Carbon Isotope and Lipid Biomarker Stratigraphy from Organic-Rich Strata Through the Neoproterozoic Shuram Excursion in South Oman

Abstract

The regulation of oxygen levels in Earth’s atmosphere and oceans is inextricably linked to the carbon cycle. Carbon isotope ratios of carbonate and sedimentary organic matter provide first order insights into the operation of the carbon cycle in the geologic past. During the Ediacaran period, the ~580 Ma ‘Shuram Excursion’ (SE) records a dramatic, systematic shift in δ^(13)C_(carbonate) values to as low as cɑ. -12‰, lasting potentially millions to tens of millions of years in duration and constitutes the largest carbon isotope excursion known in the record [1]. The extremely negative carbon isotope values in carbonate challenges our understanding of the ancient carbon cycle and is difficult to rationalise via uniform carbon cycle principles. Several hypotheses have been developed to explain this behaviour, all of which make different predictions for the abundance, structure, and isotopic composition of organic carbon through the excursion. For a direct test of these ideas, we report paired organic and inorganic stable carbon isotope ratios in addition to detailed lipid biomarker stratigraphic records from a subsurface well drilled on the eastern flank of the South Oman Salt Basin, Sultanate of Oman. This well captures thermally immature and organic-rich Nafun Group strata traversing the SE, yielding variable but primary biomarker characteristics typical of Neoproterozoic rocks from this region. Despite the high organic matter contents, the carbon isotopic compositions of carbonates do not covary with those of organic phases. Furthermore, lipid biomarker data reveal that organic matter composition and source inputs varied stratigraphically, reflecting biological community shifts in non-migrated, syngenetic organic matter deposited during this interval. Together these observations imply that carbonateorganic isotopic decoupling during the SE is not a result of mixing of fossil or exogenous carbon sources (either DOC, detrital, or migrated) with syngenetic organic matter.