Calcareous nannoplankton live in the upper surface waters of the oceans and are thus directly influenced by surface water changes. The discoaster group has long been recognized as a group that prefers warm waters, and several earlier workers have produced paleotemperature studies using the ratio of warm-water discoasters as a group to cool-water Chiasmolithus or Coccolithus (e.g., Bukry, 1978, 1981; Haq et al., 1977; Siesser 1980, 1984; Raffi and Rio, 1981). In the Neogene, however, several discoasters (D. variabilis, D. intercalaris, D. tamalis, and D. asymmetricus) are believed to have preferred cool waters (Bukry, 1981; Rio et al., 1990b). A single discoaster species, Discoaster brouweri, which has a well-established preference for warm waters (e.g., Bukry, 1978, 1981; Siesser, 1975; Müller, 1985; Wei et al., 1988) was thus selected as the warm-water proxy for this study.
Coccolithus pelagicus is used as the cool-water proxy. C. pelagicus lives in cold-temperate (6°-18°C) Northern Hemisphere waters and upwelling regions today (McIntyre et al., 1970; Raffi and Rio, 1981; Giraudeau et al., 1993; Giraudeau and Bailey, 1995; Cachao and Moita, 2000) but has apparently changed its habitat with time. In the Miocene and early Tertiary, C. pelagicus was common in tropical environments as well as in cooler waters (Bukry, 1981). Bukry (1981) made a careful analysis of the distribution of this species, concluding that by the Pliocene C. pelagicus had evolved an affinity for cool water that made it an effective proxy for determining paleotemperature trends. Raffi and Rio (1981) also concluded that C. pelagicus was a good paleotemperature indicator for the Mediterranean during the late Pliocene.
D. brouweri and C. pelagicus have additional advantages in that they are both normally significant components of nannofossil assemblages and are also less affected by diagenetic changes than many other species. The changing downhole relative abundance of these two nannofossil species should, therefore, be a good paleotemperature indicator, reflecting gross changes in water temperature with time.