Tropical Dominance of N<sub>2</sub> Fixation in the North Atlantic Ocean

Marconi, Dario; Sigman, Daniel M.; Casciotti, Karen L.; Campbell, Ethan; Weigand, M. Alexandra; Fawcett, Sarah E.; Knapp, Angela N.; Rafter, Patrick A.; Ward, Bess B.; Haug, Gerald H.

doi:10.1002/2016gb005613

Cited by 53 publications

(71 citation statements)

References 96 publications

(168 reference statements)

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The Atlantic Ocean, which is the most studied ocean basin (Figure ), displays a small coefficient of variation (CV) among models (Figure b). In contrast to the subtropical oceans, the equatorial Atlantic is estimated to harbor intense N 2 fixation rates, consistent with some of the field observations and geochemical estimates (Marconi et al, ; Montoya et al, ; Snow et al, ). However, these hot spots are not consistently identified in all models (e.g., Gulf of Guinea and northeast Atlantic with relatively large CV).…”

Section: Discussionsupporting

confidence: 83%

Machine Learning Estimates of Global Marine Nitrogen Fixation

Tang

Cassar

2019

JGR Biogeosciences

View full text Add to dashboard Cite

Marine nitrogen (N2) fixation supplies “new” nitrogen to the global ocean, supporting uptake and sequestration of carbon. Despite its central role, marine N2 fixation and its controlling factors remain elusive. In this study, we compile over 1,100 published observations to identify the dominant predictors of marine N2 fixation and derive global estimates based on the machine learning algorithms of random forest and support vector regression. We find that no single environmental property predicts N2 fixation at global scales. Our random forest and support vector regression algorithms, trained with sampling coordinates and month, solar radiation, wind speed, sea surface temperature, sea surface salinity, surface nitrate, surface phosphate, surface excess phosphorus, minimum oxygen in upper 500 m, photosynthetically available radiation, mixed layer depth, averaged photosynthetically available radiation in the mixed layer, and chlorophyll‐a concentration, estimate global marine N2 fixation ranging from 68 to 90 Tg N/year. Comparison of our machine learning estimates and 11 other model outputs currently available in literature shows substantial discrepancies in the global magnitude and spatial distribution of marine N2 fixation, especially in the tropics and in high latitudes. The large uncertainties in marine N2 fixation highlighted in our study argue for increased and more coordinated efforts using geochemical tracers, modeling, and observations over broad ocean regions.

show abstract

Section: Discussionsupporting

confidence: 83%

Machine Learning Estimates of Global Marine Nitrogen Fixation

Tang

Cassar

2019

JGR Biogeosciences

View full text Add to dashboard Cite

show abstract

“…The use of World Ocean Atlas fields for this fixed N input function (examined closely by Martin et al, ) vastly simplifies the computational requirements of the model and allows us to focus on other aspects of the N cycle with this model. Our model parameterization yields a global N 2 fixation rate of 131 Tg N/year, which is similar in magnitude to estimates from both regional and global modeling and observational studies (Table S4; Deutsch et al, ; Luo et al, ; Fonseca‐Batista et al, ; Marconi et al, ). A fixed δ 15 N value (−1‰) was also assumed for newly fixed N (Carpenter et al, ; Hoering & Ford, ).…”

Section: Methodssupporting

confidence: 77%

Assessing Marine Nitrogen Cycle Rates and Process Sensitivities With a Global 3‐D Inverse Model

Martin

Primeau

Casciotti

2019

Global Biogeochemical Cycles

Self Cite

View full text Add to dashboard Cite

We present results from a global inverse marine nitrogen (N) cycle model that include nitrate (NO3−) and nitrite (NO2−) concentrations and their N isotopic compositions as constraints on N cycle process rates in marine oxygen deficient zones (ODZs). NO2− is an important intermediate in the N cycle, particularly in ODZs where it is a substrate in the N loss processes, denitrification, and anammox. Similar to earlier work, our model yields a total water column N loss rate of 61 ± 10 Tg N/year. However, by including NO2− and its N isotopic composition, we are able to assess the relative contributions of denitrification and anammox to N loss and examine some of the potential drivers of that balance. We find that anammox contributes 60% of global water column N loss, dominating N loss along the edges of ODZs, while denitrification is more important in the anoxic ODZ cores. The decoupling of anammox and denitrification is supported by NO2− oxidation, which co‐occurs with NO3− reduction and anammox in ODZs. High rates of NO2− oxidation (up to 400 nM/day), which are tightly coupled to heterotrophic NO3− reduction, are required to match NO3− and NO2− concentration and isotope observations in marine ODZs. Lowering the rate of NO2− oxidation in ODZs by adjusting O2‐sensitive parameters results in higher rates of water column N loss, highlighting the role of NO2− oxidation in maintaining the marine fixed N inventory.

show abstract

“…Closer inspection of the Atlantic using depth and zonally averaged sections (Figs. 5 and 6) revealed that the model adequately reproduced the low δ 15 N signature of N 2 fixation at ∼4 ‰ occurring in the tropical Atlantic (Marconi et al, 2017). We found a total, basin-wide rate of Atlantic N 2 fixation equal to ∼33 Tg N yr −1 .…”

supporting

confidence: 57%

Ocean carbon and nitrogen isotopes in CSIRO Mk3L-COAL version 1.0: A tool for palaeoceanographic research

Buchanan¹,

Matear²,

Chase³

et al. 2018

Preprint

View full text Add to dashboard Cite

Abstract. The isotopes of carbon (δ13C) and nitrogen (δ15N) are commonly used proxies for understanding the ocean. When used in tandem, they provide powerful insight into physical and biogeochemical processes. Here, we detail the implementation of δ13C and δ15N in the ocean component of an Earth system model. We evaluate our simulated δ13C and δ15N against contemporary measurements, place the model's performance alongside other isotope enabled models, and document the response of δ13C and δ15N to changes in ecosystem functioning. The model combines the Commonwealth Scientific and Industrial Research Organisation Mark 3L (CSIRO Mk3L) climate system model with the Carbon of the Ocean, Atmosphere and Land (COAL) biogeochemical model. The oceanic component of CSIRO Mk3L-COAL has a resolution of 1.6° latitude × 2.8° longitude and resolves multi-millennial timescales, running at a rate of ∼400 years per day. We show that this coarse resolution, computationally efficient model adequately reproduces water column and coretop δ13C and δ15N measurements, making it a useful tool for palaeoceanographic research. Changes to ecosystem function involve varying phytoplankton stoichiometry, varying CaCO3 production based on calcite saturation state, and varying N2 fixation via iron limitation. We find that large changes in CaCO3 production have little effect on δ13C and δ15N, while changes in N2 fixation and phytoplankton stoichiometry have substantial and complex effects. Interpretations of palaeoceanographic records are therefore open to multiple lines of interpretation where multiple processes imprint on the isotopic signature, such as in the tropics where denitrification, N2 fixation and nutrient utilisation influence δ15N. Hence, there is significant scope for isotope enabled models to provide more robust interpretations of the proxy records.

show abstract

Tropical Dominance of N₂ Fixation in the North Atlantic Ocean

Cited by 53 publications

References 96 publications

Machine Learning Estimates of Global Marine Nitrogen Fixation

Machine Learning Estimates of Global Marine Nitrogen Fixation

Assessing Marine Nitrogen Cycle Rates and Process Sensitivities With a Global 3‐D Inverse Model

Ocean carbon and nitrogen isotopes in CSIRO Mk3L-COAL version 1.0: A tool for palaeoceanographic research

Contact Info

Product

Resources

About

Tropical Dominance of N2 Fixation in the North Atlantic Ocean

Cited by 53 publications

References 96 publications

Machine Learning Estimates of Global Marine Nitrogen Fixation

Machine Learning Estimates of Global Marine Nitrogen Fixation

Assessing Marine Nitrogen Cycle Rates and Process Sensitivities With a Global 3‐D Inverse Model

Ocean carbon and nitrogen isotopes in CSIRO Mk3L-COAL version 1.0: A tool for palaeoceanographic research

Contact Info

Product

Resources

About

Tropical Dominance of N₂ Fixation in the North Atlantic Ocean