1 UIUC ATMOS 397G Biogeochemical Cycles and Global Change Lecture 3: Origin of Earth’s Atmosphere Don Wuebbles Department of Atmospheric Sciences University of Illinois, Urbana, IL January 23, 2003

2 UIUC The Origin of the Atmosphere Lecture Objective: To identify how the earth’s atmospheric constituents have evolved by outlining the principal events in the evolution of the earth’s atmosphere To identify the factors that determine the capability of a planet to maintain an atmosphere

3 UIUC Origin of the Chemical Elements

4 UIUC The Early Earth The Earth is Billion years old Formed in the inner portion of solar nebula bulk of the mass comes from collision and compression of planetesimals during accretion heat generated from radiaoactive decay and collision of planetesimals Devoid of atmosphere Gravitational field too weak to retain gaseous elements

5 UIUC Stages in the Evolution of the Earth’s Atmosphere: Stage I Primitive Atmosphere Produced as a result of volcanic outgassing 4.4 – 4.0 billion years ago A “reducing atmosphere” primarily consisting of H 2 and He, with trace levels of CO, CH 4, H 2 O (v), N 2, H 2 S, NH 3, HCl, Ar, and HCN Lighter gases escaped to space CH 4  CO  CO 2 (oxidation) 2NH 3 + h  N 2 + 2H 2

6 UIUC Reduced substance: electron-rich  tendency to lose electrons H 2, NH 3, CH 4 Oxidized substance: electron-poor  tendency to gain electrons O 2

7 UIUC Principal geophysical and geochemical processes contributing to the evolution of the atmosphere

8 UIUC Stages in the Evolution of the Earth’s Atmosphere: Stage II 4.0 to 3.3 billion years ago Continued outgassing from the Earth’s interiors H 2 O, N 2, and CO 2 predominant constituents, with trace levels of CO, SO 2, Ar, He Cooling Earth resulted in condensation of water vapor and the oceans (3.8 billion years ago) Water soluble gases (CO 2, SO 2, HCl) dissolved in the primitive ocean Appearance of chemosynthetic bacteria about 3.5 bya First appearance of Oxygen (O 2 ) in the prebiotic atmosphere

9 UIUC Constituents in the Early Atmosphere

10 UIUC No accumulation of O 2 at this stage – used up for oxidation of reduced species

11 UIUC Stages in the Evolution of the Earth’s Atmosphere: Stage III Living Atmosphere 3.3 bya to present Accumulation of O 2 to its present day atmospheric level of 21% as early as 430 million years ago Development of the “ozone” layer responsible for shielding the earth’s surface from UV rays O 2 + O + M  O 3 + M Evolution of several new biochemical pathways significant to the global biogeochemical cycles, e.g., nitrification

12 UIUC Evidence for lack of free oxygen in the Earth’s atmosphere until 2 bya Banded Iron Formations (BIF) Fe 2+ oxidized to Fe 2 O 3 in the sediments of the primitive ocean Peak occurrence in rocks of 2.5 to 3.0 billion years ago Red Beds Oxidation of exposed reduced minerals, such as FeS 2, on the barren land resulted in alternating layers of Fe 2 O 3 with sediments of land origin. Earliest occurrence not before 2.0 bya Oxygen poisoning of methanogenic bacteria and sulfur bacteria Chemical building blocks of life could not have been formed in the presence of atmospheric O 2

13 UIUC Banded Iron Formations Alternating bands of red jasper and black hematite, about 2250 million years old (2.55 billion years old) Jasper Knob, Ishpeming, Michigan

14 UIUC Red Beds

15 UIUC Cumulative history of O 2 released by photosynthesis through geologic time

16 UIUC Origin of Life First sign of single-celled life 3.5 bya Abiotic synthesis aided by exogenous source of organic molecules Traditional viewpoint: life arose in the sea Important biochemical elements also abundant in seawater Methanogenesis, sulfate-reduction, and N-fixation: primitive pathways of anaerobic metabolism O 2 production by photosynthesis and the subsequent formation of O 3 layer paved way for colonization of land by higher organisms

17 UIUC Chemical Evidence for Origin of Life Miller-Urey Experiment Synthesis of simple, reduced organic molecules from constituents of primitive atmosphere and ocean (CH 4, NH 3, H 2, H 2 O) Experiment successful in abiotic conditions Building blocks of life (amino acids) could be synthesized in primitive secondary atmospheric conditions

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20 UIUC From Scientific American, 2000

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