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Ratios of Antarctic radiolarians, Spongotrochus glacialis and Lithelius nautiloides, to the subantarctic radiolarians, Lithamphora furcaspiculata and Theocalyptra bicornis, define at least 4 major cold intervals within the Brunhes Normal Magnetic Epoch, with a temperature range from less than 0 to about 5°C. In tropical areas, cooler temperatures of the Brunhes are suggested by the marked decline of Sphaeroidinella dehiscens and an increase in temperate species.
In the Antarctic area, temperate species of radiolarians such as Eucyrtidium acuminatum, Lamprocyclas maritalis, and Saturnulus planetes reflect generally somewhat warmer conditions in the Matuyama than during most of the Brunhes; variations in their abundances suggest perhaps 5 warmer cycles above the base of the Gilsa event with a range between about 5 and 15°C. In tropical areas, warmer temperatures are indicated by the common occurrences of Sphaeroidinella dehiscens and the absence or very uncommon occurrences of temperate species.
In temperate parts of the circum-Pacific area, the origin and development of Globorotalia truncatulinoides and the extinction of discoasters occurred long before the influx of polar populations of Turborotalia pachyderma. Thus, the base of the Pleistocene as defined by these events indicates that the Pliocene-Pleistocene boundary must be within the upper part of strata previously referred to as upper Pliocene in California. It is postulated that the subsequent major cooling indicated by the influx of sinistrally coiled populations of Turborotalia pachyderma is near the Brunhes-Matuyama boundary and may be near the base of the classic glacial Pleistocene.
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