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|CLIMATE CHANGE IN CONTEXT
Paleoclimate and Indicators
Paleoclimatology is another way of saying the study of past climates or paleoclimates. There have been many periods of climate changes in Earth’s History where the temperature rose or fell.
Cooler temperatures covered large areas of the surface of the globe with ice sheets or large continental glaciers. These periods can be identified by their durations. They are often named by their locations on the earth.
Long cool periods that last from tens to hundreds of millions of years are called ice ages. During this time span, glaciers advance and retreat.
Glaciations are shorter cool periods lasting thousands of years. During this time span glaciers near their maximum extent.
The term Ice Age also refers to the last major glaciation that occurred in North America and Eurasia. Our modern climate represents very short, warm periods between glacial advances.
Proxy data or data collected without direct measurement comes from rock formations, ice cores from glaciers and polar caps, pollen cores and insect jaws from lake bottom sediments, ancient trees are only some of the ways scientific evidence is collected analyzed and interpreted.
Pictographs and oral histories passed by some cultures for centuries help confirm and enhance this evidence for the more recent past.
Banded Iron Formations (BIFs) as early indicators of changing atmosphere.
Layers of glaciomarine deposits (glacier deposits in the sea), succeeded by thick layers of carbonate deposits, were found on tropical and equatorial continents during the Late Proterozoic (Timescale). This is strange because carbonates are typically warm water deposits and therefore, are not associated with glaciations.
Iron formations or BIFswere deposited into these layers of sedimentary rocks (Rock Cycle) extensively on the ocean floor over hundreds of millions of years. From the 600 trillion tons of iron ore present today, we know there was plenty of iron in Precambrian waters.
Iron normally stays dissolved in seawater but falls out of solution when it comes in contact with oxygen. The fine, bright orange particles that settle on the ocean floor are the product of a chemical reaction – rust. The alternating layers of rust and gray coloured deposits suggest that the oxygen production fluctuated over time.
Banded iron formation, illustrating the alternating layers of magnetite
and hematite (the red iron) and chert.
Image from http://www.agso.gov.au/education/factsheet/ironform.html
The oxygen revolution that led the way for evolutionary new species, took place between 2 500 and 1 900 mbp (millions of years before the present. (See Timescale). As more and more cyanobacteria (blue-green bacteria) spread across Earth, the oxygen waste they produced through photosynthesis proved toxic to most other microbes. In fact, only those sheltered in oxygen-poor habitats like the murky depths of the oceans and those with genetic mutations that somehow enabled them to tolerate oxygen, survived.
As underwater chimneys, called deep-sea vents, released dissolved iron into the ancient waters, oxygen was used up as quickly as it was produced. Once the iron supply was exhausted, however, oxygen began escaping the seas into the open air. Evidence of a buildup of atmospheric oxygen appears in rock layers dated from 2 200 to 1 900 mbp, oxidation, the reaction of oxygen with iron in exposed rocks, produced red beds of rust and by 1 900 mbp, oxygen levels reached 3% of the atmosphere (Timescale). This was still less than one-fifth of the present-day level but the level later rose to present-day levels of 21 %.
With the exception of bacteria, life on Earth had been restricted to the oceans, where the risk of exposure to ultraviolet radiation was greatly reduced. But, as the supply of oxygen increased and organisms increasingly tapped energy from it, cells grew larger and divided more quickly. But by 600 millions of years before the present (mbp), atmospheric oxygen levels were high enough to form a protective ozone layer over the planet. This ozone layer acted as shield- a critical factor in the emergence and survival of complex life on land.
BIF deposition generally ceased around 1 800 mbp, but BIFs briefly reappeared during the Late Proterozoic. These banded iron formations are the primary source of today’s global iron ore supply.
1. Check the map of Ontario and Canada and locate the Canadian Shield.
2. Where are the large iron ore mines located?