Cyanobacteria Proliferation in the Cenomanian‐Turonian Boundary Interval of the Apennine Carbonate Platform: Immediate Response to the Environmental Perturbations Associated With OAE‐2?

Abstract

Oceanic Anoxic Event‐2 (OAE‐2; Cenomanian‐Turonian) is characterized by extensive deposition of organic carbon‐rich deposits (black shales) in ocean basins worldwide as result of a major perturbation of the global carbon cycle. While the sedimentological, geochemical, and paleontological aspects of deep water expressions of OAE‐2 have been intensively studied in the last few decades, much less attention has been given to the coeval shallow water deposits. In this study, we present the results of a detailed facies and petrographic (optical microscope and scanning electron microscopy) and geochemical studies (δ13Ccarb, δ 13C org, δ 15Nbulk, TOC, and Rock‐Eval pyrolysis) on two key shallow marine sections from the Apennine Carbonate Platform (ACP; Italy). Here a continuous record of shallow water carbonates through the OAE‐2 interval is preserved, offering the unique opportunity to document the archive of paleoenvironmental changes in a neritic setting, at a tropical latitude and far from the influence of a large continental block. Two conspicuous intervals are characterized by abundant and closely spaced “dark” microbial laminites found at correlative stratigraphic horizons in the two stratigraphic sections. These laminites contain elevated concentrations of TOC (up to 1%) relative to microbial capping cycles laminites stratigraphically above and below. The organic matter preserved in these fine‐grained laminites is dominated by cyanobacteria remains, which accounted for most of the organic matter produced on the ACP in these intervals. Our study suggests that Tethyan carbonate platforms experienced significant biological changes during OAE‐2, alternating, in few kiloyears, between eutrophic phases dominated by microbial communities and mesotrophic/oligotrophic conditions favoring “normal” carbonate production/sedimentation. The synchronous occurrence of microbialite facies at different locations across the ACP and on other platforms worldwide (e.g., Mexico and Croatia) suggests a causal link between Large Igneous Province volcanism and the environmental conditions necessary to trigger cyanobacterial proliferation on shallow carbonate platforms.

Anatomy of an extinction revealed by molecular fossils spanning OAE2

“…Influx of more oxygenated waters onto shallow, restricted epicontinental seaways, driven by modest sea-level rise, has been interpreted as one of the causes for the overall increase in oxygenation during the OAE2 CIE in the WIS 50 , 51 . Evidence of such a eustatic mechanism is also found in Tethyan localities where shallow marine carbonate facies transition from peritidal successions to successions dominated by deeper subtidal lithofacies during the whole of the OAE2 CIE 52 . However, while relief from stagnation might account for the overall increase in oxygen concentrations throughout OAE2 in the WIS, biomarker data presented here indicate that during OAE2, there were punctuated episodes of drastic change in environmental conditions that affected the ecology in the water column at a much finer scale.…”

Biotic and Isotopic Vestiges of Oligotrophy on Continental Shelves During Oceanic Anoxic Event 2

“…This indicates that the compensating N isotope fractionation mechanism between epicontinental and oceanic settings continued for some time after the event, providing a continued supply of isotopically heavy nitrate. Enriched δ 15 N org during OAE2 (up to +7.5‰) have also been reported from the margins of the Tethys oceans (e.g., Frijia et al, 2019;Zhang et al, 2019), which suggests 15 N enrichment occurred globally in shallow-marine epicratonic seas during OAE2. These high δ 15 N org values have been mostly attributed to reduced oxygen depletion and increased N fixation, which we find insufficient to reconcile with such extremely high δ 15 N org values.…”

Paleoecology and proliferation of the bivalve Chondrodonta joannae (Choffat) in the upper Cenomanian (Upper Cretaceous) Adriatic Carbonate Platform of Istria (Croatia)

“…Rudist biodiversity shows three radiation phases, each of them was followed by an extinction event (Ross and Skelton, 1993). The first two extinctions occurred in the early Aptian and latest Cenomanian, in correspondence with severe Cretaceous paleoenvironmental-paleoclimatic perturbations (e.g., Anoxic Events 1a and 2; Philip and Airaud-Crumiere, 1991;Gili et al, 1995;Skelton and Gili, 2002;Masse and Steuber, 2007;Steuber et al, 2016;Skelton, 2018, Frijia et al, 2019.…”