Imprimir

PALEOBOTANY

A forest of water and fire

Fossils in a coalfield in Rio Grande do Sul reveal a swampy landscape prone to frequent wildfires 290 million years ago

Possible pollen-producing structure under a fluorescence microscope

Joalice de Oliveira Mendonça UFRJ/CCMN/IGEO/LAFOPossible pollen-producing structure under a fluorescence microscopeJoalice de Oliveira Mendonça UFRJ/CCMN/IGEO/LAFO

The landscape of southern Brazil has seen many changes in the last 290 million years. South America separated from Africa, and the Serra Geral mountain range rose up in close proximity to what is now the coastline of the states of Paraná, Santa Catarina and Rio Grande do Sul. Records preserved in the rocks suggest that, prior to those events, the region contained flooded areas with trees some 15 meters high that belonged to the pteridosperms, an ancient group of conifers that predominated in what are now coal-forming environments in the Southern Hemisphere. The group led by Margot Guerra-Sommer, a paleobotanist at the Federal University of Rio Grande do Sul (UFRGS), obtained more than just fossils on expeditions to the Faxinal Coalfield in the municipality of Arroio dos Ratos, Rio Grande do Sul. The rocks in that area have preserved paleoecological information that tells a story of recurring wildfires in an environment where scientists had presumed there was constant moisture.

“Embedded in mineral coal we found fragments of tree trunks about 20 centimeters in diameter,” says biologist Isabela Degani-Schmidt, who is conducting her doctoral research in Guerra-Sommer’s laboratory. The finding is unusual because burned organic plant matter is extremely delicate, and generally breaks up into fragments no bigger than four or five centimeters on its way to a site where it accumulates and encounters conditions favorable to preservation for posterity. That was not the case in the region studied by Gerrra-Sommer’s group. The unusually large fossils indicate that the trees in the region burned while standing. After the trunks fell, they remained in place, where they were likely found much later by miners and researchers.

Bright areas indicate that pollen was not burned

Joalice de Oliveira Mendonça UFRJ/CCMN/IGEO/LAFOBright areas indicate that pollen was not burnedJoalice de Oliveira Mendonça UFRJ/CCMN/IGEO/LAFO

The material holds important records of the environment in that region at the dawn of the geological period known as the Permian. Now, those records can be interpreted. The fossilization of the trunks and leaves indicates that the forests existed in a swampy environment. “Such conditions are favorable for fossilization because plant matter that falls into water accumulates in an acidic environment inhospitable to bacteria and fungi, the agents of decomposition,” explains Degani-Schmidt. Consequently, scientists had previously conjectured a permanent swamp in the region. “In peatlands flooded year-round, no one imagined there would be wildfires.”

The recent findings, published in the July 2015 issue of the International Journal of Coal Geology, paint a more complex picture, however. “The environment probably never stayed dry,” the researcher hypothesizes. “Instead, there was likely a period dry enough to enable wildfires to occur, which would indicate a much more oxygen-rich atmosphere than prevails at the present time.”

Samples of trunks and pollen examined via fluorescence and scanning electron microscopy also revealed that the fires were not all-consuming. The trunk wood and pollen were not charcoalified, indicating that temperatures were relatively low. Degani-Schmidt interprets the finding as an indication that the dry seasons never completely eliminated the moisture, and the soil was probably always covered by a film of water, thus creating conditions favorable to in situ fossilization and incomplete burning.

Charcoalified leaves...

Isabela Degani-SchmidtCharcoalified leaves…Isabela Degani-Schmidt

Specialized flora
The most plausible hypothesis for the origin of the wildfires, according to Degani-Schmidt, is lightning. Another suggested possibility is vulcanism—an idea reinforced by the presence of a layer of white rock rich in fossilized leaves interlayered with the coal, which was interpreted to be volcanic ash. After examining this layer of rock, Guerra-Sommer’s group concluded that the ash had fallen cold over the region, having probably originated in a distant location as yet unknown. “There are no signs of sources of volcanic activity in the area,” says Degani-Schmidt.

These findings reveal ecological dynamics far beyond charcoalified flora. The researcher believes that the forest was adapted to fire. “We found pteridosperm fossil deposition in different layers, which indicates that these plants remained there over time,” she explains. It has yet to be determined if there were resources to enable subsistence under those conditions. “We are analyzing structures in the leaves to see if they were specialized in that regard.”

... seen with the naked eye

Isabela Degani-Schmidt… seen with the naked eyeIsabela Degani-Schmidt

The newly discovered fossils, and the environmental conditions they suggest, could be a sign that plant diversity in the area was rather limited, and determined by the plants’ ability to withstand constant fires. The studies are intriguing because they reveal a landscape of which no living trace remains and whose protagonists have gone completely extinct. Pteridosperms, once viewed as an evolutionary link between samambaias (tree ferns) and conifers, belonged to a group of ancient gymnosperms whose closest present-day relatives are probably cycads and ginkgos. “There is nothing of the kind in the area now,” says Degani-Schmidt, who knows of no other landscape like the one painted by the fossils. “We can see it only by analyzing the rocks and extracting the preserved remains.”

Scientific article
DEGANI-SCHMIDT, I., et al. Charcoalified logs as evidence of hypautochthonous/autochthonous wildfire events in a peat-forming environment from the Permian of southern Paraná Basin (Brazil). International Journal of Coal Geology, V. 146, p. 55-67. July 1, 2015.

Republish