Chronostratigraphy of Bramston Reef reveals a long-term record of fringing reef growth under muddy conditions in the central Great Barrier Reef

Inshore reefs on Australia's Great Barrier Reef (GBR) are widely argued to be in decline, although recent reports suggest that some may be more resilient than traditionally assumed. Resolution of this debate requires long-term insights into past reef development and variability to provide context for the assessment of present reef condition. Long-term reef growth histories can preserve extended records of reef growth and condition, however they are rare, especially for mainland-attached fringing reefs, which are themselves uncommon on the GBR. We examined the internal structure and ecology at Bramston Reef, a mainland-attached fringing reef located in a protected bay on the central GBR. Eight reef matrix cores were collected across the reef flat. Sedimentological and palaeo-ecological analyses coupled with U–Th dating were used to develop the first reef growth history for a shore-attached fringing reef in this region. Twenty-five hard coral genera were identified in the palaeo-ecological analyses. The key reef-building genera (including Acropora, Montipora, Euphyllia, Porites and Goniopora) have contributed to reef growth since initiation and are represented in the extant coral community, despite a change in accretion ‘mode’ during the late Holocene. Sedimentological and stratigraphic investigations demonstrate Bramston Reef has always grown in a mud-rich setting. U–Th ages indicate that reef initiation occurred at or before 5400yBP in a palaeo-water depth of 2–3m. Bramston Reef reached sea level by 4256yBP when sea level was approximately 1m higher than present, after which rapid seaward progradation occurred until around 3000yBP (~19cm/yr on average). Between approximately 3000 and 1000yBP seaward progradation of the reef flat slowed (to ~9.8cm/yr on average). This deceleration of reef growth occurred long before European settlement of the Queensland coast and was driven by natural constraints, probably associated with limited accommodation space due to late-Holocene sea-level fall. Our results demonstrate that mainland-attached reef initiation and accretion can occur in muddy inshore environments over long timeframes (centuries to millennia).