Around fifteen million years ago the planet was experiencing a ‘climatic optimum’ known as the Middle Miocene Climatic Optimum (Flower and Kennett, 1993). This marked the warmest period since the late Eocene, at around 34 Ma (Flower and Kennett, 1993; Zachos et al., 2001). Evidence for this warm period can be seen in the distribution of tropical fauna, including benthic foraminifera and molluscs as far south as New Zealand’s South Island (Flower and Kennett, 1993). Furthermore, tropical corals and mangroves have also been discovered as far north as the Japanese Island of Honshu (Flower and Kennett, 1993), which today is in the temperate zone. The appearance in the fossil record of tropical palm trees and crocodiles in now temperate Northern …show more content…
Northern hemisphere forests turned from hardwood to the colder climate conifer, tropical zones in Europe (marked by mangroves) retreated, and large, tropical foraminifera in New Zealand disappeared (Hornibrook, 1992 as cited in Flower and Kennett, 1993).

This cooling culminated in the major glaciation of the northern hemisphere, which occurred around 2.5 -2.7 Ma (Bartoli et al., 2005; Haug et al., 2005; Haug and Tiedemann, 1998; Haug et al., 2004) and can be contributed to a number of factors, including the closing of the straits of Panama.

Haug et al. (2004) suggest the creation of the Panamanian land bridge between North and South America was instrumental in creating the glaciation of the Northern Hemisphere. They point to the fact that the Antarctic had an ice cap 34 Ma, whereas ice caps in the Arctic only appeared 2.5-2.7 Ma. The Antarctic ice caps coincide with the formation of the Himalayas, which stripped carbon dioxide out of the air through erosion, causing the climate to cool (Haug et al, 2004; Raymo and Ruddiman, 1992). This, as well as the separation of Gondwana and creation of the Antarctic Circumpolar Current triggered the antarctic to freeze over (Haug et al, 2004). However, the currents affecting the northern hemisphere didn’t allow for a widespread glaciation, despite the gradual cooling of the …show more content…
Haug et al. (2004) suggest that the Panama gateway affected patterns of deep water circulation through diverging salinity levels between the Pacific and Atlantic oceans, causing the intensification of the Gulf Stream, as seen in diagram