Antarctic Peninsula - Paleofloras and Paleofaunas

Paleofloras and Paleofaunas

A rich record of fossil leaves, wood, pollen, and flowers demonstrates that flowering plants thrived in subtropical climates within the volcanic island arcs that occupied the Antarctic Peninsula region during the Cretaceous and very early Paleogene periods. The analysis of fossil leaves and flowers indicate that semitropical woodlands, which were composed of ancestors of plants that live in the tropics today, thrived within this region during a global thermal maximum with summer temperatures that averaged 20°C. The oldest fossil plants come from the middle Cretaceous (Albian) Fossil Bluff Group, which outcrop along the edge of Alexander Island. These fossils reveal that at this time the forests consisted of large conifers, with mosses and ferns in the undergrowth. The paleosols, in which trees are rooted, have physical characteristics indicative of modern soils that form under seasonally dry climates periodic high rainfall. Younger Cretaceous strata, which outcrop within James Ross, Seymour, and adjacent islands, contain plants fossil of Late Cretaceous angiosperms with leaf morphotypes that are similar to those of living families such as Sterculiaceae, Lauraceae, Winteraceae, Cunoniaceae, and Myrtaceae. They indicate that the emergent parts of the volcanic island arc, the eroded roots of which now formed the central part of the Antarctic Peninsula, were covered by either warm temperate or subtropical forests. These fossil plants are indicative of tropical and subtropical forest at high paleolatitudes during the Middle and Late Cretaceous, which grew in climates without extended periods of below freezing winter temperatures and with adequate moisture for growth. The Cretaceous strata of James Ross Island also yielded the dinosaur genera Antarctopelta, which was the first dinosaur fossil to be found on Antarctica.

Paleogene and Early Eocene marine sediments that outcrop on Seymour Island contains plant-rich horizons. The fossil plants are dominated by permineralized branches of conifers and compressions of angiosperm leaves, and found within carbonate concretions. These fossils, which date to about 51.5-49.5 Ma, are dominated by leaves, cone scales, and leafy branches of araucarian conifers, very similar in all respects to living Araucaria araucana (monkey puzzle) from Chile. They suggest that the adjacent parts of the prehistoric Antarctic Peninsula were covered by forests that grew in a cool and moist, high-latitudes environment during the early Eocene.

During the Cenozoic climatic cooling, the Antarctic Peninsula was the last region of Antarctica to have been fully glaciated according to current research. As a result, this region was likely the last refugium for plants and animals that had inhabited Antarctica after it separated from the Gondwana supercontinent. Analysis of paleontologic, stratigraphic, and sedimentologic data acquired from the study of drill core and siesmic acquired during the Shallow Drilling on the Antarctic Continental Shelf (SHALDRIL) and other projects and from fossil collections from and rock outcrops within Alexander, James Ross, King George, Seymour, and South Shetland islands has yielded a record of the changes in terrestrial vegetation that occurred within the Antarctic Peninsula over the course of the past 37 million years.

This research found that vegetation within the Antarctic Peninsula changed in response to a progressive climatic cooling that started with the initiation of mountain glaciation in the latest Eocene, about 37–34 Ma. The cooling was contemporaneous with glaciation elsewhere in Antarctica and a reduction in atmospheric CO2 concentrations. Initially, during the Eocene, this climate cooling resulted in a decrease in diversity of the angiosperm-dominated vegetation that inhabited the northern Antarctic Peninsula. During the Oligocene, about 34–23 Ma, these woodlands were replaced by a mosaic of southern beech (Nothofagus) and conifer-dominated woodlands and tundra as the climate continued to cool. By middle Miocene, 16–11.6 Ma, a tundra landscape completely replaced any remaining woodlands. At this time, woodlands became completely expatriated from the Antarctic Peninsula and all of Antarctica. A tundra landscape likely persisted until about 12.8 Ma when the transition from a temperate, alpine glaciation to a dynamic ice sheet occurred. Eventually, the Antarctic Peninsula was overridden by an ice sheet, which has persisted without any interruption to this day, in the early Pliocene, about 5.3–3.6 Ma.

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