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Transcript
EAS/BIOEE 154 Lecture 6 Introduction to Oceanography Marine Sediments and Climate History Principal of Superposition Sediments get older downward. Unconformity Gap in time in the sedimentary sequence. Types of Marine Sediment Detrital particles which arise from weathering on the continents Chemical Precipitated from seawater Ions transported in solution from site of erosion Precipitation often biologically mediated Distribution of Sediments Neritic sediments Continent-derived sands and muds – mostly detrital sediments Confined to continental margins Pelagic Sediments: deep ocean Biogeneous: Calcareous and siliceous oozes Abyssal Clays: Very fine clays confined to the deep ocean basins Hydrogenous sediments: Abiologic chemical precipitates (rare) Pelagic Biogenous Sediments: Oozes Oozes are sediment composed predominantely of shells of micro-organisms Calcareous Confined to shallower regions (<4500 meters depth) Foramifera (protozoans) Pteropods (floating snails) Coccolithophorids (algae) Siliceous Shells of silica (same as quartz or glass) Radiolarians (protozoans) Diatoms (algae) Most common beneath areas of high biologic productivity Hydrogenous Sediments Direct precipitates from seawater (without biological action). Manganese Nodules the most common example. Occur only where sedimentation rates are very low. Grow at rates of 1 mm/1000 years and less Reading the History Book: Climate Changes The Ice Ages Swiss-American geologist Louis Agassiz (1807-1873) recognized that EAS/BIOEE 154 Lecture 6 glaciers larger than the present ones carved many features of Switzerland. Similar features recognized in N. America But what caused the glaciations? What was the chronology of events? Milankovitch proposed that variations in the following orbital parameters caused the Ice Ages: Axial Obliquity (Tilt) Tilt of the Earth’s rotational axis relative to the orbital plane varies from about 21 1/2˚ to about 24 1/2˚ on a time scale of 40,000 years. Orbital Eccentricity Earth’s orbit varies from more circular to less circular on a time scale of about 100,000 years. Precession Precession refers to the changing orientation of the rotational axis and its relation to orbital ellipse. Time-scale: 23,000 years. Testing the Theory Harold Urey proposed that this theory could be tested by measuring oxygen isotope ratios in calcareous oozes What are isotopes? Atoms of the same element that different numbers of neutrons in the nucleus and therefore different masses. The mass difference leads to very slight difference in chemical behavior 16 18 H2 O evaporates from the ocean more readily than H2 O; The extent of this effect increases with decreasing temperature in a predictable way. 18 When foraminifera produces their shells of calcite (CaCO3), O is used in 16 preference to O; The extent of this effect depends on temperature – it is greater at lower temperature. In the 1970’s and 1980’s oceanographers John Hayes and John Imbrie conclusively showed a correlation between O isotope ratios and Milankovitch forcing. Need for a feedback mechanism and amplification The Milankovitch insolation changes are too small to produce the climate change observed. Possible Feedbacks Albedo CO2 Changes in ocean circulation CO2 and the Greenhouse Effect Although a trace gas, CO2 strongly affects transmission of radiation through the atmosphere. In ice cores from both Antarctica and Greenland, there is a strong correlation between CO2 and temperature: CO2 was lower during glacial periods. But why? The most likely theory involves wind, dust, nutrients, plankton, and the 2 EAS/BIOEE 154 Lecture 6 biological pump. Ice Ages and Atlantic Circulation The formation of North Atlantic Deep Water (NADW) and the Gulf Stream play an important role in warming the North Atlantic (particularly Europe). Without them, Europe would have a substantially cooler climate. During glacial periods cooler temperatures in the North Atlantic may have reduced the Gulf Stream flow, and shut down associated NADW production, cutting off the flow of thermal energy to the North Atlantic. Some Conclusions about Climate Earth’s climate has varied dramatically in the past. These variations are recorded by various proxies in marine sediments This variation occurs on all time scales From a few years (El Niño) To 100 million years Some of this variation is due to external forcing (e.g., Milankovitch cycles) Some is due to other factors On all these scales, the oceans play an important role in climate change. Some Study Questions Explain why the principal of superposition should hold – in other words, why should sediments get older downward? Give an example of one kind of shell that makes up a siliceous ooze? What kind of organism produces this shell? Where are siliceous oozes typically found? Why are manganese nodules only found in areas of very low sedimentation rate? What are the Milankovitch parameters Explain how does variations in the Earth’s Axial Obliquity affect climate? Does global average insolation change as a result of Milankovitch variations? Explain how 18O and 16O atoms differ Explain how the ratio of 18O to 16O in seawater changes when glaciers build up on continents. Define the term “albedo”. Explain how albedo operates as a climate feedback mechanism. What is the evidence that atmospheric CO2 concentrations and temperature correlate over the last 150,000 years? Explain how changes in ocean circulation, particularly in the North Atlantic might result in drastic changes in climate. 3