Final Report Summary - PALEONANOLIFE (Responses of precambrian life to environmental changes)
The Precambrian spans from the formation of Earth about 4.6 billion years ago to the beginning of the Phanerozoic era, about 550 million years ago. During this period soft-shelled creatures and unicellular plankton were the dominant forms of life.
Precambrian fossil record is extremely sparse because soft fossils are not preserved in marine sediments and because rocks have been destroyed by erosion, metamorphism or are deeply buried under the present day Earth surface. For all these reasons, the evolution of life and the chemical changes of the Precambrian atmosphere and oceans are poorly known.
Contrary to the Phanerozoic era, siliceous sediments – referred to as “cherts” - were deposited at the bottom of oceans during the Precambrian. Because of their chemical nature – pure silica – these rocks seal the organic matter constituting the planktonic cells.
However, the pristine organic structures of living cells were partially destroyed because they undergone high temperature (up to 150°C) during diagenesis (the silicification in sedimentary basins) or during metamorphic episodes (up to 350°C) taking place in tectonic events. During the elevation in temperature, the fragile and soluble organic constituents are destroyed and eliminated, yielding insoluble carbonaceous residues that mimic the cell morphology.
The oldest and well preserved cherts – i.e. having been not subject to temperature higher than 200°C – containing putative fossilized cells, are 3.5 billion years old.
The aim of this ERC project was to isolate by chemical methods these carbonaceous fossils in order to reconstruct the metabolisms of the most ancient life on which we still have a geological access. Such diagnostics are possible through the determination of the structural, chemical and isotopic compositions of the insoluble organic matter left over in rocks.
During the course of this proposal we study - via laboratory simulations - the natural degradation processes of organic compounds as a function of their mineral matrices (silica of carbonates). It was shown embedded cells in cherts are preserved mainly because of a bio-encrustation mechanism that prevents the direct contact between cell walls and the dissolved silica circulating in diagenetic fluids. Micrometric spatial resolution techniques – using the light emitted by particle accelerator – show that the remaining carbonaceous structures still contain nitrogen bonds fully consistent with those constituting present day organisms. This observation confirms that the 5 micrometers carbonaceous globules observed in cherts were indeed living cells.
Improving laboratory techniques to isolate fossil cells from their silica matrix, we found an unexpected large diversity of microstructures that can be distinguished by their morphology. This suggests that several species shared the same environment and thus, that life was flourishing and formed real ecosystems.
It is admitted that around 3.0 billion years ago, the atmosphere was oxygen free. The emergence of oxygen in the atmosphere around 2.5 Gyr ago likely resulted in a complete oxidation of sediments exposed in seawater or at the surface of emerged continents. The determination of the isotopic composition of nitrogen at the scale of individual cells, reveals that – 3.0 billion years ago - some species were producing oxygen while others were using this water dissolved oxygen to breath and destroyed ammonia. This confirms the antiquity of photosynthesis along with the model of a slow process of accumulation of oxygen in seawater during the period lying between 3.5 and 2.5 billion years.
The oxygen isotopic composition of the organic matter shows a slow decrease of the seawater temperatures since 3.5 billon years (from about 50°C to 10°C). Therefore, the Earth atmosphere was much warmer in the past than today. It turns out that the sedimentary organic matters have trapped in large amounts a gaseous component of the atmosphere (Xenon). The isotopic composition of this Xenon allows a precise determination of the age at which this organic matter was isolated from the atmosphere. This age can be regarded as the age of the organic matter and coincides remarkably well with the age of chert deposition.
Precambrian fossil record is extremely sparse because soft fossils are not preserved in marine sediments and because rocks have been destroyed by erosion, metamorphism or are deeply buried under the present day Earth surface. For all these reasons, the evolution of life and the chemical changes of the Precambrian atmosphere and oceans are poorly known.
Contrary to the Phanerozoic era, siliceous sediments – referred to as “cherts” - were deposited at the bottom of oceans during the Precambrian. Because of their chemical nature – pure silica – these rocks seal the organic matter constituting the planktonic cells.
However, the pristine organic structures of living cells were partially destroyed because they undergone high temperature (up to 150°C) during diagenesis (the silicification in sedimentary basins) or during metamorphic episodes (up to 350°C) taking place in tectonic events. During the elevation in temperature, the fragile and soluble organic constituents are destroyed and eliminated, yielding insoluble carbonaceous residues that mimic the cell morphology.
The oldest and well preserved cherts – i.e. having been not subject to temperature higher than 200°C – containing putative fossilized cells, are 3.5 billion years old.
The aim of this ERC project was to isolate by chemical methods these carbonaceous fossils in order to reconstruct the metabolisms of the most ancient life on which we still have a geological access. Such diagnostics are possible through the determination of the structural, chemical and isotopic compositions of the insoluble organic matter left over in rocks.
During the course of this proposal we study - via laboratory simulations - the natural degradation processes of organic compounds as a function of their mineral matrices (silica of carbonates). It was shown embedded cells in cherts are preserved mainly because of a bio-encrustation mechanism that prevents the direct contact between cell walls and the dissolved silica circulating in diagenetic fluids. Micrometric spatial resolution techniques – using the light emitted by particle accelerator – show that the remaining carbonaceous structures still contain nitrogen bonds fully consistent with those constituting present day organisms. This observation confirms that the 5 micrometers carbonaceous globules observed in cherts were indeed living cells.
Improving laboratory techniques to isolate fossil cells from their silica matrix, we found an unexpected large diversity of microstructures that can be distinguished by their morphology. This suggests that several species shared the same environment and thus, that life was flourishing and formed real ecosystems.
It is admitted that around 3.0 billion years ago, the atmosphere was oxygen free. The emergence of oxygen in the atmosphere around 2.5 Gyr ago likely resulted in a complete oxidation of sediments exposed in seawater or at the surface of emerged continents. The determination of the isotopic composition of nitrogen at the scale of individual cells, reveals that – 3.0 billion years ago - some species were producing oxygen while others were using this water dissolved oxygen to breath and destroyed ammonia. This confirms the antiquity of photosynthesis along with the model of a slow process of accumulation of oxygen in seawater during the period lying between 3.5 and 2.5 billion years.
The oxygen isotopic composition of the organic matter shows a slow decrease of the seawater temperatures since 3.5 billon years (from about 50°C to 10°C). Therefore, the Earth atmosphere was much warmer in the past than today. It turns out that the sedimentary organic matters have trapped in large amounts a gaseous component of the atmosphere (Xenon). The isotopic composition of this Xenon allows a precise determination of the age at which this organic matter was isolated from the atmosphere. This age can be regarded as the age of the organic matter and coincides remarkably well with the age of chert deposition.