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Earth Institute News

posted 11/08/04

Contact: Mary Tobin
845-365-8607 or mtobin@ldeo.columbia.edu

Reports From the Field

The Anslope Expedition: Topography

Robin Robertson, Doherty Associate Research Scientist with the Lamont-Doherty Earth Observatory, is doing educational outreach with the 5th and 6th grade classes at George Grant Mason school in Tuxedo, NY. Robertson is sending daily updates to students while on the R/V Nathaniel B. Palmer, which is traveling between New Zealand and the Antarctic for the third ANSLOPE cruise. daily reports from Dr. Robin Robertson through December 1

penguins

November 23 , 2004
Dr. Robin Robertson aboard the R/V Nathaniel B. Palmer...

Our Science Objectives (Goals)

The average depth for the ocean is about 4000 m.  Only the upper 500-1000 m is warmed by the sun.  The rest of the ocean is cold and is referred to as deep water.  It is also the major part, three-fourths, of the ocean.  The deep water is formed in the polar regions, either around Antarctica or in the Greenland Sea.  In these regions, the air cools the ocean, which makes the water denser. Cold water is denser than warmer water and dense water sinks.  Also when ice is formed from salt water, it rejects the salt.  This makes the surrounding water saltier.  Saltier water is also denser than fresh water, which is why you float better in the ocean than in a pool.  So in the polar regions, the colder and saltier surface water becomes denser and sinks. The deep water for the entire ocean is formed in the polar regions.

We are interested in the processes that cause the deep water to form and cause mixing between the different water types: shelf waters, Circumpolar Deep Water, Antarctic bottom water (see report on Ross Sea Water Masses).  Some of these processes are internal tides, internal waves, friction, storms, etc.  

Shelf Water is very dense and flows along the bottom.  It flows off of the continental shelf and down the continental shelf.  Often it flows in a big blob, which we may call a plume, depending on its size and shape.  We are trying to observe these plumes and see how they mix as they flow down the continental slope and into the deep basin.  As they mix, they form Antarctic Bottom Water, which is the water type that makes up the global deep ocean.  One of the previous AnSlope cruises found a couple of these plumes.  Sometimes they are hard to find.  It depends on how big they are and whether they last a short time or a long time.  Our objectives for the cruise are to observe one or more of the deep plumes and also the processes causing the mixing.

Robin at the bow of the Natty B

November 20 , 2004
Dr. Robin Robertson aboard the R/V Nathaniel B. Palmer...

Ross Sea Water Masses

When you watch the weather report, they are always talking about fronts coming through.  These fronts separate different air masses.  The air has a different temperature on either side of the front.  So that once it passes, it gets cold (or warm) fast. In the ocean, there are also fronts which separate different water masses.  There are several types of water masses that exist in the Ross Sea (the site of our study).  Over the continental shelf, the wind pushes the ice away from the shore and new ice is formed. Since the salt is rejected by the ice, the water becomes very salty.  Also since it is shallow over the shelf, it is cold, because it is not deep enough to be separated from the surface.  As a result, very cold, salty water is formed over the continental shelf.  It is called Shelf Water (SW).  There are two types: High Salinity Shelf Water (HSSW), which forms in the western side and Low Salinity Shelf Water (LSSW), which forms more on the eastern side. Since density increases both with increasing salinity and decreasing temperature, this water is very, very dense and sinks to the bottom.  It also overflows off the continental shelf and flows down the continental slope.  In the deeper part of the basin, there is Antarctic Surface Water (AASW), which is warmer in the summer and colder in the winter and is located in the upper 200 m.  Below that is Circumpolar Deep Water (CDW).  This is warmer water, temperatures over 0oC (freezing for fresh water).  It flows into the basin from the Antarctic Circumpolar Current and brings heat into the basin.  CDW is generally from about 200m to 500-1000 m, depending on the location in the basin.  Below the CDW, from 1000-the bottom at 3000-4000m is another type of water called Antarctic Bottom Water.  This is an important water type, since it flows out of the Ross and Weddell Seas and fills the global deep ocean.  Between the Shelf Water and the CDW and AABW, there is a front called the Antarctic Slope Front, since it generally occurs over the continental slope off Antarctica.  There is also Ice Shelf Water (ISW), which flows out beneath the ice shelf.  It is very cold, but not as salty as the HSSW or LSSW. It is fresher since it has melted ice shelf in it.  It is cold, since some of its heat was used to melt the ice shelf.

Robin Robertson

Scuba Steve and Robin launch an XBT off the side of the Natty B (RVIB Nathaniel B Palmer).

November 6 , 2004
Dr. Robin Robertson aboard the R/V Nathaniel B. Palmer...

Under the water, there are mountains and ridges like on land. Near the continents, the water is shallow, under 500 m deep. This is called the continental shelf. The bottom then drops off rapidly with depth to about 3000 m in a short distance. This is called the continental slope. Most of the ocean is deep 3000-5000 m and is referred to as the deep basin. The exceptions are the continental slope and shelves, which are near the continents, and some islands, especially volcanic islands, like Hawaii that stick up from the ocean floor. In the middle of the Atlantic Ocean, there is a ridge that sticks up from the bottom 1000-2000 m. This is the mid-Atlantic Ridge and is where the two plates that the American continents and the European and African continents move apart. It is moving apart at about 2-3 inches/yr, which is about as fast as your fingernails grow. In 40-60 yrs, Europe and New York will be 10 ft. further apart.

We are interested in investigating what is happening at the continental slope off Antarctica. This cruise is the third one for the Anslope project. "An" stands for Antarctica and "slope" for the continental slope, so Anslope is for the continental slope off Antarctica.

CTD

The CTD is the primary instrument that goes to the bottom to take measurements.

October 24 , 2004
Dr. Robin Robertson aboard the R/V Nathaniel B. Palmer...

We are interested in finding out how much mixing is happening and what the flow is. In the waters that we are interested in, density controls much of the flow. We use the CTD to measure Conductivity, Temperature, and Depth. The CTD is our primary instrument that goes to the bottom to make measurements. To get conductivity, a small electrical field is set up and the effects of the water on it is measured. The field will react to temperature and salinity. Salinity is the saltiness of the water. Likewise temperature is measured and the pressure. Pressure increases with depth according to the amount of water above it and the density of that water. So from the conductivity, temperature, and pressure, we determine the salinity and the depth. We also measure oxygen and how cloudy the water is. All the sensors are attached to a big frame, which also has 24 water bottles (called Niskin bottles) on it. This frame with the sensors and the bottles is lowered down to within 10 m of the bottom, taking measurements as it goes down. On the way up, we send a signal to close the bottles at different depths. That way we bring water up from deep in the ocean. We then sample this water for CFC, Helium, Tritium, and O18 and also oxygen and salinity. We use the oxygen and salinity to check the sensors on the CTD. The other tracers we use to tell how old the water is and where it comes from. If the water has CFC's in it, at least part of it was at the surface since the 1960's. If the water has Tritium in it, at least part of it was at the surface during the time they were doing atom bomb testing above ground, 1950's-1970. The presence of Helium and/or O18 indicates that it comes from under the ice shelf (explained in the ice report) and they use their concentration to determine the percentage of the water that comes from the melting ice shelf.