Jurassic volcanism in southern Sweden
Volcanoes violently change the conditions of the local environment but, if large enough, may also affect the global climate. Volcanoes can make the climate warmer through water vapour and carbon dioxide release, or alternatively make it cooler by spitting out particles that block the sunlight. Adding to that, volcanic emissions can lead to acid rain and ozone depletion, and produce fires, hot mudslides and lava. This was a common situation in mid-Scania around 185 million years ago.
The rocks in in the region of Höör, central Scania, hold evidence from a time period with extensive volcanic activity. Over 100 volcanoes have been mapped in the area.
For researchers trying to reconstruct the environment at the time, there are at least two important layers: the Höör Sandstone, and an overlying layer of mud flows (lahar) that has hardened to rock. The Höör Sandstone, dated at 190–200 million years ago, is slightly older than the lahar deposits, dated to around 180–190 million years old, and both rock layers preserve fossil evidence that there was a shift in ecosystems between these periods.
Researchers Vivi Vajda, Hans Linderson and Stephen McLoughlin have looked at the characteristics of the sedimentary layers, microfossils such as spores and pollen, fossil leaves, and petrified wood containing well-preserved tree rings that can reveal detailed climatic information on annual to decennial scales. Combining the fossil and sedimentary evidence enabled them to reconstruct the ancient (Early Jurassic) environment. The fossil wood is preserved with details down to subcellular level suggesting very rapid petrification by minerals carried by hot-spring waters (hydrothermal fluids).
Deposits from the Höör Sandstone contain well-preserved macroplants, spores and pollen. The fossils are the remains of an ecosystem dominated by cycadophytes (cycads and their relatives) and seedferns. Analysis of microfossils in the lahar layer, shows that the ecosystem shifted towards a dominance of plants in the cypress family, with an understory layer of plant groups typical of environments characterized by disturbance. The researchers concluded that the volcanic activity in the area at least partly contributed to the shift in vegetation at this time.
The scientists concluded that central Scania had a Mediterranean-type climate in the Early Jurassic, with low and seasonal rainfall. It was a stressful environment for the plants, having also to adapt to the volcanic activity and hydrothermal (hot-spring) activity. These relatively harsh conditions are reflected in the growth patterns of the wood and the unusual vegetation composition.
The study is published in: Kear, B. P., Lindgren, J., Hurum, J. H., Mila`n, J. & Vajda, V. (eds) Mesozoic Biotas of Scandinavia and its Arctic Territories. Geological Society, London, Special Publications, 434