Climatic Data, Sea Floor Records

THE SEA FLOOR is blanketed with sediments composed primarily of the remains of plants and animals that live in the oceans which cover three-quarters of the Earths surface. Sea-floor sediments also include particles of soil, dust, volcanic ash, and fragments of vegetation that are washed off the land by rivers and floods, blown in by winds, or left by melting icebergs. In the deep oceans throughout the world, sea-floor sediments have piled up continuously over thousands to millions of year, silently recording the history of changes in climate and ocean conditions as far back as the time of the dinosaurs 60–80 million years ago.

Over the past 50 years, modern engineering and scientific techniques in paleoceanography have accessed these deep-water sea-floor archives and have read their messages about natural climate changes that occurred before the recent global warming started about 150 years ago. The importance of the sea-floor archives is that they allow an examination of past natural cycles of climate change and how the marine and land biological communities responded to fluctuations that include conditions hotter than the increase of 2–6 degrees C forecast for the present global warming.

Before about 1947, knowledge about climate change was mainly based on written historical records and the study of fossil plants and animals scattered over the continents in short sections that escaped destruction by glaciers during the Ice Age. In 1947, the new technology of piston coring and an international Deep Sea Drilling Program (DSDP) that enabled sampling of long [Page 246]sections of sea floor sediments revolutionized the science of climate change. Working from ships dedicated to scientific research, these new methods enabled the retrieval of unbroken sediment cores up to 2,625 ft. (800 m.) long, covering time spans of up to 10 million years. The archival data in the sea floor records consists mainly of tiny fossil marine phytoplankton and zoo-plankton called microfossils, mixed in with fine sand or mud swept off the land, and dust particles, including pollen grains from forests and grasslands on the continents. The oceanic microfossils, pollen, and sediment particles provide proxy (indirect) records of océanographie or atmospheric conditions in ancient times.

The main oceanic plant microfossils are diatoms, dinoflagellate cysts, and coccoliths; the main animal microfossils are foraminifera and radiolarians, all of which are less than a few millimeters in length. The shells of even the tiniest (pinhead-size) marine microfossils carry an imprint of the oceans temperature, salinity, and carbon production at the time they were alive.

When extracted in the thousands from the sea, floor sediments by sieving just a handful of ocean mud, the microfossils can be analyzed by chemical methods (stable isotope measurements), or by statistical analysis of their population composition, to reveal the océanographie conditions at their time of death. The kinds of pollen found in the sediment samples tell us how much forest or grassland there was on the continents surrounding the ocean, while the amount of inorganic sediment provides a proxy-signal of river flooding or sea ice.

The results of the chemical and biological studies of the sea-floor sediment cores from all the world's oceans show a zigzag pattern of alternating warm and cold climates extending back at least 5 million years. To understand the cause of this saw-tooth climate record, methods were developed for calculating the age of the warm-cold cycles. Radiocarbon dating of the youngest sediments showed that the temperature peaks corresponded to warm periods lasting about 6,000 to 10,000 years, while the dips represented longer periods of glacial conditions lasting 20,000–30,000 years.

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