What if we tested hypothesis'? - History Meta Stack …
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When the total continental land mass was small or combined into a supercontinent, there was no land to divert that diffusion of warm water toward the poles, which results in currents. During those times, the global ocean became one big, calm lake, with no currents of significance. Those oceans are called today, and they would have been anoxic; the oxygenated surface waters would not have been drawn by currents to the ocean floor, and the oceans were certainly anoxic before the GOE. The interplay of those can be incredibly complex and lead to the multitude of hypotheses posited to explain those ancient events, but a leading hypothesis today is that a combination of factors, including supercontinents, variations in volcanic output, Canfield Oceans, and ice ages prevented life from gaining ecosystem dominance until the waning of the second Snowball Earth event, which was the greatest series of glaciations that Earth has yet experienced. It is known today as the , which ended about 635 mya. The study of the Cryogenian Period, which is the subject of , resulted in the term “.”
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Kirschvink noted that reappeared in the geological record during the possible Snowball Earth times, after vanishing about a billion years earlier. Kirschvink noted that iron cannot increase to levels where they would create BIFs if the global ocean was oxygenated. Kirschvink proposed that the sea ice not only killed the photosynthesizers, but it also separated the ocean from the atmosphere so that the global ocean became anoxic. Iron from volcanoes on the ocean floor would build up in solution during the , and during the greenhouse phase the oceans would become oxygenated and the iron would fall out in BIFs. Other geological evidence for the vacillating icehouse and greenhouse conditions was the formation of cap carbonates over the glacial till. It was a global phenomenon; wherever the Snowball Earth till was, cap carbonates were atop them. In geological circles, deposited during the past 100 million years are considered to be of tropical origin, so scientists think that the cap carbonates reflected a tropical environment. The fact of cap carbonates atop glacial till is one of the strongest pieces of evidence for the Snowball Earth hypothesis. Kirschvink finished his paper by noting that the eon of complex life came on the heels of the Snowball Earth, and scouring the oceans of life would have presented virgin oceans for the rapid spread of life in the greenhouse periods, and this could have initiated the evolutionary novelty that led to complex life.