Holocene development of submerged keep‐up patch reefs on Bermuda without acroporids: A model of future reef accretion

Abstract The cover of living reef‐building corals has declined due to ocean warming, acidification, pollution and disease outbreaks. This decline endangers future reef development relying on acroporid corals, which are highly sensitive to environmental changes. Given these threats, there is a need for accretion models that do not rely on species dominance. To assess the potential of non‐acroporid framework and identify the key variables controlling their accretion, we reconstructed the internal structure and Holocene development of Bermuda patch reefs, lacking acroporids. Four rotary drill cores and two vibrocores recovered sequences composed of five facies in decreasing abundance: a framework of massive, stress‐tolerant corals, unconsolidated skeletal sand and gravel, coralline‐algal bindstone, Halimeda floatstone and peat. Radiocarbon dating of corals and peat shows that patch reefs developed in two stages. From 7 to 4 kyr B.P., during a sustained sea‐level rise, the coral framework facies accreted vertically, while the unconsolidated facies filled interstitial sand pockets in intermediate water depths. From 4 kyr B.P. onwards, during a reduced rate of sea‐level rise, vertical accretion has continued with a decline in coral diversity. Accretion was primarily controlled by massive coral framework construction and variation in environmental energy, turbidity and bioerosion, which collectively shaped the size, diversity and preservation of the framework. This led to a suppressed, steady vertical accretion rate of 2.18 ± 0.66 m/kyr on average. Although lower than that of acroporid reefs in the western Atlantic, the accretion rate did not decline throughout the Holocene, even as sea‐level rise slowed over the last 4 ka. The suppressed accretion rate resulted in submerged keep‐up reefs thriving in several metres of water during the mid‐late Holocene. This growth mode highlights the potential of coral reefs to develop under different controls beyond coral species dominance, a mode that could be applied to current and future coral reef development.