Morphometric redefinition of the component chronospecies of the Globorotalia conoidea - G. inflata lineage in DSDP Hole 29-284

Phylo-zonations (or lineage-zonations) are based upon morphological changes within individual evolutionary lineages. These zonations, although potentially of use for stratigraphic subdivision and correlation, often suffer from a lack of quantitative exactness in the definitions of chronospecies. Thus exact reproducibility is hindered for stratigraphic determinations. The potential of morphometrically defined phylo-zonations is demonstrated on a temperate South Pacific Late Cenozoic lineage of planktonic foraminifera (Globorotalia conoidea through intermediate forms to Globorotalia inflata in DSDP Site 284) exhibiting phyletic gradualism. Our sampling interval is about 0.1 m.y. during the last 8 m.y. Changes in the number of chambers in the final whorl, test conicalness, percentage of keeled forms, and test roundness or inflatedness, are used to quantitatively define the following five chronospecies: G. conoidea (Late Miocene; 6.1->8.3 m.y.), G. conomiozea (latest Miocene ; 5.3-6.1 m.y.), G. puncticulata sphericomiozea (earliest Pliocene; 4.5-5.3 m.y.), G. puncticulata puncticulata (Early-Middle Pliocene; 2.9-4.5 m.y.), and G. inflata (Late Pliocene-Quaternary; 0-2.9 m.y.). This phylo-zonation is directly applicable to temperate cool subtropical Southern Hemisphere areas where the evolution took place (Kennett, 1967, 1973; Scott, 1979). It is still not known if the lineage occurs elsewhere; thus the applicability of the phylo-zonation over broader areas is still uncertain. Trends in general size and aperture shape seem to be climatically controlled, and thus may be only of local stratigraphic utility. The practical applications of morphometric phylo-zonation for stratigraphy is to a large extent dependent upon the amount of time and effort required to statistically define the trends. Experiments with large numbers of subsamples from this lineage demonstrate that accurate stratigraphic determinations are possible from measurements on only 15 specimens per sample, except for those very close to chronospecies boundaries.

(Table 1) Age determination of sediment core MD02-2589

Sortable silt mean grain sizes together with oxygen and carbon isotopic data produced on the benthic foraminiferal species Fontbotia wuellerstorfi are used to construct high-resolution records of near-bottom flow vigour and deep water ventilation at a core site MD02-2589 located at 2660 m water depth on the southern Agulhas Plateau. The results suggest that during glacial periods (marine oxygen isotope stages 2 and 6, MIS 2 and MIS 6, respectively), there was a persistent contribution of a well-ventilated water mass within the Atlantic to Indian oceanic gateway with a d13C signature similar to present-day Northern Component Water (NCW), e.g., North Atlantic Deep Water (NADW). The records of chemical ventilation and near-bottom flow vigor reflect changes in the advection of northern source waters and meridional variability in the location of the Antarctic Circumpolar Current and its associated fronts. We suggest that during Termination II (TII), changes in chemical ventilation are largely decoupled from near-bottom physical flow speeds. A mid-TII climate optimum is associated with a low-flow speed plateau concurrent with a period of increased ventilation shown in the benthic d13C of other Southern Ocean records but not in our benthic d13C of MD02-2589. The climate optimum is followed by a period of southern cooling around 128 ka coincident with a stronger influence of NCW to interglacial levels at around 124 ka. All proxy records show a near synchronous and rapid shift during the transition from MIS 5a-4 (73 ka). This large event is attributed to a rapid decrease in NADW influence and replacement over the Agulhas Plateau by southern source waters.

(Table 1) Abundance of tunicate spicules along the Great Barrier Reef-Queensland Plateau transect

The abundance patterns of tunicate spicules are documented for the Pliocene-Pleistocene sediments at seven sites along the Great Barrier Reef-Queensland Plateau transect. The spatial distribution pattern indicates that tunicate spicules were limited to waters shallower than 900 m. The occurrences of tunicate spicules at Sites 822 and 823 that are deeper than 900 m are ascribed to downslope transport, and their distribution patterns can be used to monitor downslope transport processes. The first common occurrence of tunicate spicules at Sites 822 and 823 around 1.6 Ma may suggest the initiation of the central Great Barrier Reef at this time. The morphology of tunicate spicules varies greatly and appears to be gradational among different forms. Older tunicate assemblages are less diverse than those in younger sediments, presumably because of diagenesis. Tunicate spicules do not appear to be a promising biostratigraphic tool for the Pliocene-Pleistocene.