Anoxic brine pools are beautiful and uncanny. https://cdn.mos.cms.futurecdn.net/hB2ogu7nw4r9YZWfHoUgLi-192... https://oceanexplorer.noaa.gov/multimedia/daily-image-media-...

From the exploration vehicle Nautilus, 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,

    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 <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, https://www.nature.com/articles/s43247-022-00482-x

Nature is beautiful.

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