Abstract The Middle Eocene Climatic Optimum (MECO; 40.5–40.0 Ma) was a pronounced climatic reversal during a long-term global cooling period. Despite conspicuous signals of warming and ocean acidification in global marine sedimentary records, changes in upper ocean thermal stratification and productivity have been less comprehensively understood across the MECO. Here we examined stratigraphic records of the middle Eocene succession (43.1–38.6 Ma) at Integrated Ocean Drilling Program Site U1408 in the Northwest Atlantic, to provide a detailed local environmental reconstruction and to bring new insights on spatiotemporal evolution of coupled hydrography and productivity across the MECO based on global comparison. The present temperature–oxygen isotope calibration inferred at least ~3 °C warming in the upper ocean during the MECO at the study site. Whereas further inter-proxy calibration is required, the present estimation supports expanded meridional surface temperature gradient in both hemispheres during the late middle Eocene. Vertical gradients in planktic foraminiferal oxygen isotope ratios revealed that relatively mixed upper water column prevailed during and immediately (~150 kyr) before the MECO. The observed ~102 kyr-scale local shifts in hydrography was possibly driven by the ~2.4-myr eccentricity modulation in the development of seasonal thermocline, rather than greenhouse gas forcing. Planktic and benthic foraminiferal accumulation rates at Site U1408 decreased and increased during the MECO and adjacent cooling episodes, respectively. The relationships between carbonate productivity and local hydrography varied through time in the Northwest Atlantic. Instead, we suggest that marine carbonate productivity and ocean chemistry was controlled by multiple eccentricity-scale changes in weathering intensity and nutrient supply, widely across the major ocean basins. Hypothesized prevailing decline in productivity agrees with a possible warming mechanism of the MECO, although it is challenged by several existing micropaleontological records suggesting increased bio-silica production and eutrophication at varying oceanographic settings. Further integration of different types of proxies and quantitative assessments will help understand complex biotic changes during significant global carbon cycle perturbations in a greenhouse world.

中文翻译：

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中始新世气候最优区上层海洋水文和生产力的变化：来自西北大西洋的本地见解和全球影响

摘要 中始新世气候最适（MECO；40.5-40.0 Ma）是全球长期降温期间明显的气候逆转。尽管在全球海洋沉积记录中有明显的变暖和海洋酸化信号，但整个 MECO 对上层海洋热分层和生产力的变化的了解还不够全面。在这里，我们检查了西北大西洋综合海洋钻探计划站点 U1408 中始新世序列（43.1-38.6 Ma）的地层记录，以提供详细的局部环境重建，并为整个地区的水文和生产力耦合时空演化带来新见解。 MECO 基于全球比较。目前的温度 - 氧同位素校准推断在研究地点的 MECO 期间上层海洋至少升温约 3 °C。尽管需要进一步的代理间校准，但目前的估计支持中始新世晚期两个半球的经向表面温度梯度扩大。浮游有孔虫氧同位素比率的垂直梯度显示，在 MECO 期间和之前（~150 kyr），相对混合的上层水柱占主导地位。观察到的~102 kyr 尺度的水文局部变化可能是由季节性温跃层发展中~2.4-myr 的偏心率调制驱动的，而不是温室气体强迫。U1408 站点的浮游和底栖有孔虫积累率分别在 MECO 和相邻的冷却事件期间减少和增加。在西北大西洋，碳酸盐生产力与当地水文之间的关系随时间而变化。反而，我们认为，海洋碳酸盐生产力和海洋化学受风化强度和养分供应的多种偏心率变化控制，广泛分布于主要海洋盆地。假设普遍存在的生产力下降与 MECO 可能的变暖机制一致，尽管它受到一些现有微古生物学记录的挑战，这些记录表明在不同的海洋环境中生物二氧化硅生产和富营养化增加。不同类型代理和定量评估的进一步整合将有助于了解温室世界中重大全球碳循环扰动期间复杂的生物变化。假设普遍存在的生产力下降与 MECO 可能的变暖机制一致，尽管它受到一些现有微古生物学记录的挑战，这些记录表明在不同的海洋环境中生物二氧化硅生产和富营养化增加。不同类型代理和定量评估的进一步整合将有助于了解温室世界中重大全球碳循环扰动期间复杂的生物变化。假设普遍存在的生产力下降与 MECO 可能的变暖机制一致，尽管它受到一些现有微古生物学记录的挑战，这些记录表明在不同的海洋环境中生物二氧化硅生产和富营养化增加。不同类型代理和定量评估的进一步整合将有助于了解温室世界中重大全球碳循环扰动期间复杂的生物变化。