scholarly journals On the role of circulation and mixing in the ventilation of oxygen minimum zones with a focus on the eastern tropical North Atlantic

2014 ◽  
Vol 11 (8) ◽  
pp. 12069-12136 ◽  
Author(s):  
P. Brandt ◽  
D. Banyte ◽  
M. Dengler ◽  
S.-H. Didwischus ◽  
T. Fischer ◽  
...  
Keyword(s):  
Oxygen Consumption ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Oxygen Supply ◽  
Oxygen Minimum Zones ◽  
Oxygen Budget ◽  
Tropical North Atlantic ◽  
Oxygen Minimum ◽  
Oxygen Concentrations

Abstract. Ocean observations carried out in the framework of the Collaborative Research Center 754 (SFB 754) "Climate-Biogeochemistry Interactions in the Tropical Ocean" are used to study (1) the structure of tropical oxygen minimum zones (OMZs), (2) the processes that contribute to the oxygen budget, and (3) long-term changes in the oxygen distribution. The OMZ of the eastern tropical North Atlantic (ETNA), located between the well-ventilated subtropical gyre and the equatorial oxygen maximum, is composed of a deep OMZ at about 400 m depth with its core region centred at about 20° W, 10° N and a shallow OMZ at about 100 m depth with lowest oxygen concentrations in proximity to the coastal upwelling region off Mauritania and Senegal. The oxygen budget of the deep OMZ is given by oxygen consumption mainly balanced by the oxygen supply due to meridional eddy fluxes (about 60%) and vertical mixing (about 20%, locally up to 30%). Advection by zonal jets is crucial for the establishment of the equatorial oxygen maximum. In the latitude range of the deep OMZ, it dominates the oxygen supply in the upper 300 to 400 m and generates the intermediate oxygen maximum between deep and shallow OMZs. Water mass ages from transient tracers indicate substantially older water masses in the core of the deep OMZ (about 120–180 years) compared to regions north and south of it. The deoxygenation of the ETNA OMZ during recent decades suggests a substantial imbalance in the oxygen budget: about 10% of the oxygen consumption during that period was not balanced by ventilation. Long-term oxygen observations show variability on interannual, decadal and multidecadal time scales that can partly be attributed to circulation changes. In comparison to the ETNA OMZ the eastern tropical South Pacific OMZ shows a similar structure including an equatorial oxygen maximum driven by zonal advection, but overall much lower oxygen concentrations approaching zero in extended regions. As the shape of the OMZs is set by ocean circulation, the widespread misrepresentation of the intermediate circulation in ocean circulation models substantially contributes to their oxygen bias, which might have significant impacts on predictions of future oxygen levels.

scholarly journals On the role of circulation and mixing in the ventilation of oxygen minimum zones with a focus on the eastern tropical North Atlantic

2015 ◽  
Vol 12 (2) ◽  
pp. 489-512 ◽  
Author(s):  
P. Brandt ◽  
H. W. Bange ◽  
D. Banyte ◽  
M. Dengler ◽  
S.-H. Didwischus ◽  
...  
Keyword(s):  
Oxygen Consumption ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Oxygen Supply ◽  
Oxygen Minimum Zones ◽  
Oxygen Budget ◽  
Tropical North Atlantic ◽  
Oxygen Minimum ◽  
Oxygen Concentrations

Abstract. Ocean observations are analysed in the framework of Collaborative Research Center 754 (SFB 754) "Climate-Biogeochemistry Interactions in the Tropical Ocean" to study (1) the structure of tropical oxygen minimum zones (OMZs), (2) the processes that contribute to the oxygen budget, and (3) long-term changes in the oxygen distribution. The OMZ of the eastern tropical North Atlantic (ETNA), located between the well-ventilated subtropical gyre and the equatorial oxygen maximum, is composed of a deep OMZ at about 400 m in depth with its core region centred at about 20° W, 10° N and a shallow OMZ at about 100 m in depth, with the lowest oxygen concentrations in proximity to the coastal upwelling region off Mauritania and Senegal. The oxygen budget of the deep OMZ is given by oxygen consumption mainly balanced by the oxygen supply due to meridional eddy fluxes (about 60%) and vertical mixing (about 20%, locally up to 30%). Advection by zonal jets is crucial for the establishment of the equatorial oxygen maximum. In the latitude range of the deep OMZ, it dominates the oxygen supply in the upper 300 to 400 m and generates the intermediate oxygen maximum between deep and shallow OMZs. Water mass ages from transient tracers indicate substantially older water masses in the core of the deep OMZ (about 120–180 years) compared to regions north and south of it. The deoxygenation of the ETNA OMZ during recent decades suggests a substantial imbalance in the oxygen budget: about 10% of the oxygen consumption during that period was not balanced by ventilation. Long-term oxygen observations show variability on interannual, decadal and multidecadal timescales that can partly be attributed to circulation changes. In comparison to the ETNA OMZ, the eastern tropical South Pacific OMZ shows a similar structure, including an equatorial oxygen maximum driven by zonal advection but overall much lower oxygen concentrations approaching zero in extended regions. As the shape of the OMZs is set by ocean circulation, the widespread misrepresentation of the intermediate circulation in ocean circulation models substantially contributes to their oxygen bias, which might have significant impacts on predictions of future oxygen levels.

scholarly journals Decadal oxygen change in the eastern tropical North Atlantic

Ocean Science ◽  
2017 ◽  
Vol 13 (4) ◽  
pp. 551-576 ◽  
Author(s):  
Johannes Hahn ◽  
Peter Brandt ◽  
Sunke Schmidtko ◽  
Gerd Krahmann
Keyword(s):  
North Atlantic ◽  
Oxygen Supply ◽  
Oxygen Minimum Zone ◽  
Decadal Change ◽  
Oxygen Budget ◽  
Tropical North Atlantic ◽  
Minimum Zone ◽  
Oxygen Minimum ◽  
Density Space

Abstract. Repeat shipboard and multi-year moored observations obtained in the oxygen minimum zone (OMZ) of the eastern tropical North Atlantic (ETNA) were used to study the decadal change in oxygen for the period 2006–2015. Along 23° W between 6 and 14° N, oxygen decreased with a rate of −5.9 ± 3.5 µmol kg−1 decade−1 within the depth covering the deep oxycline (200–400 m), while below the OMZ core (400–1000 m) oxygen increased by 4.0 ± 1.6 µmol kg−1 decade−1 on average. The inclusion of these decadal oxygen trends in the recently estimated oxygen budget for the ETNA OMZ suggests a weakened ventilation of the upper 400 m, whereas the ventilation strengthened homogeneously below 400 m. The changed ventilation resulted in a shoaling of the ETNA OMZ of −0.03 ± 0.02 kg m−3 decade−1 in density space, which was only partly compensated by a deepening of isopycnal surfaces, thus pointing to a shoaling of the OMZ in depth space as well (−22 ± 17 m decade−1). Based on the improved oxygen budget, possible causes for the changed ventilation are analyzed and discussed. Largely ruling out other ventilation processes, the zonal advective oxygen supply stands out as the most probable budget term responsible for the decadal oxygen changes.

scholarly journals Water column biogeochemistry of oxygen minimum zones in the eastern tropical North Atlantic and eastern tropical South Pacific oceans

2016 ◽  
Vol 13 (12) ◽  
pp. 3585-3606 ◽  
Author(s):  
Carolin R. Löscher ◽  
Hermann W. Bange ◽  
Ruth A. Schmitz ◽  
Cameron M. Callbeck ◽  
Anja Engel ◽  
...  
Keyword(s):  
Organic Matter ◽  
Nutrient Cycling ◽  
Ocean Circulation ◽  
North Atlantic ◽  
South Pacific ◽  
Gas Production ◽  
Biological Interactions ◽  
Oxygen Minimum Zones ◽  
Tropical North Atlantic ◽  
Oxygen Minimum

Abstract. Recent modeling results suggest that oceanic oxygen levels will decrease significantly over the next decades to centuries in response to climate change and altered ocean circulation. Hence, the future ocean may experience major shifts in nutrient cycling triggered by the expansion and intensification of tropical oxygen minimum zones (OMZs), which are connected to the most productive upwelling systems in the ocean. There are numerous feedbacks among oxygen concentrations, nutrient cycling and biological productivity; however, existing knowledge is insufficient to understand physical, chemical and biological interactions in order to adequately assess past and potential future changes. In the following, we summarize one decade of research performed in the framework of the Collaborative Research Center 754 (SFB754) focusing on climate–biogeochemistry interactions in tropical OMZs. We investigated the influence of low environmental oxygen conditions on biogeochemical cycles, organic matter formation and remineralization, greenhouse gas production and the ecology in OMZ regions of the eastern tropical South Pacific compared to the weaker OMZ of the eastern tropical North Atlantic. Based on our findings, a coupling of primary production and organic matter export via the nitrogen cycle is proposed, which may, however, be impacted by several additional factors, e.g., micronutrients, particles acting as microniches, vertical and horizontal transport of organic material and the role of zooplankton and viruses therein.

scholarly journals Reviews and syntheses: Present, past, and future of the oxygen minimum zone in the northern Indian Ocean

2020 ◽  
Vol 17 (23) ◽  
pp. 6051-6080
Author(s):  
Tim Rixen ◽  
Greg Cowie ◽  
Birgit Gaye ◽  
Joaquim Goes ◽  
Helga do Rosário Gomes ◽  
...  
Keyword(s):  
Oxygen Consumption ◽  
Indian Ocean ◽  
Nitrogen Cycle ◽  
Bay Of Bengal ◽  
Arabian Sea ◽  
Oxygen Supply ◽  
Northern Indian Ocean ◽  
Minimum Zone ◽  
Oxygen Minimum ◽  
Oxygen Concentrations

Abstract. Decreasing concentrations of dissolved oxygen in the ocean are considered one of the main threats to marine ecosystems as they jeopardize the growth of higher organisms. They also alter the marine nitrogen cycle, which is strongly bound to the carbon cycle and climate. While higher organisms in general start to suffer from oxygen concentrations < ∼ 63 µM (hypoxia), the marine nitrogen cycle responds to oxygen concentration below a threshold of about 20 µM (microbial hypoxia), whereas anoxic processes dominate the nitrogen cycle at oxygen concentrations of < ∼ 0.05 µM (functional anoxia). The Arabian Sea and the Bay of Bengal are home to approximately 21 % of the total volume of ocean waters revealing microbial hypoxia. While in the Arabian Sea this oxygen minimum zone (OMZ) is also functionally anoxic, the Bay of Bengal OMZ seems to be on the verge of becoming so. Even though there are a few isolated reports on the occurrence of anoxia prior to 1960, anoxic events have so far not been reported from the open northern Indian Ocean (i.e., other than on shelves) during the last 60 years. Maintenance of functional anoxia in the Arabian Sea OMZ with oxygen concentrations ranging between > 0 and ∼ 0.05 µM is highly extraordinary considering that the monsoon reverses the surface ocean circulation twice a year and turns vast areas of the Arabian Sea from an oligotrophic oceanic desert into one of the most productive regions of the oceans within a few weeks. Thus, the comparably low variability of oxygen concentration in the OMZ implies stable balances between the physical oxygen supply and the biological oxygen consumption, which includes negative feedback mechanisms such as reducing oxygen consumption at decreasing oxygen concentrations (e.g., reduced respiration). Lower biological oxygen consumption is also assumed to be responsible for a less intense OMZ in the Bay of Bengal. According to numerical model results, a decreasing physical oxygen supply via the inflow of water masses from the south intensified the Arabian Sea OMZ during the last 6000 years, whereas a reduced oxygen supply via the inflow of Persian Gulf Water from the north intensifies the OMZ today in response to global warming. The first is supported by data derived from the sedimentary records, and the latter concurs with observations of decreasing oxygen concentrations and a spreading of functional anoxia during the last decades in the Arabian Sea. In the Arabian Sea decreasing oxygen concentrations seem to have initiated a regime shift within the pelagic ecosystem structure, and this trend is also seen in benthic ecosystems. Consequences for biogeochemical cycles are as yet unknown, which, in addition to the poor representation of mesoscale features in global Earth system models, reduces the reliability of estimates of the future OMZ development in the northern Indian Ocean.

scholarly journals Diapycnal oxygen supply to the tropical North Atlantic oxygen minimum zone

2012 ◽  
Vol 9 (10) ◽  
pp. 14291-14325 ◽  
Author(s):  
T. Fischer ◽  
D. Banyte ◽  
P. Brandt ◽  
M. Dengler ◽  
G. Krahmann ◽  
...  
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Oxygen Supply ◽  
Water Layer ◽  
Oxygen Minimum Zone ◽  
Depth Interval ◽  
Diapycnal Mixing ◽  
Tropical North Atlantic ◽  
Minimum Zone ◽  
Oxygen Minimum

Abstract. The replenishment of consumed oxygen in the open ocean oxygen minimum zone (OMZ) off West Africa in the tropical North Atlantic Ocean is studied, with a focus on oxygen transport across density surfaces (diapycnal flux). The latter is obtained from a large observational set of oxygen profiles and diapycnal mixing data from years 2008 to 2010. Diapycnal mixing is inferred from different sources: a large scale tracer release experiment, microstructure profiles, and shipboard acoustic current measurements plus density profiles. The average diapycnal diffusivity in the study area is 1 × 10−5 m2 s−1. No significant vertical gradient of average diapycnal diffusivities exists in the depth interval from 150 to 500 m. The diapycnal flux is found to contribute substantially to the oxygen supply of the OMZ. Within the OMZ core, 1.5 µmol kg−1 a−1 of oxygen is supplied via diapycnal mixing, contributing about a third of the total demand. The oxygen that is contributed via diapycnal mixing originates from oxygen that has been laterally supplied within the overlying Central Water layer by advective and eddy fluxes. Due to the existence of a separate shallow oxygen minimum at about 100 m depth throughout most of the study area, there is no direct net vertical oxygen flux from the surface layer of the study area into the Central Water layer. Thus all oxygen supply of the OMZ is associated with remote pathways.

scholarly journals Decadal oxygen change in the eastern tropical North Atlantic

2017 ◽  
Author(s):  
Johannes Hahn ◽  
Peter Brandt ◽  
Sunke Schmidtko ◽  
Gerd Krahmann
Keyword(s):  
North Atlantic ◽  
Circulation Pattern ◽  
Decadal Change ◽  
South Atlantic Central Water ◽  
Zonal Jets ◽  
Oxygen Budget ◽  
Tropical North Atlantic ◽  
Southward Shift ◽  
Minimum Zone ◽  
Oxygen Minimum

Abstract. Repeat shipboard and multi-year moored observations obtained in the oxygen minimum zone (OMZ) of the eastern tropical North Atlantic (ETNA) were used to study the decadal change in oxygen for the period 2006–2015. At the depth of the deep oxycline (200–400 m), oxygen decreased with a rate of −6.2 ± 3.8 μmol kg−1 decade−1, while below the OMZ core (400–1,000 m) oxygen increased by 4.1 ± 1.7 μmol kg−1 decade−1 on average. The inclusion of these decadal oxygen trends in the recently estimated oxygen budget for the ETNA OMZ showed a weakened ventilation of the upper 400 m, whereas the ventilation strengthened homogeneously over depth below 400 m. This resulted in a shoaling of the ETNA OMZ of −0.03 ± 0.02 kg m−3 decade−1 in density space, which was only partly compensated by a deepening of isopycnal surfaces, thus pointing to a shoaling of the OMZ in depth space as well. Shipboard, float and satellite observations of velocity and hydrography indicate different regional as well as remote changes in the circulation pattern to be responsible for the change in the ventilation of the ETNA. The reduced ventilation in the upper 400 m may have been induced by a southward shift of the wind-driven circulation or by a change of the composition of South Atlantic Central Water. There are hints that below 400 m, latitudinally alternating zonal jets have strengthened, thus contributing to the increased ventilation. Nevertheless, temporal changes in isopycnal eddy supply or diapycnal supply (diapycnal mixing as well as diapycnal advection) cannot be excluded in having contributed to the observed oxygen change.

scholarly journals Constraining oxygen consumption by nitrite oxidation in oceanic oxygen minimum zones

2020 ◽  
Author(s):  
J.M. Beman ◽  
S.M. Vargas ◽  
J.M. Wilson ◽  
E. Perez-Coronel ◽  
S. Vazquez ◽  
...  
Keyword(s):  
Oxygen Consumption ◽  
Dissolved Oxygen ◽  
North Pacific Ocean ◽  
Low Oxygen ◽  
Nitrite Oxidation ◽  
Oxygen Minimum Zones ◽  
Oxidation Rates ◽  
Low Levels ◽  
Oxygen Minimum ◽  
Oxygen Concentrations

AbstractOceanic oxygen minimum zones (OMZs) occur where microorganisms deplete dissolved oxygen (DO) to exceptionally low levels, and are globally significant sites of biogeochemical cycling. Amid the intense competition for DO and other substrates occurring in these metabolically challenging environments, aerobic nitrite oxidation may consume significant amounts of DO, but this has not been examined comprehensively. Using parallel measurements of oxygen consumption rates and 15N-nitrite oxidation rates applied to water column profiles and to oxygen manipulation experiments, we show that nitrite oxidation is a substantial sink for DO in the ocean’s largest OMZ. The contribution of nitrite oxidation to overall DO consumption increased at low DO concentrations, tracking gradients and variations within and across multiple stations in the eastern tropical North Pacific Ocean. Oxygen manipulation experiments produced highly consistent effects, with nitrite oxidation responsible for progressively more DO consumption (up to 97%) as DO was experimentally decreased. Natural abundance stable isotope data indicated coupling of nitrite oxidation and nitrate reduction, while 16S rRNA and metagenome sequencing revealed that Nitrospina ecotypes possessing high-affinity cytochrome oxidase genes were prevalent and active within the OMZ. Collectively, our results demonstrate that nitrite oxidation consumes significant amounts of DO, and that this proportion increases as DO declines—indicating that nitrite oxidation is critically important to the formation and maintenance of OMZs.SignificanceOceanic oxygen minimum zones (OMZs) are naturally-occuring regions of low oxygen found in select areas of the ocean. Lack of dissolved oxygen has important implications for both the distribution of marine organisms and global biogeochemical cycles, yet we have a limited understanding of how oxygen is depleted to such low levels. Here we comprehensively quantify the contribution of nitrite oxidation to oxygen depletion in the ocean’s largest OMZ. We observed highly consistent patterns across depth profiles, and in multiple experiments where we manipulated oxygen concentrations, finding that nitrite oxidation consumes progressively more oxygen at lower oxygen concentrations. Our findings demonstrate that nitrite oxidation plays a pivotal role in exhausting oxygen to the low levels found in OMZs.

scholarly journals Water column biogeochemistry of oxygen minimum zones in the eastern tropical North Atlantic and eastern tropical South Pacific Oceans

2015 ◽  
Vol 12 (6) ◽  
pp. 4495-4556 ◽  
Author(s):  
C. R. Löscher ◽  
H. W. Bange ◽  
R. A. Schmitz ◽  
C. M. Callbeck ◽  
A. Engel ◽  
...  
Keyword(s):  
Organic Matter ◽  
Nutrient Cycling ◽  
Water Column ◽  
North Atlantic ◽  
South Pacific ◽  
N2o Production ◽  
Oxygen Minimum Zones ◽  
Tropical North Atlantic ◽  
The Impact ◽  
Oxygen Minimum

Abstract. Recent modeling results suggest that oceanic oxygen levels will decrease significantly over the next decades to centuries in response to climate change and altered ocean circulation. Hence the future ocean may experience major shifts in nutrient cycling triggered by the expansion and intensification of tropical oxygen minimum zones (OMZs). There are numerous feedbacks between oxygen concentrations, nutrient cycling and biological productivity; however, existing knowledge is insufficient to understand physical, chemical and biological interactions in order to adequately assess past and potential future changes. We investigated the pelagic biogeochemistry of OMZs in the eastern tropical North Atlantic and eastern tropical South Pacific during a series of cruise expeditions and mesocosm studies. The following summarizes the current state of research on the influence of low environmental oxygen conditions on marine biota, viruses, organic matter formation and remineralization with a particular focus on the nitrogen cycle in OMZ regions. The impact of sulfidic events on water column biogeochemistry, originating from a specific microbial community capable of highly efficient carbon fixation, nitrogen turnover and N2O production is further discussed. Based on our findings, an important role of sinking particulate organic matter in controlling the nutrient stochiometry of the water column is suggested. These particles can enhance degradation processes in OMZ waters by acting as microniches, with sharp gradients enabling different processes to happen in close vicinity, thus altering the interpretation of oxic and anoxic environments.

scholarly journals Particle export fluxes to the oxygen minimum zone of the Eastern Tropical North Atlantic

2016 ◽  
Author(s):  
Anja Engel ◽  
Hannes Wagner ◽  
Frédéric A. C. Le Moigne ◽  
Samuel T. Wilson
Keyword(s):  
North Atlantic ◽  
Euphotic Zone ◽  
Carbon Export ◽  
Heterotrophic Activity ◽  
Oxygen Minimum Zones ◽  
Tropical North Atlantic ◽  
Water Layers ◽  
Minimum Zone ◽  
Particle Export ◽  
Oxygen Minimum

Abstract. In the ocean, sinking of particulate organic matter (POM) drives carbon export from the euphotic zone and supplies nutrition to mesopelagic communities, the feeding and degradation activities of which in turn lead to export flux attenuation. Oxygen minimum zones (OMZs) with suboxic water layers ( 100 μmol O2 kg−1), supposedly due to reduced heterotrophic activity. This study focuses on sinking particle fluxes through hypoxic mesopelagic waters (

scholarly journals Contrasting biogeochemistry of nitrogen in the Atlantic and Pacific Oxygen Minimum Zones

2012 ◽  
Vol 9 (1) ◽  
pp. 203-215 ◽  
Author(s):  
E. Ryabenko ◽  
A. Kock ◽  
H. W. Bange ◽  
M. A. Altabet ◽  
D. W. R. Wallace
Keyword(s):  
Surface Waters ◽  
Inorganic Nitrogen ◽  
Sharp Decrease ◽  
Stable Isotope Ratio ◽  
Saharan Dust ◽  
N Loss ◽  
Fractionation Factor ◽  
Oxygen Minimum Zones ◽  
Oxygen Minimum ◽  
Oxygen Concentrations

Abstract. We present new data for the stable isotope ratio of inorganic nitrogen species from the contrasting oxygen minimum zones (OMZs) of the Eastern Tropical North Atlantic, south of Cape Verde, and the Eastern Tropical South Pacific off Peru. Differences in minimum oxygen concentration and corresponding N-cycle processes for the two OMZs are reflected in strongly contrasting δ15N distributions. Pacific surface waters are marked by strongly positive values for δ15N-NO3–) reflecting fractionation associated with subsurface N-loss and partial NO3– utilization. This contrasts with negative values in NO3– depleted surface waters of the Atlantic which are lower than can be explained by N supply via N2 fixation. We suggest the negative values reflect inputs of nitrate, possibly transient, associated with deposition of Saharan dust. Strong signals of N-loss processes in the subsurface Pacific OMZ are evident in the isotope and N2O data, both of which are compatible with a contribution of canonical denitrification to overall N-loss. However the apparent N isotope fractionation factor observed is relatively low (&amp;varepsilon;d=11.4 ‰) suggesting an effect of influence from denitrification in sediments. Identical positive correlation of N2O vs. AOU for waters with oxygen concentrations ([O2] < 5 μmol l−1) in both regions reflect a nitrification source. Sharp decrease in N2O concentrations is observed in the Pacific OMZ due to denitrification under oxygen concentrations O2 < 5 μmol l−1.

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