Understanding Intrabasinal Organic Carbon Records: A New Carbon Isotope Record for the Early Cretaceous Abu Dhabi, UAE

Abstract:

The Aptian–Albian (~125–100 Ma, Early Cretaceous) is considered to be an interval of elevated greenhouse gases, intensified hydrologic cycle, limited polar ice, and elevated high latitude temperatures, with evidence for Oceanic Anoxic Events (OAE) as recorded by organic matter and carbonate δ¹³C perturbations in both the marine and continental realm (Jenkyns et al., 2004; Poulsen, 2004; Robinson & Hesselbo, 2004 and Ufnar et al., 2004). In this study four cores from Aptian­–Albian marine sediments from a single offshore basin in Abu Dhabi, UAE were examined petrographically and samples were collected to generate a high resolution organic carbon δ¹³C chemostratigraphic profile. The cores represent sediments from the Early Cretaceous upper most Thamama and lowermost Wasia Group and represent interbedded shale and limestone units from the Arabian Shelf. Three of the cores, cores A­–C, show a 3‰ negative excursion in δ¹³C_org with values of -28‰ occurring with a concurrent increase in organic carbon, likely indicative of Aptian OAE 1a. These cores also show a transition from more oxic to euxinic waters as recorded in the pyrite framboid mean diameter. The remaining core, D, shows relatively isotopically light average δ¹³C_org values of -26‰, with an intervening positive isotope excursion and values of -23‰, and may represent a different event either earlier or later in the sequence, but due to limited biostratigraphic data could not easily be correlated using C-Isotopes alone with the other three cores. The four cores come from different localities within a single basin and highlight issues pertaining to correlating cores across a single basin for the same time interval, as well as variability in organic matter burial and oxygenation within a single basin, and may provide a useful case study specifically related to separating regional signals from global carbon isotope signals for the Aptian–Albian, and understanding how ancient Mesozoic basins are affected by global marine transgression and changes in ocean oxygenation specifically during OAE1a.