Benthic nitrogen fluxes and fractionation of nitrate in the Mauritanian oxygen minimum zone (Eastern Tropical North Atlantic)

Dale, Andrew W.; Sommer, Stefan; Ryabenko, E. A.; Noffke, Anna; Bohlen, Lisa; Wallmann, Klaus; Stolpovsky, Konstantin; Greinert, Jens; Pfannkuche, Olaf

doi:10.1016/j.gca.2014.02.026

Cited by 108 publications

(94 citation statements)

References 94 publications

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“…Sediments at 18 • N are characterized by an increase of surface particulate organic carbon with water depth (0.6 wt% on the shelf and 2.7 wt% at 800 m) and a decrease of particulate organic carbon with depth in the sediments (Dale et al, 2014). While the shelf at the 18 • N upwelling region is characterized by minor sediment accumulation rates, sedimentation rates between 0.1 and 0.35 cm yr −1 were found at the deeper stations.…”

Section: Study Areamentioning

confidence: 99%

“…While the shelf at the 18 • N upwelling region is characterized by minor sediment accumulation rates, sedimentation rates between 0.1 and 0.35 cm yr −1 were found at the deeper stations. Dale et al (2014) described the 18 • N sediment as muddy sand down to 400 m water depth and as slightly sandy mud from 786 m. Surface porosity was low (0.56-0.62) at the shallow sites (<100 m) and high (0.83-0.85) at deeper sites (>786 m), with grain size observations (Sokoll, 2013) indicating permeable sediments down to 400 m water depth (Dale et al, 2014). Permeable sandy sediments were originally considered to be biogeochemically inert due to their low organic carbon content (Shum and Sundby, 1996;Boudreau et al, 2001).…”

Section: Study Areamentioning

confidence: 99%

“…However, the relevance of benthic N 2 fixation for N cycling in the Mauritanian OMZ sediments is yet unknown. Extensive bioirrigation and bioturbation by bottom dwelling macrofauna observed in this region may be a major promoting factor for benthic N 2 fixation (Dale et al, 2014). Thus, subsurface microbial activities including N 2 fixation could be stimulated here (Jørgensen, 1983;Fulweiler et al, 2007;.…”

Section: Introductionmentioning

confidence: 96%

“…The OMZ is predicted to lose more O 2 in the future at a rate of ∼0.5 µM y −1 Keeling et al, 2010). As a consequence of low O 2 , Mauritanian OMZ sediments are a net sink for dissolved inorganic N due to denitrification (Dale et al, 2014). N loss is highest on the shelf and decreases with increasing water depth, in line with particulate organic carbon flux to the seafloor.…”

Section: Introductionmentioning

confidence: 99%

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Benthic Dinitrogen Fixation Traversing the Oxygen Minimum Zone Off Mauritania (NW Africa)

et al. 2017

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Despite its potential to provide new nitrogen (N) to the environment, knowledge on benthic dinitrogen (N 2 ) fixation remains relatively sparse, and its contribution to the marine N budget is regarded as minor. Benthic N 2 fixation is often observed in organic-rich sediments coupled to heterotrophic metabolisms, such as sulfate reduction. In the present study, benthic N 2 fixation together with sulfate reduction and other heterotrophic metabolisms were investigated at six station between 47 and 1,108 m water depth along the 18 • N transect traversing the highly productive upwelling region known as Mauritanian oxygen minimum zone (OMZ). Bottom water oxygen concentrations ranged between 30 and 138 µM. Benthic N 2 fixation determined by the acetylene reduction assay was detected at all stations with highest rates (0.15 mmol m −2 d −1 ) on the shelf (47 and 90 m water depth) and lowest rates (0.08 mmol m −2 d −1 ) below 412 m water depth. The biogeochemical data suggest that part of the N 2 fixation could be linked to sulfate-and iron-reducing bacteria. Molecular analysis of the key functional marker gene for N 2 fixation, nifH, confirmed the presence of sulfate-and iron-reducing diazotrophs. High N 2 fixation further coincided with bioirrigation activity caused by burrowing macrofauna, both of which showed high rates at the shelf sites and low rates in deeper waters. However, statistical analyses proved that none of these processes and environmental variables were significantly correlated with benthic diazotrophy, which lead to the conclusion that either the key parameter controlling benthic N 2 fixation in Mauritanian sediments remains unidentified or that a more complex interaction of control mechanisms exists. N 2 fixation rates in Mauritanian sediments were 2.7 times lower than those from the anoxic Peruvian OMZ.

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Section: Study Areamentioning

confidence: 99%

Section: Study Areamentioning

confidence: 99%

Section: Introductionmentioning

confidence: 96%

Section: Introductionmentioning

confidence: 99%

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Benthic Dinitrogen Fixation Traversing the Oxygen Minimum Zone Off Mauritania (NW Africa)

et al. 2017

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“…TOU is then calculated based on the rate of oxygen decrease, accounting for the enclosed area and water volume. TOU rates have recently been measured in the upper 1000 m on the continental slope and shelf off Mauritania using benthic chambers attached to landers (Dale et al, 2014). The reported TOU rates that are quantified in terms of oxygen fluxes into the sediments were as high as 10 mmol m −2 d −1 in depths between 50 and 100 m, and decreased quasi-exponentially to about 3 mmol m −2 d −1 at a depth of 1000 m. To compare TOU rates to pelagic oxygen consumption, we have to apply the TOU to a water volume with a given in situ density: the consumption within a 1 m thick layer above the bottom due to TOU is 3 orders of magnitudes larger when compared to pelagic oxygen consumption occurring at similar depths.…”

Section: Benthic Oxygen Uptakementioning

confidence: 99%

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

et al. 2015

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Abstract. Ocean observations are analysed in the framework of Collaborative Research Center 754 (SFB 754) "ClimateBiogeochemistry 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.

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Toward a parameterization of global‐scale organic carbon mineralization kinetics in surface marine sediments

Stolpovsky

Dale

Wallmann

2015

Global Biogeochemical Cycles

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An empirical function is derived for predicting the rate-depth profile of particulate organic carbon (POC) degradation in surface marine sediments including the bioturbated layer. The rate takes the form of a power law analogous to the Middelburg function. The functional parameters were optimized by simulating measured benthic O 2 and NO 3 À fluxes at 185 stations worldwide using a diagenetic model. The novelty of this work rests with the finding that the vertically resolved POC degradation rate in the bioturbated zone can be determined using a simple function where the POC rain rate is the governing variable. Although imperfect, the model is able to fit 71% of paired O 2 and NO 3 À fluxes to within 50% of measured values. It further provides realistic geochemical concentration-depth profiles, NO 3 À penetration depths, and apparent first-order POC mineralization rate constants. The model performs less well on the continental shelf due to the high sediment heterogeneity there. When applied to globally resolved maps of rain rate, the model predicts a global denitrification rate of 182 ± 88 Tg yr À1 of N and a POC burial rate of 107 ± 52 Tg yr À1 of C with a mean carbon burial efficiency of 6.1%. These results are in very good agreement with published values. Our proposed function is conceptually simple, requires less parameterization than multi-G-type models, and is suitable for nonsteady state applications. It provides a basis for more accurately simulating benthic nutrient fluxes and carbonate dissolution rates in Earth system models.

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Benthic nitrogen fluxes and fractionation of nitrate in the Mauritanian oxygen minimum zone (Eastern Tropical North Atlantic)

Cited by 108 publications

References 94 publications

Benthic Dinitrogen Fixation Traversing the Oxygen Minimum Zone Off Mauritania (NW Africa)

Benthic Dinitrogen Fixation Traversing the Oxygen Minimum Zone Off Mauritania (NW Africa)

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

Toward a parameterization of global‐scale organic carbon mineralization kinetics in surface marine sediments

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