23 October 2014

In a story reminiscent of a tomb raider’s discovery of ancient mummified remains in the Egyptian desert, new research supported by the ARC has unveiled the preservation of intact biological molecules and their fossilised counterparts, which date back to an ancient mass extinction event brought on by the warming of the Earth’s oceans.

Researchers have unearthed these intact biomarkers from a 380-million-year-old fossil, as part of a new molecular and stable isotopic based approach which can reconstruct the Earth’s ancient environments in unprecedented detail.

The Devonian Canning Basin in the Western Kimberly is well recognised as being a source of petroleum hydrocarbons and also a record of a time when mass extinction events led to the loss of many marine species, and the rise of vascular plants led to a decline in atmospheric CO2 levels.

But until now, it was assumed unfeasible for complex biomolecules and geomolecules to survive intact through hundreds of millions of years’ entombment in rock. A new discovery of intact steroids including cholesterol from an ancient fossil crustacean from the Western Kimberley’s has opened the remarkable possibility that much more detailed information on past environments may be locked away in these rocks.

“It was actually an unremarkable fossil and we couldn’t identify it from its form alone,” said Professor Kliti Grice, Director of the Western Australia Isotope and Organic Chemistry Centre at Curtin University, and an ARC Discovery Outstanding Researcher Award recipient.

“But through the chemistry we have been able to prove it was a crustacea—as crustaceans are characteristically high in cholesterol and modify sterols from their algal diet to cholesterol. It was probably like a giant prawn.”

The researchers analysed the sample by several sophisticated wet chemical approaches and mass spectrometry tools purchased with the help of several ARC Linkage Infrastructure, Equipment and Facilities (LIEF) grants. Using this method, which analyses the 13C/12C, D/H and 15N/14N ratios of individual compounds in complex mixtures, Professor Grice’s team captured a remarkably complete picture of microbially preserved steroids and other molecules.

“We have a complete diagenetic continuum, like a moving window, from the moment the molecules sank through the water column with the organism and hit the sediment below, to the formation of the concretion around it. This means we have for the first time captured the very early stages of preservation ‘eogenesis’ occurring within the water column,” Professor Grice said.

The occurrence of biomolecules and geomolecules within a fossilised specimen was previously assumed to be unfeasible and its success is attributed to microbial driven processes which acted on the rock in-situ rather than thermal processes associated with deep burial of the limestone.

By looking at the natural abundance of 13C/12C of cholesterol and then comparing it to other steroids, researchers in Professor Grice’s team were able to recognise the diet of the organism by its isotopic signal.

“The key is the exceptional preservation conditions, fossils were preserved in water starved of oxygen; anoxic conditions. There were toxic levels of hydrogen sulfide (H2S) or rotten egg gas in the upper part of the water column within the light zone," Professor Grice said.

“This gas is likely to have killed the crustacean, but also helped to preserve its biomolecules as it sank through the water column.

“These conditions are evident from traces of organic molecules derived from strict anaerobes called Green Sulfur Bacteria (Chlorobi) that carry out photosynthesis using H2S (rather than H2O) to fix CO2 in the presence of light (referred to as Photic Zone Euxinia).

“As the organisms responsible for producing H2S in the absence of oxygen continued to degrade the crustacean’s carcass at the sediment/water interface, the alkalinity dropped and these organisms built a carbonate concretion around the crustacean, which has since trapped these fragile molecules inside without significant degradation.”

Professor Grice and her team also found evidence that the oceans became toxic as a response to warming conditions in the past. It has been suggested that this poisoning of the oceans was partly responsible for several mass extinctions, including the largest of them all, around the Permian Triassic boundary (the ‘great dying’ period occurring 250 million years ago), where as much as 95% of marine and 75% of terrestrial species on Earth became extinct.

Similar conditions have now been found for the end-Devonian and end-Triassic mass extinction events. In particular the end-Triassic event shows very similar conditions to the Permian Triassic event some 50 million years later when the supercontinent Pangea began to break apart leading to Central Atlantic Magmatic Province, flood basalts, rising CO2 levels and sea levels.

“Following the formation of the supercontinent of Pangaea, 252 million years ago, there seems to have been a cascade of events. Huge volcanism in Siberia associated with flood basalts led to rising CO2 and methane levels, which caused a global warming event.

“With the ice caps having melted there was little global temperature variance to drive the circulation of the oceans, so they stagnated and became vast anoxic zones like the inland Black Sea is today, where Green Sulfur Bacteria also thrive. These Chlorobi also use H2S gas which is a deadly poison to humans and animals like this crustacean,” Professor Grice said.

By shining a light on the molecular and stable isotopic remains of micro-organisms and other creatures that flourished in these ancient environments, and on the fossilisation process itself, Professor Grice’s team is helping us to understand organismic evolution in deep time.

Professor Grice led a team of national and international researchers resulting in many high impact journal articles (Science, Nature Scientific Reports, Gondwana Research, Earth and Planetary Science Letters, Geochimica et Cosmochimica Acta) co-authored  with ECRs and PhD scholars: including Birgit Nabbefeld, Ines Melendez, Caroline Jaraula, Ken Williford, Lindsay Hays, L. Felipe Opazo Ercin Maslen, Svenja Tulipani and collaborators including Professors Schwark, Summons, Twitchett, Boettcher, Schimmelmann and Cao and Associate Professor Trinajstic.

Samples were collected via field trips either supported by ARC funding to Professor Grice together with federal and state organisations and several petroleum industry partners also involving Professor Grice's team.


Image: The Black Sea.
Image credit: Professor Kliti Grice.