| Abstract: | Twenty-seven radiolarian species and species groups are identified and used in this study. Their stratigraphic ranges span from Late Miocene to modern times and they appear to have distributions which have changed little over this interval of time. Census data for these 27 taxa in surface sediment samples are used to define six assemblages (Q-mode factors). The distribution of these assemblages are similar to modern water-mass distributions and can be statistically related to modern sea-surface temperatures.Census data on these same 27 taxa have been collected in the Upper Miocene intervals of six Pacific sites. These faunal data are described in terms of the modern radiolarian assemblages; and based on the multiple regression equations relating the modern assemblages to modern temperatures, estimates of Late Miocene temperatures are made. The estimated temperatures indicate that there is an overall cooling trend in the later part of the Miocene (magnetic Chron 8 to Chron 5), punctuated by several distinct cooling events. In August the low-latitude sites appear to be slightly cooler, and the mid-latitude sites warmer than in modern times. In February both the low- and mid-latitude sites appear to be warmer than modern temperatures at the same locations. The relatively warm temperatures near the base of the interval studied may be associated with the passage of warm tropical Atlantic waters into the eastern Pacific via the Isthmus of Panama. The general cooling trend seen in these records is thought to be associated with the gradual closure of these straits and the increased influence of the high southern latitudes. |
