Displays

OS3.1

Ocean oxygen loss is one of the key consequences of climate change and has the potential to critically impact marine biogeochemical cycles and ecology. Current time series projections and climate models identify an unusually rapid decline in oxygen concentrations, particularly in tropical regions. However, our understanding of how stable this trend is over longer time scales, how adaptable ecosystems are, and if negative or positive feedback mechanisms exist is insufficient.

We seek to identify major gaps in knowledge helping to quantify the rate of ocean deoxygenation and its impact on both biogeochemistry and marine life. To do so, this session aims to bring together scientists from across disciplines including physical oceanography, climate modeling, biogeochemistry, and deep time experts. Our aim is not only to bring our results together but to conclude on what changes in ocean oxygen content can be identified across different ocean areas and different geological timescales.

We invite contributions that investigate ocean deoxygenation in the past, present and future ocean, and its physical, chemical and/or biological drivers, using observational or model-based approaches at regional or global scales.

Share:
Co-organized by BG4
Convener: Bastien QuesteECSECS | Co-convener: Carolin LöscherECSECS
Displays
| Tue, 05 May, 14:00–15:45 (CEST)

Files for download

Download all presentations (77MB)

Chat time: Tuesday, 5 May 2020, 14:00–15:45

D2785 |
EGU2020-22686
Angélique Hameau, Thomas Frölicher, Juliette Mignot, and Fortunat Joos
Multiple lines of evidence from observation- and model-based studies show that anthropogenic greenhouse gas emissions cause ocean warming and oxygen depletion, with adverse impacts on marine organisms and ecosystems.
Temperatures increase is a primary indicator for climate change. However, in the thermocline, changes in oxygen and other biogeochemical tracers might be detectable before warming (Hameau et al., 2019a).
Here, we compare the local time of emergence (ToE) of anthropogenic temperature and oxygen changes in the thermocline within an ensemble of Earth system model simulations from the CMIP5 dataset (Hameau et al., 2019b).
Generally, warming emerges from internal variability prior to changes in oxygen.
Yet, in 35$\pm$11\% of the global thermocline, anthropogenic deoxygenation is detectable before warming.
Earlier emergence of oxygen changes is typically related to decreasing trends in ventilation, which reduce the supply of oxygen-rich surface waters to the thermocline.
In addition, reduced ventilation slows the propagation of anthropogenic warming from the surface into the ocean interior, further contributing to the delayed emergence of warming compared to deoxygenation.
As the magnitude of simulated interval variability and of simulated anthropogenic changes vary considerably across models, we introduce the relative ToE metric. This reduces the inter-model spread, allowing for a better comparison among models.
Our results underline the importance of an ocean biogeochemical observing system and that the detection of anthropogenic impacts becomes more likely when using multi-tracer observations.
D2786 |
EGU2020-7423
Esther Portela Rodriguez, Nicolas kolodziejczyk, and Virginie Thierry

The physical transport of dissolved oxygen across the mixed layer base is the main process to oxygenate the interior ocean. This ventilation mechanism is suspected to play a dominant role in the predicted ocean deoxygenation over the 21st century, however, it has not yet been properly quantified or described at global scale. Here we show the mean distribution of the mechanisms driving the oxygen exchanges between the mixed layer and the ocean interior and their relation with water-mass formation. Most of the oxygen uptake occurs in well-defined hot-spots located in the subpolar North Atlantic (30%) which provide oxygen to deep and waters and in the Southern Ocean (37$%) that oxygenates intermediate and bottom waters. The oxygen release is concentrated within the ACC belt (37%), in the subtropical-subpolar North Atlantic (22%) and within the equatorial strip (13%). Globally, the mode waters account for about 72% of the subducted oxygen during their formation process. The oxygen uptake by the Subantarctic and Subpolar Mode Water is driven by strong currents flowing through large mixed layer depth gradients at localized hot-spots while the spatial continuity of the wind-driven vertical velocity over broad areas in the Subtropical gyres accounts for most of the oxygen subduction during the Subtropical Mode Water formation.

D2787 |
EGU2020-9451
Tomas Torsvik and Christoph Heinze

A comparative study of dissolved oxygen concentration outputs from two generations of the NorESM model, NorESM-ME (CMIP5) and NorESM-LM (CMIP6), has been carried out as part of a general model development evaluation. Model output for dissolved oxygen consist of yearly averaged historical data over the period 1850-2000. The dimensionality of this data set was reduction by computing empirical orthogonal functions (EOFs), which are eigenvectors of the spatially weighted anomaly covariance matrix defined by the spatio-temporal dissolved oxygen field. EOF analysis of the two models show similar patterns of dissolved oxygen in the upper ocean (150m depth), with pronounced anoxic conditions in the western Pacific Ocean, Indian Ocean and southern Atlantic Ocean. At 500m depth the model outputs remain mostly in agreement for the Pacific Ocean, but the EOF patterns diverge significantly for both the Indian Ocean and Atlantic Ocean, and to some extent also for the Southern Ocean. For the Indian Ocean, the EOF shift seem to reflect a general reduction of oxygen levels in NorESM-LM compared to NorESM-ME. In the Atlantic Ocean situation is more complex, with NorESM-LM showing reduced oxygen levels near the equator, and enhanced oxygen levels at higher latitudes when compared to NorESM-ME. Further studies are currently in progress to investigate to what extent the similarities and discrepancies in dissolved oxygen concentration can be attributed to ocean temperature and stratification.

D2788 |
EGU2020-18281
Angela Landolfi and Wolfgang Koeve

Ocean warming is projected to cause marine deoxygenation, reduce solubility, affect ocean circulation and enhance metabolic rates over this century. These changes, affecting oceanic N2O production and emissions, have been suggested to potentially rise atmospheric N2O concentrations and increase the positive feedback to anthropogenic climate change.However, current global model projections all suggest a decline in marine N2O emissions under global warming but the processes leading to this decline are poorly constrained. Here, using an Earth system model of intermediate complexity, we disentangle the contribution of ocean deoxygenation and the direct and indirect warming effects on oceanic N2O production and emissions changes under RCP8.5 emission scenario. We find that ocean deoxygenation and warming-reduced N2O solubility do in fact increase oceanic N2O emissions, however this increase is overcompensated by ocean circulation slow-down and reduced export production, suggesting a neglectable N2O-emssion feedback to climate on centennial timescales.

D2789 |
EGU2020-6151
Wanxuan Yao, Karin Kvale, Angela Landolfi, Wolfgang Koeve, Eric Achterberg, and Andreas Oschlies

Increasing the complexity of the representation of iron in an earth system model can lead to significant differences in surface ocean nutrient pathways in a pre-industrial climate. These differences persist even after automated calibration forces the models to achieve similar fit to the same observational data. We explore the impact of these nutrient pathway differences in the context of climate change by forcing the models (one without iron, one with a seasonally-cyclic iron mask, and one with a fully dynamic iron module) with the RCP8.5 business-as-usual atmospheric CO2 concentration scenario from years 1800 until 2100. We find that while the global oxygen inventory drops across all models over this period, different trends in the oxygen minimum zone (OMZ) volume arise. Models with iron represented simulate decreases between 60 and 80 percent in OMZ volume, while the model without iron simulates an OMZ volume increase of 10 percent. The difference is attributed to the role of iron limitation in regulating the low latitude primary production response to warming and stratification. We further quantify the corresponding denitrification trends and impact on ocean nitrate inventory. This study illustrates that model structural uncertainty further challenges predictions under a changing climate, and highlights the strong role of iron in regulating nutrient cycling and ocean deoxygenation.

D2790 |
EGU2020-13038
Wolfgang Koeve and Angela Landolfi

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.

D2791 |
EGU2020-13393
Sarah L. Wakelin, Yuri Artioli, Momme Butenschön, Jason Holt, and Jeremy Blackford

Dissolved oxygen in the ocean is an indicator of water quality and low concentrations can threaten ecosystem health. The main sources of marine oxygen are diffusion from the atmosphere and phytoplankton photosynthesis. Biological respiration and decomposition act to reduce oxygen concentrations. Under conditions of vertical stratification, the water column below the pycnocline is isolated from oxygen exchange with the atmosphere, photosynthesis may be limited by light availability and oxygen concentrations decrease. Climate change influences the oxygen cycle in two ways: 1) changing the hydrodynamic climate and 2) affecting rates of biogeochemical processes. The hydrodynamic climate affects the nutrient supply and so controls phytoplankton production while changes to water column stratification affects vertical mixing. Gas solubility decreases with increasing temperature so that oxygen uptake from the atmosphere is expected to decrease under increasing oceanic temperatures. Biological cycling rates increase with increasing temperature affecting photosynthesis, respiration and bacterial decomposition. It is not obvious whether changes in oxygen concentrations due to changing ecosystem processes will mitigate or reinforce the projected reduction from solubility changes.

The Northwest European Continental shelf (NWES) is a region of the northeast Atlantic that experiences seasonal stratification. We use the physics-biogeochemical model NEMO-ERSEM to study near-bed oxygen concentrations on the NWES under a high greenhouse gas emissions scenario (Representative Concentration Pathway (RCP) 8.5). We show that much of the NWES could experience low oxygen concentrations by 2100 and assess the relative impacts of changing temperature and ecosystem processes. Until about 2040 the impact of solubility dominates the oxygen change. The mean near-bed oxygen concentration is projected to decrease by 6.3% by 2100, of which 73% is due to solubility changes and the remainder to changes in the ecosystem. In the oxygen-depleted region in the eastern North Sea, 77% of the near-bed oxygen reduction is due to ecosystem processes.

D2792 |
EGU2020-21909
Natalia Herran and Martin Schmidt

Understanding the multilevel complexity of marine ecosystems is one of the greatest challenges on ecosystem modeling so far, due to the dualism of governing hydrodynamical processes acting on a regional scale and complex biogeochemical chain reactions that happen locally on the marine environment. A coupled hydrodynamic-ecological model based on nitrogen stoichiometry has been developed to better understand the short-term nutrient and oxygen coastal dynamics in the Benguela Upwelling System (BUS). The model shows that the effect of internal waves in the Benguela region re-shapes the benthic ecosystem due to the increased of turbulence on the ocean floor with a consequently increase of fine sediment on the water column. We show that an increase on organic-rich sediment resuspension on the water column enhance oxygen consumption and ultimately contribute to the apparent deoxygenation of the Namibian coastal shelf.

D2793 |
EGU2020-20993
Oscar Pizarro, Marcela Contreras, Nadín Ramírez, and Matías Pizarro-Koch

Modeling studies have shown that mesoscale eddies significantly contribute to modulate the variability of the oxygen minimum zone (OMZ) of the eastern South Pacific at seasonal and interannual time scales. Nevertheless, only few observations have shown the properties of these eddies. Particularly subsurface (intrathermocline) eddies may play an important role in the dynamics of the southern tip of this OMZ. In this work we analyze the characteristics of these eddies based on underwater glider observations, along with oceanographic cruises and satellite data. We also combine our observations with results from a high resolution numerical model to analyze the generation mechanism of these subsurface eddies. Observations show that the eddies are characterized by a core with high salinity (SA > 34.6 g kg-1), low oxygen (DO < 0.5 mL L-1)  and relatively low potential vorticity  (f PV < 10-13 s-4, where f is the Coriolis parameter). The eddy core is typically centered around σθ ~ 26.5 kg m3 (150-200 m depth) and their diameters are about 50 km, transporting typically ~0.2 Sv of very low-oxygen (< 0.5 mL L-1) waters offshore. The eddy core properties coincide with the water mass that is transported by the Peru-Chile Undercurrent. Our modeling study shows that the generation of the subsurface eddies is associated with the separation of the Undercurrent from the slope and current reversals (northward subsurface flow) close to the slope.

D2794 |
EGU2020-6473
Allan Devol and Wendi Ruef

 

At this point ocean deoxygenation is well documented, including in oxygen minimum zones (OMZs).  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).  How do we determine if these ODZ are losing O2?  Furthermore, denitrification occurs in oxygen minimum zones (OMZs) so one might hypothesize that denitrification is likewise expanding if oxygen is decreasing.  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  6 repeats of a 1000 km transect along 110o West in the heart of the ETNP ODZ between 1971-2019.  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.  

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.

D2795 |
EGU2020-6612
Mark Hopwood, Allanah Paul, Thomas Browning, Madeleine Freund, Toste Tanhua, Insa Rapp, Martha Gledhill, and Eric Achterberg

In regions bordering productive eastern-boundary upwelling systems, extensive O2 minimum zones (OMZs) develop along the ocean shelf. Enhanced sedimentary release of Fe and PO4 under these low O2 conditions could drive enhanced N2-fixation and primary production in the ocean, thus ultimately increasing O2 consumption, and potentially creating a positive feedback to ocean deoxygenation. A similar feedback loop has been identified in shallow and enclosed coastal regions such as the Baltic where primary production is controlled primarily by N and P bioavailability, but it is unclear to what extent a similar mechanism operates at a larger scale in the open ocean where productivity is proximally constrained by Fe and/or N availability. This is largely because of uncertainties in the fate of the Fe released from shelf sediments.

Here we present extensive Fe measurements from a series of 5 cruises and a mesocosm experiment on the Peruvian shelf which combined extensive measurements of Fe distribution with an inert tracer release experiment, to quantify off-shelf transport, and N2 fixation rates. As expected for a region with among the highest reported benthic Fe fluxes in the world, dissolved Fe concentrations were generally elevated along the inner-Peruvian shelf reaching >60 nM. Whilst concentrations rapidly declined across the shelf, reaching as low as 0.01 nM after the shelf break, ‘pockets’ of 1-2 nM elevated Fe concentrations were evident in some transects suggesting a significant role of eddies in off-shelf transport which was verified by the results of our CF3SF5 tracer release experiment. Never-the-less, benthic Fe release was rapidly attenuated close to the sediment interface, with the vast majority of Fe loss occurring on timescales of <1 day and spatial scales of <1 km. Evidence of Fe limitation was even found at some stations on the Peruvian shelf questioning the efficiency with which Fe released from sediments is able to positively influence marine primary production. Despite an excess of P across the region with respect to biological requirements, and extremely high inner-shelf Fe concentrations, N2 fixation rates remained consistently low across the region (0-0.8 nmol N L-1 d-1). Furthermore, the maximum lateral transfer of Fe was de-coupled spatially from maximum benthic Fe release, limiting the potential for a P/Fe-fueled positive feedback loop to OMZ expansion.

D2796 |
EGU2020-11803
Eike Eduard Köhn, Matthias Münnich, Meike Vogt, and Nicolas Gruber
The oxygen minimum zones (OMZs) in the Eastern Tropical Pacific (ETP)
have expanded over the past 50 years, likely leading to more frequent and more
intense low oxygen extreme events. This has potentially far-reaching implica-
tions for e.g., the production of the climate-relevant gas nitrous oxide or the
reduction of habitat for fish and zooplankton. Yet, to date our understanding
of the distribution and characteristics of low oxygen extreme events in the ETP
remains limited.
 
To fill this gap, we study low oxygen extremes in the ETP using results from
an eddy-resolution hindcast simulation with the coupled physical-biogeochemical
model ROMS-BEC for the Pacific from 1979 to 2016. Our setup permits us to
simulate oxygen variability in the ETP affected by processes on a broad range
of scales, from climate modes down to mesoscale dynamics. We detect and
track low oxygen extreme events in the upper 500 meters of the ETP, by ap-
plying temporally constant statistical thresholds to the hindcast simulation and
requiring a minimum event duration of 5 days. While most extremes last less
than 10 days and are of small volumetric extent, about 15% of the extremes
exist for over a month. The diversity of the long-lasting extremes is dominated
by westward propagating low oxygen eddies, which are mostly generated in the
near-coastal area. Superimposed inter-annual variability associated with the El
Niño-Southern Oscillation (ENSO) leads to a decrease in mesoscale extremes
during El Niño periods. Along the boundaries of the ETP OMZs transient
shoaling events of the oxycline linked to ENSO dynamics or the seasonal cycle
contribute to the generation of further pronounced low oxygen extreme events.
 
The presented detection and tracking of low oxygen extremes is an important
step towards a better understanding of extreme event occurrences and charac-
teristics and lays the groundwork for further research such as the biogeochemical
impact of such extremes.
D2797 |
EGU2020-4732
Babette Hoogakker, Christopher Day, and Melanie Leng

Intermediate waters (500 - 2000 m) from the equatorial- to North Pacific are currently hypoxic (oxygen concentrations below 120 µmol/kg), while deeper waters are well oxygenated. For the last ice-age, some proxy records suggested that this trend was reversed, with well-oxygenated Pacific intermediate waters, and lower oxygenated deeper waters associated with an increased deep carbon reservoir. Recent work however suggests that there was an overall expansion of oxygen depleted water in the eastern tropical North Pacific during the last glacial period (Hoogakker et al., 2018). To further assess the natural variability in intermediate water dissolved oxygen concentrations over longer time-scales we extend the bottom water oxygen record of ODP Site 1242 (1360 m depth located in the eastern tropical north Pacific), to 140,000 years, using  the benthic foraminifera carbon isotope gradient approach of Hoogakker et al. (2015).  Our reconstructions suggest that oxygen concentrations varied with an approximate 40 kyr period; with lowest concentration during cool periods of the penultimate glacial, MIS 5b, 4 and 2.

D2798 |
EGU2020-12973
Sami Jokinen, Karoliina Koho, Joonas Virtasalo, and Tom Jilbert

Molybdenum (Mo) and uranium (U) contents in sedimentary records are commonly used to track past changes in seafloor oxygenation. However, inadequate understanding of Mo and U sequestration mechanisms in non-euxinic coastal areas limits their use as redox proxies in these settings. Because large areas of the coastal oceans are currently undergoing partial deoxygenation due to anthropogenic nutrient inputs and increased stratification, it is critical to improve our understanding of these proxies to allow robust assessment of the trajectory of environmental change. Here, we use a comprehensive set of sediment pore water and solid-phase analyses to deconvolve the mechanisms of authigenic Mo and U sequestration in a shallow non-euxinic coastal setting in the northern Baltic Sea. Despite the permanently oxic bottom waters in the area, eutrophication over the past decades has led to establishment of a shallow sulfate-methane transition zone (SMTZ) in the sediment, which is typical for human-impacted coastal settings on a trajectory towards hypoxia. Our results demonstrate remarkably synchronous patterns of Mo and U sequestration, whereby their authigenic uptake is largely predicated upon the depth and intensity of the SMTZ. Based on sequential extraction analyses, the authigenic Mo pool is dominated by refractory Fe-S phases such as pyrite and nanoscale FeMoS4, signaling that authigenic Mo uptake largely proceeds through the Fe-sulfide pathway. In addition, we observe a pool of extremely labile Mo deep within the SMTZ, potentially denoting a transient phase in authigenic Mo uptake and/or partial switch in the mode of sequestration to the organic matter pathway at low levels of dissolved iron. Authigenic U is largely hosted by acid-extractable and refractory phases, reflecting sequestration into poorly crystalline monomeric U(IV) and crystalline uraninite, respectively. Analogously to Mo, authigenic sequestration of U proceeds at two distinct fronts within the SMTZ, which are characterized by shifts in dissolved sulfide concentrations, providing strong evidence for a link between sulfide-producing processes and U reduction. Our results imply that both Mo and U have the potential to capture temporal shifts in bottom water oxygenation indirectly, through the connection between oxygenation and the depth of the SMTZ. Of the two elements, Mo appears a more viable redox proxy because of the substantially higher share of the authigenic pool. However, temporal resolution of these proxies is restricted by the relatively deep authigenic uptake within the sediment column and the integrated character of the signal caused by vertical migrations of the SMTZ. These findings set a framework for interpreting sedimentary Mo- and U-based paleoredox archives in other non-euxinic coastal settings.

D2799 |
EGU2020-964
Bikash Ranjan Sahu, Sohom Roy, and Prasanta Sanyal

The Cenomanian-Turonian (C/T) Oceanic Anoxic Event 2 (OAE 2) at ~94 million years ago was characterized by severe depletion in marine water oxygen levels and extreme perturbations in the carbon cycle at a global scale that lasted for 5 to 6-million years. However, wealth of the data comes mainly from deep marine records, hugely limiting our understanding on the contemporaneous terrestrial environmental conditions. Here, we present major and trace element concentrations, carbon isotope composition of carbonates (δ13Ccarb) and organic matter (δ13Cbulk), organic carbon content (TOC), and biomarker composition from a ~20 m thick well-preserved shallow marine sequence from the Bagh Beds in Uchad, western India in order to investigate the nutrient dynamics, productivity variations and carbon reservoir perturbations in shallow marine as well as in terrestrial environment. Based on litho-stratigraphy, the Uchad section is divided into Lower Cenomanian, Turonian and Upper Coniacian units. A total of ~5‰ increase in the δ13Ccarb and 0.07% in TOC values and a sharp 1.7‰ decrease in the δ13Cbulk values in Lower Cenomanian suggest large changes in organic carbon recycling before the advent of OAE 2. Higher terrigenous influx and micro-nutrient supply in the lower parts is also suggested from relatively higher concentrations of Al, Ti, Th, Fe, Zn, Ni and K, although their concentrations decrease rapidly above the C/T boundary. Significant correlation observed between δ13Cbulk and δ13Ccarb (r=0.51, p=0.03) supports an authigenic organic matter production in the shallow marine environment. However, minor enrichments in redox-sensitive elements like Mo, V and U observed above the C/T boundary probably suggest that the shallow marine region was relatively less affected during the initial anoxic phases. Lack of correlation between redox-sensitive elements and Al or Ti concentrations (r <0.12) suggest that there is minimal influence of detrital supply on recycling of U, V and Mo. Interestingly, Lower Turonian units show large positive excursions in redox-sensitive elements as well as increases in U/Th, Ni/Co and V/(V+Ni) values, which are succeeded by a major decrease in δ13Ccarb values (7.6‰) and increase in the TOC values by 0.15%, thereby suggesting occurrence of a more expanded episode of anoxia in Lower Turonian that perturbed the shallow marine carbon reservoir. Ba/Al ratios are variable throughout the section, although large positive spikes preceding and succeeding the anoxic phases suggest a causal link between organic matter productivity and anoxia.