HIDDEN IN PLAIN SIGHT: UBIQUITOUS AUTHIGENIC CARBONATE IN A MODERN SLOPE SETTING

The geological record is periodically punctuated by large negative changes in the carbon isotope composition of marine carbonate sediments. Traditional interpretation of these types of excursions invokes an origin driven by the exogenic carbon cycle. Thus, the evolution of new biogeochemical metabolisms, atmospheric oxygenation, and weathering processes have previously been inextricably linked to the carbon isotope composition of marine carbonates. Recent studies have suggested that these large negative changes can be generated by a process unrelated to global carbon cycling: the formation of authigenic carbonate. The diagenetic origin of authigenic carbonates decouples their carbon isotopic composition from that of the surface oceans and atmosphere, thereby adding a new dimension of complexity to global carbon cycle models. Given the newfound importance of authigenic carbonate as a carbon sink, constraining the drivers, rate of formation, and amount of authigenic precipitation is crucial. Here we present evidence of authigenic carbonate formed in oxygenated bottom waters in sediments collected during ODP Leg 133 from the mixed carbonate-siliciclastic slope adjacent to the Great Barrier Reef in Australia. Our results produce new constraints on the timing, quantity, and drivers for authigenic carbonate formation, and identify the role of reduced sedimentation rate which provides an essentially unlimited dissolved sulfate supply for authigenic precipitation. Under such conditions, modeled precipitation of authigenic carbonate ranging from 5-15% can reproduce the observed -4‰ change in carbon isotope values during the short (~200 kyr) period of reduced sedimentation without obvious sedimentological evidence of diagenesis. These results provide a mechanism to explain asynchronous, punctuated negative shifts in carbon isotope values of marine carbonates unrelated to changes in global carbon cycling or periods of bottom water anoxia.