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 widel y distributed in several major basins across northern Australia. The succes sions are only gently deformed, and their stratigraphy is relatively contin uous, 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 v ariation in delta(13)C(carb) from approximately 1700 to 1575 Ma. In all cas es, the samples have been tied to a well established sequence stratigraphy which, along with U-Pb SHRIMP zircon dates, provides a time resolution of t he order of 1 m.y. The data presented here thus provide the most comprehens ive and best dated delta(13)C(carb) stratigraphy yet obtained from such anc ient rocks. Diagenesis occurred early in the carbonate rocks from both basi ns with the result that fluid movements were restricted and primary carbon isotopic signatures were retained. The delta(13)C(carb) values from both ba sins vary within a very narrow range around a mean of -0.6 parts per thousa nd, with extreme values seldom lying further than 1 parts per thousand from the mean. That is, the curves are essentially flat. The results of this st udy, combined with earlier studies on younger rocks, imply that the global ocean reached a state of equilibrium in the mid-Paleoproterozoic and remain ed stable for much of the following billion years. Current models of the oc ean suggest that to maintain the carbon mass balance relatively low levels of tectonic activity would be required, which in turn suggests that the ava ilability of nutrients, such as phosphorus, was stable and low. Prolonged n utrient stability may therefore have exerted a major influence upon the evo lution of the biosphere over this time interval. (C) 2000 Elsevier Science B.V. All rights reserved.