Assessing Contemporaneous Formation of Microbialites and Corals in a Post-Glacial Maximum Reef: Offshore Mozambique

Microbial structures have often been regarded as indicators of rapid environmental changes or biotic crises, where they replace phototrophic and heterotrophic organisms as carbonate producers. The last deglaciation has been interpreted as the most recent example, where microbialites are documented in several deglacial reef sequences. However, interpretations of the timing of their growth concerning the rest of the framework have been inconsistent across study sites. Here, we present the results of a combined sedimentologic, petrographic, including SEM analysis, and geochemical study of a 2 meter core drilled through a low stand reef from offshore Mozambique that grew during the last deglaciation. The framework comprises a high-diversity coral community dominated by shallow water taxa that contain microbial infill in the pore space of their aragonitic skeletons. The coral framework is typically first encrusted by calcareous red algae followed by up to 6-cm-thick microbialite crusts. A microbially influenced skeletal rudstone to grainstone infills the remaining cavities of the framework. Despite their intimate relation in the framework, the mineralogy and stable carbon and oxygen isotopic signatures of the corals and microbialites are quite different. The higher δ18O values of the microbialites suggest they grew in colder and deeper water than the corals. The microbialites in this framework are interpreted to be non-contemporary and grow with the start of Meltwater Pulse 1A, when the reef platform drowned and was overgrown by the microbialites that stabilized the reef components, added strength to the framework, and protected the coral skeletons from bioerosion.