PaleoceanographyIs a study of development of ancientocean system based on the informationavailable from the sedimentary materialincluding microfossils.
Material• Most paleo-information taken from sediment cores• Extremely expensive to collect, hence valuable• Deep Sea Drilling Program (DSDP), Ocean Drilling Program (ODP)• International Ocean Drilling Program (IODP), Glomar Explorer• Ocean sediment record limited to about 180 million years.• Study of paleoceanography is based on microfossils especially foraminifera, nannofossils and isotope (Oxygen and carbon).
History• Beginning in 1968, an enormous amount of effort in this subject, especially through the work of the CLIMAP group in Pleistocene oceanography, and through the extensive drilling in the deep sea by Glomar Challenger.
Nature of paleoceanography• Paleoceanography includes:- – Study of water circulation (surface and bottom currents) – Planktic and benthic development and evolution – History of biogenic productivity and its effect on sediment distribution – History of Carbonate and silica deposition and dissolution.
Proxies in paleooceanography• Microfossils• Remnants (inorganic structures or organic chemicals) of specific species.• Modern day activity of these species is extrapolated to the past (warm-water species vs. cold, specific diversity, coiling directions and productivity of microfossils.• Isotopic Composition of carbon and oxygen isotopes• - Measure how much of the heavy and light isotope there is in the sediments .• -The relative abundance of the heavy and light isotope (i.e. the isotopic composition) depend on ocean conditions….especially climate• -
Oxygen Isotopes and climate• Perhaps no tool has been more highly utilized for paleoclimate reconstruction than δ O18. Early work by Sam Epstein at Caltech opened the field of quantitative paleoclimate research. – O16 - 99.8% of the oxygen present – O18 accounts for most of the rest. O18/O16 ~ 1/400.• The changes in Oxygen isotope 18 is expressed by• δO18 = [(O18/O16)sample – (O18/O16)standard] X 1000 (O18/O16)standard• Standard is taken from CaCO3 from Cretaceous belemnite obtained from Pee Dee Formation, South Carolina
• The utility of oxygen isotopes as paleothermometers has to do with the fractionation that occurs during evaporation and condensation of water and in the formation of CaCO3.• During evaporation, δO18 of the vapor is less than that of the source because the heavier isotope is preferentially retained.• When this water condenses, again, the heavier isotope is preferentially condensed and the vapor becomes ever lighter.• The net movement of water vapor poleward by the atmospheric hydrological cycle produces a latitudinal gradient in δ O18 with the heavier isotope preferentially retained in the tropics. Snow falling at high latitude is very depleted in O18 and thus formation of glaciers leads to enrichment in oceanic O18 while melting of the ice caps reduces this enrichment.
Paleothermometry – using carbonates• Epstein Equation:• T= 16.9 – 4.2 (δO18c – δO18w)• where δO18c is from the calcite shells of foraminifera• δO18w is the mean value of ocean water when the shells formed.• t (ºC) = 16.9 – 4.14(δ 18Ocalcite – δ 18Owater) + 0.13(δ 18Ocalcite – δ 18Owater)2
δO18c in benthic foraminifera record the longtermcooling of the deep ocean. K.G. Miller et al.,Paleoceanography 2, 1, 1987.
Types of climate• There are two modes of climate – Warm (green-house) – Cold (ice-house) – Paleoceanographic conditions can be divided into two types. Polytaxic Oligotaxic
Polytaxic condition• Develop during the warm period, No glacier in the polar regions.• Ocean less stratified• Poorly developed thermocline• Paleogeographic provinces not well-developed• Bottom currents weak• No thermohaline circulation• Ocean bottom poorly oxygenated• Development of black shale due to reducing condition at the sea-bottom.• Relatively higher number of species
Oligotaxic condition• Develop during the glacial period the oceans are stratified warm water near the surface and cold water at the bottom.• Well developed thermocline• Paleogeographic provinces are well developed.• Strong cold dense bottom currents developed. Oxygenated bottom of the ocean.• Development of thermohaline circulation. Strong bottom currents• A lot of erosion and hiatuses• Relatively low number of species.• Nunber of species higher in Tropic and reducing towards the poles
A drastic increase in oxygen isotope values in benthic foraminifera between 15 and 13 million years ago. • This increase reflects both growth of ice on Antarctica and a world wide cooling of abyssal waters. • A substantial carbon isotope excursion toward heavy values • Synchronous with the onset of abundant sedimentation of organic-rich sediments in the margins of the Pacific.
Paleogeography All through the Tertiary, changes in geography due to plate motions affect the configuration of exchange between ocean basins. The gateways control access to the Arctic Ocean (east and west of Greenland), connnect the global ocean along the Equator ("Tethys Ocean" between Africa and Eurasia, Panama Straits, Indonesian Seaway between Borneo and New Guinea), and control the evolution of the Circumpolar Current (Tasmanian Passage, Drake Passage).
The Great PartitioningFig.9.12 Geography of the middle Eocene (ca 45 Ma) and major critical valvepoints for ocean circulation. Tropical valves are closing (filled rectangles),high latitude valves and opening up (open rectangles) throughout theCenozoic.[Base map from B. U. Haq, Oceanologica Acta, 4 Suppl.:71]
Example of using microfossils to ‘reconstruct’ change in ocean current patternWarm water surface circulation cut off between N. and S. America.
Tertiary Oxygen Isotope Record• An overall cooling trend since the Cretaceous, from the increase of oxygen-18 in benthic foraminifera.
T°C Cretaceous Pleistocene Plio- Paleo. Eocene Oligocene Miocene "ice free" -1 Antarctic Ice Sheets 14 12 0 10 N Hem . isphere Ice Sheets 8δ18O ä 1 6 possibly ice free 8 4 2 2 6 ice sheets 4 3 2 0 4 modern 70 60 50 40 30 20 10 0 Age Ma Modified after Miller et al., Paleoceanography, 2, 1-19, 1987.
(A) 1-My means for planktic foraminiferal assemblage sizes95/5 from tropical and subtropical sites(squares) and from temperate and subpolar sites (triangles). The vertical line shows the meansize95/5 (389 µm) of all assemblages. Light blue and orange shading shows ±1 standard deviations formean sizes95/5. Plio., Pliocene; Plt., Pleistocene. (B) The global deep-sea oxygen isotope recordrepresenting Cenozoic polar cooling and ice accumulation (23). (C) The total number of plankticforaminiferal species globally known per 1-My interval (18).Correlationbased onforaminifera
Atlantic and Pacific CCD Fluctuations • The CCD stood high in the late Eocene, dropped in the earliest Oligocene, rose in the Miocene when it reached a peak between 10 and 15 years ago, and then fell to its present depth near 4.3 km.Fig.9.18 Reconstructions of CCD fluctuations for various oceanic regions.Solid and dashed lines. Reconstructions of Tj. H. van Andel et al., 1977, JGeol 85:651. Dotted line Reconstructions of W. H. Berger, P. H., 1975, RevGeophys Space Phys 13:561. The reconstructions agree in the generalpatterns of the fluctuations, which appear correlated with sealevel changes
CCD Fluctuation• An overall similarity in the CCD fluctuations of Pacific and Atlantic: a sign that the chemical climate of the ocean is changing on a global scale.• Eocene and Miocene have a shallower CCD than Oligocene and Plio-Pleistocene.• A certain parallelism of this pattern with oxygen isotope variations and with sea-level variations.• Periods of high sea level are characterized by shallow CCD and warm high latitudes; periods of low sea level have a deep CCD and cold high latitudes (and deep waters).
"Anoxic Events" and Volcanism• The widespread occurrence of Cretaceous organic-rich sediments• Distinct positive excursions in the δ13C record Fig.9.21 "Oceanic Anowic Events" of Schlanger and Jenkyns, and δ 13 C record of pelagic marine limestones, showing coincidence of the "events" with positive δ 13 C excursions (generated through the lock-up of 12 C-rich carbon). Width of blank band reflects uncertainty in the value of δ 13 C . Lower part; Cretaceous stages from Berrassian (right) to Maastrichtian (left). OAEs are centered in the Aptian, Cenomanian/Turonian and Santonian/Campanian
Two Modes of Circulation• Warm Mode – N. Atlantic Deep Water – Shallow Compensation• Cold Mode – N. Atlantic Intermediate and Upper Deep Water – Deep Compensation