Siderian & Rhyacian Periods

 

Siderian Period

There are 4 periods in the Paleoproterozoic Era, the first being the Siderian Period. This period lasted from 2,500 to 2,300 million years ago.

Early in this period is when banded iron formations peaked. As I explained before, cyanobacteria produced waste oxygen, which initially combined with iron that was in the sea, forming magnetite (Fe3O4), an iron oxide, in a crystal structure rock. This process removed iron from the oceans, presumably turning the greenish water clear.

Then, with no remaining iron in the sea to serve as an oxygen sink, the oxygen escaped the ocean and built up the oxygen level in the atmosphere. This led to the oxygen catastrophe, wherein a great deal of the life then existing on the Earth was wiped out because they could not tolerate such high levels of oxygen.

Some geologists believe the high level of oxygen also triggered the Huronian glaciation. This glaciation started in mid-Siderian (2,400 million years ago) and extended into the next period, lasting until 2,100 million years ago. It was apparently caused when free oxygen combined with the methane in the atmosphere to form carbon dioxide and water, which do not retain heat as well as methane does. With far less of this greenhouse gas in the atmosphere, a great chilling took place, also known as the Huronian glaciation. I don’t know if the entire planet froze, and the article stated that this glaciation caused a mass extinction. I’m waiting for a geologist I know to clarify whether there were 2 mass extinctions so close together, or just one, with both the presence of oxygen and the cooling temperatures to blame.

 

Rhyacian Period

The Rhyacian Period is the 2nd geologic period in the Paleoproterozoic era. It lasted from 2,300 to 2,050 million years ago. And that is just about all the wikipedia article had to say on the subject. Oh, there were a bunch of rock formations created, and the Huronian glaciation lasted 100 million years, but that still leaves 150 million years at the end of the period during which, apparently, absolutely nothing of note happened.

Oof! What was the tipping point that caused the end of the glaciation? For that matter, did it cover the entire Earth? Was there an explosion of new species afterwards?

Maybe they don’t know. What about theories? Doesn’t anybody have any theories? Well, without sitting my geologist friend down and picking his brain clean, I guess that’s as far as I can go with this period. Maybe the next 2 period of the Paleoproterozoic Era will have more meat to them.

 

https://en.wikipedia.org/wiki/Siderian#:~:text=The%20Siderian%20Period%20(%20%2Fsa%C9%AA,these%20dates%20are%20defined%20chronometrically.

https://en.wikipedia.org/wiki/Rhyacian

The Paleoproterozoic Era

 Next up is the Paleoproterozoic Era, which spans from 2,500 to 1,600 million years ago. It is the first of 3 sub-divisions (era) of the Proterozoic Eon. It is the longest era of the Earth’s geological history, and is divided into 4 periods, which we will look at later. During this era, the continents first stabilized.

Paleontological evidence suggests that the Earth rotated during this era at a speed that produced days that were 20 hours long, which would have meant a year would have about 450 days long.

It was during this era that the atmosphere and shallow seas saw a great increase in free oxygen, thanks to all that cyanobacteria that had been pumping out oxygen as a waste product for so long. Before that, almost all existing lifeforms were anaerobic, meaning they did not require oxygen. In fact, free oxygen in large amounts is toxic to most anaerobic organism. Therefore, the majority of the anaerobic lifeforms died when the atmospheric free-oxygen levels soared. This was the first major and possibly the most significant mass extinction event, and is called the Great Oxidation Event.

But this was not just a time of death. Many eukaryotes lineages have been approximately dated to the Paleoproterozoic era. Eukaryotes consist of cells that have a nucleus enclosed within a nuclear envelope. To the best that I can remember my high school biology, that would mean that most plants and animals are eukaryotes. It is currently accepted that there are 3 domains of life on Earth, and the eukaryotes are one of them. Bacteria and Archaea are the other two. Neither of these types of life have cells with a nucleus within a nuclear envelope, and I think neither one of them gets large enough to be seen with the naked eye.

During this era, a number of continents collided, creating mountain belts and basins. This happened so often that it led to the assembly of the supercontinent named Columbia (or Nuna, depending on who you talk to).

Now, in doing my research about the Paleoproterozoic Era, I chanced upon a phrase called The Boring Billion. Believe it or not, that sounded interesting, so I did a little more digging (so to speak) and found that it referred to the time period between 1.8 and 0.8 billion years ago, which spans the middle of the Proterozoic eon. It would have just been starting when this era was ending, but I’ll mention it here and hope it comes up again when we get to the next era, so I can study it in more detail.

The Boring Billion section of time was characterized by a fair amount of tectonic stability, climatic stasis, and stalled biological evolution. Supposedly, it was bordered by 2 different oxygenation and glacial events, but the Boring Billion itself had very low oxygen levels and no evidence of glaciation.

Well, no doubt about it, the world is really beginning to shape up into the Earth we know. But I’m not quite ready to move in.

 

https://en.wikipedia.org/wiki/Paleoproterozoic

https://en.wikipedia.org/wiki/Boring_Billion

https://en.wikipedia.org/wiki/Eukaryote

Proterozoic Eon

 Now to move on to the next eon, the Proterozoic Eon. This eon spans the time from the appearance of oxygen in Earth’s atmosphere to just before the proliferation of complex life such as trilobites or corals. It started at 2,500 million years ago to 541 million years ago, so very nearly 2 billion years. It is divided into 3 eras; the Paleoproterozoic, the Mesoproterozoic, and the Neoproterozoic. We may or may not take a look at each of these in more detail.

The geologic record of the Proterozoic Eon is more complete than for the Archean Eon, which preceded it. The Proterozoic features rock strata that were laid down in extensive shallow inland seas. Studies of these rocks show that the eon had massive continental accretion, including the first definitive supercontinent cycle, and modern mountain building activity.

There is also evidence of glaciations taking place during the Proterozoic. The first began shortly after the beginning of the Proterozoic, with at least four others near the end of the eon. These may have climaxed with the hypothesized Snowball Earth, an idea I am thoroughly fascinated by.

One of the most important events of this eon was the accumulation of oxygen in the atmosphere. Although oxygen was probably released by photosynthesis during the Archeon Eon, it first combined with sulfur and iron in the oceans. Until roughly 2.3 billion years ago, oxygen was only 1-2% of its current level. It is stated that about 1.9 billion years ago, all the iron in the oceans had all been oxidized.

The Proterozoic Eon was tectonically active. The early part of the eon experienced a period of increasing crustal recycling, which suggests subduction. This is where one piece of crust gets shoved under another piece of crust. The bottom crust is eventually melted, while the upper crust buckles into mountains. The melting crust eventually reformed, making the upper bits of crust thick enough to endure. It is believed that 43% of the modern continental crust was formed in the Proterozoic Eon, 39% during the Archean Eon, and only 18% in the current geological eon.

It is commonly accepted that during the Precambrian SuperEon--of which the Proterozoic Eon is a part--the Earth went through several supercontinent breakup and rebuilding cycles.

The supercontinent Columbia was dominant in the early-mid Proterozoic. The article did not say much about Columbia, so we shall skip ahead to Rodinia (1,000-750 Million years ago) of the late Proterozoic. It was created when a series of continents attached to a central craton called Laurentia, which today forms the core of the North American Continent. During its construction, the mountain building processes created the Grenville orogeny located in Eastern North America, from Labrador down through Mexico. From the map, it looks like the Appalachian Mountains could be part of the mountain range that was formed.

The first advanced single celled (eukaryotes) and multi-cellular life roughly coincides with the start of the accumulation of free oxygen. This may have been due to an increase in the oxidized nitrates, which is what eukaryotes use. But the rise of eukaryotes did not preclude the expansion of cyanobacteria. In fact, stromatolites (microbial mats) reached their greatest abundance and diversity, peaking roughly 1,200 million years ago.

The earliest fossils of something like fungi date to 2,400 million years ago. These organisms lived in the deepest areas of water, and had filamentous structures capable of forming branches.

Wow, sounds like Earth was getting busy, doesn’t it? But I still don’t think we could colonize, not with the crops we grow now. There’s too many forms of other life that our crops count on to help them get the nutrients they need from the soil. Plus, I’m not sure what the climate was like, with such a low amount of free oxygen in the atmosphere. Put me back in my pod and let me sleep another million years or so, and then we’ll see.

 

https://en.wikipedia.org/wiki/Proterozoic

Mesoarchean & Neoarchean Eras

 Mesoarchean Era

We will continue studying the Archean Eon with a brief look at its 3rd part, the Mesoarchean Era. How brief a look? That will depend on how much I find.

The Mesoarchean Era lasted from 3,200 to 2,800 million years ago. There is no specific level in the rocks that designates this era, it is simply defined by the time period.

Fossils from Australia prove that stromatolites have been growing on Earth since the Mesoarchean Era. These sedimentary formations are created by photosynthetic cyanobacteria that produce adhesive compounds and cement sand and other rocky materials into mineral “microbial mats”. These multi-layered sheets of microorganisms, mainly bacteria, grow at interfaces between different types of material, mostly on submerged or moist surfaces. They have been known to colonize environments ranging in temperature from -40C to 120C (about -48F to 272F). The mats grow layer by layer and can grow to a meter or more. Although uncommon today, fossilized stromotalites record ancient life on Earth. The earliest reefs, probably formed by stromatolites, date from this era.

The article had a tantalizing statement about the Pongola glaciation occurring around 2,900 million years ago but a brief search couldn’t find any more information about it. Was it only at the poles, or was it world-wide?

At the end of this era, the first supercontinent broke up, right about 2,800 million years ago.

 

Neoarchean Era

That brings us to the 4th part of the Archean Eon, the Neoarchean Era,  from 2,800 to 2,500 million years ago. Again, this era is defined only by time, not to a specific rock level.

During this era, oxygenic photosynthesis released an abundance of oxygen, which first reacted with minerals and afterward was free to react with greenhouse gases of the atmosphere. By reacting with these greenhouse gases, the oxygen changed them into gases that trapped less heat in the Earth’s atmosphere, and the Earth began to cool off. Eventually.

Remember the microbial mats from the Mesoarchean Era? Did I mention these mats were created by cyanobacteria? Cyanobacteria give off oxygen as a waste product. We should celebrate the ancient existence of cyanobacteria as the provider of the oxygen that we need in order to live.

However, back then, what life existed could not use oxygen. In fact, it was poisonous to most forms of life of the time. So when O2 levels got too high, a lot of the existing life died off. But that happened later. The process that led to that problem was only beginning during the Neoarchean Era.

Also during the Neoarchean Era, at about 2,720 million years ago, the supercontinent Kenorland formed.

 

https://en.wikipedia.org/wiki/Mesoarchean

https://en.wikipedia.org/wiki/Stromatolite

https://en.wikipedia.org/wiki/Microbial_mat

https://en.wikipedia.org/wiki/Neoarchean

Archaen Eon

 

The Archean Eon is the second of the four geologic eons of Earth’s history. (The first was Hadean Eon.) During the Archaen Eon, which lasted roughly 4,000 to 2,500 million years ago, the Earth’s crust cooled enough to allow the formation of continents and the beginning of life on Earth. Sounds like a busy time, doesn’t it?

The oldest rock formations on Earth’s surface are Archean. They are found in Greenland, Siberia, Canada, Montana, Wyoming, Scotland, India, Brazil, western Australia and southern Africa, as well as other regions. Volcanic activity was much higher than it is today, producing many different kind of volcanic rocks.

The continents started to form during the Archean, although details are still being debated.  Although this is when the first continents formed, rock of this ages makes up only 7% of the present world’s land mass. Allowing for erosion and destruction of past formations suggests that only 5-40% of the present area of continents formed during the Archean Eon.

By the end of the Archean, plate tectonic activity may have been similar to that of the modern Earth. For those who know how to read it, evidence demonstrates that liquid water was prevalent and deep oceanic basins already existed.

The Archean atmosphere had very little free oxygen, yet temperatures appear to have been near modern levels. The moderate temperatures may be because of greater amounts of greenhouse gases. Or, the Earth may have reflected less sunlight and heat due to having less land area.

There is substantial evidence that life began either near the end of the Hadean Eon or early in the Archean Eon.

The earliest identifiable fossils consist of stromatolites, which are microbial mats formed in shallow water by cyanobacteria. The earliest were found to be 3.48 billion years old. They were found throughout the Archean and became common late in the Eon. Cyanobacteria were instrumental in creating free oxygen in the atmosphere, and created so much of it that later, there was a crisis of sorts, when the life that existed at the time could not cope with the high level of oxygen. (I read that somewhere and have included it in one of my other blogs, but at this time, I can’t remember where I got that from.)

It is generally agreed that before the Archean Eon, life as we know it would have been severely challenged by the hostile environmental conditions then found on Earth.

Life during the Archean consisted of simple single-celled organisms such as Bacteria.

However, fossilized microbes from terrestrial microbial mats show that life was already established on land as long ago as 3.22 billion years.

So, it was a busy time. Lots of water sloshing around, lots of volcanoes creating land masses, and life beginning to get a first grasp on the place. If we wound up crash-landing on a planet like that, could we survive? Could we cultivate cyanobacteria to create more oxygen for us? Doing that to any large extent might strip out some of the greenhouse gases, which could lower the temperature of the planet. Which only goes to show that you have to be careful what you do to make a place your home.

 

https://en.wikipedia.org/wiki/Archean#:~:text=The%20Archean%20Eon%20(%20%2F%C9%91%CB%90r%CB%88,beginning%20of%20life%20on%20Earth.