Nannoplankton as indicators of climatic variability in the Upper Pliocene
Discoasters are the remains of an enigmatic group of nannoplankton, whose last representatives disappeared globally at 1.89 Ma. The one million years prior to their extinction has been analysed in high time resolution and a global database has been developed from ten sites in the Atlantic, Indian and Pacific Oceans. It has been shown that the variability in Discoaster abundance at low latitudes cannot be attributed solely to changes in sea-surface temperature; discoasters were demonstrated to be suppressed at upwelling sites. In combination with global satellite images of phytoplankton blooms (Lewis, 1989) and maps of modern sea-surface temperatures (CLIMAP, 1981), Discoaster abundance variations were interpreted in terms of the interplay between productivity pressure and sea-surface temperature. Discoasters thrived in the Indian Ocean at Site 709 (4°S) in warm waters with little productivity pressure, whereas Discoaster abundance was suppressed at Sites 677 (l°N) and 662 (l°S) in the Pacific and Atlantic, which although located in warm waters, were affected by upwelling. In addition to the reduced Discoaster abundances associated with upwelling, sea-surface temperature gradients are a significant factor when comparing high and low latitude sites. A marked drop in Discoaster abundance is apparent between 41°N and 56°N (i.e., between Sites 607 and 552). Relative abundances of species in the Discoaster assemblage revealed clearly their contrasting environmental preferences; D. brouweri was produced under a wide range of conditions, favouring warm, low productivity waters; D. asymmetricus and D. tamalis were produced in higher abundances relative to D. brouweri during cooler, low productivity episodes; D. pentaradiatus showed highest absolute abundance in warm, low productivity waters, although increasing in relative abundance at higher latitudes; D. surculus increased in relative abundance in high productivity regions and in cooler waters. At all sites, though most markedly at higher latitudes, Discoaster abundances declined after 2.4 Ma, when glaciation began in the North Atlantic.