Abstract:
Antarctic Neogene deep sea sediments preserve excellent species-level records of faunal evolution that can be compared to equivalent records of environmental change from stable isotopes of carbon and oxygen, and to changes in the planktonic biota as recorded by carbonate/biosilica values in the sediment. A synthesis of paleoenvironmental data and radiolarian occurrence data from three authors analyses of Ocean Drilling Program Legs 119 and 120 to the Kerguelen Plateau shows an inverse correlation between overall radiolarian diversity and the inferred productivity of the environment. Higher diversity, spumellarian (presumed partly symbiont-bearing) faunas are seen in lower productivity, earlier Neogene carbonate phytoplankton rich sediment, but give way to lower diversity, presumably deeper dwelling, nassellarian dominated faunas (mostly without symbionts) in the later Neogene, in synchrony with the development of higher productivity in the late Neogene Southern Ocean. Major turnover events and increases in average extinction rates are associated with the mid-Miocene (ca. 15-13 Ma) and end-Miocene (ca. 7-4 Ma) increased glaciation and increased productivity shifts on or around Antarctica. Species longevities also decrease substantially during the Neogene, a phenomenon not previously reported for Cenozoic microfossils. In older sediments a bimodal distribution of taxon longevities is observed, although this latter phenomenon may be an artifact of the data analysis. Although environmental change appears to be the primary determinant of evolutionary change in these faunas, biologically mediated secondary effects caused by change in the physical environment (e.g., productivity and nutrient availability), are inferred to be the proximate causes driving evolution.