Miocene to Present Sea Level and the Origin of Modern Rimmed Atoll Morphology

Abstract:

Rising sea-level over the next century will reshape our coastlines and make low-lying islands more vulnerable to extreme events. Atolls could potentially provide unique geologic records from periods of high sea level analogous to those we might experience over the coming centuries. However, sea-level records from atolls have been largely overlooked, in part because the processes that shape coral reef and atoll form are often complex and, in many cases, remain unexplored. Darwin’s canonical model, which proposes an evolution from fringing reef to barrier reef to atoll as an ocean island ages and subsides, cannot explain the stratigraphy or morphology of many island reefs.

We will present a study that combines a numerical model of reef development with existing stratigraphic records from Pacific atolls. The model, driven by sea level, simulates the evolution of atoll morphology using parameterizations of coral growth, rim derived sediment and in situ production, dissolution, and subsidence. We use it to solve for late-Miocene to present sea level by iteratively changing the ice volume and deep-ocean temperature corrections for converting deep-ocean, benthic, δ¹⁸O to sea level and finding the best-fit between the model output and corelog stratigraphy from Enewetak Atoll. We then compare lagoon depths produced by the model for different island sizes and dissolution rates (but the same subsidence and sediment production rates) to an independent dataset of real-world observations from the Marshall, Gilbert and Caroline Island chains. Our model results suggest that a period of sustained low sea level occurred during the late Miocene before rising above present moving into the Pliocene. We propose that it was atoll exposure and enhanced lagoon dissolution during the subsequent sea-level fall since the late Pliocene, ~2.7 Ma – not crustal subsidence, as Darwin’s canonical model of atoll evolution presumes – that likely drove the development of modern rimmed atoll morphology.