A COMPARISON OF THE TREZONA CARBON ANOMALY WITH THE PLIO-PLEISTOCENE

Large and systematic shifts in the δ¹³C values (10‰) of carbonate dominated rocks preceding Neoproterozoic glacial successions are widely interpreted to record a dramatic series of global environmental and evolutionary events. The magnitude and reproducibility of these variations is often contrasted with the Phanerozoic carbon isotope record to underscore the unique nature and importance of the Neoproterozoic interval in Earth history and to reinforce a global marine origin over a potential later diagenetic origin for anomalous carbon isotopic values. In this presentation we compare δ¹³C variations associated with the Trezona anomaly with those recorded during the Plio-Pleistocene. Both the Neoproterozoic and the Pleistocene have experienced large changes in sea level caused by glaciations, but in contrast to the Neoproterozoic, Pleistocene deep-sea sediments provide an extremely well constrained record of the extent, magnitude and timing of changes in the global carbon cycle as they are recorded in the geochemical and physical record. The effect of sea-level changes upon shallow-water carbonates during the Plio-Pleistocene has been to produce an intensively altered rock, continually overprinted by repeated dissolution and precipitation reactions leaving similar δ¹³C and δ¹⁸O stratigraphic records at locations in the Atlantic and Pacific, that, while systematic and reproducible, do not record global biogeochemical events as they are at odds with the open marine pelagic record. These Pleistocene successions show similar stratigraphic patterns, sample by sample systematic variation, reproducibility and magnitude of variation in δ¹³C as the examples used to establish the Trezona anomaly. We suggest that the δ¹³C and δ¹⁸O of carbonates associated with Trezona anomaly may be explained by similar geochemical process which have been documented and are well constrained in the Pleistocene and that these examples demonstrate that the often stated robustness to diagenetic change of δ¹³C in Precambrian carbonates attributed to buffering by the dominance of marine derived C is not supported by these Pleistocene examples. It is also apparent that large negative δ¹³C excursions that are reproducible, systematic in pattern and closely timed (by sea level fall) can result from diagenetic processes.