The early Eocene climatic optimum, which constituted the peak of the long-term early Cenozoic global warming, had a significant impact on the environmental evolution of terrestrial and oceanic areas. Surprisingly, however, its influence on continental margins is poorly known. New insights are provided from a sedimentological, stable isotope, mineralogical, and micropaleontological study of an 1100-m-thick Lower–Middle Eocene deep-marine succession that accumulated on the North Iberian continental margin. The early Eocene climatic optimum is represented by a 410-m-thick interval characterized by scarcity of hemipelagic limestones, abundance of dark marls, which record a reduction in calcium carbonate content and an increase in kaolinite, and the occurrence of conspicuous red layers with high siderite and pyrite content.
Series of stratigraphically significant events frame the early Eocene climatic optimum. Based on this analysis, the environmental influence of the early Eocene climatic optimum started at 52.6 Ma and lasted ∼2.3 m.y. Its onset is marked by rapid drops in δ13C and δ18O, which record the addition of 13C-depleted carbon into the ocean-atmosphere system for 80 k.y. and a concomitant warming. A hotter climate and a perennial rainfall regime increased the supply of terrestrial clays, organic matter, and iron oxides into the sea. Eventually, these changes affected the deep-sea bottom 270 k.y. after the onset of the early Eocene climatic optimum, creating conditions in which opportunistic benthic foraminifera thrived, and leading to increased methanogenesis in the subsurface, which caused the formation of siderite. A subsequent gradual recovery culminated abruptly at 50.3 Ma with a global cooling episode, which is locally recorded by the accumulation of lowstand resedimentation deposits.