Geoscience Reference
In-Depth Information
ging from high rocky cliffs to low sand dunes and mud flats. And it is an environment into
which, for some strange reason, large numbers of humans seem to flock in hot weather. But
coastlines do not stay still. Some are eroding as the sea scours away millions of tons of ma-
terial. In other places land is building up as the sea drives up sand banks or rivers extend
their muddy deltas. Over geological timescales, the variations have been spectacular. In
some cases huge parts of continents have been flooded in what are called marine transgres-
sions. At other times the sea retreats - marine regressions. These apparent changes in sea
level can be due to a number of reasons. One of the present concerns about global warming
is that it might cause a rise in sea level. That can be due simply to the oceans warming so
the water expands slightly; this alone could raise sea level by maybe half a metre in the
next century. It could rise much further if there is significant melting of the Antarctic ice
cap. (Melting of the Arctic ice and Antarctic sea ice would have no overall effect on sea
level since the ice is already floating and thus already displacing its own weight in water.)
But all this is nothing compared to past sea level changes. Since the peak of the last Ice
Age, sea level appears to have risen by as much as 160 metres. It fluctuated dramatic-
ally with the climate during ice ages over the last 3 million years. Going back further, sea
level was at its highest between about 95 and 67 million years ago, during the upper Creta-
ceous, when shallow seas covered large areas of continents and thick deposits of chalk were
formed together with many of the deposits that are yielding oil today. One theory for these
exceptionally high sea levels is that large areas of ocean floor were being uplifted by hot
material rising in the mantle as the Atlantic Ocean began to open. The geological record of
sea level is characterized by periods of steadily rising seas followed by an apparently ab-
rupt fall in sea level. Sometimes apparent sea level falls can be linked to the tectonic uplift
of continents. In some instances it seems to happen globally and not always at the onset
of an ice age. Perhaps it is sometimes due to sudden large-scale rifting in the ocean floor,
literally pulling the floor out from under the sea.
Drilling the seas
From 1968 the ocean floor was sampled directly and scientifically by the US-led Deep Sea
Drilling Program using a drilling vessel named the
Glomar Challenger
. This was super-
seded in 1985 by the international Ocean Drilling Program using the improved
JOIDES
Resolution
. Around 200 separate two-month-long voyages or legs have taken place, with
core samples drilled at intervals along each. The deepest holes exceed 2 kilometres, and
overall thousands of kilometres of core samples have been recovered. Many of them in-
clude varying depths of sediment down to volcanic basalt beneath. They all tell stories
about their origins and the changing conditions of climate and ocean. Far from eroding
land and river deltas, sedimentation rates are much lower. At high latitudes the sediments
