How reefs drown: A case study from the Northwest Hawaiian Islands

Thousands of deeply submerged seamounts are scattered across ocean basins, many capped by thick successions of reef carbonates accumulated over millions of years prior to island drowning. How conditions sustaining shallow-water reef development at these sites deteriorated is often unclear but could lend insights into the processes that put stress on modern reef communities. Here the authors examine reef drowning in the Northwest Hawaiian Islands, brought on by low rates of carbonate accretion that often occur at mid-latitude (> 30) locations. New strontium-isotope stratigraphy from shallow water carbonates sampled from a 499 m-long boring through Midway Atoll suggest limited accumulation has occurred at the site since the early-Miocene (~ 16 Ma). The offshore morphology of Midway indicates carbonate production has often been outpaced by wave erosion during the Pleistocene, shifting the island’s centre ~ 3 km to the south-east in the prevailing wave direction and increasing its circularity. Strong wave action and slow reef accretion may also explain development of extensive reef flats on Midway and nearby islands. Similar features are shared with some other carbonate islands at mid-latitudes but are not typical of Central Pacific atolls, more generally, or guyots. Abrupt sea-level rise may therefore provide a more plausible mechanism for atoll/guyot drowning in the geologic past than poor environmental conditions for carbonate accretion as has previously been invoked.