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The late Oligocene–early Miocene interval records a discernable episode of phosphorite formation, which is considered as the first of four main phosphogenic episodes during the late early and late Cenozoic. In order to better constrain the processes leading to widespread phosphorite formation we present new radiometric, geochemical, palynological, and sedimentological data from a drill core of the Roca Fosfórica Mexicana phosphorite mine at San Juan de la Costa, Baja California Sur (Mexico). In this region, phosphogenesis was enabled by the combination of high productivity and low sediment-accumulation rates due to enhanced upwelling and low detrital input related to regionally dry climate conditions. Phosphatic particles were formed in a shallow and well-oxygenated setting, subsequently concentrated by winnowing, and transported by gravity currents, which were mostly triggered by seismic activity. Following their deposition in a deeper and less well oxygenated setting pervasive phosphogenesis contributed to cementing the accumulated phosphatic grains. Correlation with global paleoclimate records suggests that this phosphogenic episode was linked to the expansion of the Antarctic ice sheet. Glacial weathering and the establishment of large-amplitude glacio-eustatic variations enhanced phosphorus supply on a global scale. Both glacial and interglacial phases participated in enhancing primary productivity in oceans, increasing the phosphorus flux into sediments, and favoring phosphogenesis, with glaciation being the prime cause. In addition, radiometric ages obtained in this study (28.62, 28.1, 27.19, 27.08, and 26.94 Ma) indicate that the onset of the late Oligocene–early Miocene phosphogenic episode was diachronous on a global scale with 2–3 m.y. older ages in the eastern Pacific in comparison to the Mediterranean and central Atlantic. This delay is explained by regional differences in paleoenvironmental and paleoceanographic conditions.

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Upper Oligocene and lower Miocene, siliceous, organic‐, and phosphate‐rich sediments are widespread in Baja California Sur (Mexico). A representative section at La Purísima was analysed for its sedimentology, stratigraphy, geochemistry, and mineralogy. A corresponding age model was obtained by dating zircons from ash layers (27.84 ± 0.33 to 21.21 ± 0.59 Ma). The sediments were deposited in an upwelling‐dominated, hemipelagic setting, for which the presence of lamination, the scarcity of benthic organisms (except for in gravity‐flow deposits), and redox‐sensitive trace‐element enrichments indicate oxygen‐depleted conditions. Anoxic conditions were particularly strong around 27 and 24–22 Ma. Gravity‐flow deposits are frequent and predominantly composed of phosphatic‐coated grains. They were generated by seismic and volcanic activity, as is indicated by the close association with volcanic ash layers. The phosphatic‐coated particles were formed in a more proximal, better‐oxygenated shelf environment. They precipitated also in situ within the hemipelagic sediments, where they were often concentrated by subsequent winnowing. In situ phosphogenesis also partly cemented the gravity‐flow deposits. At La Purísima, phosphogenesis occurred throughout the time interval investigated and was particularly important around 28–25.5 and 23.5–21.5 Ma. These two time intervals correspond to the late Oligocene glacial maximum and the Oligocene–Miocene and early Miocene glacial intervals Mi1 and Mi1a. This provides evidence for the increasing importance of glacial denudation during the Oligocene, which led to an enhanced phosphorus flux into the ocean. Cooler climates also promoted the efficient transfer of phosphorus to thermocline waters by increased upwelling. Subsidiary phases of phosphogenesis during the intervening warm periods are explained by the weathering of glacial legacy sediments. These observations suggest that during the transition from greenhouse to icehouse conditions in the Oligocene and Miocene, new and radical changes in the global phosphorus cycle affected and partly inverted feedback mechanisms between climate, geochemical cycles and life, and profoundly influenced the biosphere and its evolution.

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