4 March, 2000
Where a Cold Tongue Isn't
74 18 s 109 22 w
Northeast of Bear Peninsula
Mt. Murphy visible in the distance
Breaking new ice @ 6 knots (7 mph)
Wind 20 knots (23 mph) out of the south
Temp 24 C (-11 F)
Barometer 995 mb, steady
Depth 940 m (3084 feet)
We just finished coring, and I'm warming up. Outside, in the predawn, the scene is weirdly beautiful. Those who live by the seashore in cold climates may know the look of a mudflat at low tide on a bitter cold winter morning. It would be, well, flat, and covered with a skim of ice. The ice we're pushing aside here looks like the mudflat ice. The only difference is that there are no trees or houses on the shore. In fact there's no shore, unless you count Mt. Murphy's dark shape in the distance. Instead of trees and houses imagine icebergs scattered about, each larger and taller than the ship, some close, some further away. Their northeast faces catch the orange glow in the sky, but the parts facing the cold south wind from the ice cap are dark. Another thing: mudflats begin at the shore and end at the water's edge. This ice surface begins at one horizon and ends at the other.
It is getting cold here, not as cold as it will get in May, June and July, but chilly nonetheless. If you figure the temperature and the wind, you get a wind chill index somewhere in the 40 to 50 range, and those temperatures are about the same in centigrade and Fahrenheit. The temperature makes coring harder. The corer breaks a hole in the slushy ice and disappears into the water. Underwater, it is 23 C (43 F) warmer than the air, but still cold enough to take your breath away if you went overboard. We go inside and wait while it is lowered, watching a TV monitor. The monitor shows the depth, how many meters of cable are out, how fast it's going out, the tension on the cable, and our position. We want to know as close as possible when and where the core hits, and how hard the winch has to pull to get it out of the mud.
When the core is back on deck, we work fast to get the outside mud cleaned off before it freezes. Even though the deck is heated, muddy slush freezes underfoot. Hoses, noses, and toes freeze. The bolts, pins and screws that hold the machine together work harder, and my insulated rubber gloves get so rigid I have to take them off and work bare handed.
We get the core free, and three people carry it into the relatively warm lab. Then we go outside, put an empty core box onto the weights, and clean up the deck as well as we can.
As soon as the corer is on deck and chained or bolted down, the ship gets underway to head to the next station. I like to watch the ice as the ship smashes through it. Each kind of ice looks and acts differently. That is something that would never have occurred to me before I got here. Ice was just ice, and ice breaking ships broke it, that's all there was to it! This morning, we are breaking through 10 cm (4 inch) new ice. "New" means it has just formed, probably within the last few days, maybe within the last few hours. You could safely walk on 10 cm freshwater ice, but I don't know about this stuff. Although it looks solid, the bow wave of the ship easily flexes the ice surface. Yesterday we came upon some new ice that had been pushed up against a larger flow by the wind, and frozen in permanent waves.
The ship tends to cut new ice, rather than break it. A few meters away cracks appear running in the direction of the ship's travel, maybe started by the bow wave. Then, pushed outward by the sides of the ship, the cut part near the ship slides under the rest of the ice. We leave a neat trail of double thickness ice on each side. Even at twice the thickness, I wouldn't try walking on it.
One thing I'd like to see that I haven't seen yet is a clear dark night with a starry sky, and maybe the Aurora Australis, the southern lights. Actually, I think it would have to be one or the other, because the aurora brightens the sky so much that many stars are not visible. On a starry night I'd see a different sky than I'm used to in Maine. Polaris, the north star, would be way below the horizon. I've got a little southern hemisphere star book with me and I hope to get a chance to use it. I'd like to see the Aurora Australis to compare it to the Aurora Borealis, the northern lights. Last September I was camping in Newfoundland and Labrador with a group of Liberty School students, and we were treated to quite a display. We sat up late, watching the sky wave and flicker.
Now comes the part about the cold tongue.
If you have a map of Antarctica that shows much detail at all, look at the area centered on 74 S, 108 30 W, about 32 km (20 miles) from the tip of Bear Peninsula. That's about where I am now, give or take a few km. You'll see an oval shaped feature about 113km (70 miles) long and 32 km (20 miles) wide. If it is named, it will be called the Thwaites Iceberg Tongue. It was the mother of all possible ice cubes, a large section of the outfall of the Thwaites Glacier. Its northern end is shown tipped slightly to the west, and its southern end close to the Thwaites Glacier Tongue.
Notice I say "was." The Thwaites Iceberg Tongue is there no longer. It's gone. It was there for at least seventy five years, since the first human beings saw these empty shores. It was noted in the thirties, mapped in the forties and again in the seventies. In 1976 there was about 5 km (3 miles) separating it from the Thwaites Glacier Tongue to the south. In 1986 or 1987, the whole Thwaites Iceberg Tongue floated off to see the world. It drifted for about 100 km (60 miles) and then once again became grounded, this time on the outer continental shelf. (Remember that in Antarctica, unlike most other areas, the continental shelf may be shallower at its outer edge.) Later still the berg broke into two pieces. The larger one floated free and followed the western edge of the Antarctic Peninsula northwards. Recently, it has made news as a hazard to shipping, floating about the Drake Passage, the open water between Cape Horn and the Antarctic Peninsula. It is somewhat reduced in size, but still voyaging far from its place of birth. I don't know what became of the smaller of the two pieces, but I suspect parts of it at least are still around.
Some questions come to my mind. Why was it there to begin with? And if it was there so long, why did it all of a sudden float off one year? Why do satellite photos show an area about the same size covered with fast ice there now? Why was the top tilted to the west, instead of lined up with the Thwaites Glacier Tongue? Does the fact that this huge chunk of ice broke off and drifted away have anything to do with global warming or sea level rise? Some I can answer, some I can't, and one I'll leave to you. The tongue was there to begin with because Thwaites Glacier is a fast moving one, as glaciers go. Satellite data indicates that the ice slide down hill and out onto the sea at as much as 2.9 km (1.8 miles) each year. That means if you built a cabin on the glacier nine miles inland, in five years it would be ocean front property. This huge mass of ice just flows out on to the continental shelf. The inland part probably floats, but by the time the iceberg tongue got to its mapped position, its northern part at least was aground. Why did it then float away? A good guess is that "warm," meaning 1-2 C (33-35 F) water melted away enough of its bottom layers so that it was thin enough to float, and did. It was still quite thick, several hundred meters, but when we say "shallow" water in this sense we mean a few hundred meters. That is well over your boots.
Do these events come from global warming, or have big pieces like this always broken off from time to time? Nobody really knows, although lots of people have opinions. That's part of what the Nathaniel B. Palmer is down here to find out.
Where the Thwaites Iceberg Tongue used to be, there is now an area of fast ice. We are following the western edge of this multi year ice north, taking cores, doing CTDs, counting seals, and studying snow as we go. Remember that "fast ice" means multiyear ice that is usually more than 2 meters (6.6 feet) thick, as compared to icebergs and ice shelves which are 100 meters (328 feet) or more thick. Another thing about fast ice: it is stuck to the shore or grounded icebergs. It's fast meaning stuck, not fast zoom zoom. Why is this new ice here? The same satellite image shows lots of little (1 km long) icebergs embedded in the fast ice. They are probably grounded where Thwaites Iceberg Tongue used to be. They serve as anchors, holding the thinner floating ice in place.
Why are the northern tips of both ice formations "leaning" west? I'll leave that for you to guess. Write me an e-mail and tell me what you think.
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