IDENTIFICATION OF CAROTENOID BREAKDOWN PRODUCTS IN THE 1.64 GA BARNEY CREEK FORMATION, McArthur BASIN, NORTHERN AUSTRALIA
Microbial communities are often highly pigmented due to organic compounds known as carotenoids. In order to study the occurrence of carotenoids in the geological record as molecular fossils, the effect of burial and heat on their molecular structure must first be determined. Burial diagenesis reduces organic molecules to their hydrocarbon equivalents. However, compounds that undergo diagenesis often retain structural information about their biological precursors.
Carotenoids are often difficult to study in the geological record because of their low preservation potential. In rare instances where intact C40 carotenoid hydrocarbons are preserved by suitable diagenetic conditions, they are often broken down into smaller fragments during catagenesis. This can be simulated in the laboratory using a method called hydrous pyrolysis.
The 1.64 billion-year-old Barney Creek Formation (BCF) in the McArthur Basin, northern Australia contains some of the world’s oldest and indisputably syngenetic biomarkers. The upper shallow sections of Glyde River 7 (GR-7) drill core contains intact carotenoid hydrocarbons. However, going deeper down the drill core, the signal for these are not detected.
The aim of this study was to determine whether the disappearance of the carotene signal was caused by a shift in basin ecology or a result of thermal degradation. This involved synthesising and pyrolysing β-carotane in a sealed quartz vessel under a cold-seal pressure regime. Ideal conditions that generated systematic cleavage products of β-carotane were heating for 1 day at 360oC under hydrous conditions.
To validate the standard that we synthesised, we looked for the occurrence of these β-carotane breakdown products in the upper sections of GR-7 where intact β-carotane was reported. Our pyrolysis standard demonstrated that a full series of β-carotane breakdown products are also present. This confirms that our pyrolysis experiments adequately simulate the breakdown of carotenoids under geological conditions.
To apply the standard in older and more mature samples from the BCF, we searched in samples where intact β-carotane was not present. We were able to detect a partial series of carotane breakdown products representing the oldest evidence for saturated carotenoids in the geological record.