A unique reworked palynological assemblage composed of 32 to 100% (average of 63%) in
Impletosphaeridium spp., was found during the study of sixteen samples recovered from piston cores taken off Seymour Island, Antarctica, during a pre-SHALDRIL study. One of the common Impletosphaeridium species recovered, I.
lorum, was previously found in Seymour Island’s La Meseta Formation. Two questions we asked are: “What is the age
of this high abundance of Impletosphaeridium spp., providing that they come from a unique source?”, and second “What is the environmental significance of such a high relative abundance?” The samples are likely to have been derived from sediments ranging from Eocene to Miocene in age, as this is the range for most of the Impletosphaeridium
species. The environmental significance of this high abundance is a difficult question, not only because the genus is extinct, but also because these species are poorly known. To try to understand the ecological significance, we used the hypothesis that the morphology of a dinocyst is linked to environmental sea-surface parameters, and looked at extant species with a similar morphology. The extant dinoflagellate cysts of Echinidinium spp, Islandinium cezare, Islandinium minutum, and Pentapharsodinium dalei were selected for their morphological similarity with Impletosphaeridium. Modern ecological parameters for the extant species listed above were derived from the DinoDatabase (940 modern sea-surface samples from around the world). The database showed that these species are all
indicators of sea-ice cover, with a minimum of 8 months for Echinidinium spp., ~ 5 months for Islandinium cezare, and
from 1 to 12 months for Pentapharsodinium dalei and Islandinium minutum. If our morphologic-similarity hypothesis is correct, and if this high abundance is indeed indicative of a paleo-environmental condition, then it is most likely to have been associated with ephemeral sea-ice development off Seymour Island, sometime between the Eocene and the
Miocene.
