Planktonic foraminiferal population dynamics and benthic foraminiferal and radiolaria distributions combined with δ¹³C and δ¹⁸O measurements of both bulk carbonate and foraminifera provide clues concerning the paleoceanographic changes across the Cenomanian-Turonian boundary interval and the Oceanic Anoxic Event 2 (OAE 2) at southern high latitudes. Samples analyzed are from Integrated Ocean Discovery Program (IODP) Expedition 369 Site U1516 in the Mentelle Basin (eastern flank of the Naturaliste Plateau, Indian Ocean, SW Australia). Site U1516 was located at 60°–62°S paleolatitude during the mid-Cretaceous, and it is the first high latitude locality in the Southern Hemisphere where planktonic foraminifera are consistently recorded across the OAE 2 interval and its associated positive δ¹³C excursion.

The sedimentary record at Site U1516 consists of a sequence of alternating black, dark greenish gray, and light greenish gray claystone in the Cenomanian that grade to white and light gray calcareous chalk interbedded with chert in the Turonian. The correlation between the δ¹³C and δ¹⁸O profiles at Site U1516 and the European reference section at Eastbourne (England) coupled with the integrated calcareous plankton biostratigraphy and stable isotopic data at Site U1516, indicate that a complete record of OAE 2 at Site U1516 was recovered.

Below and in the lower part of OAE 2, the planktonic foraminiferal assemblages are dominated by small-sized (125–38 μm) opportunistic species of Microhedbergella and radiolaria indicating a dominantly eutrophic regime. Above the onset of OAE 2, a trough in the δ¹³C profile (Plenus Carbon Isotope Event: P-CIE) coinciding with a δ¹⁸O increase may correspond to the Plenus Cold Event as observed at low latitudes, although no evidence of cooling is registered in the microfossil assemblages. At Site U1516, the middle part of OAE 2 at the initiation of the plateau phase of the δ¹³C profile is masked by absence of carbonate, by the highest TOC values, and high biogenic silica (dominance of radiolaria) indicating this interval corresponded to a time of highly stressed eutrophic conditions with possible shoaling of the Carbonate Compensation Depth (CCD). Above this interval, bulk isotopic results yield lower δ¹³C values, and the CaCO₃ increases are associated with the presence of even smaller-sized Microhedbergella showing cyclic fluctuations in absolute abundances with benthic foraminifera indicating dominantly eutrophic conditions likely affected by upwelling of nutrient-rich and δ¹³C-depleted intermediate water masses. Toward the top of OAE 2 and across the Cenomanian-Turonian boundary interval, the planktonic foraminiferal assemblages show changes in composition (e.g., Microhedbergella is replaced by Muricohedbergella), species occupying relatively deep ecological niches appear and an overall increase in diversity is observed. These features coupled with the foraminiferal species-specific δ¹³C and δ¹⁸O patterns reveal that Site U1516 occupied a paleoceanographic setting still affected by eutrophy likely related to enhanced input of nutrients but with episodes of stability with ecological/thermal separation in the surface waters. This interval also records the highest sea surface water paleotemperatures values estimated as 20°–23°C based on δ¹⁸O values of foraminiferal test and assuming seawater δ¹⁸O values of −1‰_V-SMOW. Mesotrophic to oligotrophic conditions persisted after the OAE 2 and throughout the Turonian as evidenced by a diverse planktonic foraminiferal assemblage with different species occupying separate ecological niches in the mixed layer and thermocline.

浮游有孔虫种群动态和底栖有孔虫和放射虫分布与 δ ¹³ C 和 δ ¹⁸对大块碳酸盐和有孔虫的 O 测量提供了关于跨越 Cenomanian-Turonian 边界区间的古海洋学变化和南部高纬度的海洋缺氧事件 2 (OAE 2) 的线索。分析的样本来自综合海洋发现计划 (IODP) 远征 369 站点 U1516，位于 Mentelle 盆地（自然主义高原东侧，印度洋，澳大利亚西南部）。U1516 站点位于白垩纪中期的 60°–62°S 古纬度，它是南半球第一个在 OAE 2 间隔及其相关的 δ ¹³ C正偏移中一致记录到浮游有孔虫的高纬度地区。

Site U1516 的沉积记录由一系列交替的黑色、深绿灰色和浅绿灰色粘土岩组成，这些粘土岩在 Cenomanian 阶为白色和浅灰色钙质白垩，与土伦阶燧石互层。U1516 站点的 δ ¹³ C 和 δ ¹⁸ O 剖面与伊斯特本（英格兰）欧洲参考剖面之间的相关性，再加上 U1516 站点的综合钙质浮游生物地层学和稳定同位素数据，表明站点的 OAE 2 完整记录U1516 被回收。

在 OAE 2 的下方和下部，浮游有孔虫组合以小型 (125-38 μm) 机会性物种Microhedbergella和放射虫为主，表明主要是富营养化状态。在 OAE 2 开始时，δ ¹³ C 剖面（Plenus 碳同位素事件：P-CIE）中的一个槽与 δ ¹⁸ O 增加可能对应于在低纬度观察到的 Plenus 冷事件，尽管没有冷却的证据在微化石组合中登记。在网站U1516，OAE 2在δ高原阶段的开始，中间部分¹³C 剖面被无碳酸盐、最高 TOC 值和高生物二氧化硅（放射虫占优势）所掩盖，表明该间隔对应于高压力富营养化条件的时间，可能会形成碳酸盐补偿深度 (CCD)。在此区间之上，大量同位素结果产生较低的 δ ¹³ C 值，并且 CaCO _{3 的}增加与甚至更小的Microhedbergella的存在相关，显示绝对丰度的循环波动与底栖有孔虫表明可能受养分上升流影响的主要富营养化条件 -富和 δ ¹³C-耗尽的中间水团。朝向OAE 2和整个森诺曼-土仑边界间隔的顶部，浮游有孔虫组合显示在组成变化（例如，Microhedbergella被替换Muricohedbergella），物种占据相对深的生态位出现和多样性的整体增加观察到。这些特征与有孔虫物种特有的 δ ¹³ C 和 δ ^{18 相结合}O 模式显示站点 U1516 占据仍受富营养化影响的古海洋环境，这可能与营养物质输入增加有关，但随着地表水生态/热分离的稳定发生。此时间间隔也记录基于δ估计为20℃-23℃的最高海面水古温度值^18个有孔虫测试的O值，并假设海水δ ^18个的O值-1‰ _V-SMOW。在 OAE 2 之后和整个土伦纪，中营养到贫营养的条件持续存在，这可以通过多样化的浮游有孔虫组合证明，不同物种在混合层和温跃层中占据不同的生态位。