Geoscience Reference
In-Depth Information
Messages in the mud
Sediment cores can carry a long and continuous record of past climate. The types of sedi-
ments can reveal what was going on on the surrounding land - for instance, material rafted
on icebergs or blown from deserts. But more precise records are kept by the ratios of stable
isotopes of oxygen in calcareous ooze. Oxygen in water molecules exists in different stable
isotopes, principally
16
O and
18
O. As sea water evaporates, molecules containing the light-
er
16
O evaporate slightly more easily, leaving the sea water enriched in
18
O. That is soon
diluted again by rainfall and rivers, except when large amounts of water get locked up in
polar ice caps. Then the carbonate taken up by plankton and deposited in sediments will
contain more
18
O than during the interglacial periods, so the oxygen isotopes in sediments
reflect the global climate. By matching up the changes recorded in sediments for a total of
more than 20 million years, the ocean drilling programme has shown how the climate fluc-
tuates on timescales that seem to reflect the Milankovich cycle, the wobble of the Earth's
axis, and the eccentricity of our orbit around the Sun.
In the 1970s the ocean drilling programme came to the Mediterranean. There, the drill
cores reveal something sensational. I was shown one of them where it is now stored at
the Lamont Doherty Geological Observatory of Columbia University in New York. It con-
sists of layer after layer of white crystalline material, a mixture of salt (sodium chloride)
and anhydrite (calcium sulphate). These evaporite layers can only have been formed by the
Mediterranean drying up. Even today, evaporation rates are so high that, were the Straits
of Gibraltar sealed off, the entire Mediterranean would evaporate in about 1,000 years. The
implication of the hundreds of metres of evaporite in the drill cores are that this must have
happened perhaps 40 times between 5 and 6.5 million years ago. When the scientists drilled
close to the Straits of Gibraltar, they encountered a chaotic mixture of boulders and debris.
This must have been the giant plunge pool of the world's greatest waterfall, when the At-
lantic broke through past Gibraltar to refill the Mediterranean. We can only imagine the
roar, the spray, the power of the water.
One of the most interesting of the recent legs of the ocean drilling programme involved
drilling into deposits of gas hydrate. These are sediments containing high concentrations
of methane ice, held in solid form by the low temperatures and high pressures of the deep
ocean floor. There is added excitement when a gas hydrate core is returned to the surface
as they can easily turn into gas again, sometimes explosively. That has made studying them
somewhat difficult, but there are believed to be vast deposits of them. It is possible that they
could become an economically important source of natural gas in the future. There are sug-
gestions that they played a significant role in sudden climate change in the past. They can
be quite unstable, and an earthquake can make large quantities float free of the ocean floor
