Global Deoxygenation Research Articles

Oxygen and irradiance constraints on visual habitat in a changing ocean: The luminoxyscape

Oxygen and irradiance constraints on visual habitat in a changing ocean: The luminoxyscape

Orbitally paced global oceanic deoxygenation decoupled from volcanic CO2 emission during the middle Cretaceous Oceanic Anoxic Event 1b (Aptian-Albian transition)

Abstract The ongoing oceanic oxygen loss will have a profound impact on the distribution and density of life on Earth. However, drivers of the initiation and termination of global oceanic deoxygenation are poorly understood. Here we present a thallium isotope record that reveals three rapid (50 k.y.) global deoxygenation intervals during the mid-Cretaceous that postdate massive volcanism by at least 1 m.y. New strontium isotope data reveal gradually enhanced continental weathering under a warmer climate following volcanism. However, global deoxygenation occurred only under the combined influences of a long-term increase in weathering rates in a warmer climate and short-term orbital modulation that led to atmospheric-circulation reorganization. Interactions of multiple drivers are consistent with the abrupt termination of each deoxygenation interval. Dynamic oxygenation responses in the mid-Cretaceous highlight the role of these processes for understanding the consequences and potential termination of the current oceanic deoxygenation.

Cryptic Sulfur and Oxygen Cycling Potentially Reduces N2O-Driven Greenhouse Warming: Underlying Revision Need of the Nitrogen Cycle.

Increasing global deoxygenation has widely formed oxygen-limited biotopes, altering the metabolic pathways of numerous microbes and causing a large greenhouse effect of nitrous oxide (N2O). Although there are many sources of N2O, denitrification is the sole sink that removes N2O from the biosphere, and the low-level oxygen in waters has been classically thought to be the key factor regulating N2O emissions from incomplete denitrification. However, through microcosm incubations with sandy sediment, we demonstrate here for the first time that the stress from oxygenated environments does not suppress, but rather boosts the complete denitrification process when the sulfur cycle is actively ongoing. This study highlights the potential of reducing N2O-driven greenhouse warming and fills a gap in pre-cognitions on the nitrogen cycle, which may impact our current understanding of greenhouse gas sinks. Combining molecular techniques and kinetic verification, we reveal that dominant inhibitions in oxygen-limited environments can interestingly undergo triple detoxification by cryptic sulfur and oxygen cycling, which may extensively occur in nature but have been long neglected by researchers. Furthermore, reviewing the present data and observations from natural and artificial ecosystems leads to the necessary revision needs of the global nitrogen cycle.

New Constraints on Global Geochemical Cycling During Oceanic Anoxic Event 2 (Late Cretaceous) From a 6‐Million‐year Long Molybdenum‐Isotope Record

AbstractIntervals of extreme warmth are predicted to drive a decrease in the oxygen content of the oceans. This prediction has been tested for the acme of short (<1 million years) episodes of significant marine anoxia in the Phanerozoic geological record known as Oceanic Anoxic Events (OAEs). However, there is a paucity of data spanning prolonged multimillion‐year intervals of geological time before and after OAEs. We present a Mo‐isotope record from limestones and marlstones of the Eagle Ford Group, South Texas, which was deposited in the southern Cretaceous Western Interior Seaway of North America during a 6‐million‐year period encompassing OAE 2 (Late Cenomanian–early Turonian: ∼94 Ma). Mo‐isotope compositions from deposits that formed in euxinic (sulfidic) conditions before OAE 2 allow the paleo‐seawater composition to be constrained to 1.1%–1.9%. This range of values overlaps previous estimates of up to ∼1.5% for the peak of OAE 2 determined from similarly sulfidic sediments deposited in the restricted proto‐North Atlantic Ocean. Mo‐isotopes thus varied by less than a few tenths of per mil across one of the most extreme intervals of global deoxygenation in the Late Phanerozoic. Rather than a limited change in oceanic deoxygenation, we suggest that the new data reflect changes to global iron cycling linked to basalt‐seawater interaction, terrestrial weathering and expanded partially oxygenated shallow shelf‐seas that played a key role in the burial of isotopically light molybdenum, thus acting as a counterbalance to its removal into sulfidic sediments.

Geoengineered Ocean Vertical Water Exchange Can Accelerate Global Deoxygenation

AbstractOcean deoxygenation is a threat to marine ecosystems. We evaluated the potential of two ocean intervention technologies, that is, “artificial downwelling (AD)” and “artificial upwelling (AU),” for remedying the expansion of Oxygen Deficient Zones (ODZs). The model‐based assessment simulated AD and AU implementations for 80 years along the eastern Pacific ODZ. When AD was simulated by pumping surface seawater to the 178–457 m‐depth range of the ODZ, vertically integrated oxygen increased by up to 4.5% in the deployment region. Pumping water from 457 m depth to the surface (i.e., AU), where it can equilibrate with the atmosphere, increased the vertically integrated oxygen by 1.03%. However, both simulated AD and AU increased biological production via enhanced nutrient supply to the sea surface, resulting in enhanced export production and subsequent aerobic remineralization also outside of the actual implementation region, and an ultimate net decline of global oceanic oxygen.

Major Contribution of Reduced Upper Ocean Oxygen Mixing to Global Ocean Deoxygenation in an Earth System Model

AbstractWe present a quantitative analysis of deoxygenation drivers applied to an Earth System Model and easily transposable to large model ensembles. The preindustrial ocean breathes in oxygen in polar regions and in subtropical gyres, and breathes out oxygen in the equatorial band and in subpolar gyres. Under a high‐CO2 emission scenario for the 21st century, small deviations of these large natural oxygen fluxes cause global deoxygenation. We attribute half of this trend to a decrease in oxygen solubility. The other half is explained by negative trends in subduction and respiration, which largely cancel out each other. Moreover, 75% of the subduction decrease occurs through changes in mixing across the mixed‐layer base. Our analysis also highlights strong modulations of subduction at the regional scale linked to shifts in wind patterns and associated Ekman pumping.

Projected Centennial Oxygen Trends and Their Attribution to Distinct Ocean Climate Forcings

AbstractWe explore centennial changes in tropical Pacific oxygen (O2) using numerical models to illustrate the dominant patterns and mechanisms under centennial climate change. Future projections from state‐of‐the‐art Earth System Models exhibit significant model to model differences, but decreased solubility and weakened ventilation together deplete thermocline O2 in middle to high latitudes. In contrast, the tropical thermocline O2 undergoes much smaller changes or even a slight increase. A suite of sensitivity experiments using a coarse resolution ocean circulation and biogeochemistry model show that ocean warming is the leading cause of global deoxygenation in the thermocline across all latitudes with secondary contributions from changes in hydrological cycles and wind stress modulating regional changes in O2. The small O2 changes in the tropical Pacific thermocline reflect near‐complete compensation between the solubility decrease due to warming and reduction in apparent oxygen utilization (AOU). We further quantified the changes in AOU due to contributions from changes in water mass age and biological remineralization from the sensitivity experiments. The two effects almost equally contribute to the reduction of AOU in the tropical Pacific thermocline (43% for physical circulations and 57% for biology). Our results suggest that better understanding of water mass changes in the tropical oceans is key to improving projections and reducing the uncertainties of future O2 changes.

Manifestation, Drivers, and Emergence of Open Ocean Deoxygenation.

Oxygen loss in the ocean, termed deoxygenation, is a major consequence of climate change and is exacerbated by other aspects of global change. An average global loss of 2% or more has been recorded in the open ocean over the past 50-100 years, but with greater oxygen declines in intermediate waters (100-600 m) of the North Pacific, the East Pacific, tropical waters, and the Southern Ocean. Although ocean warming contributions to oxygen declines through a reduction in oxygen solubility and stratification effects on ventilation are reasonably well understood, it has been a major challenge to identify drivers and modifying factors that explain different regional patterns, especially in the tropical oceans. Changes in respiration, circulation (including upwelling), nutrient inputs, and possibly methane release contribute to oxygen loss, often indirectly through stimulation of biological production and biological consumption. Microbes mediate many feedbacks in oxygen minimum zones that can either exacerbate or ameliorate deoxygenation via interacting nitrogen, sulfur, and carbon cycles. The paleo-record reflects drivers of and feedbacks to deoxygenation that have played out through the Phanerozoic on centennial, millennial, and hundred-million-year timescales. Natural oxygen variability has made it difficult to detect the emergence of a climate-forced signal of oxygen loss, but new modeling efforts now project emergence to occur in many areas in 15-25 years. Continued global deoxygenation is projected for the next 100 or more years under most emissions scenarios, but with regional heterogeneity. Notably, even small changes in oxygenation can have significant biological effects. New efforts to systematically observe oxygen changes throughout the open ocean are needed to help address gaps in understanding of ocean deoxygenation patterns and drivers.

Marine ecosystem resilience during extreme deoxygenation: the Early Jurassic oceanic anoxic event.

Global warming during the Early Jurassic, and associated widespread ocean deoxygenation, was comparable in scale with the changes projected for the next century. This study quantifies the impact of severe global environmental change on the biological traits of marine communities that define the ecological roles and functions they deliver. We document centennial-millennial variability in the biological trait composition of Early Jurassic (Toarcian) seafloor communities and examine how this changed during the event using biological traits analysis. Environmental changes preceding the global oceanic anoxic event (OAE) produced an ecological shift leading to stressed benthic palaeocommunities with reduced resilience to the subsequent OAE. Changes in traits and ecological succession coincided with major environmental changes; and were of similar nature and magnitude to those in severely deoxygenated benthic communities today despite the very different timescales. Changes in community composition were linked to local redox conditions whereas changes in populations of opportunists were driven by primary productivity. Throughout most of the OAE substitutions by tolerant taxa conserved the trait composition and hence functioning, but periods of severe deoxygenation caused benthic defaunation that would have resulted in functional collapse. Following the OAE recovery was slow probably because the global nature of the event restricted opportunities for recruitment from outside the basin. Our findings suggest that future systems undergoing deoxygenation may initially show functional resilience, but severe global deoxygenation will impact traits and ecosystem functioning and, by limiting the species pool, will slow recovery rates.