A brief history of Earth – part 2
Yesterday, I began a review of the recently-broadcast Australia: The Time Traveller’s Guide. However, this is such an information-rich programme, I went well over my 1000-word limit without even getting to the end of Episode 1. Also, it was clearly too much of a challenge to the unwarranted optimism of people like my (sadly-mistaken) fellow-hydrogeologist Oakwood who has, for the second time now, said “goodbye” (his first “goodbye” can be seen here).
The story so far is that with over 25% of Earth history already in the past, oxygen has only just been “invented” but, when not being combined with iron or silica to form Banded Iron Formation (BIF), it is restricted to the oceans; and is only present at a fraction of the concentration our sea creatures currently enjoy (i.e. when they’re not being dissolved-alive by ocean acidification).
I have never been to the Grand Canyon, although, yes, I intend to get there one day (and I will not be rowing across the Atlantic and walking across America to get there). If that makes me a hypocrite, so be it, but at least I am not denying the reality of the problem; or insisting that air travel should be cheap. However, I have been to Karijini National Park in the Hamersley Range of the Pilbara (WA). You would not want to visit the latter after visiting the former (it would be a major anti-climax): The gorges of the Hamersley Range may be small compared to others elsewhere in the world, and they may not be unique for being weathered to a rusty-red colour… However, they are unique for being formed of BIF that, when regularly washed clean by water, is composed of alternations of grey hematite and white silica. The juxtaposition of fresh and weathered rock alongside Spinifex grass and ghost gum trees is, it must be said, a very pretty one; and I feel very privileged to have seen it for myself (albeit 25 years ago now):
Although early photosynthesising bacteria like Stromatolites still exist today, and may dominate the foreshore of Shark Bay (WA), they could hardly be said to dominate the Earth. However, for almost half the time the Earth has existed, this would have been a legitimate description of their importance: For nearly two billion years they were the dominant life form on the planet and, as such, they slowly but surely pumped oxygen into the oceans. For the first few hundred years, the oxygen was fairly promptly removed again by being precipitated out in the form of iron or silicon oxide but, once these elements had been depleted, the Earth then had to find another use for it. Fortunately for us, it did just that.
Since there were as yet no shell-fish to make use of this oxygen stuff by combining it with carbon and calcium, it began to build-up in the atmosphere because, like CO2 today, the Earth was unable to reprocess it fast enough to stop it accumulating. By the end of the Proterozoic Era, by which time two-thirds of Australia was already a well-established landmass, all of 1% of the atmosphere consisted of oxygen (compared to 20% today). Sub-oceanic volcanic vents (like the ‘black smokers’ found on mid-ocean ridges today) were also busy pumping lots of other elements into the oceans; stuff we now find very useful – like lead and zinc.
Meanwhile, it is generally assumed that the oxygenation of the atmosphere (i.e. going from 0 to 1%) was a major contributory factor to the so-called Snowball Earth (850 to 635 Ma BP) – a bit like the opposite of the enhanced Greenhouse Effect (also known as anthropogenic climate disruption [ACD])… In the Flinders Range of South Australia (SA), finely laminated sediments laid down in equatorial oceans provide evidence that the entire surface of the oceans was covered in sea-ice. Fortunately, the venting of volcanic gases continued and, eventually, the build-up of gases in the atmosphere was sufficient to bring about an end to Snowball Earth conditions… Now, at long last, oxygen levels in the oceans and temperatures were capable of supporting complex lifeforms. The oldest of these are remarkably well-preserved in the Ediacara Fauna, which can now also be found in the Flinders Range (but only if you have been given directions to it).
Between 635 and 542 Ma BP, the convection of the Earth’s interior and continual creation and destruction of sea floor – thereby pushing early continental land masses around as well – resulted in the gradual development of the super-continent of Gondwanaland. The continental collisions involved resulted in mountain ranges as big as the Himalayas and a meandering coastline stretching from Cairns in Queensland to Kangaroo Island (SA). It is on Kangaroo Island that you will find evidence of flourishing sea life in the Cambrian Period (542 to 488 Ma BP) – a time that is associated with one species in particular – the Trilobites. However, these Cambrian rocks – and those of the same age all over the planet – also contain fossilised evidence of many other kinds of life that we would still recognise today; including arthropods, molluscs and insects. It was also in this shallow Cambrian sea that sand (the recycled product of the weathering and erosion of older rocks) was laid down. This sandstone would later become quite famous for giving rise to one of the Earth’s most readily-identifiable features – Uluru (the landmark formerly know as Ayers Rock).
So there you have it, almost 90% of Earth’s history (4540 to 488 Ma BP) is now behind us; and all we have to show for it is an abundance of marine life and whole load of sand that will one day be big, red, and famous!
In Episode 2 of Australia: The Time Traveller’s Guide, Richard Smith covers the remainder of the Palaeozoic Era (488 to 251 Ma BP); encompassing the development of fish, plants and animals – 90% of which were then wiped out by what has been called “the mother of all extinction events” at the end of the Permian Period. But that, as they say, is another story that will be covered in A brief history of Earth – part 3 next week. For now, we must just hope that humanity is not now re-writing geological history because, if we are, it would seem very clear it not going to end well (for the vast majority of the Earth’s inhabitants).