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 Particle export fluxes to the oxygen minimum zone of the eastern tropical North Atlantic

2017 ◽  
Vol 14 (7) ◽  
pp. 1825-1838 ◽  
Author(s):  
Anja Engel ◽  
Hannes Wagner ◽  
Frédéric A. C. Le Moigne ◽  
Samuel T. Wilson
Keyword(s):  
Organic Matter ◽  
North Atlantic ◽  
Organic Phosphorus ◽  
Euphotic Zone ◽  
Carbon Export ◽  
Organic Matter Quality ◽  
B Values ◽  
Chl A ◽  
Tropical North Atlantic ◽  
Particle Export

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 (O2) minimum zones (OMZs) with suboxic water layers (< 5 µmol O2 kg−1) show a lower carbon flux attenuation compared to well-oxygenated waters (> 100 µmol O2 kg−1), supposedly due to reduced heterotrophic activity. This study focuses on sinking particle fluxes through hypoxic mesopelagic waters (< 60 µmol O2 kg−1); these represent  ∼  100 times more ocean volume globally compared to suboxic waters, but they have less been studied. Particle export fluxes and attenuation coefficients were determined in the eastern tropical North Atlantic (ETNA) using two surface-tethered drifting sediment trap arrays with seven trapping depths located between 100 and 600 m. Data on particulate matter fluxes were fitted to the normalized power function Fz =  F100 (z∕100)−b, with F100 being the flux at a depth (z) of 100 m and b being the attenuation coefficient. Higher b values suggest stronger flux attenuation and are influenced by factors such as faster degradation at higher temperatures. In this study, b values of organic carbon fluxes varied between 0.74 and 0.80 and were in the intermediate range of previous reports, but lower than expected from seawater temperatures within the upper 500 m. During this study, highest b values were determined for fluxes of particulate hydrolyzable amino acids (PHAA), followed by particulate organic phosphorus (POP), nitrogen (PN), carbon (POC), chlorophyll a (Chl a) and transparent exopolymer particles (TEP), pointing to a sequential degradation of organic matter components during sinking. Our study suggests that in addition to O2 concentration, organic matter composition co-determines transfer efficiency through the mesopelagic. The magnitude of future carbon export fluxes may therefore also depend on how organic matter quality in the surface ocean changes under influence of warming, acidification and enhanced stratification.

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 Changes in the Ventilation of the Oxygen Minimum Zone of the Tropical North Atlantic

2010 ◽  
Vol 40 (8) ◽  
pp. 1784-1801 ◽  
Author(s):  
Peter Brandt ◽  
Verena Hormann ◽  
Arne Körtzinger ◽  
Martin Visbeck ◽  
Gerd Krahmann ◽  
...  
Keyword(s):  
North Atlantic ◽  
Oxygen Minimum Zone ◽  
Time Interval ◽  
Depth Range ◽  
Oxygen Levels ◽  
The Core ◽  
Zonal Jets ◽  
Tropical North Atlantic ◽  
Minimum Zone ◽  
Oxygen Minimum

Abstract Changes in the ventilation of the oxygen minimum zone (OMZ) of the tropical North Atlantic are studied using oceanographic data from 18 research cruises carried out between 28.5° and 23°W during 1999–2008 as well as historical data referring to the period 1972–85. In the core of the OMZ at about 400-m depth, a highly significant oxygen decrease of about 15 μmol kg−1 is found between the two periods. During the same time interval, the salinity at the oxygen minimum increased by about 0.1. Above the core of the OMZ, within the central water layer, oxygen decreased too, but salinity changed only slightly or even decreased. The scatter in the local oxygen–salinity relations decreased from the earlier to the later period suggesting a reduced filamentation due to mesoscale eddies and/or zonal jets acting on the background gradients. Here it is suggested that latitudinally alternating zonal jets with observed amplitudes of a few centimeters per second in the depth range of the OMZ contribute to the ventilation of the OMZ. A conceptual model of the ventilation of the OMZ is used to corroborate the hypothesis that changes in the strength of zonal jets affect mean oxygen levels in the OMZ. According to the model, a weakening of zonal jets, which is in general agreement with observed hydrographic evidences, is associated with a reduction of the mean oxygen levels that could significantly contribute to the observed deoxygenation of the North Atlantic OMZ.

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 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.

Sign in / Sign up

Export Citation Format

Share Document