Late Aptian–middle Albian bacinellid oncoid event, Apuseni Mountains, Romania: a possible link to OAE1b?

Abstract

A multi-tool investigation, including microfacies analysis, morphometric measurements, statistical analysis, SEM microscopy, biostratigraphy, and stable isotopes, was conducted on Lower Cretaceous oncoid-bearing carbonate beds that are well exposed in a limestone quarry in the Northern Apuseni Mountains, Romania. These carbonates are characterized by abundant bacinellid oncoids with thick cortices exhibiting a typical microfabric of micritic threads and spar-filled vesicles (bacinellid meshwork), that formed in a low-energy, inner-platform environment. The growth patterns of the bacinellid threads that form the meshwork of the oncoid cortices, such as constant thickness, branching modes, and bag-shaped/globular swellings, share close similarities with those developed by endolithic marine fungi. This, together with clotted micritic patches and frequent association with Lithocodium aggregatum sensu Elliott, 1956, suggests that these oncoid structures could have been generated by a microbial consortium involving bacteria, possibly fungi, and algae. Biostratigraphic and stable isotope data indicate that these bacinellid oncoid carbonates overlapped with the latest Aptian–earliest Albian Oceanic Anoxic Event 1b (OAE1b set/cluster). Our results on the bacinellid oncoid structures could support a possible relation between their extensive development and OAE1b, since bacteria, fungi, and other microbial communities are known to play an important role in carbon cycling and the restoration of oxygen levels in marine environments.