Carbon isotope stratigraphy of the interglacial Umberatana Group, Adelaide Fold Belt, South Australia
Burgess, Jamie M.
Doctor of Philosophy
University of Adelaide
The use of stable isotope geochemistry to aid the stratigraphic correlation of Neoproterozoic successions has only been in practice for the last two decades. Hitherto, rocks of this age were thought to have been too altered to allow their carbon isotopic compositions to be used for such purposes. Isotope chemostratigraphy is particularly helpful in the Neoproterozoic where biostratigraphic control is less well constrained than for Phanerozoic sequences, if available at all. This mode of geochemistry has also served to further our knowledge of the palaeoclimates and extant biogeochemical systems during this pivotal time in Earth history.
Carbon isotope signatures have been used extensively in the Phanerozoic to interpret environments in both a sedimentological and secular context. The carbon isotope record through the Neoproterozoic reveals variation and absolute δ13C values that are quite anomalous in comparison to the Phanerozoic record. Relative to PDB, hugely positive interglacial values (up to +10‰) are bracketed by negative values (as low as -5‰) near glacial horizons.
The lithostratigraphic record of the Adelaide Fold Belt is well known and has long been a reference for Neoproterozoic investigations. The latest Cryogenian Sturtian to Marinoan interglacial succession is by far the thickest and most complete such interglacial sequence in the world, attaining a cumulative thickness in excess of 4km. As such, it provides an excellent opportunity for chemostratigraphic study and the compilation of a secular C-isotope curve for use in inter-regional correlation.
Careful examination of conventional thought regarding sample quality reveals the overuse of Phanerozoic models to gauge diagenetic alteration. Indeed such models cannot be used when interpreting the unique biogeochemistry of the Neoproterozoic. Whilst interbasinal and intercontinental chemostratigraphic correlations have been attempted using very sparse data sets, until now there have been few locally intensive studies aimed at elucidating regional geochemistry.
The resultant secular C-isotope profile for the interglacial succession in the Adelaide Fold Belt exhibits an overall climb to highly positive values of +8 to +10‰, before a marked negative excursion prior to the Marinoan glaciation. This compares favourably with the fragmentary curves documented from late Cryogenian interglacial successions elsewhere, notably northwest Canada and Namibia. The increased density of C-isotope data in the Adelaide Fold Belt profile reveals higher-order perturbations of up to 4‰ that reflect changes in palaeobathymetry attributable to glacioeustatic fluctuation of sea level and local diapiric activity. The lower-order interglacial positive excursion is explicable in terms of increased marine primary productivity and high net rates of burial of organic carbon relative to carbonate carbon. A corollary of this hypothesis is a steady increase in atmospheric oxygen levels, eventually manifest in the appearance of redbed (oxidised) sediments late in the interglacial succession.
This study demonstrates the need for refinement of the established approach to interbasin stratigraphic correlation based on C-isotope geochemistry. High data resolution is desirable and correlation should take into account lithostratigraphy and sequence stratigraphy rather than solely relying on data trends or absolute δ13C values. Stable isotope signals observed through the Neoproterozoic are not well understood and their precise meaning is still open to considerable debate.