Offshore Northern California Research Articles

Calcareous Nannofossil Datum Levels Associated with Pliocene-Pleistocene Boundary: ABSTRACT

The extinction horizon of Discoaster brouweri, or datum, commonly is applied by calcareous nannofossil workers as a criterion for placement of the base of the Pleistocene in deep-sea cores. Investigators also have said that at Le Castella, Italy, gephyrocapsids having a distinct central-bar area (Gephyrocapsa caribbeanica) appear first at a horizon several meters below the extinction horizon of D. brouweri. Deep-sea cores taken on the Nicaragua Rise in the Caribbean also support this relation. Analyses of piston cores from the southern ocean, however, show that the life ranges of D. brouweri and G. caribbeanica are not concurrent at high latitudes, and here the Coccolithus doroncoides zone was erected to span the interval between the datum levels of these species. This in erval zone subsequently has been observed in southern California, the equatorial Pacific (defined there as the Emeliania annula subzone), and in deep-sea cores from the northern Atlantic and northeast Pacific Oceans, and in the Tyrrhenian basin west of Naples, Italy. The magnitude of this interval at high latitudes seemingly is greater because of an earlier last occurrence of D. brouweri in boreal regions about 2.5 m. y. ago (Deep Sea Drilling Project core 173 offshore northern California, where the interval is 75 m). Therefore, it is suggested, that the first appearance of G. caribbeanica provides a better approximation of the base of the Pleistocene than does the last occurrence of D. brouweri. End_of_Article - Last_Page 1840------------