23 February, 2000

Squeaky Boots and Diatoms

February 23, 2000

72 58 s

136 47 w

Temp 0.5 C (31 F)

Winds 38.5 km/hr (24 mph), Coming from NNE

Dark and snowing hard

Ship cruising east in light pack ice, depth 3473 meters (11,395 feet.)

Since I am on watch from twelve midnight to noon, I sleep during the day and wake up at nine or ten at night. As I lie in my upper bunk, even before I open my eyes, I can tell roughly how much cold salt water is between the ship's keel and the bottom. How do I know?

Wherever you are onboard the Nathaniel B. Palmer, but especially on the lower decks, you hear a periodic chirping sound. It goes on twenty-four hours a day, always there, so after a while you don't hear it unless you make an effort to listen. It sounds like a very large baby robin waiting for food, or like a rubber sole shoe twisting on a clean linoleum floor. At first I thought that it was my boots squeaking on the floor, because I have rubber sole boots. The floors, like the rest of this ship, are always clean and shining, thanks to the pride of the crew. I stopped walking, waited for a few seconds, and heard the sound again. Then I remembered from past voyages that the chirping noise came from the depth sounding equipment. In this case I was hearing the SeaBeam imaging equipment I told you about in a previous journal entry. (There are actually several sounding devices aboard. Each has a different sounding ping) How do I estimate depth without opening my eyes? If I could hear

the return ping, bounced back from the bottom, I could count seconds between the ping and reflection. Sound travels at about 1500 meters per second in water. If the reflected ping came back four seconds later, I'd know the total travel distance was 4 * 1500 = 6000 meters. Since the sound has to travel from the ship to the bottom and back, I'd divide that 6000 meters by 2 to get a depth of 3000 meters.

In the days of sailing ships, sailors were known to have banged iron bars underwater, then listened for the reflection, to get a rough idea of depth. I can hear the initial ping from my bunk, but the ship, with its diesels and breaking ice, is much too loud for me to hear the reflection. I can still get a rough estimate of the depth, though, from the time between successive pings. The SeaBeam needs to wait quietly a certain time (it's about 10 seconds in 3500 meters depth) so that its hydrophones can hear bottom reflections. If it pinged again during that time, the new ping would interfere with listening for the reflections. The waiting time is called the gate time, and the SeaBeam and other depth measuring devices adjust this time automatically so that they can finish listening to the reflections before the next ping.

So, in my bunk, with my covers pulled up over my head, I listen for the pings. When there is a long time between them, I know we are in deep water, beyond the shelf break. If they are three or four seconds apart, I figure we are up on the continental shelf, where depths are about 250 meters. The pings were far apart when I woke up this evening.

When I finally get out of my warm bunk and go outside, it is dark and snowing hard. The barometric pressure has been dropping, and spray blows from the tops of waves. If we were in open ocean, the waves would be a lot higher, but the broken pack ice keeps the height down. From the ship I see a dark, cold desolate, unearthly world. Once in a while a spotlight shines out from the bridge and roves around for several seconds like the alien spaceship spotlight in Close Encounters of the Third Kind. Then the ice and water go dark again. We have several hours of darkness each night. I try to imagine what it would be like here in June, in the southern hemisphere winter, when it is dark all the time, and much colder than it is now. Coming to Antarctica is the nearest I'll ever get to space travel. Yesterday I told you that there were diatoms in the drinking water at the South Pole. The person who told me this is Dr. Davida Kellogg, who received her Ph.D. from Columbia University. She is one of the three scientists who made a proposal to the National Science Foundation to study the history of glaciers here.

You probably know that diatoms are microscopic floating plants, phytoplankton, which grow in the oceans and form the base of the Antarctic food web. Davida is constantly looking for samples of diatoms. She takes them from sea water brought up by the CTD, from the bottom of ice chunks as the ship passes, from snow when Dr. Shusun Li's group goes out on the ice to dig pits, and from bottom sediment cores.

If you guessed that the marine diatoms must have gotten blown to the South Pole by the wind, you were correct. Diatoms are so light and so small that the wind can pick them up and blow them hundreds of miles. The ice cap is not frozen seawater; it comes from precipitation. The South Pole station gets its water by melting ice. Another scientist, looking for micrometeorites in the melted ice, found diatoms and sent her samples. During a very strong storm, diatoms from New Zealand or Chile may be blown to Antarctica.

There are thousands of diatom species, and Davida can recognize many of them. She and her husband have spent years studying diatoms from Antarctica, finding out which species live where. She has found that different species live in different sections of the coastline. By looking at diatoms from different levels in an ice core, she hopes in the future to be able to tell the direction and strength of past winds, and get hints about past climates. This information can help predict what is going to happen to the climate in Antarctica and the rest of the world in the future. I asked her what was most important about her work. She said that diatoms were interesting, and studying them fun, but that wasn't her principal concern. "Climate change has strong geopolitical consequences. Supposing desertification in the sahel is permanent. This interests me more than the diatoms themselves. Where are these people going to go? What will they eat? If we can predict future climate, maybe we can avoid some problems. Forewarned is forearmed."

Dr. Davida Kellogg filtering sea water to find diatoms. A vacuum pump sucks the water through a very fine filter, which traps the diatoms. The filter is then dried and examined under a microscope.

Suzanne O'Hara is a research associate and engineer at the Lamont Observatory of Columbia University. She is a veteran of numerous Antarctic trips and is our onboard expert in SeaBeam operation. She is seated at the controls of the imaging system. She knows a lot about other scientific equipment on the Nathaniel B. Palmer also.