A carbon isotope reference curve for ca. 1700-1575 Ma, McArthur and Mount Isa Basins, northern Australia

Shallow marine Paleo- and Mesoproterozoic sedimentary successions are widely distributed in several major basins across northern Australia. The successions are only gently deformed, and their stratigraphy is relatively continuous, thus offering an ideal opportunity to document secular variations in carbon isotopes. Marine carbonate intervals from two of these major basins, the McArthur and Mount Isa Basins, have been sampled to document secular variation in δ¹³C(carb) from approximately 1700 to 1575 Ma. In all cases, the samples have been tied to a well established sequence stratigraphy which, along with U-Pb SHRIMP zircon dates, provides a time resolution of the order of 1 m.y. The data presented here thus provide the most comprehensive and best dated δ¹³C(carb) stratigraphy yet obtained from such ancient rocks. Diagenesis occurred early in the carbonate rocks from both basins with the result that fluid movements were restricted and primary carbon isotopic signatures were retained. The δ¹³C(carb) values from both basins vary within a very narrow range around a mean of -0.6‰, with extreme values seldom lying further than 1‰ from the mean. That is, the curves are essentially flat. The results of this study, combined with earlier studies on younger rocks, imply that the global ocean reached a state of equilibrium in the mid-Paleoproterozoic and remained stable for much of the following billion years. Current models of the ocean suggest that to maintain the carbon mass balance relatively low levels of tectonic activity would be required, which in turn suggests that the availability of nutrients, such as phosphorus, was stable and low. Prolonged nutrient stability may therefore have exerted a major influence upon the evolution of the biosphere over this time interval.