UNRAVELLING THE MICROFOSSILS RECORD OF LATE CRETACEOUS OCEANIC ANOXIC EVENT 2 IN THE SOUTHERN HIGH LATITUDES (IODP SITES U1513 AND U1516, SE INDIAN OCEAN)

Oceanic Anoxic Event 2 (OAE 2), at the Cenomanian/Turonian boundary (93.9 Ma), was an episode of major perturbations in the global C cycle with a δ¹³C excursion of 2-3‰ documented worldwide. To investigate the response of biota and the paleoceanographic conditions at high latitudes, we present microfossils and geochemical data from Integrated Ocean Discovery Program (IODP) Sites U1513 and U1516 drilled in the Mentelle Basin (NE Indian Ocean; paleolatitude 60° S).

The distribution and variations in abundance of planktonic and benthic foraminifera, radiolaria, and calcipheres are examined in light of their known paleoecological preferences to interpret the dynamics of the water mass stratification. At both sites the middle part of OAE 2 contains an interval composed of siliceous and organic matter-rich sediments and lacking CaCO₃ that at Site U1513 spans most the stratigraphic interval δ¹³C positive excursion, whereas both the initiation and waning portion of the excursion is preserved at Site U1516.

The interval below OAE 2 is characterized by a reduced water mass stratification with alternating episodes of enhanced surface water productivity and variations of the thickness of the mixed layer. The heart of OAE 2 is entirely composed of radiolaria reflecting extremely high marine productivity and lack of CaCO₃ in this interval is consistent with marked shoaling of the Carbonate Compensation Depth which likely resulted from the emission of volcanogenic CO₂ from the Central Kerguelen Plateau Large Igneous Province making waters more corrosive.

Conditions moderate after this extreme environmental perturbation as reflected by an increase in abundance and diversity of planktonic foraminifera indicating a more stable water column. Conditions were apparently more stable at Site U1513 than at Site U1516 where episodes of enhanced eutrophic continue into the lower Turonian. At both sites the termination of OAE 2 does not correspond to modifications in the microfossils assemblages; rather, they maintain the same features observed in the underlying interval.

Overall, this study provides a complete record of the paleoceanographic changes registered by the microfossils allowing to understand causes and consequences of the paleoenvironmental perturbations associated to OAE 2 at high latitudes in the Southern Hemisphere.