QUESTION:  
What methods do Antarctic scientists use to determine what type of            
plant and animal life existed hundreds and maybe even thousands
of years ago?  
                Tiffany B., 8th grade student from Juan Morel Campos 
                Intermediate School 71 in Brooklyn NY, 

ANSWER from Scott Borg. Program Director - Office of Polar Programs
            Antarctic Geology & Geophysics Program       
            National Science Foundation
            January 31, 1997
                            
 

Scientists try to figure out what the environment was like in the past,
including finding out what plants and animals existed, using many approaches.

If you are mainly interested in finding out what kinds of plants and animals
existed, then you have to do two things.  First, you have to find some
evidence of plants and animals, and second, you have to determine the age of
the material.  When animals or plants die, the material accumulates on the
ground and is moved around by wind and water, along with sediment and soil
particles, and generally moves downhill.  When it gets to a topographically
low spot (like a pond, lake, river bed, or ocean), the material (bones,
leaves, plant stalks, or whatever, along with the sediment) can be buried by
stuff arriving later.  As long as these sediments (with their bones and
stuff) are not eroded away, then they are targets for exploring for evidence
of past life.  Geologists and paleontologists look for areas where sediments
are preserved and then they look in the sediments for material from plants
or animals of the past.  Once you find the bones or leaves (or fossils, if
the material is old), then you have to determine how old the stuff is.  If
you have a pile of sediment layers, the oldest stuff is at the bottom and
the youngest stuff is at the top, but determining the exact age of any
particular layer in years is a much more difficult problem.

In some ponds or lakes, where conditions are right, sediments accumulate in
annual cycles and you can count the yearly layers like you can count tree
rings.  If there are trees around, then scientists can use tree-ring
chronology to determine the age of material for 100's to a few 1000's of
years.  In almost all cases, it is not possible to get a complete record
from one spot and so geologists must build records from several places that
go into the past.  This means that you have to match up layers near the
bottom of a sequence in one spot to layers near the top of a sequence in
another spot.  As you might imagine this can be pretty hard, but it is a
very interesting kind of puzzle to put together.

In some places, carbon-14 dating is a method that can be used to determine
the age of the material in years.  This is most useful for things in the
range of about 1000 years to 40,000 years.  Carbon-14 cannot be used for
older times.  Also, this method has some special problems in the Antarctic
relative to more temperate latitudes.  This is a method that depends on
naturally occuring radioactive atoms.  It is commonly used because all
living things on Earth contain carbon.  There are other atoms that can be
used but the methods are very complex.

For times in the range of 100's of years, the most accurate methods rely on
tree-ring dating and counting of annual sediment layers from deposits in
lakes or ponds.  Sometimes, volcanic ash layers can be used, providing
scientists can match the volcanic ash to an eruption of known age.  There
are some other ways of figuring out the age of sediments that are pretty
interesting, like finding a chemical signal from the first atomic bomb tests
on Earth, or finding chemical signals associated with the industrial
revolution.  However, these ways of dating sediments are more difficult to
use because so many other things can mask or contaminate the chemical
signals that might be present.