INTRODUCTION

Dinoflagellates are predominantly single-celled organisms (protists) with two characteristic flagella and a special type of eukaryotic nucleus, termed a dinokaryon (Fensome et al., 1993). They form a major component of the marine plankton; there are about equal numbers of autotrophic and heterotrophic dinoflagellates, including some species that use both feeding strategies. Some dinoflagellates are symbionts or parasites. Although most dinoflagellates are marine, they can occur in brackish and freshwater environments, ice, snow, and wet sand. Dinoflagellates have a simple to complex life cycle, which typically includes a motile stage. Nonmotile cells may be resting, temporary, vegetative, or digestion cysts. Those resting cysts (hereafter termed dinocysts) that have been studied are hypnozygotes, most being distinguished by a resistant wall and a predetermined excystment opening, the archeopyle.

Almost all dinocysts are assignable to the subclass Peridiniphycidae, with a few representing the subclasses Gymnodiniphycidae and Dinophysiphycidae (Fensome et al., 1993). The Late Cretaceous–Tertiary taxa discussed in this paper are almost all representatives of the Peridiniphycidae, which contains siliceous, calcareous, and organic walled cysts: this paper deals with the organic walled forms. The only non-peridiniphycideans treated here are the dinogymnioids Alisogymnium euclaense and Dinogymnium spp., which appear to represent fossilizable shed pellicles of gymnodiniphycidean dinoflagellates (Fensome et al., 1993). For simplicity and convenience, they are treated here as dinocysts.

The use of fossil dinocysts in biostratigraphy and paleoecology has been discussed in detail in several papers including Williams and Bujak (1985), Powell (1992), and Stover et al. (1996). Pioneer studies, primarily by palynologists working for petroleum companies, were based largely on subsurface sections. The need for more precise correlations led these and other palynologists to undertake extensive studies of classical surface sections, including stratotypes, and to the publication of formal zonations (e.g., Clarke and Verdier, 1967).

Biostratigraphic and paleoecologic studies of subsurface sections are dependent upon utilization of microfossils. Such studies were not only the domain of the petroleum exploration companies in the twentieth century: the Ocean Drilling Program (ODP) and its precursor, the Deep Sea Drilling Project (DSDP), focused attention on the utilization of planktonic foraminifers and nannofossils for correlation and zonation of deep-sea sediments and provided a much-needed framework for assessing the stratigraphic ranges of dinocysts. And a major bonus of ODP has been the widespread use of paleomagnetic ages, the ultimate degree of sophistication for defining the first and last occurrences of fossils. In Upper Cretaceous sequences, however, the use of the paleomagnetic polarity timescale is limited because there are fewer reversals.

The success of planktonic foraminifer and nannofossil dating led many dinocyst workers to also develop zonations based on the concept of index species. Comprehensive zonations for the Cretaeous–Cenozoic of the Northern Hemisphere have been published by numerous authors, including Monteil (1985, 1992) for the Early Cretaceous, Pr÷ssl (1990) for the Hauterivian–Turonian, Kirsch (1991) for the Turonian–Maastrichtian, Schi°ler and Wilson (1993) and Schi°ler et al. (1997) for the Maastrichtian, Powell (1992) for the Tertiary, Bujak and Mudge (1994) and Mudge and Bujak (1994) for the Eocene, Brinkhuis and Biffi (1993) for the Eocene–Oligocene transition, Wilpshaar et al. (1996) for the Oligocene, Zevenboom (1995) for the Oligocene–Miocene, and Head (1998) for the Pliocene. These works have been major sources for our compilations.

Several detailed zonations for Late Cretaceous–Paleogene dinocysts of the Southern Hemisphere or circum-Antarctic realm have been published in recent years. These include Helby et al. (1987) and McMinn (1988) for the Late Cretaceous and Wilson (1988) for the Paleocene–Eocene. Other notable contributions highlight assemblages from Argentina, southeastern Australia, New Zealand, the Ross Shelf, and Seymour Island, as well as from several DSDP/ODP sites in the region (see, for example, Haskell and Wilson, 1975; Goodman and Ford, 1983; Askin, 1988a, 1988b; Wilson, 1985, 1988; Wrenn and Hart, 1988; Marshall, 1988, 1990; Mohr, 1990; Mao and Mohr, 1995; Crouch and Hollis, 1996; Truswell, 1997; Hannah, 1997; Hannah et al., 1997; Hannah and Raine, 1997; Levy and Harwood, 2000; Guerstein et al., 2002). These studies have documented Southern Ocean Paleogene dinocyst distribution and taxonomy in great detail, but there are difficulties with the chronostratigraphic calibration (Brinkhuis, Sengers, et al., this volume).

There are fewer studies of Oligocene–Neogene dinocysts from the Southern Hemisphere. McMinn (1992b) proposed a zonation for the Pliocene–Holocene of southern Australia. Other authors have described assemblages from various ODP sites (McMinn, 1992a, 1993; McMinn et al., 2001; Brinkhuis, Sengers, et al., this volume) and the Antarctic shelf (Wrenn et al., 1998; Hannah et al., 1998). One of the difficulties in developing a Neogene dinocyst zonation for the higher latitudes of the Southern Hemisphere is the general paucity of specimens. It appears that the organic wall of the dinocyst is not resistant to the oxygen-rich waters in the Antarctic region, and/or winnowing at depth and low sedimentation rates mean that these microfossils are not preserved (McMinn, 1995; Brinkhuis, Munsterman, et al., this volume). McMinn (1995) believes that the absence of specimens reflects the disappearance of cyst-producing dinoflagellates from the Antarctic–Southern Ocean region since the Oligocene. This exclusion has resulted from the geographic and thermal isolation of Antarctica.

Independent chronostratigraphic control is essential to constrain a proposed biostratigraphic calibration. This is the case at Leg 189 Sites 1168 and 1172, where the Late Cretaceous (Maastrichtian) to Quaternary succession has a clear magnetostratigraphy, calibrated by biotic events (Stickley et al., this volume; Schellenberg et al., in press). Hence, for the first time, we are able to tie Southern Ocean dinocyst events to the geomagnetic polarity timescale and, for the middle Eocene, to a calibrated Milankovich cyclostratigraphy (R÷hl et al., in press). For further discussion of these topics, see also Brinkhuis, Munsterman, et al. (this volume), Brinkhuis, Sengers, et al. (this volume) and Sluijs et al. (this volume).

By compiling the dinocyst data from Leg 189 with the nannofossil, foraminiferal, and paleomagnetic results, we established a sequence of calibrated dinocyst events that can be compared with dinocyst events keyed to magnetostratigraphy and other fossil groups from type and other precisely dated surface or subsurface sections in both hemispheres. Thus for the first time, we can fully document selected dinocyst events in the Late Cretaceous–Cenozoic with a surprising degree of precision. The brevity of this paper does not denote a constrained database. Rather, it represents the culmination of a decade-long study, commencing with the 1-week workshop on Paleogene dinocysts, 6–10 June 1994. Subsequent workshops, input into the revised version of Haq et al. (1987; see Williams et al., 1998b), and the results from Leg 189 have led to the culmination of the results presented here. Background data for those desiring further clarification are provided in the workshop manuals, which are available from the author upon request.

NEXT