ABSTRACT:

The sub-tropical marginal reefs of Moreton Bay were characterized by diverse mid-Holocene communities that dramatically declined in the absence of major anthropogenic disturbance. We examined the nature and timing of changes in surficial coral death assemblages from Moreton Bay reefs to gain insight to the interplay of local and global changes. Corals were selected throughout the Bay across a depth range from 1 m above high tide to 5 m below low tide. Uranium-series age determinations (n=57) were made by Uranium-series thermal ionization mass spectrometry. The oldest sample was found atop in situ reef accretions at the highest elevation sampled, so Moreton bay reefs initiated before 6.82 ka and sea level must have reached its Holocene maximum prior to that time. Since the oldest cohort of corals was not buried by younger ones we conclude that coral accretion since 5.5 ka has been minimal. Coral accretion occurred in discrete episodes rather than continuously, and depths of accretion, corrected for palaeo-sea-level, became deeper through the Holocene. Major coral accretion episodes are 0-0.3 ka (19% of samples dated), 1.3-1.7 ka (11%), 4.3-4.7 ka (11%), and 5.5-6.8 ka or older (51%). Conspicuous episodes with no accretion are 0.4-1.1 ka and 4.7-5.5 ka. Sea-level fell from its Holocene high in a series of oscillations which coincide with episodes of accretion (stable/rising) and no accretion (falling). Greater extremes of temperature and salinity would result from a lower volume of water in the Bay, and decreased tidal volume would increase the residence time of river-discharge amplifying the sea-level-driven episodes. The hyposaline wedge associated with increased flooding since ENSO re-establishment ~3.5 ka likely forced younger accretions into deeper water. Because European colonization coincides with an episode of accretion we have an excellent opportunity to isolate historical causes of reef decline from novel anthropogenically-induced causes of decline.