ABSTRACT

Based on nannofossils a carbonate core from the South Pacific (Eltanin 21-5; 36°41S; 93°38W; length 480 cm; depth 3121 m.) is Upper Pliocene to Lower Pleistocene in age. The Pliocene-Pleistocene boundary (325cm) is placed at the last appearance of most discoaster species in the core including D. pentaradiatus and D. surculus. Discoaster brouweri extends higher, to 225 cm where it also becomes extinct. Above the top of the Pliocene, the presence of D. brouweri and absence of Gephyrocapsa oceanica indicates a lower Pleistocene age, with the middle Pleistocene and much of the Upper Pleistocene missing in unconformity near the core top.

This core, which lies south of the tropics, is significant in showing alternations of dominantly keeled and dominantly non-keeled populations of the Globorotalia truncatulinoides-G. tosaensis plexus. The lower (425-480 cm) and upper (0-130 cm) core sections contain populations dominated (>78%) by keeled forms referable to G. truncatulinoides, while intermediate intervals between 198 and 400 cm contain populations dominated (>80%) by non-keeled forms which agree well with topotypes of G. tosaensis. Transitional populations occur between 145 and 180 cm.

Globorotalia truncatulinoides is associated in the core only with marginal tropical foraminiferal faunas including Globorotalia menardii, Globigerinoides conglobatus and "Globigerina" dutertrei while G. tosaensis is associated with a cooler-water planktonic foraminiferal assemblage lacking these species and with higher frequencies of Globorotalia inflata and right coiling Globigerina pachyderma. Likewise, the warm-water coccolith Umbilicosphaera leptopora exhibits marked increases in frequency in the upper and lower core sections containing G. truncatulinoides.

Although not decisive, this sequence suggests that during the Upper Pliocene to Lower Pleistocene, at least in this area, G. truncatulinoides and G. tosaensis were either phenotypic variants or separate subspecies or species with distinct environmental preferences. It also provokes speculation as to whether the G. tosaensis to G. truncatulinoides evolutionary bioseries reported by a number of workers near the Pliocene-Pleistocene boundary in tropical deep-sea areas, including the Gulf of Mexico, is instead the result of ecological or oceanographic change.

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