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I am wondering what kind of catastrophe could contribute to affecting the average dissolved oxygen level in the ocean to become less than 1 milligram per liter and as a result making 90% of the ocean a no-go zone within a period of 1 year?

It would be better if the cause is man-made using existing technology!

btw climate change is ruled out because of the timing.

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    $\begingroup$ Atmospheric oxygen comes mostly from the ocean. As a reader I would be very annoyed with a book which kills off the ocean and doesn't consider what will happen to the rest of the living world. $\endgroup$ – AlexP Sep 22 '17 at 8:46
  • $\begingroup$ Circulation in the deep ocean is so slow that I doubt it would be possible with in 1 year $\endgroup$ – Slarty Sep 22 '17 at 14:35
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Removing 90% of the free oxygen from the oceans would be very difficult:

1) It is constantly being replenished -- oxygen comes into the water due to simply diffusion, but it is accelerated by plants releasing oxygen and wind and wave action.

2) You have to remember that fish have been actively depleting free oxygen for a long time, yet there is still free oxygen.

3) The methods to remove free oxygen can be chemical (something that reacts with oxygen), physical (a argon or nitrogen sparge to bubble out the free oxygen), or biological a.k.a. plants.

4) Because of the size of the oceans, chemical and physical means are impractical and biological means run into diminishing returns, i.e., once levels drop too large, biology can't remove more oxygen.

5) You would also kill off most life on earth -- perhaps you are ok with that.

It is very hard to envision a catastrophe that would disrupt with ocean oxygen cycle, without being far more disruptive in other ways.

However, though I can't see it being likely, removing the oxygen using sonic vibration of the upper layers might work. I don't know if you could get the oxygen level as low as you request, but sonic vibration will remove free oxygen, and presumably not destroy the whole earth in the process. Don't know the optimal frequency and sound level, but an evil genius should be able to get some mad scientists to grok this out.

Perhaps some bio-engineered super water-cricket is a good bet. As a bonus it can eat the plants, and feed the fish, both helping to decrease oxygen levels.

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  • $\begingroup$ I like your answer better than mine and I like that we ended up in roughly the same place. $\endgroup$ – DPT Sep 22 '17 at 3:22
  • $\begingroup$ @dpt -- just read yours. Agreed. $\endgroup$ – Gary Walker Sep 22 '17 at 3:26
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I've worked in oxygen minimum zones. I think you have a few problems with those numbers and that scale. One is wave action, which physically brings oxygen into the surface water. Another problem is the photosynthesis of surface algae, etc, which provides the main source of oxygen (I think 70%) to the entire planet, including water down several meters or more. Those oxygenated waters are pulled into deeper waters through mixing events, which happen a number of ways. Upwelling is one way that marine layers mix.

Ways to draw O2 down:

  1. Oxygen is depleted through eutrophication from run off. OMZs have expanded greatly (hugely) due to human action (agriculture) but we are nowhere near 90% OMZs in our oceans. Even in OMZ areas, most of the water column has some oxygen in it. 1 mg/liter is pretty low, if I am thinking clearly.

  2. Oxygen is depleted when we load the water with carbon, like during oil spills. (Or methane. You could destabilize all the methane clathrate to load up the water column.) Anyway, some source of hydrocarbon will cause bacterial bloom, which will consume the oxygen.

  3. You could have an asteroid strike and years-long darkening of the sky. In the absence of photosynthesis, you'd throw things out of balance and I think the O2 would be consumed.

  4. So, the above are all science-based responses. Fictionally, using a man made device? That's a toughie, but perhaps we have decided to sequester the oxygen for some odd reason (to kill the kraken?) and have seeded the oceans with a compound that reacts with the O2. Precipitates it out as carbonate or some such.

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  • $\begingroup$ I like precipitates, because it moves the oxygen to the deep ocean. Don't know that it would make any real difference. $\endgroup$ – Gary Walker Sep 22 '17 at 3:25
  • $\begingroup$ Your second point might work if the spill is large enough and we let waves distribute it leaving fish and other water organisms to fight for the left over oxygen and then everything under water dies. This spill could be from a crack (100s of km long) on the ocean ground leaking crude. $\endgroup$ – dawyda254 Sep 22 '17 at 9:01
  • $\begingroup$ @davyda254 I think it unlikely. Not sure we have enough buried oil for that kind of effect, (oceans are huge) and such a leak still doesn't impact waves, photosynthesis, and mixing events. the criteria in the question are too extreme. $\endgroup$ – DPT Sep 22 '17 at 13:22
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Melt the ice caps. (This builds on DPT's answer about Oxygen Minimum Zones). If you want a quick man-made way of doing this... huge solar mirrors? Spraying black dust over the ice to enhance absorption of heat? This is straying into mad scientist super-science territory for the mechanism.

So here's the real science. The polar ice caps are the pumping mechanism to get oxygen down to the bottom of the oceans. Ice forms, and the extra-saline (salty), highly oxygenated water sinks to the ocean floor. Here is an article about Antarctic Bottom Water which carries oxygen along the seabed all the way to the equator and beyond.

So on modern Earth, we have oxygen minimum 'zones' - a band of low oxygen in midwater.

  • The water above gets oxygen from the atmosphere and phytoplankton. It diffuses and mixes downwards. As you go deeper, it gets used up by creatures breathing and decaying.
  • The water below gets oxygen from the Antarctic Bottom Water and other such currents. It diffuses and mixes upward. As you go shallower, it gets used up by creatures breathing and decaying.
  • There is therefore a band in the middle of low oxygen. The Oxygen Minimum Zone. Here's a handy diagram of the oxygen levels at different depths

In a prehistoric Earth, such as the Cretaceous Period, there were no ice caps. Thus there is little to zero oxygen carried to the deep ocean floor. So, look again at that diagram I linked to... everything below the "anoxic zone" on that picture becomes anoxic. Basically, the bulk of the oceans becomes deoxygenated.

The average dissolved oxygen will therefore have gone down, since:

  1. The oxygen level for pretty much anything deeper than 200 to 500m is zero.
  2. To have no ice caps, the global temperature has to be higher. Warm water can hold less dissolved oxygen than cold water.

However, the shallow seas are doing fine, and have indeed become larger, since the water from all that melted ice will have raised sea level. Life in them can survive by adapting to a little less oxygen.

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  • $\begingroup$ To melt the ice caps, it might be more realistic to have something that somehow changes the jet stream, so it brings hot tropical air in to melt them pretty quickly (maybe freezing some other unsuspecting area/continent too). How to move the jetstream... asteroid? Moon or tides change? Solar flare messing with the atmosphere? I'm not too sure, but at least you won't need to spray-paint the ice caps black $\endgroup$ – Xen2050 Sep 22 '17 at 11:57
  • $\begingroup$ @Xen2050. Yes I think that would work for the Arctic. Not sure about the Antarctic. IIRC some of the cooling down there is because currents of the Southern Ocean can go completely round Antarctica in a circle, and there is limited heat exchange at the surface with the Atlantic, Pacific and Indian oceans. Not sure that reversing 30 million years of continental drift to 'correct' this is feasible! :-) $\endgroup$ – DrBob Sep 25 '17 at 15:05
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A fast acidification of the ocean might be able to achieve that : the plankton that generates most of the oxygen needs a shell to survive. And that shell cannot form if the water is too acid. All that organic mass of dead plankton will decompose in water lacking oxygen, releasing hydrogen sulfide that will acidify the water even more... and it becomes a vicious circle. The initial acification could be :

  • huge quantities of industrial wastes, which are known to cause "deadzones" which is quite what you're looking for.

  • The Clathrate Gun Hypothesis (mentionned by DPT) could be interesting. Humans looking for new fossil fuel sources could easily accelerate the process.

Deadzones have algea that don't produce oxygen, if some kind of algea similar to these, but even more invasive would develop and replace the oxygen producing ones, the deadzones could expand much faster.

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