Southern Ocean pliocene paleotemperature variation from high-resolution silicoflagellate biostratigraphy

Applying the modern biogeographic distribution of silicoflagellate genera Dictyocha and Distephanus and calcareous nannoplankton as an analog, the biostratigraphic record of these phytoplankton can be used qualitatively to examine past changes in surface temperature in the Southern Ocean. The Antarctic Polar Frontal Zone (PFZ) is a major biogeographic barrier, limiting Dictyocha and calcareous nannoplankton to areas north of this zone. Ocean Drilling Project (ODP) holes 748B and 751A on the southern Kerguelen Plateau, 900 km south of the present location of the PFZ, contain expanded Pliocene sections through four time intervals: ∼4.5 to 4.2 Ma (Hole 751A), ∼3.7 to 3.1 Ma (Hole 748B), ∼3.6 to 3.5 Ma (Hole 751A), and ∼3.2 to 3.0 Ma (Hole 751A). Three distinct peaks in Dictyocha and calcareous nannofossil abundance reflect surface-water warming of at least 4°C through either southward migration of the PFZ or a weakening of the thermal gradient across this zone. Peak warming in the intervals studied is centered near ∼4.3 Ma, with other brief, yet significant, warming events near ∼4.5, ∼4.2, and ∼3.6 Ma. Cooling as a result of northward migration of the PFZ, or intensified thermal gradient, is indicated by the absence of Dictyocha and calcareous nannofossils in other stratigraphic intervals. These data support previous studies indicating Pliocene oceanic and climatic variability in the southern high latitudes. Silicoflagellate abundance patterns from Hole 751A identify taxonomic relationships between several morphotypes. The simultaneous appearance and proportional occurrence of five-sided and four-sided Dictyocha specimens suggests a close relationship between these two forms. Distephanus crux s.l. and Dictyocha aspera var. pygmaea also show a near mutually exclusive abundance pattern in Hole 751A and are similar in basal ring size and morphology, indicating they are conspecific. They differ only in apical structure, whereby Distephanus crux s.l. may have formed an apical bar under fluctuating environmental conditions in the Early Pliocene. These variations in silicoflagellate architecture appear to be related to paleoenvironmental changes indicating that silicoflagellate morphology may represent a more useful paleoceanographic tool that has been noted previously.