Brachiopods in Epeiric Seas as Monitors of Secular Changes in Ocean Chemistry: A Miocene Example from the Murray Basin, South Australia

Abstract

Abstract Early and middle Miocene cool-water carbonates from the Murray Basin, South Australia, preserve an excellent stable-isotope record of ocean-climate change. These variably fossiliferous heterozoan deposits accumulated on a low-energy, mesotrophic, centripetal epeiric ramp during a gradual shift in climate from cool, wet conditions and abundant continent-derived nutrients to a seasonal, arid climate with reduced delivery of trophic resources to the marine realm. Temporal trends in δ13C and δ18O from unaltered brachiopods record an epeiric sea response to this warming. The globally recognized middle Miocene Monterey Event (~ 17 to 13.5 Ma) dominates the carbon isotope record, albeit with higher (~ 0.5‰) than open-ocean δ13C values. Such higher δ13C values are attributed to an increase in benthic carbonate production that accompanied climate change and the relatively short seawater mixing times characteristic of epeiric-sea systems. The Murray Basin oxygen isotope curve contains lower δ18O values (~ 2.0‰ lower) than those of the deep-sea record. This difference is ascribed to the warmer seawater temperatures (~ 17 to 22°C) that prevailed across the Miocene Murray Basin. These results show that the isotope chemistry of epeiric-sea brachiopods can be a reliable gauge of regional and global environmental evolution. Although diagenetic overprinting from meteoric cement-filled punctae and local forcing factors introduce noise that mutes isotopic signals, the open-ocean secular record is clearly discernible.