Metre-scale cycles in shallow water carbonate successions: Milankovitch and stochastic origins

Metre-scale cycles are a common feature in Precambrian and Phanerozoic shallow water carbonate successions, and astronomically forced changes in sea-level (Milankovitch cycles) may have been an important driver controlling their deposition. Nevertheless, the degree to which potentially low amplitude astronomically paced sea-level oscillations may have controlled carbonate accumulation in deep time is unclear. In this study, a stochastic model of carbonate accumulation demonstrates how metre-scale exposure-bound sequences can be generated under conditions of random sea-level change. These sequences have characteristic durations close to Milankovitch cycles, despite the absence of any astronomical control on their formation. Metre-scale sequences with sub-Milankovitch (millennial-scale) durations can also be generated by the model, potentially shedding light on the origin of sub-Milankovitch sequences such as those recorded on the Middle Triassic Latemar platform of Northern Italy. Sensitivity tests demonstrate how shallow water carbonates may be very sensitive to weak (i.e. low amplitude) astronomically forced sea-level oscillations. Notably, strong statistical evidence (P < 0.01) for astronomical cycles can be preserved in modelled successions even when astronomical forcing contributes <1% of the sea-level variance on million year timescales. Taken together, metre-scale cycles with Milankovitch-scale durations in ancient carbonate successions may reveal very little about the amplitude, or even the existence, of astronomical forcing as a sea-level driver.