STABLE CARBON ISOTOPES OF AMORPHOUS KEROGENS FROM PHANEROZOIC SEDIMENTARY ROCKS

Abstract

Amorphous kerogens from Phanerozoic sedimentary rocks may be divided into two groups on the basis of their δ13C values. h-amorphous kerogens have isotopic values in the range of −24 to −20 per mil, and l-amorphous kerogens have isotopic values in the range of −35 to −26 per mil. The occurrence of amorphous kerogens in the intermediate isotopic range from one rock unit in the data set suggests a weak continuum may exist between these two groups. Phytoplankton and, to a lesser extent, zooplankton and bacteria are considered to be the precursors for both kerogen types. A plausible explanation for this dichotomy may be found in the source of CO2 utilized by phytoplankton, which is considered to be controlled by environmental factors. h-amorphous kerogens are expected to occur in silled basins that are overlain by expansive well-circulated deep (> 500 m) water, where the prevailing source of carbon in the photic zone is atmospheric-derived CO2. Conversely, l-amorphous kerogens are expected to occur in restricted basins that are overlain by stratified shallow (< 200 m) water, where the prevailing source of carbon in the photic zone may be organic-derived CO2.