Cosmology: snowball Earth advanced life (Introduction)

by David Turell , Thursday, August 17, 2017, 17:42 (2654 days ago) @ David Turell

Earth had two episodes of being a giant snowball, covered with ice. The last one starting over 700 million hears ago and lasting about 60 million years set the stage for algae to appear and supply oxygen to the atmosphere:

https://cosmosmagazine.com/geoscience/how-snowball-earth-gave-rise-to-complex-life

"Now research from scientists at the Australian National University published in Nature has an answer to the when and why, dating the “rise of algae” to 659–645 million years ago. It is a surprisingly narrow interval, falling between the Earth’s second-last and last great ‘snowball’ glaciation episodes, when the oceans froze and the planet’s surface was covered in ice up to several kilometres thick.

"As much as ice-age conditions would seem inhospitable to life, the ANU research team led by Jochen Brocks and Amber Jarrett suggest the Earth’s penultimate snowball phase, known as the Sturtian glaciation, set the scene for the proliferation of algae through the grinding work of glaciers, which moved mountains into the oceans, pouring into the water a feast of mineral nutrients – notably phosphorus – that was too much for bacteria to devour. On the leftovers, in a way, did algae thrive.

"The Sturtian glaciation – the longest in Earth’s history, lasting about 60 million years between 717 million and and 660 millions years ago – is named after geological evidence unearthed from Sturt River Gorge in South Australia. This new evidence comes from Australia’s Northern Territory. The scientists analysed the ancient sedimentary rocks for molecular evidence of steroid alcohols, or sterols, which only eukaryotes can produce. They discovered a marked Increased in steroid diversity and abundance that pointed to a rapid rise of marine planktonic algae, known as archaeplastida.

“'In these rocks we discovered striking signals of molecular fossils,” says Jarrett, “We immediately knew we had made a ground-breaking discovery that snowball Earth was directly involved in the evolution of large and complex life.”

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"Algae did not, however, conquer the oceans immediately following the end of the Sturtian. “In the post-snowball greenhouse world, tropical sea surface temperatures may have reached 50 to 60 °C, which is above the growth optimum even of extremely thermophilic algae, but well within the range of cyanobacterial picoplankton. Thus, we propose that cyanobacteria persisted in the tropics as dominant primary producers, and algae were only able to radiate once temperatures dropped after several million years.”

"Drawing on the modelling of modern planktonic ecosystems, the authors suggest the immediate response to rising phosphate probably saw cyanobacterial picoplankton densities rise to levels sufficient to sustain bacterivorous grazers, which then capped cyanobacterial cell numbers so that accessory nutrients became available for the eventual expansion of the larger algae.

"The radiation of algae, in turn, would have created a “more efficient biological pump” to drive up organic carbon and nutrients in the ocean: “We posit that permanent burial of hydrocarbon-rich algal biomass caused a step-increase in the sedimentary ratio of organic carbon to phosphorous, a key parameter ultimately controlling the net release of oxygen to the atmosphere. Thus, higher nutrient levels, a more efficient biological pump and the emergence of degradation-resistant algal biopolymers would have caused permanently increased rates of carbon burial and oxygen release to the atmosphere.”

"Says Brocks: “These large and nutritious organisms at the base of the food web provided the burst of energy required for the evolution of complex ecosystems, where increasingly large and complex animals, including humans, could thrive on Earth.'”

Comment: Obviously massive photosynthesis conversion of the Earth' atmosphere to a large porton of oxygen was crucial to the development of advanced forms of life.