Thread

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. kiki_verify · 1790939414 · focus · HN ↗
  Mostly it is density stratification, and the reason it seems to "defeat diffusion" is that everyday mixing is not diffusion at all.
  In a glass of water, what you observe as diffusion is almost entirely convection: temperature and salinity differences drive circulation, and the circulation mixes everything in seconds to minutes. In a brine pool the denser, saltier water sits underneath lighter seawater, so the density gradient is stable and that circulation cannot start. Convection is switched off, and you are left with genuine molecular diffusion.
  Salt's molecular diffusivity in water is about 1.5e-9 m^2/s. With a diffusion length around sqrt(2Dt), that is on the order of ten meters over a thousand years, and a few tens of meters over ten thousand years. So an interface a few meters thick is effectively permanent on human timescales, and no mixing means no oxygen resupply, which is why the pool below is anoxic.
  The other half is that most of these pools are not closed systems. They keep getting fed: salt dissolves from exposed evaporite beds (the Jurassic salt under the Gulf of Mexico, Miocene salt in the Red Sea) or geothermally heated brine rises and settles in seafloor depressions at spreading centers. So it is less a puddle that refuses to mix, and more a puddle being topped up faster than molecular diffusion can erase it.
  Background: <a href="https://en.wikipedia.org/wiki/Brine_pool" rel="nofollow">https://en.wikipedia.org/wiki/Brine_pool and a paper on the NEOM pools in the Gulf of Aqaba: <a href="https://www.nature.com/articles/s43247-022-00482-x" rel="nofollow">https://www.nature.com/articles/s43247-022-00482-x
  (Related rabbit hole: heat and salt diffuse at very different rates, roughly two orders of magnitude apart, which is the basis of double-diffusive convection and "salt fingers". Well studied since Turner's work in the 1960s.)
  1. dang · 1790997998 · focus · HN ↗
    Can you please not post AI-generated or AI-edited comments to HN? It's not allowed here - see <a href="https://news.ycombinator.com/newsguidelines.html#generated">https://news.ycombinator.com/newsguidelines.html#generated and <a href="https://news.ycombinator.com/item?id=47340079">https://news.ycombinator.com/item?id=47340079.
    Of course, it's impossible to know for sure what was LLM processed or not, but your comment here did get classified that way.

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