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History of Geology

History of Geology

What rocks tell and how we came to understand it

How Plants survived the Ice Age

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"No such hypothesis is sufficient to explain either the cataclysms or the glacial phenomena; and we need not hesitate to confess our ignorance of this strange, this mysterious, episode in the history of the globe...."

BRISTOW, H.G. (1872): The world before the deluge by Louis Figuier - Newly edited and revised by H.W. Bristow. 2nd. edition - Cassel, Petter, Galpin & Co.

The published description of the resuscitated "32.000 year old Ice Age plant", in fact a phenotyp variation of the extant Silene stenophylla, is an ulterior intriguing glimpse in the ancient ecosystem of the Ice Age steppe, an environment characterized by unusual climatic conditions and a particular plant community with species that today grow in very different environments.

Fig.1. The period of the Diluvium, or Ice Age, with a glacier invading the land, from UNGER, F. (1851): Ideal Views of the Primitive World, in its Geological and Palaeontological Phases. Taylor and Francis, London (image in public domain, from the U.S.G.S. library).

There are various methods to reconstruct the plant community of a past landscape. Flowering plants produce pollen grains composed by a chemically very stable substance named Sporopollenin, therefore pollen grains usually are well preserved in sediments (but as correctly noted in the comments not in soils). Identifying and counting the pollen deposited over time on the bottom of a lake or conserved in the layers of a bog we can infer the vegetation that once surrounded these sediment traps. In such sediments also plant detritus can be conserved.

Many animals transport and store plant detritus in their burrows. The conserved seeds of S. stenophylla were found in an ancient ground squirrel cache. Apart trying to grow the seeds, the fragments of leafs, blossoms, buds and seeds can often be identified to species-level and help to reconstruct the vegetation inside the range of activity of the former burrow-occupier.

Plant remains or pollen can also be found in the gut content of mummified animals or in fossilized dung, also referred as coprolithes (or dung-stones).

Based on such fossils a particular ice age plant community was reconstructed that is often referred as steppe, arctic-steppe, steppe-tundra, herb-tundra or mammoth steppe. Today the term steppe is applied to grassland or shrubland with a relative dry and temperate climate, meanwhile tundra applies to an environment where the growth of trees and shrubs is inhibited by the low temperatures and species of herbaceous plants or grass dominate. The words steppe-tundra or mammoth steppe seems therefore at first an odd combination of contradicting terms.

Studying the preserved content in the intestine of the mummified mammoth calf "Lyuba", we can try to imagine the plant community that dominated the continents of Europe-Siberia and North America some 40.000 years ago.

The preserved material is dominated by species of two grass-families (Poaceae and Cyperaceae), indicating an open environment. Other plant groups identified are Artemisia, a group found today especially under dry or semi-dry conditions, the rushrose Helianthemum, also found under dry conditions, and the Jacob's ladder Polemonium, native today to cool temperate and arctic regions. Unlike as in the modern treeless tundra there existed apparently spots of forest, indicated by the presence of pollen grains of Pine (Pinus), Spruce (Picea), Birch (Betula), Willow (Salix) and Alder (Alnus), all trees that can tolerate snow or low temperatures and grow in part under dry conditions (pine and spruce) and in part under humid conditions (birch, willow, alder). It is unlikely that all these plants grow directly on one spot, considering also that a mammoth herd moved from grazing spot to grazing spot and pollen grains could be transported by wind, however all these plants nevertheless existed in a relative restricted space. The term mammoth steppe is therefore not too inappropriate, as species of warm and dry habitats did coexist with species of cold and humid habitats, resulting in a plant community with a uniquely rich biodiversity.

To understand such a community it is important to note the climatic differences of the modern tundra and the ice age steppes. The overall cold temperatures in an ice age world reduced significantly the evaporation of water from the oceans, resulting in a drier atmosphere and lack of precipitation (snow or rain) on large areas of the continents. This caused dry conditions in the summer and also reduced snow cover during winter time; and snows inhibits photosynthesis and therefore limits plant growth.

The modern tundra is today found at a northern latitude of more than 65°, characterized by short summers with the sun reaching only a low altitude above the horizon in this season. During the ice age the mammoth steppe spread until 45° N. Here the sun climbs much higher above the horizon and the insolation is more intense, photosynthetic activity is more productive and plants can grow well despite cold temperatures.

The particular plant community of the mammoth steppe was an adaption to the particular combination of environmental factors during the ice age. Little is known about adaptions of the single plants to this environment. The described specimens of the ice age S. stenophylla produced more buds and the roots developed slower than modern specimens of the same species. In an environment with a short period of growth and limited availability of insects as pollinators investing in more flowers could be an advantage - the discovery of living tissue is therefore a unique opportunity to study possible physiological adaptations to the lost world of the mammoth steppe.

Bibliography:

CRAWFORD, R.M.M. (2008): Plants at the Margin - Ecological Limits and Climate Change. Cambridge University Press: 496

GAGLIOTI, B.V.; BARNES, B.M.; ZAZULA, G.D.; BEAUDOIN, A.B. & WOOLLER, J.M. (2011): Late Pleistocene paleoecology of arctic ground squirrel (Urocitellus parryii) caches and nests from Interior Alaska's mammoth steppe ecosystem, USA. Quaternary Research 76: 373-382

GEEL, v.B.; FISHER, D.C.; ROUNTREY, A.N.; ARKEL, v.J.; DUIVENVOORDEN, J.F.; NIEMAN, A.M.; REENEN, B.A.v.; TIKHONOV, A.N.; BUIGUES, B. & GRAVENDEEL, B. (2011): Palaeo-environmental and dietary analysis of intestinal contents of a mammoth calf (Yamal Peninsula, northwest Siberia). Quaternary Science Reviews 30: 3935-3946

NAGY, L. & GRABHERR, G. (2009): The Biology of Alpine Habitats. Oxford University Press: 389

The views expressed are those of the author and are not necessarily those of Scientific American.

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