Magnetobiostratigraphy of Planktonic Foraminiferal Datums: Deep Sea Drilling Project Leg 94, North Atlantic

Abstract

The six Leg 94 sites have produced a unique data set from a north-south transect of sites, all with good paleomagnetic records, and all but one with above-average accumulation rates. We show that it is necessary to have high accumulation rates to obtain good quality magnetobiostratigraphies, and that it is difficult to correlate geological sequences over distances of several hundred kilometers without independent age control. Using the paleomagnetically derived ages for species ranges, we show several examples of diachronic first and last appearance datums between the core sites. This has enabled us to test the existing zonal schemes for the Pliocene-Pleistocene and to identify new zonal markers. INTRODUCTION There are still relatively few Neogene sites with good paleomagnetic control in the North Atlantic. Site 397 off northwestern Africa (Mazzei et al., 1979) has a good paleomagnetic record, and Berggren et al. (in press) used data from that site in their assessment of the ages of planktonic foraminiferal datums. More recently, Site 548 on the Goban Spur, southwest of Ireland, produced a good paleomagnetic record for the Quaternary and late Pliocene (Pujol and Duprat, 1985). Other paleomagnetically controlled datums in the North Atlantic have been derived mostly from piston cores (e.g., Saito et al., 1975; Haq et al., 1977). The six sites drilled on Leg 94 (Fig. 1) all have good paleomagnetic records through continuous Pliocene-Pleistocene sections (Clement and Robinson, this volume), and at all sites except Site 608 those sections accumulated at above average rates. These sites considerably extend our data base, and comparisons of species ranges with the magnetic stratigraphy have made possible not only the testing of existing zonations but also the recognition of new zonal markers. The north-south transect of holes has made possible identification of species showing diachronic first and last appearance datums (FADs and LADs respectively), and has shown the latitudinal extent of some species. Comparison with the ages of the datums in other oceans shows that in some cases evolution was centered in one ocean, or one oceanographic province, with the new species subsequently migrating to other areas. The migratory first appearance of some species (e.g., Globorotalia puncticulata) was synchronic throughout the North Atlantic, and for other species (e.g., Globorotalia truncatulinoides) it was progressively later in higher latitudes. The data suggest that great care must be exercised in atRuddiman, W. R, Kidd, R. B., Thomas, E., et al., Init. Repts. DSDP, 94: Washington (U.S. Govt. Printing Office). 2 Addresses: (Weaver) Institute of Oceanographic Sciences, Brook Rd., Wormley, United Kingdom, (Clement) Ocean Drilling Program, Texas A&M University, College Station, TX. 70°N • Leg 94 sites OLeg 12 sites 49 sites 80°W 70° 50° 40 30° 20° 10° Figure 1. Locations of DSDP North Atlantic sites mentioned text. 10°E in the tempting fine-scale stratigraphy using microfossils alone. Ideally, magnetobiostratigraphy should be carried out in all oceans at a variety of latitudes before species FADs and LADs are used in worldwide correlation. Far from diminishing the value of microfossils, this would lead to a greater understanding of evolution and migratory patterns. Assessment of the ages of datums at all six sites has enabled us to test existing zonal schemes—particularly the PL zonation of Berggren (1973, 1977), which is the most widely used Pliocene zonation for the Atlantic. We have also identified several datum levels which give synchronic age determinations throughout the six sites studied (Weaver and Clement, in press), and we show how these fit within the PL zones. In this work we adopt the time scale of Berggren et al. (in press), and all ages have been calculated—or in the case of previously published