Terrestrial paleoecology of the Cretaceous (Early Aptian) Cerro Negro Formation, South Shetland Islands, Antarctica: a record of polar vegetation in a volcanic arc environment
Nothing is known about the community-scale plant ecology of early Cretaceous volcanic environments in the southern high latitudes. The paleoecology of the early Aptian (119–120 Ma) volcaniclastic Cerro Negro Formation of South Shetland Islands, Antarctica, has been investigated to improve understanding of these enigmatic ecosystems. This unit was deposited in an intra-arc setting at the edge of the paleo-polar circle (66°S). Taphonomic analysis of megaflora in seven volcanic facies indicates the existence of a spatially complex vegetation mosaic co-dominated by conifers, bennettites, and ferns. A lower stratigraphic unit was deposited by high-magnitude, low-frequency silicic eruptions, partially reworked by fluvial processes. Following ash falls, shrubby communities of ferns and bennettites dominated for short periods in low-altitude paleovalleys before being replaced by araucarian-podocarp-fern conifer forests. Forests of podocarp conifers and bennettites covered adjacent mid-altitude slopes of active volcanic cones. These were destroyed periodically by hot pyroclastic flows, and their charred remains washed down into paleovalley fluvial systems. Analysis of architectural and phenological data from both major forest communities indicates the predominance of evergreen trees with a canopy height of c. 20–30 m. Growth-ring analysis indicates that, despite occasional catastrophic eruptions, growing conditions were mostly favorable and uniform, although in riparian paleovalley niches flooding locally produced more stressful growing conditions. An upper stratigraphic unit was deposited by low-magnitude, high-frequency basaltic eruptions. Shrubby, fern and bennettite-dominated vegetation, together with local conifer stands, colonized this environment. Growth of more widespread arborescent vegetation in the upper unit was inhibited by high-frequency volcanic disturbances. Bennettites are much more abundant and diverse in the Cerro Negro Formation compared with coeval, non-volcanic sites in the southern high latitudes, indicating that this group may have favored volcanic environments.