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Precambrian Climates. A Quick Review. Archean Tectonics - Small Protocontinents Proterozoic - Amalgamation into larger continents Rodinia, Iapetus, and Mirovia. Rodinia ~ 1 Ga. Rifting of Rodinia. Initial Rifting ~700 Ma Iapetus ~650 Ma. GRENVILLE Orogeny 1.3-1.0 billion years ago.
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A Quick Review • Archean Tectonics - Small Protocontinents • Proterozoic - Amalgamation into larger continents • Rodinia, Iapetus, and Mirovia
Rifting of Rodinia • Initial Rifting ~700 Ma • Iapetus ~650 Ma
GRENVILLE Orogeny 1.3-1.0 billion years ago • Prior to this orogeny, a westward-dipping subduction zone existed along the east coast of Proto-North America, adjacent to the Pre-Grenville Ocean. • As subduction of the Pre-Grenville Ocean occurred, a huge continent from the opposite site of this ocean slammed into Proto-North America, creating a Grenville supercontinent. • This was a very active tectonic environment, affecting an area from Labrador, Canada, south through Georgia and Texas into Mexico, forming mountains as high as the Himalayas. • The resulting mountains, the Grenville Plateau, were a source for sediment all over North America.
RIFTING of the Grenville Supercontinent 660-550 million years ago • A rift zone formed along the suture zone where continents had joined during the Grenville Orogeny. • The splitting of the Grenville Supercontinent formed the Iapetus Ocean. • About 550 million years ago, the Taconic Island Arc formed in the Iapetus Ocean, as a result of a subduction zone that developed to the east.
Iapetus Ocean • In Roman mythology, Iapetus was the father of Atlas. • Atlas is the namesake for today's Atlantic Ocean.
Panthalassic Ocean • Great Ocean that covered the most of the planet. • Extension of the Mirovia
Archean & Proterozoic Climates • Rocks • Sediments • Glaciers • Ice Sheets
Archean & Proterozoic Climates • Rocks • Sediments • Glaciers • Ice Sheets
Rocks • Tell us that the Earth’s surface was cool enough for rocks to solidify • Prior to the rock formation, may have had magma ocean
The story of zircon The Jack Hills region of Western Australia, where the zircons were discovered. Microscopic view of a zircon (zirconium silicate) crystal determined to be 4.4 billion years old.
Earth's Oldest Sedimentary Rocks Earth's oldest sedimentary rocks, found in Greenland, are about 3.9 billion years old. Unusual chemical traces in these rocks may suggest that life existed when they formed. Image courtesy of Dr. Graham Ryder.
The Gowgonda Glaciation • Tillite • Varved Sediments • 2300 Ma • Overlays 2600 Ma igneous rock and is cross cut by 2100 Ma dike
Snow Ball Earth Hypothesis When the Earth Froze Over
Earth’s Climate • 18,000 years ago were experienced one of the largest glaciations of the last 500 million years.
Earth’s Climate Over the past million years we have experience 10 of these glacial-interglacial cycles.
The Evidence • While these changes are dramatic, they pale in comparison to glacial intervals that occurred during the late Neoproterozoic. • The following is based largely on a Scientific American article by Paul Hoffman and Dan Schrag at Harvard.
The Evidence • Dropstones • Paleomagnetics • Sedimentology • Scenario
Glaciations • In 1964, Brian Harland at Cambridge University postulated that the Earth had experienced a great Neoproterozoic ice age. • He pointed out that Neoproterozoic glacial deposits, similar in type to those of the Pleistocene, are widely distributed on virtually every continent.
Glaciations • Harland could only speculate on the positions of continents in Neoproterozoic time and could not rule out the possibility that various continents were glaciated at different times as they drifted close to the poles. • Nevertheless, he inferred that ice lines penetrated the tropics from the occurrence of glacial deposits within types of marine sedimentary strata characteristic of low latitudes.
Glaciations • In response to the Nuclear Winter Hypotheses in the 1960s, Mikhail Budyko made some calculations concerning climate. These had interesting implications for the postulated tropical Ice sheets.
Climate Notes: • The Earth's climate is fundamentally controlled by the way that solar radiation interacts with the Earth's surface and atmosphere. • We receive ~343 watts per square meter of radiation from the Sun. • Some of this is reflected back to space by clouds and by the Earth's surface, but approximately two thirds is absorbed by the Earth's surface and atmosphere, increasing the average temperature.
Climate Notes: • Earth's surface emits radiation at longer wavelengths (infrared), balancing the energy of the radiation that has been absorbed. • If more of the solar radiation were reflected back to space, then less radiation would be absorbed at the surface and the Earth's temperature would decrease.
Climate Notes: Surface albedo = radiation is reflected Snow has a high albedo (~0.8) Seawater has a low albedo (~0.1) Land surfaces have intermediate values that vary widely depending mainly on the types and distribution of vegetation. When snow falls on land or ice forms at sea, increased albedo causes greater cooling, stabilizing the snow and ice. This is called ice-albedo feedback.
Glaciations • Where were the continents? • Tropics • Sediments are carbonates • paleomagnetism
Where were the continents? • Namibia
Paleomagnetic - Problem NRMs are acquired when rocks formed, so long as they had not subsequently been heated above their Curie temperature (580-680 degrees Celsius). However, it was subsequently learned that many rocks, particularly sedimentary rocks, may be chemically remagnetized at much lower temperatures if subjected to prolonged groundwater percolation. Many of the early paleomagnetic measurements were shown to be from remagnetized samples and the rest were suspect.
Paleomagnetic - Problem Solved Joe Kirschvink reexamined South Australian Neoproterozoic glacial deposits giving shallow inclinations. These had the least chance of being remagnetized because South Australia was never at low latitude in the last 400 million years.
Paleomagnetic - Problem Solved Another reason to look at South Australia: Glacial deposits formed close to sea level; sediments of tidal origin. Modern tropical glaciers are found but not below 5000 meters above sea level. During LGM, equatorial ice lines in the Andes were no lower than 4000 meters above sea level.
Paleomagnetic - One more Point • Linda Sohl at Lamont-Doherty Earth Observatory documented as many as six polarity reversals in the South Australian glacial deposits. The frequency of polarity reversals of the Earth's magnetic field is such that the glacial deposits must represent a minimum of several 100,000s and more likely millions of years. • Consistent with the time scale of a "snowball" Earth
Glaciations • Budyko's model results helped stimulate interest in the science of climate modeling, but few believed that the Earth had ever actually experienced a runaway ice albedo. • Extinctions • Escape clause
Glaciations • The first of these objections began to fade in the late 1970s with the discovery of remarkable communities of organisms living in deep-sea hydrothermal (hot water) vents, and later in the extremely cold, dry, mountain valleys of East Antarctica.
Glaciations • Some of these organisms appeared capable of surviving global glaciation and their existence in the Neoproterozoic was unquestioned — molecular studies showed that they disproportionately represent the oldest branches in the universal tree of life.
Glaciations The key to the second problem— reversing the ice-albedo feedback —is plate tectonics.
Glaciations • In the late 1980s, Joe Kirschvink at the California Institute of Technology pointed out that during a global glaciation, what he termed a "snowball" Earth, the supply of carbon dioxide to the atmosphere and oceans from volcanism would continue because of plate tectonics.
