VIZUALIZING EARTH HISTORY

1. VIZUALIZING EARTH HISTORY By Loren E. Babcock Chapter 9 Proterozoic World

2. Evolution of the Cratons and Proterozoic Supercontinents About 2.5 billion years ago the Earth entered a new phase of its history, which is why this period was chosen as beginning of the Proterozoic Eon. During the Archean, the Earth experienced its mobile crust phase. Between 2.7 and 2.3 billion years ago, most small crustal bodies were amalgamated through orogenesis into larger cratons, which are the continental cores.

3. Evolution of the Cratons and Proterozoic Supercontinents

4. Evolution of the Cratons and Proterozoic Supercontinents Contrast the nature of the crust in the Proterozoic Eon with that of the Archean Eon. Platform - The part of a continent covered by flat-lying or gently tilted, mostly sedimentary strata. Platform phase - Interval of Earth history, beginning with the Proterozoic Eon, characterized by relatively stable, amalgamated continental cores.

5. Evolution of the Cratons and Proterozoic Supercontinents Contrast the nature of the crust in the Proterozoic Eon with that of the Archean Eon.

6. Evolution of the Cratons and Proterozoic Supercontinents Rodinia - An early supercontinent, assembled in the Mesoproterozoic and separated in the Neoproterozoic. This supercontinent was considerably different in its configuration from Pangea, which would become assembled more than a billion years later

7. Evolution of the Cratons and Proterozoic Supercontinents Orogenic belt - A linear or arcuate region subjected to folding and other deformation during a mountain building cycle. Also known as an orogen. Grenville orogenic belt - An arcuate orogenic region that developed 1.3 to 1.0 billion years ago and that affected an extensive area of present-day North America and adjacent regions.

8. Evolution of the Cratons and Proterozoic Supercontinents Supercontinent cycle - A tectonically driven cycle defined by the assembly of a supercontinent and later fragmentation and dispersal of its pieces. Pannotia - A hypothesized late Neoproterozoic supercontinent. A supercontinent cycle begins with the collision and welding of tectonic plates to form an enormous mass of continental crust. It ends with the breakup and dispersal of fragments of the supercontinent. The Proterozoic Eon witnessed at least two supercontinent cycles.

9. Formation of Paleozoic continents from Neoproterozoic supercontinents Gondwana - The Paleozoic to mid-Mesozoic landmass that included South America, the Falkland Islands, Africa, Madagascar, India, Australia, and Antarctica. Continental breakup stimulated a substantial rise in global sea level during the Neoproterozoic as high rifting rates produced a large volume of warm, relatively expanded, oceanic lithosphere. The eustatic rise lasted into the Cambrian Period.

10. Oxygenation of the Atmosphere-Ocean System Understand the ways that free oxygen can be released to the atmosphere. Since early in Earth’s history, free oxygen (O2) was released in small amounts from the breakdown of water vapor in the upper atmosphere by the Sun’s ultraviolet radiation. By about 3.5 billion years ago, photosynthetic prokaryotes (especially cyanobacteria) also began releasing oxygen to the atmosphere-ocean system.

11. Oxygenation of the Atmosphere-Ocean System Explain banded iron formations and what they signify about oxygen levels in the atmosphere and ocean. Banded iron formations (or BIFs), which are composed of iron minerals interlayered with silica, offer further evidence of oceanic oxygen levels. Most banded iron formations were deposited between 3.6 and 1.9 billion years ago. A few Neoproterozoic and Phanerozoic examples exist but they tend to be small in size compared to Archean and Paleoproterozoic examples. Banded iron formations serve as the world’s major sources of iron ore.

12. Oxygenation of the Atmosphere-Ocean System Banded Iron Formations

13. Redbeds, which contain well-oxidized, iron-bearing sediments, show a clear relationship to atmospheric oxygenation. The occurrence of redbeds has an inverse relationship to band on formations— they are almost non-existent in strata older than 2 billion years, and are prevalent in strata younger than 1.9 billion years. Oxygenation of the Atmosphere-Ocean System Redbeds and what they tell us about atmospheric oxygen levels.

14. Proterozoic Glaciations Multiple episodes of glaciation occurred during the Paleoproterozoic and Neoproterozoic eras. Glacial conditions are so much a part of the sedimentary record of the middle part of the Neoproterozoic that the interval between 850 and 630 million years ago has been named the Cryogenian Period (from Greek, kryos, ice, and genesis, birth).

15. Proterozoic Glaciations Snowball Earth hypothesis During the Neoproterozoic, the Earth experienced at least two episodes of global glaciation. One glacial episode was in the Cryogenian Period, between 745 and 725 million years ago, and the other was in the Ediacaran Period, between 590 and 550 million years ago. Cryogenian and Ediacaran periods are associated with banded iron formations and capped by carbonate rocks. Snowball Earth hypothesis - The concept that during the Proterozoic Eon, the entire surface of the Earth was repeatedly plunged into freezing conditions.

16. Proterozoic Life Forms The most important steps in the history of life on Earth. The Proterozoic Eon witnessed some of the most pivotal changes in the history of life on Earth. During the preceding Archean Eon, life was dominated by prokaryotes. As early as 3.5 billion years ago, organisms had adopted three major strategies for acquiring nutrients: chemosynthesis (in archeans), photosynthesis (in cyanobacteria), and heterotrophy (predation, scavenging, herbivory, and breakdown of detritus in eubacteria).

17. Proterozoic Life Forms The most important steps in the history of life on Earth. Photosynthetic activity eventually led to evolution of an oxygenated atmosphere-ocean system. Heterotrophy gained in importance as the probable means by which the eukaryotic cell initially evolved (through the symbiotic association of predator and undigested prey). Later, heterotrophy became a major driving force in the evolution of skeletons and behavioral strategies in animals.

18. Proterozoic Life Forms Proterozoic prokaryotes At the start of the Proterozoic, prokaryotes were the dominant life forms on Earth. Molecular clock evidence suggests that origination times of eubacteria and archaebacteria were before the end of the Archean Eon, although we do not have much supporting data from fossils. The earliest prokaryotic specimens are preserved in chert, and comprise small rounded cells and filaments formed of linked cells. They have been reported from Archean strata as old as 3.5 billion years. Biogenic-sedimentary structures constructed by cyanobacteria include stromatolites and thrombolites.

19. Proterozoic Life Forms Proterozoic prokaryotes

20. Proterozoic Life Forms Early eukaryotes Eukaryotes so far are known from rocks as old as the Proterozoic, although molecular clock evidence suggests they originated during the Archean. Distinguishing the cells of simple, single-celled eukaryotic organisms from similar-appearing prokaryotic organisms is not always easy. Size and structure are the basic means of identifying cell type. Prokaryotic cells typically range up to 10 micrometers (μm) in diameter, whereas eukaryotic cells are usually larger than 10 μm.

21. Proterozoic Life Forms Early eukaryotes Inside eukaryotic cells are distinct masses presumed to be remains of cell nuclei or other organelles. Eukaryotic cell - A cell type having a true nucleus. Symbiosis - Condition in which two or more dissimilar organisms live together.

22. Proterozoic Life Forms Early eukaryotes Eukaryotic cells evolution - All eukaryotic cells contain mitochondria, (extractor of energy from food). Mitochondrial precursors could have been independent prokaryotic organisms captured by other cells but resistant to digestion inside the predator cells. Minor alteration allowed captured cells to adapt to a symbiotic life, transforming into cell organelles.

23. Proterozoic Life Forms Early eukaryotes

24. Proterozoic Life Forms Early eukaryotes The oldest known multicellular eukaryotes are algae resembling modern seaweed. Coiled, ribbonlike Grypania fossils, some as much as 50 cm long, occur in rocks estimated to be 1.4 to perhaps 2.1 billion years old. Accompanying the multicellular condition is the implication of sexual reproduction, although algae can also reproduce asexually.

25. Proterozoic Life Forms The Ediacaran biota and the radiation of animals Until the Ediacaran Period, most organisms were microscopic. That changed about 570 million years ago with the appearance of a remarkable collection of Neoproterozoic organisms that mark an important step toward the multicellular eukaryote-dominated world of the Phanerozoic. Ediacaran biota - Fossils dating from the Ediacaran Period, including the earliest putative animals. Metazoan - A multicellular animal.

26. Proterozoic Life Forms The Ediacaran biota and the radiation of animals

27. Proterozoic Life Forms The Ediacaran biota and the radiation of animals The Ediacaran Period derives its name from a conspicuous assortment of fossils having flattened zipper-like, concentric, frondlike, radial, and other miscellaneous shapes. Most are between 1 and 30 cm in size, but a few reach lengths of 50 cm or more. These fossils, collectively referred to as the Ediacaran biota, are so named for an early discovery site in the Ediacara Hills of South Australia. Today Ediacaran-type fossils are known from localities on all modern continents except Antarctica.

28. Proterozoic Life Forms The Ediacaran biota and the radiation of animals Today microbial mat-stabilized sediments are relatively restricted in their distribution, but during the Neoproterozoic microbial mats may have stabilized sedimentary surfaces across the continental shelves. Microbial mat - Layer of microscopic bacteria and fungi growing at the sediment surface.

29. Proterozoic Life Forms The Ediacaran biota and the radiation of animals The beginning of a trace fossil record coincides with the beginning of a body fossil record of animals at about 570 million years ago. Scratches on the mud surface, near-surface horizontal traces, and simple tubular burrows are about all that Ediacaran strata have to offer as evidence of animal behavior.

30. Proterozoic Life Forms The Ediacaran biota and the radiation of animals Most Ediacaran animals had relatively soft, pliable, but fairly durable external coverings, useful for locomotion but offered little protection from predators. Before the end of the Ediacaran Period, the cloudiniids, evolved small conical skeletons of hard calcium carbonate, to reduce the threat of predation. One of the most remarkable indications of animal life during the Ediacaran comes from carbonate strata deposited in China and elsewhere just before the end of the period.