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Environmental Science
Nick Eyles

EESA06 Lecture March 14, 2009 How are fossils formed? -shells buried and preserved unaltered (<100 million years) -cavities filled with silica, calcium carbonate, iron – a process called mineralization Methods: Fossilization -freezing -drying -asphalt -amber -carbonization -permineralization (most common) -peat bogs -paraffin -volcanic ash Permineralization -die in or near water -soft parts consumed by bacteria -sedimentation (fine grained = more detail; chemical makeup of rock = colour) -more sed’s pile on = pressure = rock -mineral rich water = changes to the shell -uplift -erosion Ammolite – an organic gemstone -ammonite fossils -iridescent aragonite with trace elements (Fe, Mn, Al, Ba) Trace fossils -preserved tracks, trails, burrows, borings -ichnology – study of trace fossils -bioturbation – process of disturbing sediment Fossils as clues to ancient environments -palaeoecology – study of ancient organisms and their environments -clues from: fossil types, assemblages, fossil morphology, trace fossils Crinoid aka. “sea lily”, loose like a mop head on a tall stem made up of rings & plates, found in the shallow seas. Fossils and stratigraphy -how do we use fossils in stratigraphy? -to establish relative age of rock units, correlate units (rocks that represent the same time period in different locations) -what information do we need to do this? -relative age of rock units -fossil species present in each unit -establish time of first and last appearance of each species geologic range Note: certain species only live in certain time periods What kinds of fossils are most useful for stratigraphic work? -short-lived, widespread -assemblages (groups) of species -e.g. ammonites The fossils of short-lived species are more useful than fossils that lived over long periods of time Trilobite growth -trilobites molted regularly -they is a very abundant type of fossil Evolution on Earth -intense meteorite bombardment from 4.5 to 4 Ga prevented life become established -at the same time, there was no magnetic field to prevent oceans and atmosphere being stripped off the planet’s surface by the solar wind (without the magnetic field, we’d be toasted) Sir James Dawson - said he found the oldest representation of life on earth (fossil), it turns out he was totally wrong, it was not a fossil at all, it was just a mineral Step 1. The earliest bacteria, 3.8 Ga (3.8 billion yrs) -prokaryotes some are photosynthetic (called cyanobacteria: blue/green algae) -microscopic, single celled, lacking a nucleus -the earliest prokaryotes (the Archaea) lived around the superheated water near submarine volcanic vents (hyperthermophilic). -simple bag-like cells -only life forms between 3.8 and about 2 Ga Evidence of earliest life on earth is oxygen isotope evidence, these rocks were found in Isua in west Greenland and they were 3.8 Ga old. -Living things incorporate more C12 into their tissue -this results in water and air enriched in the larger heavier C13 isotope -the first positive C13 excursion (spike) is recorded at 3.8 Ga and is thought by some to indicate organic activity -the same isotopes are used in rocks that are c. 600 Ma old, to argue for “virtual shutdown” of the biosphere (giving a negative C13 excursion) due to exceptionally cold climate worldwide (the so-called “snowball earth hypothesis” of Paul Hoffman). When life resumed it is argued to have expanded rapidly with many new species (the cambrian explosion) this idea is no longer believed as true and has been di
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