Paleoenvironmental trends of late Pliocene benthic foraminifera from DSDP Site 548, northeast Atlantic
Benthic foraminfera from 60 samples of the Late Pliocene section at DSDP Site 548 in the NE Atlantic were studied to examine paleoenvironmental responses of the species during major climatic changes caused by the onset of Northern Hemisphere glaciation. Factor analysis of the species abundance data delineated two different patterns of variation. The first pattern illustrates a long-term trend in which a Globocassidulina subglobosa dominated assemblage is replaced in steps by an assemblage dominated first by Bulimina mexicana and then, finally by Nonion barleeanum. This trend correlates to a late Pliocene shift in surface waters (and the resultant change in surface ocean productivity),from warm-temperate to subpolar. The second pattern is of an episodic nature with no overall trend. This pattern appears related to an onset of glaciation and the associated glacial/interglacial cycles. Using the oxygen isotope record, five major glacial/interglacial periods were recognized. Each exhibits a unique assemblage with no two cycles showing similar faunal abundance patterns. The constantly changing nature of the assemblages indicates that environemntal conditions of successive cycles were not repetitive or constant. All the dominant species (with the exception of N. barleeanum) attain maximum abundance at least once during a peak glacial pulse. Hence, "glacial" and "interglacial" fauna are not consistently different from one another. Environmental parameters were monitored using δ¹⁸0 (temperature), sediment size analysis (substrate and calcium carbonate content) and planktonic assemblage diversity (food availability). A correlation analysis between the factor trends and these environmental parameters illustrates the inability of any one particular variable to explain the faunal abundance patterns. Therefore, benthic foraminifera species abundances at Site 548 must result from a complex interplay of any and/or all of these environmental parameters. Other biological parameters such as competition, predation and organic carbon content of the substrate undoubtedly affect distribution in, as yet, undetermined ways.