−1, 50 % of the DFe inventory has not been regenerated in the past and 85 % will not be regenerated in the future. The globally averaged mean number of past and future regenerations scale with the bulk iron lifetime τ ~ σ−1tot and have ranges of 0.05–2.2 and 0.01–1.4, respectively. Memory of birth location fades rapidly with each regeneration, and DFe regenerated more than ~ 5 times is found in a pattern shaped by Southern Ocean nutrient trapping. We quantify the natural fertilization efficiency at any point r in the ocean as the global export production resulting from the DFe at r, per iron molecule. We show that this efficiency is closely related to the mean number of future regenerations that the iron will experience. At the surface, the natural fertilization efficiency has a global mean in the range 0.7–7 (mol P) (mmol Fe)−1 across our family of state estimates and is largest in the central tropical Pacific, with the Southern Ocean having comparable importance only for high iron-source scenarios.
Publisher: Springer Science and Business Media LLC
Date: 17-10-2022
Publisher: Copernicus GmbH
Date: 07-07-2017
Publisher: Copernicus GmbH
Date: 16-06-2022
Abstract: Abstract. Spatially distant sources of neodymium (Nd) to the ocean that carry different isotopic signatures (εNd) have been shown to trace out major water masses and have thus been extensively used to study large-scale features of the ocean circulation both past and current. While the global marine Nd cycle is qualitatively well understood, a complete quantitative determination of all its components and mechanisms, such as the magnitude of its sources and the paradoxical conservative behavior of εNd, remains elusive. To make sense of the increasing collection of observational Nd and εNd data, in this model description paper we present and describe the Global Neodymium Ocean Model (GNOM) v1.0, the first inverse model of the global marine biogeochemical cycle of Nd. The GNOM is embedded in a data-constrained steady-state circulation that affords spectacular computational efficiency, which we leverage to perform systematic objective optimization, allowing us to make preliminary estimates of biogeochemical parameters. Owing to its matrix representation, the GNOM model is additionally amenable to novel diagnostics that allow us to investigate open questions about the Nd cycle with unprecedented accuracy. This model is open-source and freely accessible, is written in Julia, and its code is easily understandable and modifiable for further community developments, refinements, and experiments.
Publisher: Zenodo
Date: 2019
Publisher: Copernicus GmbH
Date: 16-10-2018
Publisher: Zenodo
Date: 2019
Publisher: Zenodo
Date: 2018
No related grants have been discovered for Benoît Pasquier.