Methane plumes in the South China Sea?

This piece is pure speculation on my part, so please read accordingly. 

There is an article today titled [... How a Captain Stopped His Submarine from Falling Into a 3,000-Meter Trench] in which is reported:

"... Wang Hongli, captain of the submarine number 372,... calmly reversing the disaster during a routine patrol in deep waters... the submarine encountered a sudden fall in water density... The sub started to plunge straight into a deep trench in the ocean. / Wang, of the South Sea Fleet, calmly gave out orders and within three minutes, steered them out of danger (Subs usually operate down to 500 meters only; 3000 meters would be absolutely fatal.).... / Its submersible system went back to normal three hours later and 372 continued its patrol, which lasted for 20 days... The South Sea Fleet's area of responsibilities are northern regions of the Taiwan strait and southern areas from James Shoal including the Paracel Islands...]

http://www.scmp.com/news/china/article/1584450/pla-reveals-how-captain-stopped-submarine-falling-3000-metre-trench

My question is: What caused this "sudden fall in water density"? As far as I can reason, there could be only 3 possible causes:

1. The sub moving from a body of water with a high salinity (salt water) into a body of water with a low salinity (e.g. fresh water). If so, where does the fresh water come from? The melting of the Himalayan glaciers (2000 of them, all melting) would be a good guess. Or

2. The sub moving from colder water into warmer water. However, the temperature gradient is usually vertical, i.e., water at shallow depths being usually warmer than water at greater depths. Presumably, the sub is moving more or less horizontally. Or

3. The sub entering a body of water containing a lots of gas bubbles, which would have a lower density. This is the point I will speculate on.

The first question to ask is: What kind of gas is it? My best bet would be METHANE.

The map shows the concentrations of methane hydrate in today's oceans. The orange/yellow patch on the extreme right mid-way down is in the South China Sea, where the Paracel Islands are located. It is one of just 3 major methane hydrate deposits in the tropical/subtropical regions (the other two being just east of India and at the mouth of the Persian Gulf. Ocean warming could melt some of the deposit in this region and release large volumes of methane gas into the water.

The average depth of the South China Sea is 1,500 meters (5,000 feet), whereas the trenches in that region reach depths of over 5,000 meters (16,500 ft). The general direction of the ocean current in that region is from SW to NE, that is, from the equator, and that is, warm. During the previous cooler period, whatever methane hydrate that could be melted, i.e. those at shallower depths where the temperature is higher and the pressure is lower. With ocean warming, the methane hydrate deposits at greater depths could begin to melt, and this might include those in the trenches.

In general, the closer to the equator, the deeper a deposit has to be to stay solid and intact. Conversely, the deposit currently out-gassing in the Arctic Ocean is at a much shallower depth than that in the South China Sea, if the latter is out-gassing at all.

Let's imagine a scenario where methane is out-gassing from a trench. When a submarine passes over the trench, it would indeed encounter a body of water with a lower density. So, the sub, as trimmed for the surrounding higher-density water, i.e. water without bubbles, would tend to sink. Further, if the bubbles are absorbed by the water on their way up, the upper layer of the sea would have a higher density than the lower layer, and the two layers would tend to invert, with the descending denser water carrying the sub down with it.

This, as I said in the beginning, is by and large speculation. But if the methane hydrate in the South China Sea is out-gassing at a depth, but the gas won't make it to the surface, indeed only submarines would know about it.

Some would argue that since the methane doesn't make it to the surface, it wouldn't contribute to atmospheric warming. But when dissolved by the sea water, the methane would turn to CO2, which would increase OCEAN ACIDIFICATION.

Anthony Marr, founder
Heal Our Planet Earth (HOPE)