Graham L. Williams

Eocene circulation of the Southern Ocean: Was Antarctica kept warm by subtropical waters?

[1]xa0Near the Eocenes close (∼34 million years ago), the climate system underwent one of the largest shifts in Earths history: Antarctic terrestrial ice sheets suddenly grew and ocean productivity patterns changed. Previous studies conjectured that poleward penetration of warm, subtropical currents, the East Australian Current (EAC) in particular, caused Eocene Antarctic warmth. Late Eocene opening of an ocean gateway between Australia and Antarctica was conjectured to have disrupted the EAC, cooled Antarctica, and allowed ice sheets to develop. Here we reconstruct Eocene paleoceanographic circulation in the Tasmanian region, using (1) biogeographical distributions of phytoplankton, including data from recently drilled Ocean Drilling Program Leg 189 sites and (2) fully coupled climate model simulations. We find that the EAC did not penetrate to high latitudes and ocean heat transport in the region was not greater than modern. Our results do not support changes in “thermal isolation” as the primary driver of the Eocene-Oligocene climatic transition.

Dinoflagellate diversity through time

Abstract Updated plots show that fossil dinoflagellate species diversity is a more sensitive parameter than generic diversity. Following a Norian species diversity of 33, a marked decrease in the early Jurassic probably reflects lack of data. Apart from a decline in the Portlandian (75) and Neocomian, there was then a continuous increase which reached a peak of 279 species in the Albian, resulting from an influx of 150 species in this stage. The Campanian —Maastrichtian was characterized by a large number of extinctions. Tertiary species diversity reached a peak in the early Eocene (206), followed by a reduction to the 44 dinoflagellate species known from the Pleistocene. The preferential preservation of the encysted over the motile stage in dinoflagellates suggests that the diversity data may primarily represent cyst-forming species.

A revision of the Paleogene dinoflagellate genera Areosphaeridium Eaton 1971 and Eatonicysta Stover and Evitt 1978

Northern Hemisphere specimens of Areosphaeridium diktyoplokus (Klumpp 1953) Eaton 1971, the type of Areosphaeridium Eaton 1971, and Eatonicysta ursulae (Morgenroth 1966) Stover and Evitt 1968, the type species of Eatonicysta Stover and Evitt 1978, have a gonyaulacalean sexiform tabulation pattern. Other Northern Hemisphere species of Areosphaeridium, such as Areosphaeridium arcuatum Eaton 1971, have a gonyaulacalean partiform tabulation pattern and are more closely akin to Southern Hemisphere specimens of Areosphaeridium diktyoplokus. Accordingly, we emend the diagnosis of Areosphaeridium and Eatonicysta and propose the genus Enneadocysta for those species formerly included in Areosphaeridium but with a partiform tabulation pattern. The genus Cooksonidium is proposed for forms with a sexiform tabulation pattern and both simple intratabular processes and process complexes. The morphologic reinterpretations have resulted in some revisions to the morphogenetic series defined by Eaton (1971). The stratigraphic and geographic ranges of the four genera are distinctive. Areosphaeridium and Eatonicysta have been recorded only from the Eocene of the Northern Hemisphere. Cooksonidium is restricted to the Late Eocene of the Southern Hemisphere. Enneadocysta has a confirmed Mid Eocene to Early Oligocene range and occurs in both hemispheres. We erect the following new species: Eatonicysta sequestra, Enneadocysta deconinckii, Enneadocysta harrisii, Enneadocysta robusta and Enneadocysta partridgei. Eatonicysta ursulae subsp. furensis is raised to the rank of species as Eatonicysta furensis.

Distribution patterns of some North Atlantic Cenozoic dinoflagellate cysts

Abstract The lateral distributions of Cenozoic dinoflagellate cyst species in Scotian Shelf-Grand Banks wells, and elsewhere in the North Atlantic region show distinct geographic patterns that may be attributable to ecologic factors. Eocene-Oligocene species of Areosphaeridium and Distatodinium are selected to demonstrate these patterns. Lateral variations in the stratigraphic ranges of certain other Cenozoic species in the North Atlantic are attributed in part to water temperature differences, probably resulting from oceanic currents. Such examples confirm the potential of fossil dinoflagellate cysts as indicators of paleoceanographic parameters.

The Cretaceous/Paleogene Transition on the East Tasman Plateau, Southwestern Pacific

Ocean Drilling Program Leg 189 recovered a potentially complete shallow marine record of the Cretaceous-Paleogene boundary (KPB) at Site 1172 on the East Tasman Plateau. Here we present high-resolution (cm-scale) data from micropaleontology, geochemistry, sedimentology, and paleomagnetism that provide no evidence for a complete KPB, but instead suggest a boundary-spanning hiatus of at least 0.8 Ma. We interpret this hiatus to represent the sequence boundary between the uppermost Maastrichtian Ta1.1 and lowermost Danian Tal.2/ Da-1 3 rd -order sequence stratigraphic cycles. Microfloral assemblages indicate generally shallow paleodepths, restricted circulation, and eutrophic conditions through the section. Paleodepths progressively shallow through the late Maastrichtian, while more oceanic and warmer conditions dominate the early Danian. The Site 1172 KPB section is broadly comparable to other southern high-latitude sections in Antarctica and New Zealand, but appears to record a shallower and more restricted environment that permitted a eustatically-driven hiatus across the KPB mass extinction event.

Hypocystal archaeopyles in the dinoflagellate cyst genus Caligodinium Drugg

Abstract The unique archeopyle in the dinoflagellate cyst (dinocyst) genus Caligodinium Drugg has previously been regarded as apical gonyaulacalean with the operculum consisting of three partially or completely separated pieces, despite there being a problem with determination of the paraplate relationships. A more logical interpretation would be to regard the opercular pieces as representing the three fundital plates in a peridinialean tabulation, making the archeopyle hypocystal. The apparent stratigraphic range of the Caligodinium type of archeopyle is Albian to Early Miocene, although there is some uncertainty over the archeopyle morphology in the Cretaceous forms.