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Introduction to Palaeoceanography

Palaeoceanography is the study of the geological history of the the oceans.

The geological record of the Earths oceans is preserved mainly in sediments that accumulate on ocean floors. These sediments can be obtained by coring or drilling to recover sections of ocean sediment. Marine sediments can also be studied in places where natural geological processes have exposed on land sediments that formed on the sea floor.

The ancient Greeks realised that fossil shells were formed in ancient seas, and modern geologists study successions of fossil organisms to determine changing ecologies through geological time. The remains studied include those of microscopic marine animals and plants.

Some of the most dramatic of oceanographic changes have occurred during the last million years, when huge ice sheets have waxed and waned over the north and south polar regions - the "ice ages". Water that is evaporated from the oceans is relatively impoverished in the heavier isotope of oxygen, oxygen-18. During glacial episodes the oceans are relatively enriched in oxygen-18, because so much oxygen-16 has been evaporated and "locked up" in the ice sheets. Sea water is therefore richer in the heavier oxygen isotope, oxygen-18, during ice ages.

These changes in the isotopic composition of the oceans can be followed using the sequences of microscopic marine fossils of tiny unicellular marine organisms called "foraminifera". Some of these organisms live near the ocean surface (planktonic foraminifera) and some in the sediment on the ocean floor (benthic foraminifera). Since they build their shells using the sea water in which they live, a study of the isotopic composition of a small group of foraminifera can indicate the chemistry of the ocean in which the organisms lived. A series of measurements on such fossils from different depths in the core can provide a history of changing isotope ratios in the oceans, and thus a record of the changing size of the polar ice caps. The planktonic and benthic foraminifera record changes in the chemistry of the surface and deep water, respectively. Sediments cores are recovered by drilling operations from a drill ship; several techniques are used. A piston corer, for example, punches its way through the sediment to retrieve a record less affected by the rotation of rotary drilling. Besides recording the depth below the sea floor surface from which samples are recovered (MBSF, or "meters below sea floor") a composite depth sequence for a particular site can be made; this provides depth as mcd or meters composite depth.

The stable isotopes of carbon can also be used to help to trace the history of carbon cycling in the oceans.

 

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