Abstract
Use of coral skeletons to determine growth histories of reefs situated in warm, clear tropical waters is well established. Recently, however, there has been increasing awareness of the significance of reefs occurring in environments that are considered as marginal for coral growth, such as turbid inshore settings characterized by episodes of elevated turbidity, low light penetration, and periodic sediment burial. While these conditions are generally considered as limiting for coral growth, coral reefs in these settings can exhibit high live coral cover and species diversity, and thus can be both ecologically and geologically significant. Turbid-zone reefs are also commonly concentrated along eroding shorelines with many analogues to erosional shorelines developed during the Holocene transgression. A growing number of studies of these previously undocumented reefs reveal that the reef deposits are detrital in nature, comprising a framework dominated by reef rubble and coral clasts and set within a fine-grained terrigenous sediment matrix. In addition to the recognized effects of diagenesis or algal encrustations on the radiocarbon signature of coral samples, episodic high-energy events may rework sediments and can result in age reversals in the same stratigraphic unit. As in other reef settings, the possibility of such reworking can complicate the reconstruction of turbid-zone reef growth chronologies. In order to test the accuracy of dating coral clasts for developing growth histories of these reef deposits, 5 replicate samples from 5 separate coral clasts were taken from 2 sedimentary units in a core collected from Paluma Shoals, an inshore turbid-zone reef located in Halifax Bay, central Great Barrier Reef, Australia. Results show that where care is taken to screen the clasts for skeletal preservation, primary mineralogical structures, and δ13C values indicative of marine carbonate, then reliable 14C dates can be recovered from individual turbid reef coral samples. In addition, the results show that these individual clasts were deposited coevally.
Highlights
IntroductionWhile it is documented that corals can grow and form reef structures in turbid environments (see Sanders and Baron-Szabo 2005; Browne et al 2012 for reviews), coral reefs are typically associated with warm, clear tropical waters
There has been increasing awareness of the significance of reefs occurring in environments that are considered as marginal for coral growth, such as turbid inshore settings characterized by episodes of elevated turbidity, low light penetration, and periodic sediment burial
In order to test the accuracy of dating coral clasts for developing growth histories of these reef deposits, 5 replicate samples from 5 separate coral clasts were taken from 2 sedimentary units in a core collected from Paluma Shoals, an inshore turbid-zone reef located in Halifax Bay, central Great Barrier Reef, Australia
Summary
While it is documented that corals can grow and form reef structures in turbid environments (see Sanders and Baron-Szabo 2005; Browne et al 2012 for reviews), coral reefs are typically associated with warm, clear tropical waters. In the past decade or so, there has been increasing awareness and understanding of the spatial significance of reefs that occur in a range of settings considered more marginal for coral growth such as those developed along the inner Great Barrier Reef (GBR), Australia (Larcombe et al 2001; Smithers et al 2006) These include nearshore areas influenced by elevated turbidity and high sedimentation regimes and which are of particular interest because they are abundant in modern locations and have numerous analogues in the geological record (Kleypas et al 1999; Perry and Larcombe 2003; Sanders and Baron-Szabo 2005). The long-term influence exerted on these reefs by terrigino-clastic sediments means that they represent useful analogues for those reefs systems that are subject to the effects of increasing terrigenous sediment influence (widely cited as a major threat to reef ecosystems globally, McLaughlin et al 2003)
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