Later upon a transgression sea level rise sedimentation may resume

Later upon a transgression sea level rise

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existing sediment may be eroded away. Later, upon a transgression (sea level rise), sedimentation may resume. Disconformities are subtle. -Sea level drops: no new sediment accumulates, and some of the old sediment is eroded. Paleontology:study of ancient life forms based on the fossil record. Principle of faunal succession: species evolve, exist for a time, and then disappear forever. -Index fossils: fossils that are widespread geographically and are limited to a short span of geological time The age of a mineral is determined by measuring the ratio of parent to daughter isotopes. The age can be calculated from a knowledge of the parent half-life. The half-life is the time it takes for half of an unstable nuclei to decay. The half-life is a unique characteristic of each isotope. Other absolute dating methods: -Numerical ages are impossible without radioactive isotopes, but they typically have a very limited range. Annual growth rings from trees or shells can be counted to establish dates. -Rhythmic layering—annual layers in sediments or ice—can be counted to establish numerical dates The geologic time scale (Eons):
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-Phanerozoic: “visible life”, starts at the Pre-Cambrian-Cambrian boundary. Marks the first appearance of hard shells. Life diversified rapidly afterwards. Includes Cenozoic, Mesozoic, Paleozoic. -Proterozoic:“before life”, development of tectonic plates like those of today. Buildup of atmospheric O2, multicellular life appears. -Archean:“ancient”, birth of continents, appearance of the earliest life forms. -Hadean:“Hell”, “beneath the Earth”, internal differentiation, formation of the oceans and secondary atmosphere. Phanerozoic Eon: “visible life”-Cenozoic: “recent life”, higher primates and humans -Mesozoic:“middle life”, dinosaurs, primitive mammals, flowering plants -Paleozoic:“ancient life”, marine invertebrates, fish, amphibians, reptiles, early land plants. Ended with the Permian extinction, which eradicated 90% of all marine species. Astronomical calculations indicate that the probability that objects 10 km in diameter will hit our planet is about once every 100 million years. The K-T boundary: -Evidence of an impact includes a thin layer of plankton-free clay that separates planktonrich chalk at the K-T boundary. This suggests that plankton were shut off for a short time. -Analysis of the clay revealed that it is highly enriched in iridium, an element rare on Earth and abundant in meteorites. Geologists began to find iridium enrichment in K-T boundary clay worldwide. -The clay contained other unusual material, including pressure-shocked quartz and tiny glass spheres called microtektites, features found elsewhere associated with impacts. Deep timeis the immense span of geologic time. Human history is miniscule when compared against deep geologic time.
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