Response of microfossils to the Late Cretaceous Oceanic Anoxic Event 2 at southern high latitudes

Oceanic Anoxic Event 2 (OAE 2), spanning the Cenomanian/Turonian boundary (93.9 Ma) was an episode of major perturbations in the global carbon cycle with a δ¹³C excursion of 2-3‰ documented worldwide. To investigate the response of biota to OAE 2 at high latitudes, we present data from Integrated Ocean Discovery Program (IODP) Sites U1513 and U1516 drilled in the Mentelle Basin (Indian Ocean, offshore SW Australia; paleolatitude of 59°-60° S in the mid-Cretaceous).

The distribution and variations in abundance of planktonic and benthic foraminifera, radiolaria, and calcipheres and their paleoecological preferences permit interpretation of their response to the dynamics of the water mass stratification during the OAE 2.

The lower OAE 2 interval is characterized by the abundance of the opportunistic Microhedbergella that alternates with common eutrophic radiolaria in the surface waters. The concomitant high abundance of benthic foraminifera, especially gavelinellids, might suggest episodes of enhanced oxygenation at the sea floor. The eutrophic feature of the water masses seems more pronounced at Site U1513 because of the rarity of the intermediate dwelling planispiral taxa and the consistent occurrences of the opportunistic and eutrophic calcispheres. In this interval biota indicate a reduced water mass stratification with alternating episodes of enhanced surface water productivity and variations of the thickness of the mixed layer.

Lithologies in the middle OAE 2 interval are characterized by a very low CaCO₃ content and are marked by the near absence of calcareous foraminifera. Radiolaria are the sole microfossils present except for a couple of isolated samples that contain three benthic foraminifera at Site U1516 and calcispheres at Site U1513.The high abundance of radiolaria and the rarity of calcareous microfossils indicate very high fertility conditions and, possibly, shoaling of the CCD which resulted from the emission of volcanogenic CO₂ from the Kerguelen Plateau Large Igneous Province making waters more corrosive.

Conditions moderate after this extreme perturbation as reflected by the sudden increase in abundance of foraminifera and return of carbonate deposition presumably after deepening of the CCD. At Site U1513 the dominance of Microhedbergella over the other planktonic foraminiferal genera, the presence of lower mixed layer planispiral and scattered keeled taxa, and the radiolaria distributions and abundances might suggest the presence of a thick mixed layer with significant thermal differences between surface and thermocline waters. Benthic agglutinated taxa, the epifaunal Gavelinella and Stensioeina and the opportunist Praebulimina show a significant increase in abundance that suggests the presence of oxic-dysoxic bottom waters. On the contrary at Site U1516 Microhedbergella is progressively replaced in abundance by Muricohedbergella to indicate a relatively stable water column with a thick mixed layer and a thin thermocline and frequent episodes of eutrophy toward the top.

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.

Finally, this study provides a complete record of the microfossil response to the palaeonvironmental perturbation associated to the OAE 2 at southern high latitudes.