Dissolved benthic phosphate, iron and carbon fluxes in the Mauritanian upwelling system and implications for ongoing deoxygenation

Schroller-Lomnitz, Ulrike; Hensen, Christian; Dale, Andrew W.; Scholz, Florian; Clemens, David; Sommer, Stefan; Noffke, Anna; Wallmann, Klaus

doi:10.1016/j.dsr.2018.11.008

Cited by 18 publications

(23 citation statements)

References 62 publications

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“…Dissolved Mn, Co and Fe are micronutrients, and in Figure 8 fall on the trajectory of the other nutrient‐type elements (e.g., Cd), but less of their variance is explained by PC1, indicating that processes other than biological uptake and remineralization influenced their distributions. While these redox sensitive elements have elevated concentrations in oxygen minimum zones as a result of benthic supply (Bruland et al, 2005; Rapp et al, 2019; Schroller‐Lomnitz et al, 2019), no association with low oxygen is notable in Figure 8 and their variance is not explained by the oxygen/salinity correlated PC2. As might be expected, the dissolved concentrations of Mn, Co, and Fe therefore appear to be determined by a combination of factors including supply strength, biological uptake and remineralization, characteristics and abundance of particles, and water mass age and mixing.…”

Section: Resultsmentioning

confidence: 99%

“…Cd), but less of their variance is explained by PC1, indicating that processes other than biological uptake and remineralization influenced their distributions. Whilst these redox sensitive elements have elevated concentrations in oxygen minimum zones as a result of benthic supply (Bruland et al, 2005;Rapp et al, 2019;Schroller-Lomnitz et al, 2019), no association with low oxygen is notable in Fig. 8 and their variance is not explained by the oxygen/salinity correlated PC2.…”

Section: Ocean Sections Of Dissolved and Total Dissolvable Trace Elementsmentioning

confidence: 95%

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Trace Element Biogeochemistry in the High‐Latitude North Atlantic Ocean: Seasonal Variations and Volcanic Inputs

Achterberg

Steigenberger

Klar

et al. 2021

Global Biogeochemical Cycles

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We present dissolved and total dissolvable trace elements for spring and summer cruises in 2010 in the high-latitude North Atlantic. Surface and full depth data are provided for Al, Cd, Co, Cu, Mn, Ni, Pb, and Zn in the Iceland and Irminger Basins, and consequences of biological uptake and inputs by the spring Eyjafjallajökull volcanic eruption are assessed. Ash from Eyjafjallajökull resulted in pronounced increases in Al, Mn, and Zn in surface waters in close proximity to Iceland during the eruption, while 3 months later during the summer cruise levels had returned to more typical values for the region. The apparent seasonal removal ratios of surface trace elements were consistent with biological export. Assessment of supply of trace elements to the surface mixed layer for the region, excluding volcanic inputs, indicated that deep winter mixing was the dominant source, with diffusive mixing being a minor source (between 13.5% [dissolved Cd, DCd] and −2.43% [DZn] of deep winter flux), and atmospheric inputs being an important source only for DAl and DZn (DAl up to 42% and DZn up to 4.2% of deep winter + diffusive fluxes) and typically less than 1% for the other elements. Elemental supply ratios to the surface mixed layer through convection were comparable to apparent removal ratios we calculated between spring and summer. Given that deep mixing dominated nutrient and trace element supply to surface waters, predicted increases in water column stratification in this region may reduce supply, with potential consequences for primary production and the biological carbon pump.

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Section: Resultsmentioning

confidence: 99%

Section: Ocean Sections Of Dissolved and Total Dissolvable Trace Elementsmentioning

confidence: 95%

Trace Element Biogeochemistry in the High‐Latitude North Atlantic Ocean: Seasonal Variations and Volcanic Inputs

Achterberg

Steigenberger

Klar

et al. 2021

Global Biogeochemical Cycles

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“…This in turn results in increased organic carbon export flux, leading to higher oxygen demand, OMZ expansion and intensification, and a positive feedback with benthic phosphorus release (Slomp and Van Cappellen, 2007;Wallmann, 2010). By contrast, other studies suggested that intensified phosphorus burial occurs under anoxic conditions in shallow water environments based on observations of calcium fluorapatite (CFA) precipitation in present-day shallow water oxygen-depleted upwelling areas (Schulz and Schulz, 2005;Arning et al, 2009a, b;Goldhammer et al, 2010;Ingall, 2010;Cosmidis et al, 2013). In the long term, the formation of CFA is approximately in balance with enhanced phosphorus release from anoxic sediments, implying that the dissolved oceanic phosphorus inventory is largely unaffected by regional changes in oxygen concentrations (Delaney, 1998;Anderson et al, 2001;Roth et al, 2014).…”

Section: Oae1amentioning

confidence: 99%

Cretaceous oceanic anoxic events prolonged by phosphorus cycle feedbacks

et al. 2020

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Abstract. Oceanic anoxic events (OAEs) document major perturbations of the global carbon cycle with repercussions for the Earth's climate and ocean circulation that are relevant to understanding future climate trends. Here, we compare the onset and development of Cretaceous OAE1a and OAE2 in two drill cores with unusually high sedimentation rates from the Vocontian Basin (southern France) and Tarfaya Basin (southern Morocco). OAE1a and OAE2 exhibit remarkable similarities in the evolution of their carbon isotope (δ13C) records, with long-lasting negative excursions preceding the onset of the main positive excursions, supporting the view that both OAEs were triggered by massive emissions of volcanic CO2 into the atmosphere. However, there are substantial differences, notably in the durations of individual phases within the δ13C positive excursions of both OAEs. Based on analysis of cyclic sediment variations, we estimate the duration of individual phases within OAE1a and OAE2. We identify (1) a precursor phase (negative excursion) lasting ∼430 kyr for OAE1a and ∼130 kyr for OAE2, (2) an onset phase of ∼390 and ∼70 kyr, (3) a peak phase of ∼600 and ∼90 kyr, (4) a plateau phase of ∼1340 and ∼200 kyr, and (5) a recovery phase of ∼380 and ∼440 kyr. The total duration of the positive δ13C excursion is estimated at 2700 kyr for OAE1a and 790 kyr for OAE2, and that of the main carbon accumulation phase is estimated at 980 and 180 kyr. The long-lasting peak, plateau and recovery phases imply fundamental changes in global nutrient cycles either (1) by submarine basalt–seawater interactions, (2) through excess nutrient inputs to the oceans by increasing continental weathering and river discharge, or (3) through nutrient recycling from the marine sediment reservoir. We investigated the role of phosphorus in the development of carbon accumulation by analysing phosphorus speciation across OAE2 and the mid-Cenomanian Event (MCE) in the Tarfaya Basin. The ratios of organic carbon and total nitrogen to reactive phosphorus (Corg∕Preact and Ntotal∕Preact) prior to OAE2 and the MCE hover close to or below the Redfield ratio characteristic of marine organic matter. Decreases in reactive phosphorus resulting in Corg∕Preact and Ntotal∕Preact above the Redfield ratio during the later phase of OAE2 and the MCE indicate leakage from the sedimentary column into the water column under the influence of intensified and expanded oxygen minimum zones. These results suggest that a positive feedback loop, rooted in the benthic phosphorus cycle, contributed to increased marine productivity and carbon burial over an extended period of time during OAEs.

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“…Enhanced O 2 consumption is a result of elevated surface productivity caused by upwelling of nutrient-rich subsurface waters in eastern boundary regions of the oceans through Ekman divergence and intense remineralization of sinking particles (e.g., Helly and Levin, 2004). Elevated organic matter supply and water column O 2 depletion lead to enhanced benthic release of redox-sensitive elements by influencing sediment diagenetic processes (Noffke et al, 2012;Severmann et al, 2010). Elevated concentrations of sediment-derived dissolved Fe, Co, and Mn have been associated with lateral offshore advection in O 2 -depleted waters in the Arabian Sea and Pacific and Atlantic oceans (Biller and Bruland, 2013;Hatta et al, 2015;Hawco et al, 2016;Milne et al, 2017;Moffett et al, 2015;Noble et al, 2012).…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2019

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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 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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Dissolved benthic phosphate, iron and carbon fluxes in the Mauritanian upwelling system and implications for ongoing deoxygenation

Cited by 18 publications

References 62 publications

Trace Element Biogeochemistry in the High‐Latitude North Atlantic Ocean: Seasonal Variations and Volcanic Inputs

Trace Element Biogeochemistry in the High‐Latitude North Atlantic Ocean: Seasonal Variations and Volcanic Inputs

Cretaceous oceanic anoxic events prolonged by phosphorus cycle feedbacks

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

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