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 Earth's 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 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 zooplankton 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 oceano-graphic 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 ocean's 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 oceanographic 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.

Paleomagnetic records of periodic reversals in the Earth's magnetic field, together with dating from the decay of radioactivity in rock fragments containing uranium or potassium and argon, allowed a time-

scale to be placed against the sea floor proxy-climate records. Cambridge University scientist Sir Nicholas Shackleton then showed that the oxygen in the marine microfossil shells came in two forms, one lighter, one heavier, called isotopes, and he showed that when ice sheets develop on the continents, the lighter oxygen-16 isotope diminishes in the oceans because it is locked up in the frozen water on land. When the ice sheets melt, however, the light oxygen returns to the oceans and again appears in the microfossil shells.

Using this proxy-climatic data from the sea floor records, in 1976, Nick Shackleton, Jim Hays, and John Imbrie were able to decipher the encoded climate messages in the sea-floor sediments and confirmed that the ice-volume cycles correspond to seasonal and geographical changes in the amount of the sun's energy received at the Earth's surface, because of shifts in the position of the Earth's movement around the Sun, as predicted in the early 20th century by Russian scientist Milutin Milankovitch. The sea floor records of climate change revealed a history of 40-50 ice age cycles in the Northern Hemisphere, completely revising the old idea that there were just four glacial intervals during the Pleistocene Ice Age.

The proxy-climate records from fossil phytoplank-ton (dinoflagellates) in sea-floor sediments in the Canadian Arctic region west of Greenland also confirm climate model estimations of summer ice-free conditions and sea-surface temperature increases of 4-6 degrees C that followed the end of the last glacial cycle. These arctic sea floor records show that the natural warming cycles took place over one to two centuries, long enough for marine and land plants and animals to adjust to the climate change.

Less is known about the older pre-Ice Age sea floor records, but oxygen isotope data from 40 DSDP and ODP sites now extend back over the past 124 million years. These show that several times 50-120 million years ago, temperatures as much as 12 degrees C warmer than today. These proxy-temperature data come from measurements of the ratio of magnesium to calcium (Mg/Ca) in fossil foraminifera. The boron isotopes in these microfossils also suggest that very high levels of carbon dioxide accompanied this hothouse ocean. The sea floor records also reveal several times, just before and after the extinction of the dinosaurs, when vast quantities of organic carbon were buried in some of the ocean basins during periods called Oceanic Anoxic Events (OAEs).

The OEAs seem to be related to times of extreme warmth and great variability of surface water in the subtropical to tropical latitudes of the Atlantic Ocean. The OEAs lasted about .5-1 million years and were accompanied by maj or extinctions of marine biota and wholesale changes in the composition and structure of marine animal populations. Overall, the sea-floor climate data show that very warm temperatures and hothouse conditions prevailed until about 55 million years ago, after which there was a slow drift toward the Pleistocene Ice Age and the oscillations between greenhouse and icehouse conditions.

SEE ALSO: Climatic Data, Oceanic Observations; Greenland Cores; Ice Ages; Ocean Component of Models; Oceanic Changes; Oceanography; Paleoclimates; Vostok Core.

BIBLIOGRApHY. John Imbrie and K.P. Imbrie, Ice Ages: Solving the Mystery (Enslow, 1979); Dick Kroon, "Exceptional Global Warmth and Climatic Transients Recorded in Oceanic Sediments," JOIDES Journal (v.28/1, 2002); Peta Mudie, Andre Rochon, and Elizabeth Levac, "Decadal-Scale Sea Ice Changes in the Canadian Arctic and the Impact on Humans during the Past 4,000 Years," Environmental Archaeology (v.10, 2005); W.F. Ruddiman, Plows, Plagues and Petroleum (Princeton University Press, 2005).

Peta Mudie

Geological Survey of Canada Atlantic

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