Pliocene Paleo-Oceanographic Perturbations and Ecosystem Response: Implications for Planktonic Foraminiferal Datums

Ronald E. Martin, Brenda Spotz

ODP hole 625B (Legg 100, northeastern Gulf of Mexico) represents a nearly continuous record of Pliocene paleo-oceanographic development. Planktonic foraminiferal species abundances document the effect of physical and biotic processes on the operational utility of industrial biostratigraphic datums.

Early Pliocene water mass structure was characterized by a shallow thermocline, high productivity, and low species diversity. Deep-dwelling species (e.g., Globorotalia margaritae) were packed into the environment as the niches of dominant species (e.g., Neogloboquadrina humerosa) shrank. Hence, operationally less reliable first appearance datums (FADs) dominated this ecosystem. Rare specimens of a Globorotalia limbata/Globorotalia miocenica intergrade appear near the base of a broad abundance peak in Neogloboquadrina pachyderma, which may reflect water mass reorganization (and allopatric speciation?) in the Caribbean associated with the closure of the Isthmus of Panama. The Globigerina nepenthes last appearance datum occurs abruptly as the middle Pliocene thermocline deepens and productivity declines. Characteristic early Pliocene surface and intermediate-water species (e.g., Sphaeroidinellopsis spp.) were replaced by Dentoglobigerina altispira and Globigerinoides spp., and deep-water Globorotalia margaritae gave way to Globorotalia multicamerata, G. crassula, and G. crassaformis. The G. menardii complex was nearly excluded from the middle Pliocene.

Niches vacated by extinctions about 2.8 Ma were filled by new intermediate and deep-dwelling species (e.g., Neogloboquadrina dutertrei, Globorotalia inflata, G. ungulata, G. hirsuta, and G. tosaensis). Hence, FADs again predominated in the late Pliocene. G. miocenica eventually gave way to the G. menardii complex.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.