Diapycnal diffusivity in the core and oxycline of the tropical North Atlantic oxygen minimum zone

Köllner, Manuela; Visbeck, Martin; Tanhua, Toste; Fischer, Tim

doi:10.1016/j.jmarsys.2016.03.012

Cited by 8 publications

(7 citation statements)

References 29 publications

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“…Steinfeldt et al, 2015): (Osborn, 1980), where is stratification and Γ is the mixing efficiency for which a value of 0.2 was used. The use of this value has recently shown to yield good agreement between turbulent eddy diffusivities determined from microstructure measurements and from tracer release experiments performed in our study region (Köllner et al, 2016). The 95% confidence intervals for station-averaged K ρ values were determined from Gaussian error propagation following Schafstall et al (2010).…”

Section: Turbulence Measurements and Vertical Flux Calculationsmentioning

confidence: 73%

Controls on redox-sensitive trace metals in the Mauritanian oxygen minimum zone

Rapp¹,

Schlösser²,

Barraqueta³

et al. 2018

Preprint

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Abstract. The availability of the micronutrient iron (Fe) in surface waters determines primary production, N2 fixation and microbial community structure in large parts of the world's ocean, and thus plays an important role in ocean carbon and nitrogen cycles. Eastern boundary upwelling systems and the connected oxygen minimum zones (OMZs) are typically associated with elevated concentrations of redox-sensitive trace metals (e.g. Fe, manganese (Mn) and cobalt (Co)), with shelf sediments typically forming a key source. Over the last five decades, an expansion and intensification of OMZs has been observed and this trend is likely to proceed. However, it is unclear how trace metal (TM) distributions and transport are influenced by decreasing oxygen (O2) concentrations. Here we present dissolved (d; <&#8201;0.2&#8201;&#956;m) and leachable particulate (Lp; >&#8201;0.2&#8201;&#956;m) TM data collected at 7 stations along a 50&#8201;km transect in the Mauritanian shelf region. We observed enhanced concentrations of Fe, Co and Mn corresponding with low O2 concentrations (<&#8201;50&#8201;&#956;mol&#8201;kg&#8722;1), which were decoupled from major nutrients and nutrient-like and scavenged TMs (cadmium (Cd), lead (Pb), nickel (Ni) and copper (Cu)). Additionally, data from repeated station occupations indicated a direct link between dissolved and leachable particulate Fe, Co, Mn, and O2. An observed dFe decrease from 10 to 5&#8201;nmol&#8201;L&#8722;1 coincided with an O2 increase from 30 to 50&#8201;&#956;mol&#8201;kg&#8722;1 and with a concomitant decrease in turbidity. The changes in Fe (Co and Mn) were likely driven by variations in their release from sediment pore water, facilitated by lower O2 concentrations and longer residence time of the water mass on the shelf. Variations in organic matter remineralization and lithogenic inputs (atmospheric deposition or sediment resuspension) only played a minor role in redox-sensitive TM variability. Vertical dFe fluxes from O2-depleted subsurface to surface waters (0.08&#8211;13.5&#8201;&#956;mol&#8201;m&#8722;2&#8201;d&#8722;1) were driven by turbulent mixing and vertical advection, and were an order of magnitude larger than atmospheric deposition fluxes (0.63&#8211;1.43&#8201;&#956;mol&#8201;m&#8722;2&#8201;d&#8722;1). Benthic fluxes are therefore the dominant dFe supply to surface waters on the continental margins of the Mauritanian upwelling region. Overall, our results indicated that the projected future decrease in O2 concentrations in OMZs may result in increases in Fe, Mn and Co concentrations.

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Section: Turbulence Measurements and Vertical Flux Calculationsmentioning

confidence: 73%

Controls on redox-sensitive trace metals in the Mauritanian oxygen minimum zone

Rapp¹,

Schlösser²,

Barraqueta³

et al. 2018

Preprint

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show abstract

“…The strongest poleward flow is observed during the relaxation period between May and July when alongshore, upwelling-favorable winds weaken but wind stress curl is at its maximum (Klenz et al, 2018). During the upwelling season in boreal winter, the circulation more closely resembles the classical eastern boundary circulation regime, with a weak PUC flowing beneath an equatorward coastal jet (Klenz et al, 2018;Kounta et al, 2018). At deeper levels (300-500 m depth), flow was found to be equatorward during both seasons.…”

Section: Oceanographic Settings Of the Study Areamentioning

confidence: 93%

Controls on redox-sensitive trace metals in the Mauritanian oxygen minimum zone

et al. 2019

View full text Add to dashboard Cite

Abstract. The availability of the micronutrient iron (Fe) in surface waters determines primary production, N2 fixation, and microbial community structure in large parts of the world's ocean, and thus it plays an important role in ocean carbon and nitrogen cycles. Eastern boundary upwelling systems and the connected oxygen minimum zones (OMZs) are typically associated with elevated concentrations of redox-sensitive trace metals (e.g., Fe, manganese (Mn), and cobalt (Co)), with shelf sediments typically forming a key source. Over the last 5 decades, an expansion and intensification of OMZs has been observed and this trend is likely to proceed. However, it is unclear how trace-metal (TM) distributions and transport are influenced by decreasing oxygen (O2) concentrations. Here we present dissolved (d; <0.2 µm) and leachable particulate (Lp; >0.2 µm) TM data collected at seven stations along a 50 km transect in the Mauritanian shelf region. We observed enhanced concentrations of Fe, Co, and Mn corresponding with low O2 concentrations (<50 µmol kg−1), which were decoupled from major nutrients and nutrient-like and scavenged TMs (cadmium (Cd), lead (Pb), nickel (Ni), and copper (Cu)). Additionally, data from repeated station occupations indicated a direct link between dissolved and leachable particulate Fe, Co, Mn, and O2. An observed dFe (dissolved iron) decrease from 10 to 5 nmol L−1 coincided with an O2 increase from 30 to 50 µmol kg−1 and with a concomitant decrease in turbidity. The changes in Fe (Co and Mn) were likely driven by variations in their release from sediment pore water, facilitated by lower O2 concentrations and longer residence time of the water mass on the shelf. Variations in organic matter remineralization and lithogenic inputs (atmospheric deposition or sediment resuspension; assessed using Al as indicator for lithogenic inputs) only played a minor role in redox-sensitive TM variability. Vertical dFe fluxes from O2-depleted subsurface-to-surface waters (0.08–13.5 µmol m−2 d−1) driven by turbulent mixing and vertical advection were an order of magnitude larger than atmospheric deposition fluxes (0.63–1.43 µmol m−2 d−1; estimated using dAl inventories in the surface mixed layer) in the continental slope and shelf region. Benthic fluxes are therefore the dominant dFe supply to surface waters on the continental margins of the Mauritanian upwelling region. Overall, our results indicated that the projected future decrease in O2 concentrations in OMZs may result in increases in Fe, Mn, and Co concentrations.

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“…The coefficients of the horizontal and the vertical eddy diffusion are chosen based on previous observational studies. We use a vertical diffusivity of k z = 10 −5 m 2 /s (Banyte et al, 2012;Fischer et al, 2013;Köllner et al, 2016). Hahn et al (2014) suggested a meridional diffusivity k y of 500-1,400 m 2 /s between 100 and 300 m depth.…”

Section: 1029/2018jc014760mentioning

confidence: 99%

Interannual Variability of the Atlantic North Equatorial Undercurrent and Its Impact on Oxygen

et al. 2019

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The North Equatorial Undercurrent (NEUC) has been suggested to act as an important oxygen supply route toward the oxygen minimum zone in the eastern tropical North Atlantic. Observational estimates of the mean NEUC strength are uncertain due to the presence of elevated mesoscale activities, and models have difficulties in simulating a realistic NEUC. Here we investigate the interannual variability of the NEUC and its impact onto oxygen based on the output of a high‐resolution Ocean General Circulation Model (OGCM) and contrast the results with an unique data set of 21 ship sections along 23° W and a conceptual model. We find that the interannual variability of the NEUC in the OGCM is related to the Atlantic meridional mode with a stronger and more northward NEUC during negative Atlantic meridional mode phases. Discrepancies between the OGCM and observations suggest a different role of the NEUC in setting the regional oxygen distribution. In the model a stronger NEUC is associated with a weaker oxygen supply toward the east. We attribute this to a too strong recirculation between the NEUC and the northern branch of the South Equatorial Current in the OGCM. Idealized experiments with the conceptual model support the idea that the impact of NEUC variability on oxygen depends on the source water pathway. A strengthening of the NEUC supplied out of the western boundary acts to increase oxygen levels within the NEUC. A strengthening of the recirculations between NEUC and the northern branch of the South Equatorial Current results in a reduction of oxygen levels within the NEUC.

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Diapycnal diffusivity in the core and oxycline of the tropical North Atlantic oxygen minimum zone

Cited by 8 publications

References 29 publications

Controls on redox-sensitive trace metals in the Mauritanian oxygen minimum zone

Controls on redox-sensitive trace metals in the Mauritanian oxygen minimum zone

Controls on redox-sensitive trace metals in the Mauritanian oxygen minimum zone

Interannual Variability of the Atlantic North Equatorial Undercurrent and Its Impact on Oxygen

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