[Rhodes22-list] Expanding Dead Zone in the Gulf of Mexico - some ideas

[DCLewis1 at aol.com]

JW,
 
Thanks very much for your post.  From the description, Ride Tide is  sort of 
like sperm - a flagella etc.  It’s a strange world.
 
Also, thank you for the link to the CRS document discussing hypoxia.   It 
gave a good description of the problem - where it occurs, frequency, causes -  
but not much about the chemistry/physics of the capping mechanism where fresh  
water overlays salt water.  It did say the fresh water overlay capped the  salt 
water.  I understand the density and oxygen solubility differences,  but I don
’t understand why that interface should act as a diffusion barrier for  free 
oxygen coming down from the surface.  Seems to me the fresh water  overlay 
should actually enhance the concentration of oxygen at any depth  because the 
overlay carries a greater concentration of oxygen down to whatever  it's depth is 
than oxygenated salt water would.
 
But maybe diffusion of oxygen from the surface doesn’t drive the  problem.  I’
ve taken a quick look at the diffusion coefficient of oxygen in  water and it’
s remarkable.  The diffusion coefficient of oxygen in air (our  air) is 0.20 
cm2/sec, in water it’s 0.000018 cm2/sec; that’s a difference of  about 4 
orders of magnitude!  Looks to me as though diffusion of oxygen  down from the 
surface happens, but it might be a verrrry slooow  process.  If something began 
to use oxygen at depth (decomposition  subsequent to eutrophication) diffusion 
of oxygen from above might not be able  to keep up - hence hypoxia.
 
Perhaps the most important mechanism short time mechanism in getting free  
oxygen to deeper water and preventing hypoxia is bulk transport of oxygenated  
water from the surface to the bottom?  The oxygen might be entrained with  the 
water and just carried along.  The buoyancy difference between fresh  and salt 
water might prevent that mechanism - hence the fresh water cap  description.
 
So maybe I do understand the barrier after all: the fresh water overlay is  
about stratification, the flip side of stratification is no mixing, mixing  is 
(maybe) what carries the oxygen (short term), no mixing means no oxygen  at 
depth and that means hypoxia. If I get time, I’ll look at a  model to better 
assess the diffusion rate of oxygen in water. 
 
If the problem were simply mixing the oxygen rich fresh water and oxygen  
poor salt water, you’d think there’d be simple ways to accomplish that.   I’m 
sure people are working on that.  Clearly though, issues with excess  nutrients 
etc have to be addressed. The CRS document identifies farm  runoff, sewage, 
etc as a material contributors to some, but not all, hypoxic  events.
 
Finally, it occurred to me, the decomposition involved here is really  
oxidation of dead material which ties up free oxygen, right?  What's this  going to 
do to the long term availability of free oxygen for those of us, like  myself, 
that are air/oxygen breathers?  Is all the free oxygen going  to wind up 
bound to carbon dissolved in the ocean?  I did a quick  check of the relative 
molecular abundance of carbon and oxygen, it's  roughly 1:2 - as in CO2.  
Hmmmm.....
 
Thanks again for your post, it’s fascinating stuff.  Get on your  soapbox 
anytime.
 
Dave
 
 
 
 



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