Platform-wide shift to microbial carbonate production during the late Aptian

In the aftermath of major Phanerozoic biocrises, diverse metazoan-dominated reef ecosystems were commonly replaced by microbial carbonate-producing communities. Apart from the loss of metazoan competitors, the factors causing pervasive microbial carbonate production in shallow-water platform settings are not completely understood. Amongst others, outstanding warm temperatures coupled with low-oxygen waters were proposed as possible triggers. This study focuses on late Aptian shallow marine carbonates deposited on the Apennine carbonate platform (ACP) in the central Tethys. By establishing an integrated high-resolution chemostratigraphic framework for two sections of the ACP, the coeval onset of pervasive bacinelloid growth is discovered, indicating a platform-wide shift from a metazoan-dominated ecosystem to microbial carbonate production. The initial phase of microbial proliferation coincides with the final stage of the so-called late Aptian "cold snap" and the subsequent temperature increase, which was paralleled by a significant sea-level rise. Our results contrast with observations from the early Aptian Oceanic Anoxic Event la, where a similar shift toward microbial "bacinelloid" carbonate production has been linked to exceptionally warm conditions and hypoxia.