|Icy Inverts 2004|
|Daily Journal of the R/V Laurence M. Gould|
|Dec. 10, 2004 --- Position Lat/Long: S063° 50.577 W062 37.729 (midnight) |
[Wind: SW 18kn / Air Temp: -0.1C / Wind Chill: -15.5C /Depth: 252 m]
|A remarkable sight this morning is that of the tall peaks of the Brugmann Mountains on Liège Island. We are traveling toward Pasteur Peninsula on Brabant Island. Did you know that Antarctica is the tallest continent?|| Photo1 || |
It has a majority of the highest peaks in the world, making it average higher than any other continent. In the winter, the continent doubles its size due to the freezing of the waters surrounding it (and this is considered to add to its land mass). Conversely, the continent shrinks to half it’s winter size due to the ‘melt down’ in the summer months. When that water melts, it makes the ocean water more like fresh water, or less salty, on top. More salty means more dense, and the more dense water sinks to the bottom. This would imply that that cold water would still be cold down there, right? Well, here’s an interesting theory that has come along based on some data collected here. Raul Reta (1) and Sandy Williams (2) explain these initial findings:(1) It is very common in the sea water column that the temperature decreases with depth. But… What had happened here?? We had found warm water below the surface on the shelf near the coast, called CIRCUMPOLAR DEEP WATER by physical oceanographers. Above it, there is colder and fresher water but only near the surface in a layer 50-100 meters in depth, called ANTARCTIC SURFACE WATER. This water comes from the melting of continental ices. In the deepest oceanographic station we also found a third kind of water mass, ANTARTIC BOTTOM WATER.
The water masses are distributed in the water column as its density permits. So, in the surface we can see the lighter waters and below, the denser ones. All these water masses are laid out like layers in a sandwich.
The Circumpolar Deep Water is the more important water mass around the Antarctic, flowing (moving) from West to East. Its maximum temperature (+1.5°C) is due to the incoming water from the Pacific, Atlantic and Indian Oceans.
The surface waters are often below 0°C. We have measured temperatures as low as -1.5°C. But… are these waters “below freezing” in a liquid state? The answer is: YES. The freezing point of sea water is -1.8 °C and that is due to the presence of salt in the water. Remember…: The saltier the sea water is, the lower the freezing point (temperature needed to freeze it). You can make a simple experiment in your home to test this. Put water in different cups with different amounts of salt and put them into the freezer. What do you think will happen? This morning I pieced together several pieces of temperature measurements I have been making with XBTs (Expendable Bathythermographs, probes with fine wires that measure temperature as they fall to the bottom). There was warm water at 300 meters depth at the an area near Deception Island. This was unusual for water at this depth – it should be cold! I checked to see if the warm water was more salty, as it would have to be if this measurement was real. It has to be saltier to keep the warmer, otherwise less dense water down (warm water wants to rise). And it was saltier, as shown from the CTD (Conductivity, Temperature, Depth) instrument. So the measurements of temperature made some sense, but so far we haven't found where this warm salty water comes from. It is just possible that it is from a new hydrothermal (hot water) vent. After all, Deception Island is an active volcano and there could be hydrothermal activity nearby on the sea floor. What happens then, is that water bubbles into the hot rocks in the vent, is heated and dissolves minerals, and then comes out as a hot spring.[Now, when I say it is hot, these are relative terms. It is +0.5oC to +1.5oC against a normal temperature of -1.0oC!] The warm plume water rises and sucks in bottom water as it does. So it gets saltier until it matches the density of the water in its neighborhood.
Finally it spreads out sideways (which is the picture that my graph drew). This could be the plume that we saw, and we see it each time we come by. We’ll look into it more closely before we put it on the front page of the Antarctic Times, though.