Atmospheric Evolution: Earth s s Oxidation

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1 Earth s s Atmosphere Thematic Questions about the Atmosphere Observations of the Modern Atmosphere What is its structure and composition? What controls atmospheric dynamics? Information from the Rock Record What clues describe ancient atmospheres? What approaches can be employed? Origin and Rise of O 2, CO 2 and other Gases How has the atmosphere changed over time? What factors have effected these changes? Atmospheric Evolution: Earth s s Oxidation Topics: Characteristics of Earth s Atmosphere heated by solar radiation, climate controls physical features and chemical composition dynamic behavior: wind belts and patterns Atmospheric History of Earth modifiers of atmospheric composition early Earth atmosphere, rise of oxygen temporal variations in carbon dioxide and oxygen: geochemical assessments as proxies of atmospheric composition

2 Atmospheric Heat: Solar Radiation Variation and Effects Decreases with latitude obliquity, absorption Reflected, absorbed, reradiated, transferred wavelength dependent Earth s surface atmosphere solar radiation budget for solar radiation: role of atmosphere and ocean Earth s s Modern Atmosphere Modern Characteristics Physical properties transparent, odorless, colorless Chemical composition nitrogen (N 2 ), 78.08% oxygen (O 2 ), 20.95% argon (Ar) 0.93% carbon dioxide (CO 2 ), 0.036% nitrogen oxygen inert gases: neon (Ne), helium (He), kypton (Kr), xenon (Xe), and hydrogen (H) argon

3 Atmospheric Structure System of Layers and Boundaries Defined by temperature Troposphere 90%, clouds tropopause (jet stream) Stratosphere ozone, stratopause Mesosphere mesopause Thermosphere Atmospheric Circulation Wind Belts and Climatic Controls Six cells Controls: Solar heat Earth s rotation (Coriolis force)

4 Surface Winds: Annual Average >10.5 westerlies 10 9 m/s 8 trade winds westerlies 2 <1.5 Seasonal Monsoons Wind Patterns ITCZ shift changes wind direction Summers: wet: (low pressure over land) Winters: dry (high pressure over land) seasonal changes in wind direction ITCZ shifts seasonally, changing wind patterns

5 Atmospheric Evolution: Modifiers Agents for Change in Atmospheric Composition Volcanic Outgassing Contributes CO 2, varies over time, dependent on seafloor spreading (faster rates: more CO 2 ) Biological Processes Uptake of CO 2, release of O 2 by photosynthetic bacteria and plants (reverse in respiration) O 2 leads to build-up of O 3 in stratosphere Geochemical Cycling and Tectonics Weathering, mineral reactions absorb CO 2, O 2 Sequestration of organic matter and carbonate in sediments influences atmospheric O 2 Atmospheric Proxies Assessment of Atmospheric Composition Rock Types, Fossils, Geochemistry formation of banded iron formations (BIFs), red beds, paleosols related to levels of O 2 evidence from paleontological record, evolution of photosynthetic bacteria and animals atmospherically coupled geochemical cycles: recorded in inventories of rocks through time carbonate/silica: weathering and burial of CO 2 oxygen/sulfur: weathering and burial of O 2 geochemical proxies of discrete characteristics Direct Measurements in Ice Cores

6 Atmospheric History Compositional Changes Hydrogen: initially lost to space Carbon dioxide: incorporated into rock record Oxygen: produced by biology Prebiotic Earth Contrasts Oxidizing: N 2, H 2 (tr.), CO 2, H 2 O Reducing: H 2, NH 3, CH 4 gas concentration (%) unknown water nitrogen carbon dioxide Time (Ma) temporal changes in Earth s atmosphere oxygen Oxygen in Earth s s Atmosphere Thematic Questions about Oxygen Information from the Rock Record What differences are attributable to O 2? What features record O 2 thresholds? Evidence of Origins for Oxygen What controls O 2 production and consumption? How have they varied over geological time? Controls on Oxygen Levels How important is oxidation of organic matter? Does enhanced burial of organic matter increase atmospheric O 2?

7 Evidence for Oxidation Banded Iron Formations Common until 2.0Ga, intermittent afterwards Deposition in ocean settings until metals exhausted Red Beds Blouberg red beds 1.9Ga Beginnings of atmospheric oxidation: iron oxide coatings on rocks History of Atmosphere Oxidation Rise of Oxygen Coupled to biotic evolution and chemical thresholds

8 Atmosphere Oxygen Geochemical Cycles and Oxygen Levels O 2 increase from C org burial (e.g. during continental growth) Oxygen/sulfur CO 2 + H 2 O CH 2 O + O 2 inventories describe O 2 levels through time C org burial (10 18 mol/ma) Time (Ma) Also: Corg 2Fe 2 O HCO Ca SO 2-4 4FeS CaCO 3 + 8H 2 O + 15O 2 O 2 concentration (%) CO 2 in Earth s s Atmosphere Thematic Questions about Carbon Dioxide Information from the Rock Record What differences are attributable to CO 2? What features record CO 2 thresholds? Evidence of Controls on Carbon Dioxide What affects CO 2 production and consumption? How have they varied over geological time? Effects of Changes in Carbon Dioxide Levels How important is CO 2 in climate regulation? How do levels of atmospheric CO 2 (pco 2 ) affect biology? How are they measured?

9 Changes in Carbon Dioxide Geochemical Cycles and CO 2 Budgets (reservoirs, fluxes and interactions) of carbon and silica related to rock weathering Also from 13 C record of paleosols Estimates CO 2 + CaSiO 3 CaCO 3 + SiO 2 Cenozoic Atmospheric pco 2 Estimates Based on Independent Proxies Surface Ocean ph (controlled by pco 2 ) based on Boron isotopes of planktonic foraminifera Alkenone! 13 C record from phytoplankton

10 Measurement of Past Atmosphere Gases in Ice Cores Gas bubbles trapped in ice Variations in CO 2 and CH 4 concentrations reflect ice ages Vostok Ice Core Atmospheric Perturbations Methane Hydrate Structure Water: molecules linked by H-bonds Ice: rigid network structure with cavity CH 4 water for CH 4 hydrogen bonds ice clathrate

11 Methane Hydrates Formation and Stability CH 4 formed by bacterial processes (low in 13 C). Clathrate structures stable at low temperatures. Occurrence: permafrost, ocean floor sediments. Accumulate below surface in ocean sediments. Appear as BSR in seismic profiles. seismic image of seafloor sediments BSR hydrates in sediment cores bottom simulating reflector (BSR) Late Paleocene Thermal Maximum Methane Emission to Atmosphere Massive flux of light carbon to atmosphere generated by global warming 55Ma ago, event lasted ~50ka Evidence from carbon isotopic excursion Oceans warmed by ~1-4 C Site 527 Walvis Ridge

12 Late Paleocene Thermal Maximum Global Phenomenon 527: S. Atlantic 690: W. Pacific Records influx of 2600 Gt of CH 4 Now PETM: Paleocene/Eocene thermal maximum (i.e. a boundary event). Paleocene/Eocene Thermal Maximum Sediment Records from West Pacific Shatsky Rise: marked changes in carbonate content of sediment cores associated with event