Mesoproterozoic Era

This era lasted for about 600 million years, from about 1.6 billion to 1.0 billion years ago.

This era is marked by the further development of continental plates and plate tectonics. At the end of this era, the continental plates that had developed were more or less the same we have today. This is the first era of which a good geological record still exists today. Some of the highlights of this era include:

The mesoproterozoic is neatly divided into 3 equal chunks of 200 million years each, which goes to show that it is chronometrically, rather than stratigraphically or biologically dated. Chronometry is used when there are no widely occurring geological or biological events that can form significant boundaries between periods. Chronometry isn't the best way to assign ages, since it typically has a random error of 1-2%. For dating an era in this range (~ 1.5 billion years ago), this can result in random errors of 20-50 million years, which is a significant portion of each period. This is why periods are chosen to be large enough (e.g., 200 million years) to accommodate these errors and still define events within these ranges. These are just random errors, not even accounting for systematic errors. This problem will diminish as radiometric dating improves, but ultimately we want geological or biological dating, based upon significant developments that happened widely across the Earth. This is the field of bio- or geo-stratigraphy, where dates can be much more precise.

The Mesoproterozoic is divided into three periods as follows:

Bangiomorpha pubescens fossils, 1.2 billion years old.

Calymmian

This period lasted from about 1.6 billion to 1.4 billion years ago. The name derives from the Greek word for "cover", which indicates the development and expansion of "platform covers", which are basically just deposits or layers (igneous or sedimentary) that accumulate over cratons. This is part of the process of continent building, where the Earth's crust thickens due to the deposition of rock on top of older rock. The continental plates expand as sediments are washed into the sea and deposited along the coast, making the sea shallower. The movement of continents also creates shallow seas between landmasses, which are also gradually filled up due to the deposition of sediment, forming new land.

The Calymmian was marked by the breakup of the first continent Columbia, which started somewhere around 1.6 billion years ago, and continued through the Calymmian to end in the Ectasian, around 1.3-1.2 billion years ago. Rifts appeared along the western margin of Laurentia (the north American plate), the east margin of India, southern margin of Baltica, southeastern margin of Siberia, northwestern margin of South Africa, and the northern margin of the South China Block. Rifting was accompanied by volcanism along the rift sites, which led to large igneous deposits of rock in these regions.

Ectasian

The second period of the Mesoproterozoic era, which lasted from about 1.4 billion to 1.2 billion years ago. The name derives from the Greek word for "extension", signifying the extension of the platform covers and consequent enlarging and further thickening of the continental landmasses. The continents continued to drift away as they grew. This trend was to continue through the middle of the Ectasian, before they started drifting towards each other again to form the super continent Rodinia during the next period, the Stenian.

This period is interesting for the first evidence of sexual reproduction. The 1.2 billion year old Hunting Formation on Somerset Isle, north Canada, dates from the end of the Ectasian. It contains the microfossils of the multicellular filaments of Bangiomorpha pubescens (type of red algae), the first taxonomically resolved eukaryote. This was the first organism that exhibited sexual reproduction, which is an essential feature for complex multicellularity. Complex multicellularity is different from "simple" multicellularity, such as colonies of organisms living together. True multicellular organisms contain cells that are specialized for different functions. This is, in fact, an essential feature of sexual reproduction as well, since the male and female gametes are specialized cells. Organisms that reproduce sexually must solve the problem of generating an entire organism from just the germ cells.

Grenville Orogeny, showing the extent of the Orogeny as the continents collided to create Rodinia. From Karlstrom, K.E., 1999.

So long as reproduction simply means one cell copying itself, true multicellularity cannot exist. Therefore, sexual reproduction and the ability of gametes to develop into an organism is the necessary antecedent to true multicellularity. In fact, we tend to think of sexual reproduction and true multicellularity as occurring at the same time, and true multicellularity is often taken as a marker for sexual reproduction.

The photographs show two Bangiomorpha fossils from the late Ectasian. The fossil on the right shows a clear differentiation of the lower cells, which were adapted to anchor the organism to some substrate. The photograph on the left shows (particularly in the upper part) how the disc shaped cells have divided into multiple wedge shaped cells on each disc, which is very similar to how existing Bangiomorpha species divide.

Stenian

This is the last period of the Mesoproterozoic, roughly dated to 1.2 billion to 1.0 billion years ago. This was the period in which the first well-recognized super continent of Rodinia first appeared. The name is derived from the Greek word for "narrow", referring to the narrow polymetamorphic belts that formed during this period.

This is the period during which the super continent Rodinia (Russian for "homeland") was assembled, during an event known as the Grenville Orogeny. Evidence of the Grenville Orogeny still survives today, in the mountain ranges of western Europe and eastern North America (see figure).

Rodinia's landmass was centered south of the equator. The continents clustered around Laurentia, which included the North American plate and Greenland.

To the north was the ancient continent of Ur (India, Australia, Madagascar, East Antarctica), which was formed about 3 billion years ago. In the roughly 2 billion years since its formation, it had grown significantly, mostly during the Proterozoic. After the breakup of Rodinia, this continent later became East Gondwana.

Configuration of Rodinia during the Stenian From USGS.

To the southwest were the remains of the continent of Atlantica, consisting of West Africa, Congo and Amazonia (Brazil, the Sao Francisco and Rio Plata cratons). These areas were later to become West Gondwana after the breakup of Rodinia.

Siberia was to the northwest of Laurentia, and Baltica to the southwest. The figure below shows a diagram of what Rodinia may have looked like about 1.2 - 1.0 billion years ago during the Stenian.

Rodinia must have been a stark and desolate place. Life had not yet colonized land. The only signs of life near the land were the profuse growths of stromatolites along the coastlines. The atmosphere, which had contained about 1% of oxygen at the start of the Mesoproterozoic, had increased the oxygen content to no more than 2-4% at the time of the formation of Rodinia. Consequently, there was no ozone layer.

Note: However, there is some recent research that indicates that microbes may have colonized land much, much ealier, as early as 2.75 billion years ago. This evidence comes from the record of how sulfur and molybdenum levels increased in the oceans around this time. The simultaneous increase of both indicates that microbes on land may have been breaking down rock, which washed into the sea. The evidence is not clear and it may be some time before we understand the extent of early colonization of land by microbes.

The center of the continent was a vast floodplain, accumulating silts and sedimentary deposits over the years. With no plant life, there was little to obstruct the floods, which must have been massive. A giant basin of sedimentary rocks known as the "Belt Supergroup" extends across Alberta, British Columbia, Montana, Idaho and Washington. These are mostly sandstone, siltstone, and limestone. They show beautifully preserved features like cracks, ripples, and stromatolites. Remember, the super continent wasn't all land, it must have contained many rivers and inland seas, which were abundantly covered with stromatolites where the water was shallow enough. The belt grew very thick, reaching a maximum of 50,000 feet near the Washington-Idaho line.

Rodinia was surrounded by a single ocean, known as Mirovia. The figure on the left shows a commonly proposed configuration for Rodinia, around 1 billion years ago. Note that the bulk of the continent is south of the Equator. The pink zones are the ancient continents, with Ur at the north, Laurentia in the middle, Atlantica and Baltica to the south. The brown areas show the Grenville Orogeny, with new mountainous land created in zones where two landmasses were drifting together.

The white striped areas are the rift zones, which appeared later during the Neoproterozoic when Rodinia started to tear itself apart. Laurentia drifted eastwards, opening up an ocean on its western edge, which was to become the Panthallasic Ocean, the proto-Pacific. Baltica and Amazonia drifted southwards about 600-550 million years ago, opening up the Iapetus Ocean between them and Laurentia.

The eight continents which made up Rodinia later reassembled briefly into another super continent Pannotia, and again into Pangaea.

Rodinia produced some significant changes in the Earth. It was the largest landmass to have existed up till that time. It significantly changed ocean currents, which may have led to snowball Earth later in the Cryogenian.

 

Continue reading about the Neoproterozoic Era, or return to the index.