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oceans. For
of precious clues.
1. The ocean’s memory
Variations in solar radiation, linked to the position of the Earth in relation to the sun,
explain the occurrence of alternating glacial (cold) and interglacial episodes (warm)
over the course of the past million years of our planet’s history. These changes in
climate had an impact on sea level, temperature and the circulation of marine
currents. They are recorded in the features of marine sediment composition and
certain fossil organisms.
We are now living in an interglacial period. This is following on from the last glacial episode which
peaked about 20,000 years ago. The average temperature on Earth at the time was 4 to 5°C lower
than it is now; ice covered an extensive part of northern Europe and North America. However, the
tropical oceans were locally a little warmer than at present (+ 1 to 2°C). During the last glacial
maximum, the average sea level was 120 m lower than it is now.
, contain
The corals of barrier reefs, marine landscapes typical of the Pacific
precious archives of past climates.
2. At the ocean floor
The first palaeo-oceanographic research investigated the composition of the different layers of sediment that accumulated
with time at the bottom of the oceans. Sediments brought up in ocean drilling cores contain different species
of Foraminifera (shell-bearing marine organisms). These species are distinguished by the specific
type of environment they inhabit (according to temperature especially); they live either at the
surface (planktonic species), or at the bottom (benthic species). Distribution analysis of the
different species of these two types of Foraminifera and the geochemical composition
of their shells yield valuable information on the marine temperature (at the surface
and at the bottom of seas respectively) and on the circulation of ocean water masses.
Certain fossil shells yield useful information for analysing the El
Niño events that have occurred over the past 9 millennia. Shells
of Protothaca thaca and growth disturbances associated with
excessively high sea water temperatures that prevailed in certain
summers (A, B, C). Dotted line: section line.
3. When corals tell their history
The skeletons of some coral species provide information on the marine environment and past
climatic conditions in tropical regions. Scientists have used such data to determine the sea surface
temperature, the salinity and variations in precipitation over periods of tens of years or of one or two
centuries, after drilling surveys conducted on living coral heads. Research on older fossil corals has resulted in reconstruction
of the climatic conditions prevailing in the tropics during particular periods of the past. Events characteristic of El Niño
episodes have been recognised, for example at the end of the last glacial period.
In 2005, an international scientific expedition launched a wide-ranging drilling campaign on coral reef
heads in the South Pacific. Data from analysis of the fossil corals of the barrier reef off Tahiti allowed the
230 °
220 °
210 °
trace to be drawn of the sea level variations over a period covering the last 20,000 years !