Wave and current controls on the formation and steady-state maintenance of deep reef flats on atoll rims

Abstract:

The rims of active atolls typically extend to close to sea level, and are comprised of reef flats that have accreted typically 10’s of meters of Holocene reef on top of Pleistocene unconformities. However, not all atoll rims approach sea level, and deep reef flats (5-20m) are prevalent in the South China Sea (SCS) and also can be found along atoll rims worldwide. Traditional models suggest that these deep reef flats are either relict features abandoned during sea-level rise (‘give up’) or should be actively accreting towards sea level (‘catch up’). However, hydrodynamic observations across four of the numerous deep reef flats at depth (10-20 m) of the Nansha Islands, SCS, the longest spanning > 8 months, indicate consistently strong wave orbital motions that, combined with persistent moderate currents, should be capable of entraining and transporting significant quantities of sediment off the reef flat for much of the year. These observations, along with the presence of reef-building corals, suggests that these deep reef flats may be active features. Here, we present a model of reef accretion including the effects of wave- and current-driven transport of sediment as bedload off the reef flat. Similar to models that include an arbitrary “wave stress”, our combined accretion/sediment transport model suggests that reef accretion may not be possible for shallow depths in high-energy environments. Modeling Holocene reef growth, the model can develop both catch-up and give-up reefs, but also can develop deep reef flats that maintain themselves at depths greater than 5 m, implying that a dynamic keep-up deep reef flat can develop. Exploring a wide parameter space of gross accretion rate, wave height, current velocity, and initial basement depth, we explore the broader controls on the potential formation of these deep reef flats. We then compare model results to geologic and modern observational data from atolls in both the SCS and the Pacific Ocean. These results could be further tested with focused coring of deep reef flats and increased in situ observations of hydrodynamic conditions, reef coverage, and sediment transport.