The Forest Out of Time

An artist’s impression of Antarctica as a swampy rainforest between 92m and 83m years ago. (Credit: Alfred-Wegener-Institut/James McKay)

 

Past fluctuations in global temperatures are crucial to understand Earth’s climatic evolution. During the mid-Cretaceous, Earth’s climate was extraordinarily warm with temperatures in the tropics as high as 35 degrees Celsius, particularly during the Turonian to Santonian stages (93.9–83.6 Ma), with increasingly high sea levels and numerous epicontinental seas. The interval was characterized by extensive deposition of organic carbon (OC) rich black shales across a wide range of marine settings. Because marine proxies dominate records of past temperature reconstructions, our understanding of continental climate is relatively poor. Now, researchers from the UK and Germany discovered evidence of a temperate rainforest in West Antartica. The new finding offers a window into the terrestrial conditions of the extreme southern latitudes during this period.

The evidence comes from a core of sediment drilled into the seabed near the Pine Island and Thwaites glaciers in West Antarctica. One section of the core revealed a network of fossil plant roots, and countless traces of pollen and spores from plants, including the first remnants of flowering plants ever found at these high Antarctic latitudes. Pollen assemblage is dominated by the conifer tree families Podocarpaceae and Araucariaceae. The abundant tree ferns includes Cyathea. The presence of Sterisporites antiquasporites (Bryophyta, Sphagnum) suggest the temporary existence of a peat swamp. This coincides with increasing Peninsulapollis pollen.

 

West Antarctica. Image: Unsplash/Henrique Setim

Pollen and other palynomorphs proved to be an extraordinary tool to palaeoenvironmental reconstruction. In 1921, Gunnar Erdtman, a Swedish botanist, was the first to suggest this application for fossil pollen study. Like spores, pollen grains reflects the ecology of their parent plants and their habitats and provide a continuous record of their evolutionary history. Based on the palynomorph assemblage, the researchers found that the annual mean air temperature was around 13 degrees Celsius, and the average temperature of the warmest summer month was 18.5°C, whereas the amount and intensity of rainfall were similar to those in today’s Wales.

The mid-Cretaceous was an interval of intense climatic, tectonic and biotic changes across Gondwana. The break-up of the supercontinent and the rise of angiosperms caused a global floral turnover. Antarctica is particularly important because it preserves rock sequences that record the climate during the break-up of the supercontinent and the climate changes during the onset of continental-scale glaciation.

 

References:

Klages, J.P., Salzmann, U., Bickert, T. et al. Temperate rainforests near the South Pole during peak Cretaceous warmth. Nature 580, 81–86 (2020). https://doi.org/10.1038/s41586-020-2148-5

Forster, A. et al. Tropical warming and intermittent cooling during the Cenomanian/Turonian Oceanic Anoxic Event (OAE 2): sea surface temperature records from the equatorial Atlantic. Paleoceanography 22, PA1219 (2007). DOI:10.1029/2006PA001349

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