08 - PALEOCEANOGRAPHY How has the Earth's ocean changed through time and how do we know Heat exchange between ocean and atmosphere control oceanic

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Unformatted text preview: PALEOCEANOGRAPHY How has the Earth's ocean changed through time and how do we know? Heat exchange between ocean and atmosphere control oceanic and atmospheric circulation patterns. Reliable past seawater temperature estimates are crucial to any reconstruction / modeling of past ocean salinity, density, stratification, thermohaline circulation, and ice volume. Need paleoceanography proxies... proxies... records of what the ocean was like Last 800,000 years Imbrie and others (1984) Very cyclic pattern... Glaciers, Chamonix, France Milankovitch cycles As the Earth spins around its axis and orbits around the Milankovitch cycles (named after mathematician who Sun, several quasi-periodic variations occur. quasidescribed them, 1911) are the collective effects of changes in Earth's astronomical movements on climate Earth' Changes in movement and orientation change the amount and location of solar radiation reaching the Earth (solar forcing) 1- Eccentricity 2- Obliquity 3- Precession Zachos et al. 2001 Milankovitch cycles do explain long-term climate change Milutin Milankovitch (1879-1958) 1 Shape of Earth's orbit Earth' The Earth's orbit is an ellipse The eccentricity is a measure of the departure of The shape of the Earth's orbit varies from being this ellipse from circularity nearly circular (low eccentricity of 0.005) to being mildly elliptical (high eccentricity of 0.058) and has a mean eccentricity of 0.028. The present eccentricity is 0.017. If the Earth were the only planet orbiting our Sun, the eccentricity of its orbit would not vary in time. The Earth's eccentricity varies primarily due to interactions with the gravitational fields of Jupiter and Saturn. These variations occurs on a period of ~100,000~100,000year cycle. 1. Eccentricity circular orbit- no eccentricity orbit with 0.5 eccentricity When the orbit is at its most elliptical, the amount of solar radiation at perihelion (point closest to sun) is about 23% greater than at aphelion (point furthest from sun) 2. Obliquity Axial tilt The angle of the Earth's tilt on its axis varies Earth' tilt: cause of seasons from between 22.1 and 24.5 degrees, shifting between these ~ every 41,000 years When the obliquity increases, the amplitude of the seasonal cycle in insulation increases, with summers in both hemispheres receiving more radiative flux from the Sun, and the winters less radiative flux. As a result, it is assumed that the winters become colder and summers warmer. Currently the Earth is tilted at 23.4 degrees, ~half way between its extreme values. The tilt is in the decreasing phase of its cycle, and will reach its minimum value around the year 10,000 (7,992 years from now). 3. Precession wobble of axis Precession is the change in the direction of the Earth's axis of rotation relative to the fixed stars, with a period of ~ 26,000 years. years. 12,000 years from now the Northern Hemisphere will experience summer in December and winter in June because the axis of the Earth will be pointing at the star Vega instead of it's current alignment with Polaris. 2 Milankovich cycles cause Ice Ages The episodic nature of the Earth's glacial and interglacial periods within the present Ice Age (the last couple of million years) have been caused primarily by these cycles These variables are only important because the Earth has an asymmetric distribution of landmasses, with virtually all (except Antarctica) located in the Northern Hemisphere. At times when Northern Hemisphere summers are coolest (farthest from the Sun due to precession and greatest orbital eccentricity) and winters are warmest eccentricity) (minimum tilt), snow can accumulate on and cover tilt) broad areas of northern America and Europe (= Ice Age). At present, only precession is in the glacial mode, with tilt and eccentricity not favorable to glaciation Ruddiman (2001) Last 800,000 years Sea level The rise and fall of sea level can be local or global. The shorelines of the world serve as our barometer of sea level change. During periods of lowered sea level, areas that were offshore are exposed to agents of erosion. When sea level rises, coastal areas are flooded and river valleys drowned. Some causes of sea level change Local short-term changes: Imbrie and others (1984) Ice volume change = sea level change Trade winds can pile up water at one end of an ocean; tides. Local long-term changes: Tectonic activity can cause shorelines to sink into the sea or to be pushed upward. Global sea level changes: Continental glacial melting or growth Ice volume today? About 10% of Earth's land surface is covered by ice. These include: valley glaciers and continental glaciers, which include ice caps and ice sheets. Ice sheets and ice caps are the largest accumulation of ice completely covering the underlying topography of the land. Currently, most of the ice on Earth is stored in the ice sheets covering Greenland and Antarctica. Melting Antarctica... Antarctic ice 20,000 years ago ~peak of the last ice age Sea level is not expected to rise significantly from melting of sea ice or icebergs because the ice is already displacing water. For sea level to rise significantly, ice from a grounded ice sheet has to flow rapidly into the sea and that's most likely to happen in West Antarctica first and then East Antarctica, where the ice sits on bedrock below sea level. Antarctic ice now During the last 20,000 years, the west Antarctic ice sheet lost two-thirds of its mass and raised the sea level 10 m. It still contains enough ice to raise the sea level by ~8 m if it were to lose the remainder of its mass. Potential sea-level rise after seamelting the entire Antarctic ice sheet is estimated to be 73 m. m. 3 Future change? The Carbon Cycle What goes around comes around... Carbon is the 4th most abundant element in the The movement of carbon, in its many forms, C in > C out = net carbon sink C out > C in = net carbon source Geological processes: long term flux Biological processes: short term flux http://earthobservatory.nasa.gov/Library/CarbonCycle/carbon_cycle4.html universe, and is essential to life on Earth between the atmosphere, oceans, biosphere, and geosphere is described by the carbon cycle "biogeochemical cycle" cycle" Biogeochemical cycles The Earth can be viewed as a system with a set of interacting "spheres", the atmosphere, spheres" hydrosphere, biosphere, and lithosphere. Being open systems, energy and mass is constantly cycled between them. The transport and transformation of substances through the Earth system are known collectively as biogeochemical cycles. cycles. These include the hydrologic, nitrogen, carbon, and oxygen cycles 4 The Carbon Cycle starring Brenda and Kelly! http://www.epa.gov/climatechange/kids/carbon_cycle_version2.html The ocean's role in the C cycle ocean' On land: Plants draw about one quarter of the carbon dioxide out of the atmosphere and photosynthesize it into carbohydrates. Some of the carbohydrate is consumed by plant respiration and the rest is used to build plant tissue and growth. Animals consume the carbohydrates and return carbon dioxide to the atmosphere during respiration. Carbohydrates are oxidized and returned to the atmosphere by soil microorganisms decomposing dead animal and plant remains (soil respiration). (soil respiration). In the ocean: Another quarter of atmospheric carbon dioxide is absorbed by the world's oceans through direct air-water exchange. Surface world' airwater near the poles is cool and more soluble for carbon dioxide. dioxide. The cool water sinks and couples to the ocean's thermohaline circulation which transports dense surface water toward the ocean's interior. Marine organisms form tissue containing reduced carbon, and some also form carbonate shells from carbon extracted from the air. C reservoirs an accounting of where different forms of carbon are located on Earth (note that 1015 g = 1 billion tons = 1 gigaton): gigaton): Rocks: 65,000,000 Oceans: 39,000 Soils: 1,580 Atmosphere: *750 Land plants: 610 *In the atmosphere, CO2 is 99.6% of the total (i.e., the amount of CH4 is small). LOCATION: Amount (x1015 g C) 5 C flux Although the largest storage of carbon is in rocks, the largest magnitudes of fluxes don't involve the rock pool directly on short time periods. The magnitudes of C fluxes are as follows (all in 1015 g of C per year): Ocean uptake = 1.7 (x 1015 g C / yr) Photosynthesis = 111 Respiration = 110 Fossil fuels = 6.3 Biomass burning = 1.6 The "missing sink" we can use biogeochemical principles to construct a mass balance for the atmosphere by knowing that the amount of C in the atmosphere increases increases by 3.2 x 1015 g C /yr, by knowing that the internal change in the atmosphere is zero, and by knowing the other fluxes into and out of the atmosphere: NET CHANGE = INPUT + OUTPUT + INTERNAL CHANGE 3.2 = (110 + 6.3 + 1.6) + (-111 -1.7) + (0) ( 3.2 = 5.2 Given this analysis we find that there is an imbalance -- we need an additional "sink" of -2.2 (3 - 5.2) x1015 g C to balance the global C budget. In other words, we are "missing" over 2 billion tons of C each year; this shows how incomplete our understanding of the global carbon cycle is at present. Although it is possible that this missing sink could be found in any of the major pools of carbon on earth, it is most likely that the likely pools with a shorter residence time such as vegetation, soils or the ocean (compared to rocks) are most important. Recent models indicate that terrestrial ecosystems are the most likely repository of this carbon. carbon. Geologic time scales of C cycle There is actually very little of the total carbon cycling through the Earth system at any one point in time. Most of the carbon is stored in geologic deposits - carbonate rocks, petroleum, and coal formed from the burial and compaction of dead organic matter on sea bottoms. The carbon in these deposits is normally released by rock weathering. The rock cycle volcanic activity rock weathering 6 Rock weathering and C cycle CO2 concentrations have been limited over geologic time to a range CO2 in the atmosphere is consumed in the weathering of many rocks. of about 200 - 6000 ppm in the atmosphere due to weathering of rocks and the formation of carbonate minerals. This weathering produces bicarbonate (HCO3-), a form of inorganic carbon, and calcium (Ca2+) that are then transported in river water to the oceans. Once in the oceans the calcium and bicarbonate are combined by organisms to form calcium carbonate, the mineral that is found in in shells. This calcium carbonate mineral is buried in the sediments, where eventually it comes under great temperature and pressure and is melted during the process of "subduction", when one tectonic plate "subduction", moves under another. The melted rock rises to the surface in the form of magma and is released back to the surface of the earth through volcanoes. This high-temperature process also converts some of the calcium highcarbonate back to CO2, which is released to the atmosphere to begin the cycle over again. Volcanic activity and C cycle CO2 concentrations in the atmosphere are related to The concentration of CO2 has varied over a large range When volcanic activity is high the CO2 concentration in the atmosphere is also high due to the release of CO2 from volcanoes into the atmosphere. Large, single volcanic eruptions today have much less effect on the atmospheric CO2 concentration, but can release particles into the atmosphere that can cause a slight (but temporary) cooling of the Earth's surface. in our atmosphere in the geologic past. volcanic activity. Earth's energy budget Earth' Everything, from an individual person to Earth as a whole, The Atmosphere Moves in Response to Uneven Solar Heating and Earth's Rotation Earth' emits energy. Scientists refer to this energy as radiation. As Earth absorbs incoming sunlight, it warms up. The planet must emit some of this warmth into space or increase in temperature. Two components make up the Earth's outgoing energy: heat (or thermal radiation) that the Earth's surface and atmosphere emit; and sunlight (or solar radiation) that the land, ocean, clouds and aerosols reflect back to space. The balance between incoming sunlight and outgoing energy determines the planet's temperature and, ultimately, climate. Both natural and human-induced changes affect this balance, humancalled the Earth's radiation budget. earthobservatory.nasa.gov An estimate of the heat budget for Earth. On an average day, about half of the solar energy about arriving at the upper atmosphere is absorbed at Earth's surface. Light (short-wave) energy Earth' (shortabsorbed at the surface is converted into heat. Heat leaves Earth as infrared (long-wave) Earth (longradiation. Since input equals output over long periods of time, the heat budget is balanced. 7 "Scientists have concluded more energy is being absorbed from the sun than is emitted back to space, throwing the Earth's energy "out of balance" and warming the globe" -NASA 4/28/05 Also a physical energy budget in addition to the thermal / radiation budget Ex: solar energy powers temperature gradients winds move surface ocean if the wind is "harnessed" for an energy source harnessed" Clouds and the Earth's Radiant Energy System (CERES) measurements show the reflected solar radiation (left) and emitted heat radiation (right) for January 1, 2002. In both images, the lightest areas represent thick clouds, which both reflect radiation from the Sun and block heat rising from the Earth's surface. Notice the clouds above the western Pacific Ocean, where there is strong uprising of air, and the relative lack of clouds north and south of the equator. http://www.nasa.gov/centers/goddard/news/topstory/2005/earth_energy.html that drive winds for human use, is it then available for natural Earth processes?? 8 ...
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This note was uploaded on 04/29/2008 for the course EAS 104 taught by Professor Brown during the Spring '08 term at Purdue University-West Lafayette.

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