Reef growth history at intermediate to mesophotic depths since the end of the Last Glacial period along the Great Barrier Reef shelf edge

Reef communities at intermediate (10 to 30 m) and mesophotic (~ 30 to 150 m) depths occupy large areas of sea floor but little is known about their potential to accrete vertically, their response to sea-level change and other environmental perturbations. In this study, the authors have examined cores from two holes, M0040A and M0041A, drilled by the International Ocean Drilling Program Expedition 325 along the shelf edge of the modern GBR at 131 m water depth. The objective was to investigate reef growth history at palaeo-water depths > 20 m over a time period spanning 30,000 years, from the end of the Last Glacial period through the last deglaciation. Based on changes in lithologies and biotic components, and a robust chronostratigraphic framework supported by 47 radiometric ages, the authors have identified two episodes of reef growth, one during Marine Isotope Stage (MIS) 2 and the other at the onset of the deglaciation, both characterised by abundant microbialite crusts and distinct coral assemblages. Palaeo-water depths range from 30 to 60 m and from 20 to 30 m for the MIS 2 and early deglacial reef sections, respectively. The first episode of reef growth documented in the cores initiated at 27 to 25 ka, possibly in response to increased light availability and change in sedimentation resulting from falling sea-level between 32 and 29 ka from MIS 3 and also to low atmospheric pCO2 at the end of the Last Glacial period. Reef accretion was reduced or ceased sometime between 24 and 19 ka, coinciding with the rapid 20 m sea-level fall of the peak Last Glacial Maximum and minima of SSTs. Reef growth resumed at 19.5 to 18.5 ka, influenced by a period of moderate sea-level rise and increasing sea surface temperatures at the onset of the deglaciation. Reef growth termination at ca. 17 ka correlates with a major episode of reef demise previously identified in adjacent mid and outer terrace cores and linked to reduced water quality combined with rapid deglacial sea-level rise. Vertical accretion (VA) rates were calculated based on two methods: linear visual fitting and Bayesian modelling. The findings show that the highest VA rates are associated with microbialite boundstone. Reef ecosystems dominated by microbialite and corals developed at intermediate and mesophotic depths, and grew vertically at maximum rates of 2 to 5 mm yr-1 depending on the method used, over a period of rapid environmental change during the transition from MIS 3 to MIS 1. Further study needs to explore the potential of modern-type deep coralgal communities to cope with higher rates of sea-level rise predicted this century.