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Oxygen-deprived underwater zones may not be “dead zones” but clue to early life

124 points · 21 comments · gumby

areoform · 1790890930 · focus · HN ↗
Anoxic brine pools are beautiful and uncanny. <a href="https://cdn.mos.cms.futurecdn.net/hB2ogu7nw4r9YZWfHoUgLi-1920-80.jpg" rel="nofollow">https://cdn.mos.cms.futurecdn.net/hB2ogu7nw4r9YZWfHoUgLi-192... <a href="https://oceanexplorer.noaa.gov/multimedia/daily-image-media-20200720/" rel="nofollow">https://oceanexplorer.noaa.gov/multimedia/daily-image-media-...
From the exploration vehicle Nautilus, <a href="https://www.youtube.com/watch?v=nGLtMWx28hs" rel="nofollow">https://www.youtube.com/watch?v=nGLtMWx28hs
Their existence isn't intuitive. You'd expect diffusion to dilute the pools and yet they seem to last a very long time somewhere on the order of thousands of years. It's not immediately obvious to me how the physics of these pools functions because there is a persistent interface as if the brine is another liquid entirely. From another paper, "Discovery of the deep-sea NEOM Brine Pools in the Gulf of Aqaba, Red Sea,"
```
    The CTD measurements from 1200 m depth, through the brine pool interface (1769.46 m) and 2 m below (1771.50 m) revealed that the bathyal water column above the brine pool has a stable 21.33 °C temperature, the salinity of 40 PSU, and dissolved oxygen of 180 μmol L−1.
```
but if you sample the pool itself,
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    Within 15 cm beneath the brine interface, salinity rose from 40 PSU to values higher than the limits of the conductivity probe (120 PSU). Subsequent lab analysis of the brine provided actual salinity values of 160 PSU. Dissolved oxygen values fell more than 75% at the interface, reaching 50 μmol L−1 by 20 cm submergence into the brine, and further falling to a minimum value of &lt;10 μmol L−1 by 50 cm below the interface.
```
The brine is also hotter than the water just above it,
```
    Unlike salinity and dissolved oxygen, the temperature did not abruptly change across the brine interface. Instead, the temperature gradually increases below the brine interface at a rate of 0.1 °C per 20 cm depth increase, reaching an increment of 1.0 °C above the temperature of ambient seawater by 2 m depth into the brine (Fig. 4). The temperature differential to ambient did not increase with deeper submergence. Such a modest warming increment suggests the lack of proximal hydrothermal heating of the brine.
```
from, <a href="https://www.nature.com/articles/s43247-022-00482-x" rel="nofollow">https://www.nature.com/articles/s43247-022-00482-x
Nature is beautiful.
1. andrewflnr · 1790908355 · focus · HN ↗
  > You'd expect diffusion to dilute the pools and yet they seem to last a very long time somewhere on the order of thousands of years.
  What I've learned from things like this is that my intuition for when diffusion should work, and how fast, is just busted.
  The biggest example in my mind is pyroclastic flows from a volcano. For years (mostly as a kid), I didn't understand what they were because I kept thinking roughly "if it was just gas it couldn't hold together into a coherent flow, right?" Well, as far as I can tell, yeah, they're basically just gas, but really hot and full of rocks. I still don't really get why they hold together instead of poofing out into clouds, except that it has something to do with how they're denser than the surrounding atmosphere. They're sometimes also referred to as "pyroclastic density currents", go figure. <a href="https://en.wikipedia.org/wiki/Pyroclastic_flow" rel="nofollow">https://en.wikipedia.org/wiki/Pyroclastic_flow
  Another big one that threw me for a loop as a kid was "warm/cold air masses" interacting in a way that kept their identity. "why does the warm air mass ride up on top of the cool air instead of just mixing?" I guess the answer here is that they're just too big for the mixing to happen faster than the bulk motion.
  Anyway, it seems like fluids, especially fluids of very different properties, especially different density, take their time mixing, sometimes long enough to let them act like separate objects in contact for much longer than I, for one, would think. Thousands of years in the case of these brine pools (though in this case, they're being at least partially refreshed). I wish I understood it better.
  PS: I was half expecting this EV Nautilus brine pool video. Grim comedy of a sort. <a href="https://youtu.be/9ZYJAmAmFPw" rel="nofollow">https://youtu.be/9ZYJAmAmFPw
  1. HPsquared · 1790935713 · focus · HN ↗
    The ratio of volume to surface area changes with scale.

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