Wednesday, July 13, 2016

Neoproterozoic Atmospheric Oxygen

Captured from air bubbles trapped in 815 million year old Halite crystals:

We present a new and innovative way of determining the oxygen level of Earth's past atmosphere by directly measuring inclusion gases trapped in halite. After intensive screening using multiple depositional, textural/fabric, and geochemical parameters, we determined that tectonically undisturbed cumulate, chevron, and cornet halite inclusions may retain atmospheric gas during crystallization from shallow saline, lagoonal, and/or saltpan brine. These are the first measurements of inclusion gas for the Neoproterozoic obtained from 815 ± 15–m.y.–old Browne Formation chevron halite of the Officer Basin, southwest Australia. The 31 gas measurements afford us a direct glimpse of the composition of the mid- to late Neoproterozoic atmosphere and register an average oxygen content of 10.9%. The measured pO2 puts oxygenation of Earth's paleoatmosphere ∼100–200 m.y. ahead of current models and proxy studies. It also puts oxygenation of the Neoproterozoic atmosphere in agreement with time of diversification of eukaryotes and in advance of the emergence of marine animal life

Oxygen is tied to the evolution of complex life on earth. Early earth contained little free oxygen in the atmosphere and the oceans. Life was made up of two divisions of prokaryote cell types, the Archaea and the Bacteria. The activities of one type of aerobic bacteria, Cyanobacterial photosynthesis, eventually triggered the Great Oxygenation Event by 2.5 to 2 billion years ago and began raising the levels of free oxygen in the atmosphere and the oceans. Cyanobacteria are cells capable of burning fuel in this free oxygen.  Between 2 billion and 1.5 billion years ago this type of oxygen utilizing cell merged with an Archaea cell to form a large complex cell type known as the eukaryote cell. The oxygen utilizing cell evolved into the mitochondria. The other partner evolved into the larger host which contains our genome and undertakes other physiological functions.  All complex multicellular life forms are descendants of this symbiotic cell type.

Another boost to oxygen levels was needed to ratchet up the evolution of more diverse and large life forms. That happened by the Neoproterozoic beginning about 1000 million  years ago. This study pins down the oxygen content of the atmosphere to about 10%-11% by 815 million years ago, about 200 million years before fossil evidence of multicellular animals (Ediacaran biota) first appears.

Still, there may not be a simple and direct causal link between enhanced oxygen levels and the evolution of multicellular animals.  Two good reviews, on the Neoproterozoic by Nicholas Butterfield and on the Cambrian Explosion by Derek Briggs make a case that the transformation of the biosphere from the Cryogenic to the Cambrian, from simpler eukaryote to the diversification of multicellular marine animal life, was engineered partly by life itself. For example, pumping of sea water due to filter feeding action of sponges would have dispersed oxygen more efficiently through the water column, ventilating the marine shelf environment where physiologically demanding predator prey evolutionary arms races began to be played out. The evolution of grazing invertebrates destroyed the microbial mats that covered large areas of the shallow sea floor. This gave access to the sediment column to the burrowing activity of the earliest bilaterans which would have likewise allowed oxygen to reach deeper into the sediment pile, thus opening up new ecosystems where an infaunal biota evolved. The activities of animals thus created new ecological opportunities for other creatures.

This finding though aligns well with the thinking that threshold levels of oxygen required for multicellular animal life were already present in the atmosphere and the oceans. With the right climatic (ending of the Cryogenic Period and the warming of the earth) and tectonic (breakup of Rodinia resulting in formation of wide shallow shelf areas) triggers, evolutionary opportunism took over.

Open Access

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