Paleoceanography

paleoceanography, scientific study of Earth’s oceanographic history involving the analysis of the ocean’s sedimentary record, the history of tectonic plate motions, glacial changes, and established relationships between present sedimentation patterns and environmental factors.

Prior to the breakup of Pangea, one enormous ocean, Panthalassa, existed on Earth. Currents in this ocean would have been simple and slow, and Earth’s climate was, in all likelihood, warmer than today. The Tethys seaway formed as Pangea broke into Gondwana and Laurasia. In the narrow ocean basins of the central North Atlantic, restricted ocean circulation favoured deposition of evaporites (halite, gypsum, anhydrite, and other less abundant salts). Evaporites also were deposited some 100 million years ago in the equatorial regions of the South Atlantic during the early opening of this ocean.

Sequences of organic-rich, black shales were deposited during the early phases of spreading in the North and South Atlantic. These sediments indicate anoxic conditions in the deep ocean waters. The oceans must have been well stratified into dense layers to prevent the overturning and mixing required to replace depleted oxygen. Black shales also were deposited in the older areas of the eastern Indian Ocean.

During the time interval between 200 and 65 million years ago, but especially from about 100 to 65 million years ago, microplankton abundance and diversity increased enormously in the oceans. This resulted in increased deposition of biogenic sediments in the ocean basin. During the Cretaceous Period (145.5 to 65.5 million years ago), sea level was often high, and shallow seas lapped onto the continents. This may have provided an environment favourable to the explosion in the numbers of species of foraminiferans, diatoms, and calcareous nannoplankton (single-celled, photosynthetic organisms with shells made up of calcium carbonate plates called coccoliths). Increased abundance of calcareous nannoplankton shifted the locus of carbonate sedimentation from shallow seas to the deep ocean. The end of the Cretaceous Period is marked by a sudden extinction of many life-forms on Earth, and marine organisms were no exception (see K–T extinction). Coccolithophores (calcareous nannoplankton) and planktonic foraminiferans were particularly affected, and only a few species survived. Ocean sediments were suddenly less biogenic, and clays became widespread.

After the Cretaceous Period Earth underwent a gradual cooling, especially at high latitudes. Deep-sea sedimentation changed as thermohaline bottom-water circulation became fully developed. The Calcite Compensation Depth (CCD), the level at which the rate of carbonate accumulation equals the rate of carbonate dissolution, rose in the Pacific and dropped in the Atlantic as a result of changes in thermohaline circulation. An event of major significance was the spreading away of Australia from Antarctica beginning about 58 million years ago. This separation initiated limited circum-Antarctic circulation, which isolated Antarctica from the warmer oceans to the north, and led to cooling, which set the stage for later major glaciation.

At the boundary between the Eocene and Oligocene epochs (33.9 million years ago), Antarctic Bottom Water (AABW) began to form, resulting in greatly decreased bottom-water temperatures in both the Pacific and Atlantic oceans. Bottom-living organisms were strongly affected, and the CCD suddenly dropped from about 3,500 metres (about 11,500 feet) to approximately 4,000 to 5,000 metres (13,000 to 16,000 feet) in the Pacific. Bottom-water temperatures were generally warm, 12 to 15 °C (54 to 59 °F), during the time preceding this event. In a study of deep-sea sediment core material from near Antarctica, New Zealand Earth scientist J.P. Kennett and American oceanographer Lowell D. Stott discovered that there was a period between roughly 50 and 35 million years ago when deep waters were very warm (20 °C [68 °F]) and salty. The origin of these ocean waters was most likely in the low latitudes and resulted from high evaporation rates there.

The modern oceans are distinguished by very cold bottom water. The gradual changes toward this condition began 10 million years after the origination of AABW. Particularly significant among these changes was the closing of the Tethys seaway as Australia and several microcontinents moved north into the Indonesian region. Also, Australia moved far enough north that circum-Antarctic surface circulation became fully established.

The modern ocean circulation patterns and basin shapes were mostly in place by the beginning of the Miocene Epoch (about 23 million years ago). An exception was an ocean connection between the Pacific and Caribbean Sea in Central America that persisted until about three million years ago. Major and probably permanent ice sheets on Antarctica formed during the Miocene Epoch, and glacial sediments began to dominate the seafloor surrounding the continent shortly thereafter. Siliceous oozes also became widespread around Antarctica. Siliceous sedimentation increased in this area at the expense of siliceous sedimentation in equatorial regions. Ocean circulation became more vigorous, global climate became cooler, and sedimentation rates in the ocean basins increased. Planktonic microorganisms were segregated into latitudinal belts. Bottom-water flow north through the Drake Passage between South America and Antarctica began in the Miocene Epoch, resulting in erosion and nondeposition of sediments in the southwest Atlantic and southeast Pacific oceans. Also during the Miocene Epoch rifting between Greenland and Europe had progressed to a point where a connection was established between the North Atlantic and the Norwegian Sea. This resulted in the formation of North Atlantic Deep Water, which began flowing south along the continental rise of North America at this time. Sediments redistributed and deposited by this deep current are called contourites and have been extensively studied by American geologists Bruce Heezen, Charles D. Hollister, and Brian E. Tucholke, among others.

Sudden global cooling set in near the end of the Miocene Epoch some six million years ago. The strength of ocean circulation must have increased, as evidence of increased upwelling and biological productivity is present in ocean sediments. Diatomaceous sediments were deposited in abundance around the rim of the Pacific. This cooling event is synchronous with a drop in sea level, thought to be about 40 or 50 metres (130 to 165 feet) by various authorities, and probably corresponds to the further growth of the Antarctic ice sheet. This lowered sea level, coupled with the closure of narrow seaways probably due to plate movements, isolated the Mediterranean Sea. Subsequently, the sea dried up, leaving evaporite deposits on its floor. The Swiss geologist Kenneth J. Hsü and the American oceanographer William B.F. Ryan have concluded that the Mediterranean probably dried up about 40 times as seaways opened and closed between six and five million years ago. This evaporation removed about 6 percent of the salt from the world ocean, which raised the freezing point of seawater and promoted further growth of the sea ice surrounding Antarctica.

Enormous ice sheets emerged in the Northern Hemisphere between three and two million years ago, and the succession of Quaternary glaciations began at 1.6 million years ago. The exact cause of the glacial period is unclear, but it is most likely related to the variability in solar isolation, increased mountain building, and an intensification of the Gulf Stream at three million years ago due to the closing off of the Pacific-Caribbean ocean connection in Central America. The Quaternary glaciations, of which there were probably 30 episodes, left the most dramatic record in ocean sediments of any event in the previous 200 million years. Terrigenous sedimentation rates greatly increased in response to fluctuations in sea level of up to 100 metres (about 300 feet) and a more extreme climate. Biogenic sedimentation also increased and fluctuated with the glacial episodes. Deep-sea erosion began in many places as a result of intensified bottom-water circulation.