It came from the land
I have just spent the first half of my morning avoiding the rain in session 22c travelling back in time to the Precambrian. This is a pretty weird time period for me, being so drastically different to modern-day Earth, but the questions asked about it are so big and far-reaching, I can’t help but be drawn in.
The highlight of this morning for me was Stefan Lalonde’s keynote talk, attempting to reconcile evidence for the presence of free oxygen as early as 3 billion years ago with evidence simultaneously suggesting that the atmosphere was anoxic. He turns to microbial ecosystems like stromatolite microbial mats for an answer. These are things I know nothing about so I was fascinated to hear that 1 cm of microbial mat can produce similar amounts of oxygen to 100 m of pelagic water column – in his own words, a veritable oxygen oasis.
Apparently there are loads of oxygen oases like these in modern-day Earth, such as biological soil crusts and microbial mats in estuaries and riverbeds, which produce free oxygen. If we go back 3 billion years, together they would have created enough free oxygen to mobilise redox-sensitive trace metals and sulphate from land into the oceans (which would produce the evidence for oxidative weathering that we see in the geological record), but because the percentage of the Earth covered by colonisable land area was so small, this could happen without affecting global atmospheric oxygen. As continents developed between 3 and 2.5 billion years ago, cyanobacteria would have colonised more land and benthic oxidative weathering would have become more significant leading up to the Great Oxidation Event – neat.
I’m looking forward to the Bob Berner memorial session this afternoon, where there’ll be some more great talks on global weathering and oxygen evolution – check it out. I’m off to learn about neo-plumbotectonics in today’s plenary session now – see you all there!