Scenarios of Deoxygenation of the Eastern Tropical North Pacific During the Past Millennium as a Window Into the Future of Oxygen Minimum Zones

Toward a global calibration for quantifying past oxygenation in oxygen minimum zones using benthic Foraminifera

10.5194/bg-2020-482 ◽

2021 ◽

Author(s):

Martin Tetard ◽

Laetitia Licari ◽

Kazuyo Tachikawa ◽

Ekaterina Ovsepyan ◽

Luc Beaufort

Keyword(s):

North Pacific ◽

Transfer Functions ◽

Late Quaternary ◽

Global Climate ◽

Single Species ◽

Pore Waters ◽

Oxygen Minimum Zones ◽

Global Calibration ◽

Circularity Index ◽

Oxygen Minimum

Abstract. Oxygen Minimum Zones (OMZs) are oceanic areas largely depleted in dissolved oxygen, nowadays considered in expansion in the face of global warming. Their ecological and economic consequences are being debated. The investigation of past OMZ conditions allows us to better understand biological and physical mechanisms responsible for their variability with regards to climate change, carbon pump and carbonate system. To investigate the relationship between OMZ expansion and global climate changes during the late Quaternary, quantitative oxygen reconstructions are needed, but are still in their early development. Here, past bottom water oxygenation (BWO) was quantitatively assessed through a new, fast, semi-automated, and taxonfree morphometric analysis of benthic foraminiferal tests, developed and calibrated using Eastern North Pacific (ENP) and the Eastern South Pacific (ESP) OMZs samples. This new approach is based on an average size and circularity index for each sample. This method, as well as two already published micropalaeontological approaches based on benthic foraminiferal assemblages variability and porosity investigation of a single species, were here calibrated based on availability of new data from 23 core tops recovered along an oxygen gradient (from 0.03 to 1.79 mL.L−1) from the ENP, ESP, AS (Arabian Sea) and WNP (Western North Pacific, including its marginal seas) OMZs. Global calibrated transfer functions are thus herein proposed for each of these methods. These micropalaeontological reconstruction approaches were then applied on a paleorecord from the ENP OMZ to examine the consistency and limits of these methods, as well as the relative influence of bottom and pore waters on these micropalaeontological tools. Both the assemblages and morphometric approaches (that is also ultimately based on the ecological response of the complete assemblage and faunal succession according to BWO) gave similar and consistent past BWO reconstructions, while the porosity approach (based on a single species and its unique response to a mixed signal of bottom and pore waters) shown ambiguous estimations.

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Anoxic metabolism after the 21st century in oxygen minimum zones

10.5194/egusphere-egu2020-13038 ◽

2020 ◽

Author(s):

Wolfgang Koeve ◽

Angela Landolfi

Keyword(s):

Organic Matter ◽

Ocean Circulation ◽

Large Scale ◽

Deep Ocean ◽

Sulphate Reduction ◽

Oxygen Minimum Zones ◽

The Future ◽

Ocean Models ◽

The Impact ◽

Oxygen Minimum

Global models project a decrease of marine oxygen over the course of the 21th century. The future of marine oxygen becomes increasingly uncertain further into the future after yr 2100 , partly because ocean models differ in the way organic matter remineralisation continues under oxygen- and nitrate-free conditions. Using an Earth system model of intermediate complexity we found that under a business-as-usual CO2-emission scenario ocean deoxygenation further intensifies for several centuries until eventually ocean circulation re-establishes and marine oxygen increases again. (Oschlies et al. 2019, DOI 10.1038/s41467-019-10813-w).In the Pacific Ocean the deoxygenation after yr 2100 goes along with the large scale loss of nitrate from oxygen minimum zones. Here we explore the impact on simulated ocean biogeochemistry of three different process formulation of anoxic metabolism, which have been used in other ocean models: (1) implicit sulphate reduction (organic matter degradation continues without oxidant), (2) no sulphidic metabolism (organic matter is not degraded under anoxic conditions), and (3) explicit sulphate reduction (with H2S as explicit model tracer). The model with explicit sulfphate reduction supports larger regional organic matter fluxed into the deep ocean and an increase in respired carbon storage, compared with the model applying implicit sulphate. We discuss the impact of anoxic metabolism on the coupling between export production and respired carbon stored in the ocean interior.

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Expansion of the eastern North Pacific OMZ and the associated denitrification regime

10.5194/egusphere-egu2020-6473 ◽

2020 ◽

Author(s):

Allan Devol ◽

Wendi Ruef

Keyword(s):

Pacific Decadal Oscillation ◽

North Pacific ◽

Arabian Sea ◽

Thermocline Depth ◽

Fixed Nitrogen ◽

Potential Density ◽

Hurricane Intensity ◽

Oxygen Minimum Zones ◽

Marine Areas ◽

Oxygen Minimum

&#160;At this point ocean deoxygenation is well documented, including in oxygen minimum zones (OMZs).&#160; Within the large OMZs of the Arabian Sea and eastern Pacific are imbedded areas where oxygen concentrations are so low that they are undetectable by routine CTD sensors (oxygen deficient zones, ODZs).&#160; How do we determine if these ODZ are losing O2?&#160; Furthermore, denitrification occurs in oxygen minimum zones (OMZs) so one might hypothesize that denitrification is likewise expanding if oxygen is decreasing.&#160; This is important because the ocean's fixed nitrogen inventory limits the productivity over large marine areas.We have investigated these questions in the largest OMZ, the eastern tropical North Pacific (ETNP) through an analysis of &#160;6 repeats of a 1000 km transect along 110o West in the heart of the ETNP ODZ between 1971-2019.&#160; We use N*, a stoichiometric parameter calculated from nitrate and phosphate, as our indicator of denitrification. The more Negative N* the more denitrification has occurred. After secondary QC the values of O2 concentration between potential density 24.75 and 1000m along with N* were integrated across the transect and over the depth of the ODZ. &#160;The results show a clear decrease in oxygen inventory along with an increase in N*, suggesting deoxygenation and intensification of denitrification over during the 50 year period. We discuss potential mechanisms for denitrification signal increase including ENSO, Pacific Decadal Oscillation, tropical hurricane intensity, and variations in thermocline depth.

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Toward a global calibration for quantifying past oxygenation in oxygen minimum zones using benthic Foraminifera

Biogeosciences ◽

10.5194/bg-18-2827-2021 ◽

2021 ◽

Vol 18 (9) ◽

pp. 2827-2841

Author(s):

Martin Tetard ◽

Laetitia Licari ◽

Ekaterina Ovsepyan ◽

Kazuyo Tachikawa ◽

Luc Beaufort

Keyword(s):

North Pacific ◽

Transfer Functions ◽

Late Quaternary ◽

Global Climate ◽

Single Species ◽

Shape Index ◽

Pore Waters ◽

Oxygen Minimum Zones ◽

Global Calibration ◽

Oxygen Minimum

Abstract. Oxygen minimum zones (OMZs) are oceanic areas largely depleted in dissolved oxygen, nowadays considered in expansion in the face of global warming. To investigate the relationship between OMZ expansion and global climate changes during the late Quaternary, quantitative oxygen reconstructions are needed but are still in their early development. Here, past bottom water oxygenation (BWO) was quantitatively assessed through a new, fast, semi-automated, and taxon-independent morphometric analysis of benthic foraminiferal tests, developed and calibrated using WNP (western North Pacific, including its marginal seas), ENP (eastern North Pacific), and ESP (eastern South Pacific) OMZ samples. This new approach is based on an average size and shape index for each sample. This method, as well as two already published micropalaeontological techniques based on benthic foraminiferal assemblages' variability and porosity investigation of a single species, was calibrated here based on availability of new data from 45 core tops recovered along an oxygen gradient (from 0.03 to 2.88 mL L−1) from the WNP, ENP, EEP (eastern Equatorial Pacific), ESP, SWACM (southwest African continental margin), and AS (Arabian Sea) OMZs. Global calibrated transfer functions are herein proposed for these methods. These micropalaeontological reconstruction approaches were then applied to a palaeorecord from the ENP OMZ to examine the consistency and limits of these methods, as well as the relative influence of bottom and pore waters on these micropalaeontological tools. Both the assemblage and morphometric approaches (which are also ultimately based on the ecological response of the complete assemblage and faunal succession according to BWO) gave similar and consistent past BWO reconstructions, while the porosity approach (based on a single species and its unique response to a mixed signal of bottom and pore waters) showed ambiguous estimations.

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Centennial changes in North Pacific anoxia linked to tropical trade winds

Science ◽

10.1126/science.1252332 ◽

2014 ◽

Vol 345 (6197) ◽

pp. 665-668 ◽

Cited By ~ 83

Author(s):

Curtis Deutsch ◽

William Berelson ◽

Robert Thunell ◽

Thomas Weber ◽

Caitlin Tems ◽

...

Keyword(s):

Climate Warming ◽

North Pacific ◽

Sediment Cores ◽

Equatorial Pacific ◽

Biological Productivity ◽

Anoxic Zone ◽

Trade Winds ◽

Oxygen Minimum Zones ◽

Oxygen Minimum ◽

The Tropical Pacific

Climate warming is expected to reduce oxygen (O2) supply to the ocean and expand its oxygen minimum zones (OMZs). We reconstructed variations in the extent of North Pacific anoxia since 1850 using a geochemical proxy for denitrification (δ15N) from multiple sediment cores. Increasing δ15N since ~1990 records an expansion of anoxia, consistent with observed O2 trends. However, this was preceded by a longer declining δ15N trend that implies that the anoxic zone was shrinking for most of the 20th century. Both periods can be explained by changes in winds over the tropical Pacific that drive upwelling, biological productivity, and O2 demand within the OMZ. If equatorial Pacific winds resume their predicted weakening trend, the ocean’s largest anoxic zone will contract despite a global O2 decline.

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