|Decaying modern carcasses can provide evidence as to how fossils might preserve delicate organic molecules.|
Thursday, 25 July 2013
Nature's Chemical Laboratory
The brilliant 18th Century chemist, Antoine Lavoisier (1771-1794), is quoted as saying, 'I consider nature a vast chemical laboratory in which all kinds of composition and decompositions are formed.' It is often folly to paraphrase the accepted genius of Lavoisier, but the work that we undertake at particle accelerators (such as Diamond and SSRL) has enabled us to extend this chemical insight to life through deep time. It might now be fair to say, 'We consider nature, both past and present, a vast chemical laboratory in which all kinds of composition and decompositions are formed, which are occasionally preserved as fossils'.
The study of decaying bodies has provided useful data on the various stages of decomposition that might give clues to the time and type of burial environment. It is remarkable that as soon as a heart stops beating, oxygen that was so critical to life process ceased to circulate and internal anoxic conditions rapidly develop. In the absence of oxygen cells began their auto-destruct sequence, this is called autolysis. Here the cells self-destruct courtesy of enzymatic digestion. While the tissues of an organism are still relatively soft, prior to rigor mortis setting in, blow and flesh flies all take there chance to colonise a corpse with their eggs and soon to be voracious larvae (who we know and love as maggots).
If there are no scavengers around to chomp through our dead body, fly maggots will soon compensate for this. Within 24 hours flesh can often be moving again, not in a zombie-like state, but with the writhing bodies of maggots, feasting on rotting flesh. Maybe I should have suggested at the beginning of this blog that you not read this close to meal times? Whilst the maggots are munching from within a carcass, soil microbes start the process of multiplying on the new food source beginning to ooze from the various orifices of a body into the underlying soil. Microbial communities that were once symbiotic with an animal in life, living in the respiratory system, gut and intestine, also begin to multiply and consume their host. This is the ultimate in organo-recycling systems. The organic acids and gases (including methane and hydrogen sulphide) generated as by-products of the microbes metabolism soon begin to change carcass colour and start to bloat the body, accompanied by the distinctive rotting smell...as my anatomist friend Dr. Dino Frey might say, 'It has all gone soft and soupy'.....we all have days like that!
The combined efforts of insects, microbes and possibly scavengers would make short work of our deceased beastie. Most of the body fluids would soon be 'released' into the surrounding soil, causing an initial dye-off of vegetation (due to nitrogen toxicity), leaving a deathly halo around the body. The maggots would pupate soon after having eaten their fill. All is quiet on the recycling front to the causal observer, but not to the geochemist, as things have only just started to get interesting. Depending on the soil porosity and permeability beneath a body, the sediments would also be directly affected by the gentle ebb of decay juices from the body...yum! As the soil microbes get a free dinner, they also begin to process the vast influx of nutrients from the carcass. Metabolic by-products of the microbes also begin to alter pore-water chemistry, giving rise to the precipitation of early mineral cements to bind the soil particles. The stink of decay marks the availability of reactive chemistry. The once living animal is now entering the immortal realm of fossilisation. The bodies of all living organisms are a wonderful store for elements that once released (or partly released) from their biological bonds can complex to form different species of mineral. Here is the paradox. Release too many of the organic building blocks to the inorganic processes and you reduce the amount of organism that remains to be fossilised. This is a bizarre 'two-horse' race between decay and fossilisation...one that we rely on being a close draw.... especially if we are to search any fossil remains for a whiff of original biochemistry that might still lurk within the mineralogical straight-jacket. This is precisely why we need to use some of the most sensitive imaging technology in the world to tease-out this astoundingly dilute organic signal, through working at synchrotrons light sources such as Diamond and SSRL. Splendid fun!