BIOTIC RESPONSE DURING TIMES OF SEVERE ECOLOGICAL STRESS: HOW PALEOCEANOGRAPHIC CHANGES IN UPPER CENOMANIAN-EARLIEST TURONIAN OCEAN INDUCED STEP-BY-STEP EXTINCTION AND DWARFISM

Multidisciplinary investigations in North-western Tunisia indicate that the reference southern tethyan margin Bahloul Formation contains a complete sedimentary record across the global ocean anoxic event (OAE-2) near the Cenomanian–Turonian boundary. The first recorded Δ¹³C shift occurred over a period of about 60 ky and has induced the total extinction of the deep-dwelling specialized keeled rotaliporids. During this global major sea level transgression, the Bahloul deposition was marked by oxygen-deficient waters creating a benthic total absence interval and a simultaneously thriving of the Nassellarian community just following the second isotopic excursion that lasted about 35 ky. This interval is characterized by the appearance of dwarf individuals. CaCO₃ values and the relative abundance changes of the guembelitriids (planktonic foraminifera) and nassellarians (radiolarian) reveal cyclic variations in surface eutrophy that may be related to intensified upwelling, whereas dominance of the low-oxygen tolerant heterohelicids suggests the development of oxygen minimum zone (OMZ) almost during total Bahloul deposition. The third isotopic excursion, lasting less than 20 ky, precedes several biological and ecological events including filaments appearance, Turonian ammonites (Watinoceras), several ichnofossils levels, the worldwide Heterohelix shift.

Obvious faunal turnover accompanied these paleoceanographic events that induced step-by-step extinction including 37 % of assemblage species extinction and 28 % of appearance (evolution/radiation). Dealing with relative abundances of outgoing and incoming radiolarian species, 55 % in loss is recorded for the total assemblage. The stepwise faunal turnover began with the global sea level rise and rapid increase in d¹³C values, coinciding with laminated black shales onset, and accelerated within maximum organic carbon records and during increased water mass stratification. The oxygen-rich deep waters incursion that favoured new ecological niches creation and new foraminiferal keeled specialized genera appearance proves the end of severe ecological and paleoceanographic stress times reigning near the Cenomanian-Turonian boundary.