REDOX BIOGEOCHEMISTRY OF MID-TRIASSIC REEFS

Geochemical signatures highlight the contrasts between two distinctive mid-late Triassic reef types in the Dolomite Alps, Italy. In the first type, sponges, bryozoans, calcified cyanobacteria, and problematica (Archaeolithoporella, Shamovella), together with precipitated seafloor crusts, formed dense reef frameworks in high energy shallow-water at the margins of high-rise ~238 Ma Ladinian-Carnian carbonate platforms. Debris from these margins created steep foreslopes, and some allochthonous blocks (Cipit Boulders) of reef rock were buried in basinal shales that protected them from subsequent regional dolomitization. In the second type, small Carnian (~234 Ma) patch reefs in the same area formed low energy, but also shallow water, environments and were also surrounded by shales that protected them from alteration. They were constructed mainly by scleractinian corals, sponges, and red algae, and have framework cavities with distinctive clotted-peloidal micrite fabrics. They are early examples of reefs biotically comparable with present-day tropical coralgal reefs, whereas the Cipit Boulders have much more in common with Late Permian reefs.

Both of these reef types preserve primary microfabrics and biomarkers. The Cipit Boulder samples contain bacterial, including cyanobacterial, biomarkers, lack specific molecules typical of sulfate-reducing bacteria (SRB), and have Rare Earth Element (REE) values indicative of oxic conditions. These signatures are consistent with the high-energy platform margin setting of these reefs, their relatively tight framework structure, and the presence of calcified cyanobacteria such as Cladogirvanella. In contrast, the coralgal patch reefs contain SRB biomarkers and REE values indicative of sub-oxic conditions, consistent with their lower energy environment and well-developed skeletal frameworks that created localized protected low-oxygen conditions. The SRB biomarkers can also be linked to the clotted-peloidal cavity-lining crusts, which resemble those that commonly occur in much younger coral-reef frameworks. These details further emphasize the radical evolutionary and environmental developments that significantly altered reefs during the early Mesozoic.