Changes in upper ocean hydrography and productivity across the Middle Eocene Climatic Optimum: Local insights and global implications from the Northwest Atlantic

摘要

The Middle Eocene Climatic Optimum (MECO; 40.5-40.0 Ma) was a pronounced climatic reversal during a longterm 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 similar to 3 degrees 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 (similar to 150 kyr) before the MECO. The observed similar to 10(2) kyr-scale local shifts in hydrography was possibly driven by the similar to 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.