The End Permian Extinction is the most severe extinction event
recorded in the fossil record, with the loss of around 96% of all known
species, and many dominant Palaeozoic groups of organisms. The event led to the
effective resetting of the Earth’s biosphere, enabling the emergence of a
radically different Mesozoic Biota. The event is thought to have been caused by
massive volcanism in the Siberian Traps, which not only produced huge emissions
of lava and volcanic gasses at the surface, but ignited vast areas of buried
Palaeozoic coals and hydrocarbons, leading to massive emissions of acidic and
halogenic gas, which in turn resulted in bouts of severe acid rain and the
breakdown of the ozone layer, allowing harmful ultraviolet light to reach the
Earth’s surface.
A widespread increase in the mutation rate seen in Lycopsid
microspores at the end of the Permian has long been seen as evidence of the
mutagenic influence of ultraviolet radiation on End Permian ecosystems, but
widespread soil acidification is harder to detect. Such an event would be
expected to wash acid-soluble metallic plant nutrients such as aluminium,
calcium and magnesium out of soils, as well as cause a rapid increase in
erosion (and marine sedimentation) rates, due to a loss of soil cohesion;
however while both of these have been recorded, they could both also be caused
by a variety of other phenomena, making them inconclusive evidence.
In a paper published in the journal Geology on 7 January 2015, Mark Sephton
and Dan Jiao of the Department of Earth Science and Engineering at Imperial College London, Michael Engel of the School of Geology and Geophysics at The University of Oklahoma, Cindy Looy of the Department of Integrative Biology and
Museum of Paleontology at the University of California–Berkeley and Henk Visscher
of the Laboratory of Palaeobotany and Palynology at the Department of EarthSciences at Utrecht University, describe the results of an investigation into
the breakdown of lignin at the Vigo Meano Section in northern Italy.
The Vigo Meano Section is thought to provide the most detailed record
of the molecular composition of solvent-extractable organic matter from across
the Permian/Triassic boundary. The section comprises organic-rich marls
(calcium-rich clays, likely to have been formed in an inshore marine
environment with high ground runoff) that are not thought to have been
influenced by any subsequent metamorphic heating. This section has been used in
several prior studies of geochemistry across the Permian/Triassic boundary, and
has extremely well constrained dates.
Lignin (the major component of plant fibres) is largely broken down
by enzymes excreted into the environment by Fungi and Bacteria. This breakdown
process results in lignin breaking down to vanillin (4-hydroxy-3-methoxybenzaldehyde),
which then breaks down to vanillic acid (4-hydroxy-3-methoxybenzoic acid),
which is further broken down into protocatechuic acid (3,4-dihydroxybenzoic
acid), which can be broken down further into a variety of products by a range
of soil microbes. Because vanillin is widely used as flavouring in the food
industry (vanilla), this breakdown process has been extensively studied in
investigations into food spoiling and is very well understood.
Soil bacteria are known to be very sensitive to fluctuations in pH
(in chemistry the pH is a reflection of the acidity or alkalinity of a
substance, with neutral substances such as pure water having a pH of 7 and more
acid substances having lower pHs), with even small changes in acidity leading
to major differences in species composition, and much poorer and less diverse
bacterial flora found in acid soils. Fungi are far more tolerant, with many
common soil species able to survive large fluctuations in soil pH. However the
enzymes used to break down vanillin are at their most effective when the soil
pH is about 8, and cease activity at about pH 4, with the effect that few soil
fungi can survive long at pHs below about 4.5.
Sephtonet al. reasoned
that since this is the case, it should be possible to detect any sudden and
dramatic increase in soil acidity acid during the end-Permian biotic crisis due
to an increase in vanillin and vanillic acid in sediments at this time.
Moreover since both compounds are readily biodegradable even under anaerobic
conditions, they should not persist for long in the marine environment, making
for a close relationship between spikes in soil acidity and sediment
composition.
Results mass spectrography analysis for vanillin and vanillic acid
across the Permian/Triassic boundary at Vigo Meano show a number of peaks in
presence of the two chemicals, including a sustained peak across the major extinction episodes and Permian/Triassic boundary, which suggests the occurrence of pulses of soil
acidification so severe that an almost complete cessation of biodegradation
must have occurred not just within the soil, but during the transportation and
sedimentation process, strongly supporting the idea that the soils were exposed
to bouts of rainfall with pHs as low as 4 and possibly lower than 2 (strong
enough to cause acid burns to exposed skin) during the end Permian biotic
crisis.
Ratios of vanillic acid to vanillin [acid to aldehyde
ratio, (Ad/Al)v] in latest Permian and earliest Triassic organic matter
assemblagesfrom the VigoMeano section (southern Alps, Italy), providing proxy
evidence for pulses of soil acidification (pH < 4 ).VG, Val Gardena Formation;
PTB, approximate positionof the Permian-Triassic boundary; extinctions, interval
of principal marine extinction and floral turnover in southern Alps;
d13C, position ofend-Permian negative carbon-isotope shift in southern Alps.
Letters at bottom of stratigraphic column, from left to right, correspond to
clay, silt, fine sand, medium sand, and coarse sand, respectively. Sephton et al. (2015).
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