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).
See also...
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Seeking Earth’s earliest fossils. In the nineteenth century the origin of life seemed an intractable
problem for palaeontologists, with large complex animal fossils appearing in
the Cambrian explosion, but scientists having access to neither examples of
earlier fossils nor the means with which to examine them...
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