COLLAPSE AND RECOVERY DYNAMICS OF EARLY JURASSIC REEF ECOSYSTEMS FROM THE CENTRAL HIGH ATLAS OF MOROCCO

Reefs are key marine ecosystems but today, many keystone species are threatened by various anthropogenic and natural threats. Reef crises and recoveries related to ancient environmental perturbations are important analogues for modern conservation. One of the most severe reef collapses of the Phanerozoic occurred during the Pliensbachian–Toarcian transition (Early Jurassic). Lithiotid bivalve and coral reefs flourished in the Sinemurian and Pliensbachian Stages but were decimated by the environmental changes associated with the Pliensbachian/Toarcian boundary event and Toarcian Oceanic Anoxic Event (T-OAE, ~183 million years ago).

Recent interdisciplinary research in the Central High Atlas Mountains of Morocco has identified a dynamic record of reef proliferation and collapse. During the Late Pliensbachian, coral and lithiotid reefs thrived in shallow water depositional environments. These reefs and biostromes were home to diverse communities with abundant macro and microfauna. Just after the Pliensbachian/Toarcian boundary, the climate regime shifted, resulting in a pulse of terrigenous clastic material and nutrients that poisoned carbonate ecosystems. Interestingly, lithiotid bioherms and biostromes were extremely quick to return to shallow water settings and were often the first colonizing carbonate fauna. Conversely, corals and foraminifera did not recover quickly, with large benthic foraminifera being absent from early Toarcian reefs and corals only occupying minor roles in these lithiotid communities. It was not until just before the onset of the T-OAE that corals reappeared as key reef builders, producing small patch reefs in several locations just before the onset of the event. Reef recovery was short-lived as the T-OAE wiped out the lithiotid bivalves and caused a second reef eclipse. Despite the severity of the T-OAE, coral reefs were quick to recover, albeit with an extremely depauperate foraminifera community and no lithiotids. This high-resolution record of multi-phased reef crisis and recovery highlights both the severity and uniqueness of these events. Although globally they are often linked mechanistically via the eruption of the Karoo and Ferrar large igneous provinces, in Morocco, there is clearly a distinction in the effect of these two extinctions on reef communities.