Controls on the spatio-temporal distribution of microbialite crusts on the Great Barrier Reef over the past 30,000 years

Calcification of microbial mats adds significant amounts of calcium carbonate to primary coral reef structures that stabilizes and binds reef frameworks. Previous studies have shown that the distribution and thicknesses of late Quaternary microbial crusts have responded to changes in environmental parameters such as seawater pH, carbonate saturation state, and sediment and nutrient fluxes. However, these studies are few and limited in their spatio-temporal coverage. In this study, we used 3D and 2D examination techniques to investigate the spatiotemporal distribution of microbial crusts and their responses to environmental changes in Integrated Ocean Drilling Program (IODP) Expedition 325 (Great Barrier Reef Environmental Changes) fossil reef cores that span 30 to 10 ka at two locations on the GBR reef margin. Our GBR microbialite record was then combined with a meta-analysis of 17 other reef records to assess global scale changes in microbialite development (i.e., presence/absence, thickness) over the same period. The 3D results were compared with 2D surface area measurements to assess the accuracy of 2D methodology. The 2D technique represents an efficient and accurate proxy for the 3D volume of reef framework components within the bounds of uncertainty (average: 9.45 +/- 4.5%). We found that deep water reef frameworks were most suitable for abundant microbial crust development. Consistent with a previous Exp. 325 study (Braga et al., 2019), we also found that crust ages were broadly coeval with coralgal communities in both shallow water and fore-reef settings. However, in some shallow water settings they also occur as the last reef framework binding stage, hundreds of years after the demise of coralgal communities. Lastly, comparisons of crust thickness with changes in environmental conditions between 30 and 10 ka, show a temporal correlation with variations in partial pressure of CO2 (pCO(2)), calcite saturation state (Omega(calcite)), and pH of seawater, particularly during the past similar to 15 kyr, indicating that these environmental factors likely played a major role in microbialite crust development in the GBR. This supports the view that microbialite crust development can be used as an indicator of ocean acidification.