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Oceanic Anoxic Event 2, spanning the Cenomanian/Turonian boundary (93.9 Ma), was an episode of major perturbations in the global carbon cycle. To investigate the response of biota and the paleoceanographic conditions across this event, we present data from International Ocean Discovery Program sites U1513 and U1516 in the Mentelle Basin (offshore SW Australia; paleolatitude 59°–60°S in the mid‐Cretaceous) that register the first complete records of OAE 2 at southern high latitudes. Calcareous nannofossils provide a reliable bio‐chronostratigraphic framework. The distribution and abundance patterns of planktonic and benthic foraminifera, radiolaria, and calcispheres permit interpretation of the dynamics of the water mass stratification and provide support for the paleobathymetric reconstruction of the two sites, with Site U1513 located northwest of the Mentelle Basin depocenter and at a deeper depth than Site U1516. The lower OAE 2 interval is characterized by reduced water mass stratification with alternating episodes of enhanced surface water productivity and variations of the thickness of the mixed layer as indicated by the fluctuations in abundance of the intermediate dwelling planktonic foraminifera. The middle OAE 2 interval contains lithologies composed almost entirely of radiolaria reflecting extremely high marine productivity; the low CaCO3 content is consistent with marked shoaling of the Carbonate Compensation Depth and ocean acidification because of CaCO3 undersaturation. Conditions moderated after deposition of the silica‐rich, CaCO3‐poor rocks as reflected by the microfossil changes indicating a relatively stable water column although episodes of enhanced eutrophy did continue into the lower Turonian at Site U1516.

<p>At Site U1516 (Mentelle Basin, southeast Indian Ocean, offshore western Australia), the International Ocean Discovery Program (IODP) Expedition 369 recovered an almost complete record of the Upper Cretaceous, including the Oceanic Anoxic Event 2 (OAE 2).<em> </em>Benthic foraminiferal assemblages indicate a distal, outer neritic to bathyal depositional environment. For a better understanding of the paleoenvironmental changes across the OAE 2, we analysed 32 samples for benthic foraminiferal abundance data. The dataset represents one of the few deep water benthic foraminiferal records of the OAE 2 in the southern high latitudes.<em> </em></p> <p>The record of the OAE 2 can be subdivided in pre OAE 2, pre max-CIE, low CaCO<sub>3</sub>, and post low CaCO<sub>3</sub> intervals.<em> </em>The interval of low CaCO<sub>3</sub> content covers a prominent positive Carbon Isotope Excursion (CIE). The Cenomanian-Turonian boundary records an extreme decline in benthic foraminifera during OAE 2. The post low CaCO<sub>3</sub> interval documents a strong repopulation event in benthic foraminifera.<em> </em></p> <p>During the pre OAE2 and pre-max-CIE intervals, taxa like the calcareous deep-water gavelinellids, lingulogavelinellids and gyroidinids are dominant. In the overlying low-carbonate interval, the microfossil record documents a substantial increase in Radiolaria while foraminifera become a rare faunal element<em>.</em> Remarkable changes in benthic foraminiferal assemblage composition are recorded in the initial low CaCO<sub>3</sub> interval directly underlying the maximum CIE. Rather than acting as the trigger, it appears that the max CIE merely interrupted environmental change in the bottom waters.  Furthermore, the comparison of pre- and post-CIE benthic foraminiferal assemblages highlights a distinct repopulation event during the post max- CIE interval mainly represented by a profound increase in the abundance of agglutinated foraminifera and the calcareous epibenthic taxon <em>Osangularia</em>. The dataset collected at Site U1516 documents one of the most complete benthic foraminiferal records across the OAE 2 that registers the Late Cretaceous environmental changes in the Southern Hemisphere.</p>

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<p>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 δ<sup>13</sup>C 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).</p> <p>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.</p> <p>The lower OAE 2 interval is characterized by the abundance of the opportunistic <em>Microhedbergella</em> 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.</p> <p>Lithologies in the middle OAE 2 interval are characterized by a very low CaCO<sub>3</sub> 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 CO<sub>2</sub> from the Kerguelen Plateau Large Igneous Province making waters more corrosive.</p> <p>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 <em>Microhedbergella</em> 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 <em>Gavelinella</em> and <em>Stensioeina</em> and the opportunist <em>Praebulimina</em> show a significant increase in abundance that suggests the presence of oxic-dysoxic bottom waters. On the contrary at Site U1516 <em>Microhedbergella </em>is progressively replaced in abundance by <em>Muricohedbergella </em>to indicate a relatively stable water column with a thick mixed layer and a thin thermocline and frequent episodes of eutrophy toward the top.</p> <p>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.</p> <p>Finally, this study provides a complete record of the microfossil response to the palaeonvironmental perturbation associated to the OAE 2 at southern high latitudes.</p>