Modern Earth has an extensive biosphere, which is both dependent on and helps to maintain an oxygen rich atmosphere. However when this atmosphere developed remains a difficult question to answer. The appearance of a diverse animal fauna in at the beginning of the Cambrian strongly implies an oxygen-rich atmosphere had evolved by this time, but it is thought likely that for much of the Earth's history prior to this time the planet had an atmosphere depleted in oxygen, as most of the rocks laid down in this time appear to have formed in a reducing (oxygen-poor) atmosphere, and the earliest life-forms are thought to have been anaerobic, Bacteria-like organisms for which oxygen would have been highly toxic. However higher oxygen levels are thought to have developed at least twice during this time, the Great Oxidation Event, which occured about 2.3 billion years ago and which saw the first appearance of oxygen in the atmosphere, and the Neoproterozoic Oxidation Event, towards the and of the Precambrian that gave rise to the oxygen-rich atmosphere that enabled the Cambrian Explosion. Directly measuring ancient atmospheres is difficult however; the most reliable methods are considered to be those which utilize biogenic limestone minerals (Shells, Coral etc.), but these are not available for Precambrian rocks, so most estimates of oxygen content in this ancient atmosphere rely on using metal compounds as proxies for the presence of oxygen in the air.
In a paper published in the journal Geology on 8 July 2016, Nigel Blamey of the Department of Earth Sciences at Brock University, the Department of Earth and Environmental Science at the New Mexico Institute of Mining and Technology, and the Department of Geology and Petroleum Geology at the University of Aberdeen, Uwe Brand, also of the Department of Earth Sciences at Brock University, John Parnell, also of the Department of Geology and Petroleum Geology at the University of Aberdeen, Natalie Spear of the Department of Earth and Environmental Science at the University of Pennsylvania, Christophe Lécuyer of the Laboratoire de Géologiede Lyon at the University of Lyon and Institut Universitaire de France, Kathleen Benison of the Department of Geology and Geography at the University West Virginia, Fanwei Meng of the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences and Pei Ni of the School of Earth Sciences and Engineering at Nanjing University, discuss a new method for sampling ancient atmospheric samples trapped as bubbles inside halite (rock salt) crystals.
Halite typically forms as an evaporite mineral; that is to say as salt crystals formed when bodies of seawater become trapped in shallow basins and evaporate. Bubbles trapped in crystals of halite can therefore potentially come from the ancient atmosphere, from the sediment beneath the ancient pool in which the halite was forming (marsh gas or similar) or from some subsequent diagenetic process (i.e. gas that has formed during subsequent alteration of the minerals after they were initially formed.
When halite crystals first begin to form on bodies of trapped salt water they initially form a crust on the surface. Once this crust becomes sufficiently thick and dense it then sinks to the bottom of the pool, where it continues to accumulate more salt. The crystals that form in the initial, floating, phase of formation have a distinctive 'chevron and cornet' texture, which enables them to be distinguished from later crystals. Blamey et al. reason that any gas bubbles trapped in these 'chevron and cornet' crystals within an evaporite deposit are therefore likely to have come from atmospheric rather than sediment gas, and furthermore diagenetic gases can also be ruled out, as diagenetic alteration would be expected to destroy the distinctive 'chevron and cornet' texture of the crystals.
This being the case, Blamey et al. then set about finding suitable examples of gas bubbles in ancient and modern halite crystals. These crystals were then cleaned with isopropanol to remove any surface contaminants, then dried under air and placed under a vacuum overnight. The crystals were then crushed incrementally in a mass spectrometer to produce small bursts of gas which could be analysed for their nitrogen, oxygen and argon contents.
Firstly modern halite crystals were chosen, from ponds at the Mosaic Salt Mine near Carlsbad in New Mexico and from Lake Polaris at Southern Cross in Western Australia. When subjected to the technique these produced results typical of a modern atmosphere (expand), with the exception of a single crystal from the Mosaic Salt Mine, which was depleted in oxygen by 9.2%.
Secondly Blamey et al. selected halite crystals from the Late Miocene Racalmuto Salt Mine of Sicily. These also produced gas with a ratio matching the modern atmosphere, which is in line with predictions; the composition of the atmosphere is not thought to have changed significantly in the six million years since the Late Miocene.
Next Blamey et al. examined halite crystals from the Middle Cretaceous Mengyejing Formation of Tibet. Unlike the Miocene atmosphere, the composition of the Cretaceous atmosphere is still hotly debated, with some researchers n the field believing that oxygen levels were much higher at that time, helping to support the extensive megafauna of the period. These samples yielded gas with an oxygen lever of 25.8% (compared to about 20% today), and since there is no obvious method by which additional oxygen is likely to have become incorporated into these crystals, Blamey et al. accept this as evidence for a higher oxygen level in the Cretaceous.
Finally Blamey et al examined halite Crystals from the Empress 1A and Lancer 1 drill cores from the Officer Basin of southwestern Australia. These cores contain halites from the Tonian (the first geological period in the Neoproterozoic era, roughly 1000 to 720 million years ago) Browne Formation, known to be between 800 and 830 million years old. These samples yield oxygen levels at a far lower level than today, yet far higher than expected, with one sample from the Empress 1A core yielding an oxygen level of 1.64%, with all the other samples cluster around 10.5%.
Thin section view of halite sample 1502 from the Empress 1A core (Officer Basin, Australia). Undisturbed chevron bands with gas/fluid inclusions are clearly visible in photograph. Upward crystal growth is in direction of chevron tip (upper left corner), and geochemical concentrations (e.g., Br) are in parts per million. Blamey et al. (2016).
There are currently several different models of the Neoproterozoic Oxidation Event being put forward by different groups of researchers. However all of these essentially predict a fairly rapid rise in oxygen levels from less than 5% of the atmosphere to something similar to modern levels, sometime between the Late Tonian, about 800 million years ago, and the beginning of thew Cambrian, about 540 million years ago. The discovery of intermediate level atmospheric samples earlier in the Tonian is outside all of these models, and possibly suggests a more gradual increase in atmospheric oxygen during the Late Neoproterozoic than has previously been thought.
Atmosphere and ocean oxygenation trends during Neoproterozoic. Dashed line is proposed atmospheric oxygen trend by Canfield (2005), red unidirectional curve represents traditional viewpoint (e.g., Kump, 2008), and blue curve represents emerging model presented by Lyons et al. (2014). Green field shows Blamey et al.'s atmospheric oxygen measurements for Tonian time during Neoproterozoic. C., Cambrian; O., Ordovician. Blamey et al. (2016).
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