CAROTENOIDSAS ENVIRONMENTAL BIOMARKERS IN NEOPROTEROZOIC SEDIMENTS AND OILS

The C₁₂-C₂₁aryl isoprenoids, degradation products derived from the aromatic C₄₀carotenoid precursors of green and purple sulfur bacteria, have been recognized and utilized as biomarkers for photic zone euxinia (PZE) in source rock depositional environments. However, their distributions can be very complex and suffer from multiple interferences that limit their wide use as interpretive tools. More recent work has shown that simpler and more consistent patterns for the intact saturated and aromatic C₄₀carotenoids can be readily acquired by gas chromatography-mass spectrometry using a triple quadrupole mass analyser (GC-QQQ-MS). These compounds are even preserved in mature samples and show trends that are diagnostic for environmental conditions in a wide range of sedimentary environments (French et al., Geobiology, 13, 139-151, 2015) In this study we analysed a suite of Neoproterozoic samples in order to search for features that might be diagnostic for marine or non-marine settings of that period.

C₄₀carotenoid derivatives are widely distributed in crude oil and sediment samples across geological time and in all types of paleoenvironmental settings. A primary control on their absolute abundances was found to be thermal maturity with progressive loss of the aromatic carotenoids at vitrinite reflectance equivalent values above 1.1% Ro limiting the number of amenable Neoproterozoic samples.

The presence of saturated and aromatic carotenoids, minimally isorenieratane and 2,3,6-trimethyl aryl isoprenoids is typical of many marine sediment and petroleum samples deposited during the Phanerozoic. This is rather surprising since, as far as we can determine, there are no regions of the open oceans of today where the oxygen minimum zones are sufficiently shallow and sulfidic to support significant populations of green sulfur bacteria.

In contrast to Phanerozoic samples, those so far examined from Neoproterozoic settings are more similar to those of lacustrine paleoenvironments and are dominated by the fully saturated carotanes together with lower abundances of carotenoids that are typically associated with purple sulfur bacteria. In this presentation we explore a number of possible explanations for this enigma.