Laurentia

1. Laurentia Laurentia, or the Laurentian Shield, or the Canadian Shield is the geological term for the North American Craton. The North American Craton (brown) has comprised a portion of a series of supercontinents, and has remained stable for about 600 million years. This cratonic region comprises a basement of Precambrian metamorphic and igneous rock, exposed in the north as the Canadian Shield, and covered by a relatively thin cover of younger sedimentary rock on the Interior Platform. The craton is named after the Laurentian Shield, which in turn is named after the Laurentian Mountains, which were named after the Saint Lawrence River which in turn was named after Lawrence of Rome. Harry Williams, Historical Geology

2. HISTORICAL GEOLOGY LECTURE 7. EARLY LIFE. The Early Atmosphere. Recap: The Earth's early atmosphere was formed by the release of gases by volcanic activity (4.6 - 3.6 BYBP) - termed OUTGASSING. Evidence = marine sedimentary rock 3.8 BYBP - proves existence of oceans. Composition = carbon dioxide, water vapor, carbon monoxide, hydrogen, hydrogen chloride (similar to modern volcanic gas). There was very little OXYGEN - when it was emitted it probably combined very quickly with iron and precipitated as iron oxide. No oxygen meant no OZONE (ozone is created by the breakdown of oxygen in the atmosphere) and therefore ULTRAVIOLET RADIATION bathed the surface of the Earth (deadly now). Harry Williams, Historical Geology

3. The Earliest Life Life originated sometime prior to 3.5 BYBP - the exact process is still not known; however experiments have shown that carbon + oxygen + hydrogen + nitrogen + phosphorus + sulfer + UV + electrical discharge (lightning) + heat --> AMINO ACIDS, one of the major building blocks of PROTEINS - a basic component of life. It has also been suggested that conditions required to form organic molecules may also have existed at mid-oceanic ridges. Today hyperthermophiles thrive around these vents. They use chemosynthesis to get energy from substances such as hydrogen sulfide and ammonia. Other microbes – lithotrophs – have been found in hot rocks 3 km below the surface of the earth. They gain energy from hydrogen, iron, magnesium and sulfur. There is evidence, then, that life can thrive in harsh environments – we don’t know what came first (or if life originated in several different places). You can read about the latest theories of how life originated here: http://evolution.berkeley.edu/evolibrary/article/origsoflife_04 Harry Williams, Historical Geology

4. Feeding Strategies: • HETEROTROPHS: at some point early in the history of life, single-celled marine organisms developed, which consumed organic molecules for food and, in the absence of oxygen, used fermentation (breakdown organic molecules and rearrange the parts, releasing energy) to convert the food into energy. • AUTOTROPHS: manufacture their own food (useful if organic molecules are scarce). E.g. Sulfur bacteria make their food from carbon dioxide and hydrogen sulfide; nitrifying bacteria make food from ammonia). • PHOTOAUTOTROPHS: use photosynthesis to breakdown carbon dioxide into carbon (for growth) and oxygen (which escaped into the air). Billions of algae-like photoautotrophs called cyanobacteria (formerly called blue/green algae) developed in the oceans and acted like oxygen factories, slowly transforming the Earth's atmosphere to one rich in oxygen. This was a crucial change that allowed life as we know it to develop…. Harry Williams, Historical Geology

5. Harry Williams, Historical Geology

6. Evidence of this is found in the form of banded iron formations. These are chert layers with alternating rust-red and gray bands. The rusty bands are colored by ferric iron oxide (Fe2O3), which shows that enough 02 was present to oxidize iron at the earth’s surface. BIF’s are common in rocks 3.4 - 2 billion years old. Michigan Egypt Proterozoic Banded Iron Formations. Harry Williams, Historical Geology

7. The cyanobacteria (Blue/green algae) responsible for adding oxygen to the atmosphere, created mat-like colonies in intertidal areas which collected layers of calcium carbonate mud. The algae grew through the layer and the process was repeated to form laminations, each layer representing a day's growth. Modern stromatolites are also found in the intertidal zone. Modern stromatolites in Australia Ancient stromatolites in South Africa Harry Williams, Historical Geology

8. Section through 2 billion year old stromatolite, Michigan, showing laminated structure. Colony of cyanobacteria from Proterozoic rocks in Canada. Harry Williams, Historical Geology

9. The earliest known group of these simple (bacteria-like or prokaryotic – cells lack a nucleus and organelles) fossils are from western Australia and are about 3.5 billion years old. This suggests that life originated about 3.6 billion years ago. Harry Williams, Historical Geology

10. More advanced forms of prokaryotes evolved - a famous group are from the Gunflint Chert of Canada (1.9 billion years old). Some resemble modern types of algae. Harry Williams, Historical Geology

11. The continuing build up of oxygen in the atmosphere by photosynthesis led to a protective ozone shield (which blocked UV radiation) and the development of AEROBIC (“oxygen-burning”) multicellular organisms (more than one type of cell; cells organized into tissues, organs etc., used oxygen to convert food into energy - more efficient). These are termed “Metazoans”. These are soft-bodied organisms e.g. soft corals, worms, jellyfish. Harry Williams, Historical Geology

12. Famous group = Ediacaran Fauna of Southern Australia. These are about 630 million years old (“Vendian” time - named after the final period of the Proterozoic in Russia). Harry Williams, Historical Geology

13. Ediacaran Fauna Harry Williams, Historical Geology

14. Reconstruction of Ediacaran sea floor about 580 - 630 million years ago. The metazoans were the forerunners of the great expansion of life that occurred in the Phanerozoic.These organisms were largely soft-bodied (no hard parts), which greatly reduced the chances of fossilization and preservation. This changed dramatically around the beginning of the Cambrian Period when shelled organisms became abundant (a lot more fossils). Harry Williams, Historical Geology