Turbid-zone and terrigenous sediment-influenced reefs

Abstract

Recent studies are providing an improved understanding not only of the controls on, but also the dynamic nature of, terrigenous sedimentation, resuspension, and turbidity within nearshore marine environments, and how these parameters fluctuate in and around coral reefs. The key issue that arises from such studies is the need for consideration of the marked variability that occurs spatially (between and across individual reef systems) and temporally (within reef systems/sites). The implications of this are that all turbid-zone and terrigenous sediment-influenced reefs are subject to varying degrees of sediment/turbidity stress. Consequently, the responses exhibited by coral communities and the facies that accumulate are highly site specific. Although the dynamics of sedimentation are often complex, short-term wave-driven sediment resuspension can generate turbidity regimes that far exceed perceived threshold levels for coral survival, while over longer (centennial to millennial) timescales, the landward transgression of inner-shelf terrigenous sediments can control regional patterns of reef establishment. It is, however, clear that turbid-zone reefs can develop within a wide range of nearshore and coastal depositional environments, with the earliest coral colonizers clearly exploiting a wide range of, often unconsolidated, sediment substrates, including clays, alluvial sands/gravels, and intertidal and subtidal sands and muds (see Smithers et al., 2006; Perry and Smithers, 2009). However, once established, these reefs demonstrate the potential for relatively rapid growth, exhibiting both rapid accretionary and progradational growth modes. Reef facies in these environments typically demonstrate that reef accretion has occurred under conditions of net (often fine-grained) terrigenoclastic sediment accumulation (and are characterized by detrital coral rubble set within a mixed carbonate–siliciclastic matrix). Recent studies also demonstrate that reef building in these environments can be dominated by persistent (but taxonomically restricted) suites of coral taxa, with marked faunal transitions only occurring once the reefs reach sea level and reef flat communities evolve. The long-term tolerance of corals to, at least episodically, high turbidity levels is probably aided by the ability of some corals to switch from predominantly phototrophic to autotrophic feeding modes. These issues are clearly of fundamental importance to understanding the long-term ecological stability of turbid-zone coral reefs and for understanding coral community level responses to sediment flux. Much remains to be learnt, however, about the interactions that occur between reef growth, sea-level fluctuations, and shoreline morphodynamics in these environments, and how these interact to control rates, styles, and the longevity of reef building.