Widespread Ocean Anoxia to be Noticeable by 2030

[Paladiea]

Came across this the other day, how credible is it, and apologies if this has come up elsewhere.

https://www.sciencedaily.com/releases/2016/04/160427150914.htm

[AbruptSLR]

Paladiea,

Thanks for starting this thread in the Consequences folder.  For other information on hypoxia in the linked Carbon Cycle thread (in the Science folder), see (at least) Replies: 290, 291, 297, and 299:
http://forum.arctic-sea-ice.net/index.php/topic,77.0.html

For other information about hypoxia & the Canfield Ocean, see also:
http://forum.arctic-sea-ice.net/index.php/topic,1380.0.html
&
https://en.wikipedia.org/wiki/Canfield_ocean

Best,
ASLR

[A-Team]

When that article first appeared, I recall one of the (thoroughly disgusted) authors telling the press that the onset was first noticed by another research group in 2007-08 'but the scientists involved didn't say anything' not wanting to alarm anyone or draw attention to themselves. I have not yet been able to relocate the exact source for that.

The article here is just talking about too little dissolved oxygen, not euxinic like the Black Sea, not toxic sulfide like Canfield ocean. Same thing that happens in a home aquarium if the bubbler stops ... consumption exceeds replenishment.

[Paladiea]

What I find shocking is that this wasn't front page news everywhere...

Thank you SLR, by the way. I will be reading that topic thoroughly, but I want to pick your brain for a quick second...

Given that the rate of warming we're seeing is technically more than twice that of the P-Tr extinction on average, and looking at the temperature records from about 300 mya to 250 mya, we see a disturbingly similar global average temperature spike from about -2C to about 11C (1961-1990 baseline):

1) How likely is it that we're recreating the conditions that led to to the P-Tr extinction?
2) Will we see things that characterized that extinction in my lifetime (I'm a millennial)?

The article here is just talking about too little dissolved oxygen, not euxinic like the Black Sea, not toxic sulfide like Canfield ocean. Same thing that happens in a home aquarium if the bubbler stops ... consumption exceeds replenishment.

Could you clarify the difference for me?

[AbruptSLR]

1) How likely is it that we're recreating the conditions that led to to the P-Tr extinction?
2) Will we see things that characterized that extinction in my lifetime (I'm a millennial)?

Paladiea,

First, I am not an expert on this topic; but nevertheless, my biggest concern about a possible Canfield Ocean event would be if the West Antarctic Ice Sheet, WAIS, partially collapses this century, leading to an ice-climate feedback that slows the thermohaline circulation ala Hansen et al (2016), see the following link:

https://forum.arctic-sea-ice.net/index.php/topic,1327.0.html

However, I think that if the WAIS collapses the other impacts (sea level rise and violent storms) will be a much bigger concern than hypoxia in your lifetime (due to the size of the ocean).  Nevertheless, if the WAIS does start to collapse in the next couple of decades, then you might experience large areas of the tropical seas subjected to hypoxia before you die.

Best,
ASLR

[Paladiea]

How would the slowing down of the thermohaline circulation cause a Canfield ocean event?

Why would the WAIS cause hypoxia if it collapsed?

I'm confused as to the mechanism and asking genuinely. From what I understand, Canfield ocean events are caused by anaerobic marine organisms...

[AbruptSLR]

I do not have much spare time now so if you want more information you can read the following linked thread:

https://forum.arctic-sea-ice.net/index.php/topic,1380.0.html

But in short, if the WAIS collapses it injects cold freshwater into the Southern Ocean & the injection of so much cold water could lead to a stratification of the water column, with warm water buried underneath cold surface water.   As Hansen stated: “Instead of emerging at the surface, much of that heat is melting the ice shelves,” thus producing more fresh water and amplifying the feedback.  This results in a slowing of the thermohaline circulation as this circulation is partially driven by cold Antarctic Bottom Water, AABW, sinking along the coast of Antarctica.  As the circulation slows  the ventilation of the intermediate waters (in the Oxygen Minimum Zones, OMZ, from 200 to 1000m depth) also slows down; which deprives these depths from adequate re-oxygenation that leads to hypoxia.

[Paladiea]

Alright, I think I understand the mechanism.

Lots of fresh water = themohaline circulation stoppage = stagnation at ocean depths.

However, reading the abstract from the blog post you linked to, the authors don't actually link the anoxia to that particular mechanism; instead they point out that if the planet warms by 6C the oceans will be too warm for phytoplankton to photosynthesize, leading to anoxia through depletion...

https://link.springer.com/article/10.1007%2Fs11538-015-0126-0

Is it possible that a Canfield ocean could have multiple triggers, and the WAIS is one of several? If so, could we be on the verge of a trigger that would preempt it?

[AbruptSLR]

Yes, multiple different factors would likely be necessary to even regionally reach Canfield Ocean conditions in your lifetime and changes in ocean circulation patterns (see attached image & the following link to Scribbler's August 2015 article) is only one of many including increasing ocean temperature, and biological causes.

https://robertscribbler.com/2015/08/28/shades-of-a-canfield-ocean-hydrogen-sulfide-in-oregons-purple-waves/

As Hansen et al (2016) is a recent paper, most other researchers have not yet fully considered the impacts of the ice-climate feedback mechanism; and for discussions of other issues that other researchers have not yet fully considered you might want to scan the contents of the following threads.  The first link is to a thread on "Human Stupidity" & the second is entitled: "Conservative Scientists & its Consequences":

http://forum.arctic-sea-ice.net/index.php/topic,1548.0.html
&
http://forum.arctic-sea-ice.net/index.php/topic,1053.0.html

Edit: For a general discussion on this topic see the following linked SkS article:

https://www.skepticalscience.com/Ocean_Oxygen_another_climate_shoe_dropping.html

[AbruptSLR]

I think that coming hypoxia will contribute to regional sea desertification, as is currently occurring of the coast of Vietnam:

http://english.vietnamnet.vn/fms/environment/160066/risk-of--sea-desertification--challenges-vietnam.html

Extract: "'Sea desertification’ is a new terminology used to describe the area where all sea creatures cannot live because of poor natural conditions, water quality and land.

 According to experts from the General Department of Sea and Islands, there are six factors which can lead to sea desertification. First, climate change.

 Second, the discharging of hazardous waste to the sea, causing greenhouse gas emissions. This causes an increase in the acid concentration in the sea, disrupts the supply of nutrients to the sea, reduces biodiversity and interrupts the periodic operation of oxygen, nitrogen and phosphorous.

 The ocean acidification causes coral bleaching, habitat for sea creatures changing, the water quality degrading and nutrients decreasing, which then causes desertification.

The third reason lies in ocean currents.

 Fourth, the destruction of important marine ecosystems such as coral ecosystems, mangrove and sea grass ecosystems by explosives and toxic chemicals. Fifth, the rapid development of coastal urban areas and industrial zones which spoils the air quality, surface water and sea water.

 Finally, the air transportation, which concentrates in some certain areas."

[Paladiea]

The problem with a Canfield ocean is the massive production of hydrogen sulphide. That is produced by anaerobic bacteria. Do those bacteria need anything beyond "ocean deserts" to thrive?

[AbruptSLR]

The problem with a Canfield ocean is the massive production of hydrogen sulphide. That is produced by anaerobic bacteria. Do those bacteria need anything beyond "ocean deserts" to thrive?

It is not clear to me whether you are looking at any of the information at the links that I provide, but the attached image (from the following link) shows that the biological pump brings plenty of nutrients for the anaerobic bacteria.  That said, we are currently a long ways away from a Canfield Ocean conditions (except in local areas):

https://www.skepticalscience.com/Ocean_Oxygen_another_climate_shoe_dropping.html

[Paladiea]

I think there's some miscommunication going on.

When you refer to the Canfield Ocean event, you're referring to deep ocean anoxia, the shutdown of the thermohaline etc.

What I mean when I asked the question about anaerobic bacteria is that if ocean surface layers go anoxic I don't see any reason they wouldn't be able to thrive in those dead zones and be just as detrimental to life on land.

[AbruptSLR]

I think there's some miscommunication going on.

When you refer to the Canfield Ocean event, you're referring to deep ocean anoxia, the shutdown of the thermohaline etc.

What I mean when I asked the question about anaerobic bacteria is that if ocean surface layers go anoxic I don't see any reason they wouldn't be able to thrive in those dead zones and be just as detrimental to life on land.

It is my understanding that generally, there is so much oxygen (which kills anaerobic bacteria) in the oceans surface layers that they will not become anoxic; and the risk to life on land happens in zones of the ocean with upwelling of deeper (intermediate) anoxic water.

[A-Team]

The paper under discussion considers a subtle new effect of climate change: slightly lowered oxygen levels in the ocean, especially at depth, coming this century. Its focus is really on attribution to global warming against the strong background noise of natural variability.

This is not a paper to conflate with anoxia, contemporary dead zones, meromictic lakes, euxinic waters or ocean acidification. It never mentions the Canfield paleo ocean hypothesis.The slight changes in dissolved O₂ under discussion would have no discernible effect on multi-cellular ocean species viability during this century. It's a distant third to near-surface warming and acidification effects which are already well underway.

The paper consists solely of model runs. Experimental attribution to global warming is currently untestable because scarcely any baseline data exists and no instrumentation is in place or planned. Ocean oxygenation is affected by a great many other processes.

However as the authors say, a small effect on ocean oxygenation attributable to global warming might have occurred already in certain regions of the ocean. The paper discusses its signature.

The history of oxygen in the earth's atmosphere and ocean is very complex and like all paleo, rapidly becomes more uncertain the farther back in time, degenerating to barely supported but strongly voiced opinions. The area suffers from a lack of funding and a resulting small community of active researchers.

Oxygen cannot be considered in isolation. It is all mixed in with hydrogen, hydrogen sulfide, nitrogen, nitrogen fixation, methane, iron oxidation states, iron sulfides, insoluble calcium and magnesium salts, on and on, in an evolving atmosphere and biosphere. 

The broader context makes for an excellent forum topic worthy of a decent exposition. However that takes a strong background in biochemistry, microbiology, oxidation-reduction chemistry, photosynthesis, quantum chemistry of molecular bonds, free radical chemistry in the upper atmosphere, oceanography and especially biogeochemistry -- before even throwing climate change models into the mix.

I am ok writing a lot this up -- starting with contemporary dead zones, photosynthetic anaerobes that thrive on hydrogen sulfide, and the connections between sulfate oxidation, methane release and CO₂ acidification -- but who is going to read it?

An alternative is a forum dedicated to stratification, overturning and mixing of ocean waters and the coming impacts from climate change. That is important for oceanography and heat redistribution but is a very different topic from ocean oxygenation and marine biogeochemistry.

http://butane.chem.uiuc.edu/pshapley/GenChem1/L9/web-L9.pdf intro to N₂ and O₂

[Paladiea]

The broader context makes for an excellent forum topic worthy of a decent exposition. However that takes a strong background in biochemistry, microbiology, oxidation-reduction chemistry, photosynthesis, quantum chemistry of molecular bonds, free radical chemistry in the upper atmosphere, oceanography and especially biogeochemistry -- before even throwing climate change models into the mix.

Not sure you would need that much expertise, perhaps to document the nitty gritty about what's happening via metrics, but a general discussion wouldn't need to be that in depth.

I am ok writing a lot this up -- starting with contemporary dead zones, photosynthetic anaerobes that thrive on hydrogen sulfide, and the connections between sulfate oxidation, methane release and CO2 acidification -- but who is going to read it?

I would love to read that!

It is my understanding that generally, there is so much oxygen (which kills anaerobic bacteria) in the oceans surface layers that they will not become anoxic; and the risk to life on land happens in zones of the ocean with upwelling of deeper (intermediate) anoxic water.

The paper mentions that ocean photosynthesis constitutes 70% of atmospheric oxygen production, with phytoplankton being the largest producer; contributing 50% of atmospheric oxygen.

We know that oxygen is highly reactive, and if the oceans get too warm for phytoplankton photosynthesis, wouldn't a 50% reduction of oxygen production not cause widespread anoxia?

[Paladiea]

As an aside, my background is botany, with more than average experience with thermodynamics and quantum interactions between light and molecules.

[sidd]

There is some discussion by Kidder and Worsley in this series of papers
doi:10.1016/S0031-0182(03)00667-9
doi:10.1016/j.palaeo.2010.05.036
doi: 10.1130/G131A.1

although in a larger context. I like these papers.

[AbruptSLR]

It is my understanding that generally, there is so much oxygen (which kills anaerobic bacteria) in the oceans surface layers that they will not become anoxic; and the risk to life on land happens in zones of the ocean with upwelling of deeper (intermediate) anoxic water.

The paper mentions that ocean photosynthesis constitutes 70% of atmospheric oxygen production, with phytoplankton being the largest producer; contributing 50% of atmospheric oxygen.

We know that oxygen is highly reactive, and if the oceans get too warm for phytoplankton photosynthesis, wouldn't a 50% reduction of oxygen production not cause widespread anoxia?

By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases.  So by volume there is about 537 times as much oxygen in the atmosphere as there is CO₂.  So the changes in CO₂ concentrations have much more impact than do the changes in O₂.  Per the linked website, O₂ concentration is decreasing at a rate of about 19 molecule out of every 1 million molecules per year. So in about 100-years the percent of oxygen in the atmosphere might decrease from about 20.95% to about 20.946% by volume.  Thus your concern that meaningful portions (besides local/temporary cases) of the surface layers of the ocean will become anoxic in your lifetime are unfounded:

http://scrippso2.ucsd.edu/

Extract: "Oxygen levels are decreasing globally due to fossil-fuel burning. The changes are too small to have an impact on human health, but are of interest to the study of climate change and carbon dioxide. These plots show the atmospheric O₂ concentration relative to the level around 1985. The observed downward trend amounts to 19 'per meg' per year. This corresponds to losing 19 O₂ molecules out of every 1 million O₂ molecules in the atmosphere each year."

[Paladiea]

Yes, but that's with phytoplankton photosynthesis still occurring, my question was under the circumstance that the oceans were too warm for the phytoplankton.

In such a case, atmospheric oxygen concentration may plummet much faster than it currently is.

We're already seeing large declines of phytoplankton, a 2010 paper mentioned about 40% of the global phytoplankton population has died since 1950.

http://www.nature.com/nature/journal/v466/n7306/full/nature09268.html

Recent phytoplankton declines have been observed as well.

https://www.sciencenews.org/article/phytoplankton-rapidly-disappearing-indian-ocean

There is some discussion by Kidder and Worsley in this series of papers
doi:10.1016/S0031-0182(03)00667-9
doi:10.1016/j.palaeo.2010.05.036
doi: 10.1130/G131A.1

Thanks! I'll see if I can read them today.

[sidd]

That 40% drop in phytoplankton was challenged later, as I recall.

[Paladiea]

I also heard that, I also heard it was from denier sites, who cherry picked some gains in phytoplankton populations in the North Atlantic, not sure though.

NASA has been documenting declines in the Northern Hemisphere as well.

https://www.nasa.gov/feature/goddard/nasa-study-shows-oceanic-phytoplankton-declines-in-northern-hemisphere

[AbruptSLR]

Yes, but that's with phytoplankton photosynthesis still occurring, my question was under the circumstance that the oceans were too warm for the phytoplankton.

In such a case, atmospheric oxygen concentration may plummet much faster than it currently is.

We're already seeing large declines of phytoplankton, a 2010 paper mentioned about 40% of the global phytoplankton population has died since 1950.

http://www.nature.com/nature/journal/v466/n7306/full/nature09268.html

Recent phytoplankton declines have been observed as well.

https://www.sciencenews.org/article/phytoplankton-rapidly-disappearing-indian-ocean

There is some discussion by Kidder and Worsley in this series of papers
doi:10.1016/S0031-0182(03)00667-9
doi:10.1016/j.palaeo.2010.05.036
doi: 10.1130/G131A.1

Thanks! I'll see if I can read them today.

If you are interested you could investigate the linked reference; however, personally I have little concern that atmospheric oxygen will be a significant problem in the next hundred years.

https://www.researchgate.net/publication/284728921_Mathematical_Modelling_of_Plankton-Oxygen_Dynamics_Under_the_Climate_Change


Yadigar Sekerci, Sergei Petrovskii. Mathematical Modelling of Plankton–Oxygen Dynamics Under the Climate Change. Bulletin of Mathematical Biology, 2015; DOI: 10.1007/s11538-015-0126-0


Extract: "Ocean dynamics is known to have a strong effect on the global climate change and on the composition of the atmosphere. In particular, it is estimated that about 70 % of the atmospheric oxygen is produced in the oceans due to the photosynthetic activity of phytoplankton. However, the rate of oxygen production depends on water temperature and hence can be affected by the global warming. In this paper, we address this issue theoretically by considering a model of a coupled plankton–oxygen dynamics where the rate of oxygen production slowly changes with time to account for the ocean warming. We show that a sustainable oxygen production is only possible in an intermediate range of the production rate. If, in the course of time, the oxygen production rate becomes too low or too high, the system’s dynamics changes abruptly, resulting in the oxygen depletion and plankton extinction. Our results indicate that the depletion of atmospheric oxygen on global scale (which, if happens, obviously can kill most of life on Earth) is another possible catastrophic consequence of the global warming, a global ecological disaster that has been overlooked."

Also see:

https://www.sciencedaily.com/releases/2015/12/151201094120.htm

[Paladiea]

I linked to one of those papers earlier,

My concern is that there are synergistic effects going on that may link the two and accelerate both.

[sidd]

Re: putative phytoplankton decline

see comments on 40% phytoplankton decline Nature paper doi:10.1038/nature09268

http://www.nature.com/nature/journal/v472/n7342/full/nature09950.html
http://www.nature.com/nature/journal/v472/n7342/full/nature09951.html
http://www.nature.com/nature/journal/v472/n7342/full/nature09952.html

To me, Boyce et al. have not made the case.

Check for citations on the Boyce paper, i think there was at least one full length publication in refutation. I think I have it, but I have not now the time to find it, I shall post a reference if i do.

sidd

[RoxTheGeologist]

If you look at stratified seas, such as the Black Sea, then the lower layers are anoxic, they don't mix with the surface waters that are replenished with runoff.

I fear the same can happen to the worlds oceans. I was taught that one of the features of, say, the AMOC is that the very cold briny water contains a high proportion of dissolved gas, including oxygen. This oxygenated water  is circulated through the entire ocean.

As the AMOC decreases in strength, the supply of oxygen to the lower parts of the Atlantic will diminish. Without the AMOC, or some form of vertical mixing I fear there will be extensive extinctions within deep ocean ecosystems, ecosystems that we are only now discovering.

[Paladiea]

To me, Boyce et al. have not made the case.

Check for citations on the Boyce paper, i think there was at least one full length publication in refutation. I think I have it, but I have not now the time to find it, I shall post a reference if i do.

Fair enough, I'll note that the first paper in rebuttal pointed to the North Atlantic gaining phytoplankton as a rebuttal to Boyce, but I also pointed out that NASA and others have confirmed a net loss for the northern hemisphere.

[Paladiea]

As an update to the original thread, I'd like to share some articles on what's currently happening concerning ocean hypoxia/anoxia.

https://science.sciencemag.org/content/359/6371/eaam7240

https://serc.si.edu/media/press-release/ocean-losing-its-breath-heres-global-scope

https://science.sciencemag.org/content/362/6419/eaat1327

Apparently recent research has formed a link between the End Permian and temperature dependent anoxia. Further, large sections of our oceans have been found to be currently hypoxic... This wasn't supposed to happen till 2030...

[mitch]

Thanks for link to oxygen paper. If you glance through the paper, 15% of the oxygen change is because of changes in O2 solubility and higher kinetics of organic matter degradation in warmer water. The other 85% is caused by less exchange with the surface, because the ocean is heated from above. The warmer surface waters have a higher density gradient with those below. These gradients will get weaker in a few centuries because the subpolar regions are warming as well and they provide the waters underneath the mixing zone. The likelihood that we will drive the whole oceans anoxic is very small. 

[Paladiea]

I forgot to include this paper, which is actually the most alarming.

https://www.pnas.org/content/115/23/5896

And yes, I know that this is definitely not a congruent situation, but abrupt anoxia is possible.

[mitch]

Thanks for the second paper as well. I need to look into the systematics of U-isotopes, but would expect that the isotope anomaly would also match a low in U/Ca.  The main problem with the Paleozoic studies is that essentially all the preserved material is from shallow water--plate tectonics has wiped out Ordivician age sea floor.  So, significant confirming evidence is needed before the hypothesis is accepted. 

It is very hard to make the whole ocean anoxic, since cutting off oxygen to the deep prevents recycling of nutrients to the surface. Shelf areas are most susceptible, like the anoxia that now appears off Oregon, e.g.:
https://journals.ametsoc.org/doi/full/10.1175/JPO-D-15-0119.1

[SteveMDFP]

I forgot to include this paper, which is actually the most alarming.

https://www.pnas.org/content/115/23/5896

And yes, I know that this is definitely not a congruent situation, but abrupt anoxia is possible.

This mass extinction event is quite interesting.  Along with the extinction, the earth went from greenhouse conditions to glaciation.  I recently watched a lecture on a theory that this particular mass extinction might have been precipitated by a gamma ray burst:

Did Gamma Rays Cause Ordovician Mass Extinction?
https://www.aps.org/publications/apsnews/200407/extinction.cfm

Interestingly, it's possible that the star system WR 104 in the constellation Sagittarius could do this to earth all over again, at any time over the coming centuries:

Is Earth in Danger of Being Hit With a Gamma-ray Burst?
https://futurism.com/is-earth-in-danger-of-being-hit-with-a-gamma-ray-burst-2

Just another disaster possibility to keep us up at night.

[Paladiea]

One new paper, confirming phytoplankton declines coinciding with warming, and an article about plastic pollution poisoning photosynthetic bacteria (grain of salt time).

https://phys.org/news/2019-05-phytoplankton-decline-coincides-temperatures-years.html

https://www.wired.com/story/ocean-plastics-bacteria/

One of the reasons I'm particularly concerned about this is the fact that water has a very low solubility for O2, which is why most of the oxygen produced by oceanic photosynthesis ends up in the atmosphere.

[Lord M Vader]

Nasa Giss is in. May 2019 was the third warmest behind 2016 and 2017 with an anomaly of 0,87° above the 1951-1980 mean. 2016 had an anomaly of 0,95° while 2017 had 0,89°.

[b_lumenkraft]

The next El Nino will be unpleasant...

[Paladiea]

Two new articles, the first focusing on the Atlantic ocean and its declining oxygen... Or more accurately outlining a study that aims to study that as well as other metrics.

https://www.bbc.com/news/uk-scotland-48666091

The second talking about how the permafrost is a full 70 years ahead of schedule in terms of melting.

https://www.livescience.com/65709-arctic-permafrost-melts-decades-early.html

And one very fascinating video outlining the mechanism of the P-Tr extinction.

[vox_mundi]

Dark Carbon May Be Hiding the True Scale of Ocean 'Dead Zones'
https://phys.org/news/2019-12-dark-carbon-true-scale-ocean.html

Dead zones within the world's oceans—where there is almost no oxygen to sustain life—could be expanding far quicker than currently thought, a new study suggests.

...Researchers measured the stable isotopes of organic carbon in sediment cores taken from the floor of the Arabian Sea, one of the world's large natural dead zones, in order to get a clear understanding about what is contributing to the organic matter contained within them.

This value is a mixture of all the distinct signatures from all the organisms that produced this carbon—thought to be mostly algae and bacteria living in the oxygen-rich, light, surface ocean where it sinks from.

However, using a distinct biomarker produced by anaerobic bacteria, they suggest that around one fifth of the organic matter on the seafloor could in fact stem from bacteria living in or around these dead zones.

They in fact believe the dead zones could be expanding much faster than previously thought, and that future calculations must take the bacteria into account in order to accurately predict the full impacts of climate change and human activity on the marine environment.

... Our study shows that organic matter that sinks to the seafloor is not just coming from the sea surface, but includes a major contribution from bacteria that live in the dark ocean and can fix carbon as well. Existing models could be missing out on a key contribution as a result of which people have underestimated the extent of the oxygen depletion we are to expect in a future, warming world.

Sabine K. Lengger et al, Dark carbon fixation in the Arabian Sea oxygen minimum zone contributes to sedimentary organic carbon (SOM), Global Biogeochemical Cycles (2019)

[Paladiea]

Paper from January of this year focusing on the productivity of phytoplankton in relation to species diversity and warming, and the results don't look good.

Abrupt declines in marine phytoplankton production driven by warming and biodiversity loss in a microcosm experiment
https://onlinelibrary.wiley.com/doi/full/10.1111/ele.13444

As species were lost from the communities, the probability of taxa remaining that could tolerate warming diminished, resulting in abrupt declines in ecosystem production. Our results highlight the potential for synergistic effects of warming and biodiversity loss on marine primary production.

[Paladiea]

Biogeochemical evidence for euxinic oceans and ecological disturbance presaging the end-Permian mass extinction event
Earth and Planetary Science Letters
Volume 281, Issues 3–4, 15 May 2009, Pages 188-201
https://www.sciencedirect.com/science/article/abs/pii/S0012821X09001083

Photic Zone Euxinia During the Permian-Triassic Superanoxic Event Science  04 Feb 2005:
Vol. 307, Issue 5710, pp. 706-709
DOI: 10.1126/science.1104323

https://science.sciencemag.org/content/307/5710/706.abstract

^ Another couple of papers with evidence that ocean anoxia/euxinia was the kill mechanism in the P-Tr.

And one paper that says it wasn't ocean stagnation that was the cause of anoxic events, but merely ocean warming.

Oceanic Euxinia in Earth History: Causes and Consequences  Annual Review of Earth and Planetary Sciences Volume 36, 2008 Meyer, pp 251-288

https://www.annualreviews.org/doi/10.1146/annurev.earth.36.031207.124256

"We conclude that periods of global warmth promoted anoxia because of reduced solubility of oxygen, not because of ocean stagnation. "

[Paladiea]

News piece on the dead zone off the west coast of North America. This year seems especially bad and they're talking about complete anoxia in the area by the end of summer.