How Pleistocene sea-level change and wave erosion have shaped the morphology of Midway Atoll

Thousands of seamounts are found scattered across deep ocean basins, many capped by thick successions of reef carbonates accumulated over millions of years prior to island drowning. How conditions sustaining reef development at these sites deteriorated over time is often unclear, but could offer potential insights into the stressors facing modern reefs. Here we examine the process of reef drowning at Midway atoll and other nearby islands in the Northwest Hawaiian hotspot chain, brought on by marginal growth conditions at mid-latitude (~30 °N). A new chronology developed using the ⁸⁷Sr/⁸⁶Sr values of shallow water carbonates sampled from a 504-meter long boring through Midway Atoll suggest limited accumulation has occurred there since the mid-Miocene (~15 Ma), with a depositional hiatus occurring until the Mid-Pleistocene (~1 Ma). During the Pleistocene, carbonate production at Midway has often been outstripped by wave erosion, shifting the island's center ~3 km to the southeast, increasing its circularity, and promoting the development of wide reef flats. Similar features are shared with other carbonate islands at high latitudes but are not typical of atolls, in general, or Pacific guyots. These differences suggest that guyot formation may not have been driven by the gradual deterioration of environmental conditions that sustain carbonate production as seen at Midway today. Abrupt sea-level rise may provide a more plausible mechanism for reef drowning in the geologic past.