The photo above is the upper Miocene sandstone outcrop in Yokosuka, Kanagawa, Japan.

The Cenozoic Era is characterized by a long-term (~10^6 to 10^7 year) global cooling trend from the early Paleogene ice-free “greenhouse” conditions towards the Neogene “ice-house” regime, as inferred based on the globally compiled deep-sea benthic foraminiferal oxygen stable isotope records. The long-term Cenozoic climatic trend is punctuated by three major cooling steps around the Eocene/Oligocene boundary (ca. 33.4 Ma), the middle Miocene (ca. 15 Ma), and the late Pliocene (ca. 3 Ma). 

Middle Miocene climatic optimum and the subsequent transition into icehouse

Of the three Cenozoic benthic foraminiferal oxygen isotope events pointed above, the middle Miocene positive shift of ~0.9‰ between ca. 14.5 and 12.7 Ma is generally interpreted to indicate major expansion and permanent establishment of the East Antarctic ice sheet (EAIS) accompanied by some effect of deepwater cooling. Before the middle-late Miocene EAIS/cooling shift, the benthic foraminiferal oxygen isotope record shows a brief but distinct negative excursion in the early middle Miocene that represents the most depleted values of the entire Neogene. This oxygen isotope minimum is considered to reflect an event of global climatic warmth, generally referred to as the early middle Miocene “climatic optimum” (MMCO; ca. 17-15.4 Ma).

Paleoclimate reconstructions based on paleontological records from onshore sections around the circum-Pacific margin reasonably well correspond to the Miocene deepwater oxygen isotope trends. In particular, a wealth of shallow-marine molluscan faunas along the coasts of Japan documents the effects of the MMCO in the mid-latitude western North Pacific, as represented by the “Kadonosawa” fauna, which comprises taxa indicative of subtropical to tropical climatic regimes. The Kadonosawa fauna appear to occur within the period of the deepwater MMCO event. Furthermore, molluscan assemblages similar to the Kadonosawa fauna that are interpreted as warm-temperate to subtropical are recognized in higher latitudes in Sakhalin, Kamchatka, and the south Alaskan Pacific coasts. These early middle Miocene “warm” molluscan faunas in the North Pacific are replace by a relatively “cool” faunas, the timing of which likely corresponds to the deepwater EAIS/cooling oxygen isotope positive shift.

The occurrence of warm-temperate and subtropical molluscan faunas in the high-latitude sections can be interpreted as evidence of an intensive warming of the subpolar waters and a significant thermal redistribution in the North Pacific in response to the global MMCO event. Furthermore, the subsequent retreat of the “warm” molluscan faunas in the North Pacific and the worldwide benthic foraminiferal oxygen isotope positive shift after the MMCO require an effective decrease in the poleward heat transport and reorganization of the global thermal budget.