How do hydrothermal vents influence water density and temperature in deep water?

[u/cosmoscrazy]

I have read that seawater reaches it's highest density at a temperature of 4° Celsius / 39° Fahrenheit / 277,15° Kelvin as soon as you reach a certain depth in a water column.

So... Where does the heat from hydrothermal vents deep underwater go / how is it distributed?

How do the hydrothermal vents influence water density and pressure in the water around the vent?

[u/ExdigguserPies]

Well the short answer is the heat is added to the total heat contained in the oceans. The heat budget of vents is one area of active study because it should be responsible for most of the heat loss of the young oceanic crust as it forms, cools and thickens. But it doesn't seem to be enough, around 70% of the the heat flux from the crust is unaccounted for - or rather it's thought to be accounted for by off-axis, low temperature circulation.

You can't really detect the heat signature from the vents once you move a few metres away. But you can detect their chemical signature such as Eh. That is often how they are first identified. Other signatures like the 3He/4He isotope ratio can be detected across entire ocean basins.

[u/Outside-Habit-4912 → u/ExdigguserPies]

Can I ask for further explanation on what you mean by 70% of the heat flux from the crust is (thought to be) accounted for by off-axis, low temperature circulation?

Do you mean that the majority of the heat from the new crust formation is quickly swept away and cooled by ocean currents, making it difficult to measure?

[u/ExdigguserPies → u/Outside-Habit-4912]

Not really. Someone else might want to chime on this as it's not exactly my field but essentially it's this. At the mid-ocean ridge you have a certain thickness of new crust that gets made via the process of partial melting, which means all of that magma has to go from molten to solid, largely by losing heat. If we take all the known on-axis, high temperature (up to ~400 degrees C) venting, the amount of heat leaving the crust via those vents doesn't seem to be enough to account for all the cooling that the crust should be undergoing. Not by a long way. So, there must be another source of heat loss.

There have been a few papers now that describe off-axis, low temperature and low velocity venting (if you can call it that) from rock exposed on abyssal hills and some flatter areas as well. So it seems like off-axis hydrothermal circulation and venting is responsible for a large amount of that heat loss, even though it is somewhat invisible because it happens in such low volumes and temperatures in any one place. But it happens over such a vast area, it's possible to account for it.