The NeoproterozoicJarðsaga 1

-Saga Lífs og Lands –

Ólafur Ingólfsson

The Neoproterozoic EraA time period when a variety of global environmental changes led to the development of the fungi and animal kingdoms...

• The Varangian Ice Age, ca. 850-600 MY:

There is evidence from all continents except Antarctica of a major Ice Age during late Precambrium. During this ice age there were at least four major glaciations, of which the latest (~650-600 MY) was extremely widespread.

The origin of Fungi

Based on the fossil record, fungi are presumed to have been present since M-L Proterozoic, 1.000-900MY. As the sister group of animals and part of the eukaryotic crown group that radiated about a billion years ago, the fungi constitute an independent group equal in rank to that of plants and animals.

While fungi are not uncommon fos-sils, their fossils have not received a great deal of attention compared to other groups of fossils. Their fossils tend to be microscopic; very few large fungal bodies, such as mushrooms, have ever been found as fossils.

Rodina (Russian: Homeland) SupercontinentThe oldest known supercontinent: Rodinia. It was formed about 1100 MY ago and started braking apart ~750 MY ago. Though its exact size andconfiguration are not known, it appears that North America formed the core of this supercontinent. At that time, the east coast of North America was adjacent to western South America and the west coast of North America lay next to Australia and Antarctica.

• Rodinia split into 2 halves ~ 750 MY ago, opening the Panthalassic Ocean. N America rotated towards the South Pole.• The northern half of Rodinia, (Antarctica, Australia, India, Arabia, China), rotated across the North Pole.• Between the two halves of Rodinia, a third continent - the Congo craton -was caught in the middle as the two halves of Rodinia came crashing down on it.• By the end of the Precambrian, ~ 550 MY ago, the three continents collided to form a new supercontinent called Pannotia. The mountain-building event associated with this collision is called the Pan-African orogeny.

The split-up of Rodina

Rodinia was surroundedby a single ocean, called the Iapetus Ocean.

Towards the end of theProterozoic, Rodiniafragmented, giving rise tothe late Vendiancontinents of Pannotia, Siberia, and North China. From Pannotia in turn came the diverse continents of Laurentia, Gondwana, and Baltica. Later these in turn came together to form Pangea...

Mjög góð vefsíða, þar sem hægt er að skoða legu meginlanda frá frumlífsöld (700 MY) og fram til nútíma:

http://www.uwgb.edu/dutchs/platetec/plhist94.htm#700my

The Rodina Supercontinent

http://www.scotese.com/climate.htm

Glacial deposits from the Varangian

Ice AgeThey occur as far apart as in Africa (Namibia), Finland, Australia, Svalbard, N America. Even areas which at the time were close to the Equator (Australia) were extensively glaciated. It is difficult to understand how the glaciation could have been so widespread.

Sites where ~750-600 MY old tills occur

Glacial Deposits (Tillites) overlain by Carbonates

carbonates

glacial depositsSkeleton Coast, Namibia

A 600 MY old tillite in Namibia

A 600 MY old tillite on Svalbard

The Snowball Earth

Snowball Earth Model:Runaway Icehouse EffectPaul Hoffman and Daniel Schrag, 1998

In most extreme scenario, almost entire Earth frozenOver

Up to 4 major snowball eventsbetween 750 and 600 Ma

The Snowball Earth

• Between ~750 and 600 m.y. ago, Earth froze completely (or almost) about four times

• Global freezing alternated with extremely rapid sea-level rise and global warming

• Evidence:– Glacial deposits on all continents, even at

low latitudes– Glacial deposits immediately succeeded

by thick deposits of carbonate rocks

The Snowball Earth

The Snowball Earth

Before the Snowball (about 770 MY)

• Breakup of a single landmass leaves small continents scattered near the equator. • Formerly landlocked areas are now closer to oceanic sources of moisture. Increased rainfall scrubs heat-trapping carbon dioxide out of the air, eroding continental rocks more quickly.• Global temperatures fall, and large ice packs form in the polaroceans. White ice reflects more solar energy than darker seawater, driving temperatures even lower. • Feedback cycle triggers strong cooling effect.

Into the Ice House

• Average global temperatures plummet to -50 oC shortly after the runaway freeze begins.• Oceans freeze ice to average depth of more than 1 km, limited only by heat slowly released from Earth's interior.• Most microscopic marine organisms die, but a few cling to life around volcanic hot springs.• Cold, dry air arrests the growth of land glaciers, creating vast deserts of windblown sand.• With no rainfall, carbon dioxide emitted from volcanoes is not removed from the atmosphere. • As carbon dioxide accumulates, Earth warms and sea ice slowly thins.

Snowball to Slushball

• Concentrations of carbon dioxide in the atmosphere increase 1,000x due to 10 million years of normal volcanic activity• Ongoing greenhouse warming effect pushes temperatures to the melting point at the equator• As Earth heats up, moisture from sea ice sublimating near the equator refreezes at higher elevations and feeds the growth of land glaciers.• Open water in the tropics absorbs more solar energy and initiates a faster rise in global temperatures

From Freeze to Fry

• As tropical oceans thaw, seawater evaporates and works along with carbon dioxide to produce even more intense greenhouse conditions.• Surface temperatures soar to more than 50 oC driving an intense cycle of evaporation and rainfall. • Torrents of carbonic acid rain erode the rock debris left in the wake of the retreating glaciers.• Swollen rivers wash bicarbonate and other ions into the oceans,where they form carbonate sediment.

The Snowball EarthPossible reasons: • Fainter early sun • Biological changes- Global ice cover- Weathering and erosion shut down- Volcanoes continue to erupt CO2- At 10% CO2, abrupt warming begins- Go from –50 C to +50 C in 10,000

years?• Implications for life?

Iron gives clues as to how the environments changed...

1. 2.

3. 4.

Snowball Earth

http://www.sciam.com/article.cfm?articleID=00027B74-C59A-1C75-9B81809EC588EF21&pageNumber=1&catID=2

Did the Varangian Ice Age cause

explosion of life?

ANIMALS descended from the first eukaryotes, which appeared ~2.000 MY ago. By the time of the Varangian Ice Age, more than one billion years later, eukaryotes had not developed beyond unicellular protozoa and filamentous algae. All 11 animal phyla ever to inhabit the Earth emerged within a narrow window of time in the aftermath of the glaciation. The prolonged genetic isolation and selective pressure intrinsic to a long ice agemight be responsible for this explosion of new life-forms...

The Varangian Ice Age set the pattern for global

temperature through geological time

There have been two basic modes of global climate: a steaming ”greenhouse” and cool to cold ”icehouse”. The overall control lies in the configuration of the continents and the global distribution of energy through the Oceanic and Atmospheric systems...

Possibilities/questions posed by timing of earliest complex metazoan (vefdýr, fjölfrumungar) and Snowball Earth

1. Metazoans evolved from unicellular eukaryotes REALLY fast(580 to 575 million years provides only 5 million years for this to occur)

2. Metazoans evolved before or during Snowball Earth (perhaps just no record up until 575 Ma) ? If so, how did they survive on ice-covered Earth ?

3. Perhaps Snowball Earth wasn’t as severe as originally asserted by Hoffman and Schrag (to permit survival of life) ?Perhaps a fair amount of open ocean at equator ?

Result: Yeepee! Snowball Fight ! Many geologists in favour of Snowball Earth concept

BUT many against !

Momentous evolutionary changes as the Proterozoic Eon neared its end

Organisms for the first time evolve to be larger than of microscopic size, displaying a wide variety of forms

Causes:

• Origin of some kind of thermohaline Oceanic circulation?

• Chemical changes in the Oceans (oscillations in carbon isotope ratios, oxygenation of the Oceans)?

• A build-up of atmospheric oxygen to to a critical level?

• The great Varangian glaciation caused led to the evolution of new species adapted to new climatic conditions?

• Maybe the evolutionary radiation was triggered by initial evolution of a key adaptive features, such as muscle or nerve cells.

Latest Proterozoic(650 to 544 Million Years Ago)

Oldest Metazoa (complex multicellular organisms)well established

“Ediacaran” or “Vendian” fauna

The Precambrian Edicara (Venedian) Fauna

Venedian Fauna Sites

What is the Edicara Fauna all about?

100 million years before the end of Proterozoic a group of marine animals, so-called Edicaran fauna, appeared. They were named after a locality in South Australia - Edicara, where they were first discovered.

Edicara/Venedian Fauna...

For a long time it was thought that this fauna consisted of invertebrates like jelly fishes, Annelide worms, arthropods and many forms that aren't numbered among any known groups. It was also assumed that these organisms were ancestry to Cambrian and even some contemporary animals.

Some more Venedian fossils

What are we seeing preserved?

Not easy to interpret; alternative explanations for many fossils...

One alternative interpretation

These animals were "highly flattened fronds, sheets and circlets composed of numerous slender segments quilted together" that lay on the bottom of the ocean. Their plan of the body was different from any fossil or contemporary group of animals. "They may constitute a separate and failed experiment in animal life, or they may represent a full range of diploblastic (two-layered) organization, of which the modern phylum Cnidaria (corals, jelly fishes and their allies) remains as a small and much altered remnant" (Stephen J. Gould 1994).

But what do they represent?They represent the oldest forms of complex, multi-celluar animals to live on Earth.

When the Edicara Biota appeared, it is certain that some kind of threshold had been crossed. Animals had arrived on the scene...

Are they monsters, hopeful ones that were seminal to to what followed, or doomed experiments committed to

oblivion?

Some intresting observations on the Venedian fauna...

•No animal of the Edicara fauna has shell, mineralized skeleton or armor – these appeared in the Cambrian.

• No animal of the Edicara fauna has developed eyes –these developed in the Cambrian.

• No evidence of predation has been found in the Edicara fauna.

• Was the Venedian the last geological period of innocence; before wide-spread predation where animals either are eaten or eat other

animals or both?Why did the Venedian fauna become extinct?

Summing up the Precambrian

References, web resources etcStanley, Earth System History, chapter 12 (Mjög gott yfirlit, LESIÐ!!)

Fortey, Life. Vintage Books, New York, 346 p. (Besta bók á markaðnum um þróun lífsins. Jólagjöfin í ár!)Snowball Earth. Feature Article in January 2000 issue of Scientific American

http://www-eps.harvard.edu/people/faculty/hoffman/snowball_paper.html (NB: Hér finnið þið grein Hoffman & Schrag “The Snowball Earth”).

http://www.uwgb.edu/dutchs/platetec/plhist94.htm#700my

http://www.palaeos.com/Earth/Geography/palaeogeography.htm

http://www.pbs.org/wgbh/evolution/library/index.html

http://www.ucmp.berkeley.edu/index.html

http://www.science501.com/PTProteroz.html

http://www.scotese.com/Rodinia3.htm