Reviewed by Alex SmithJun 1 2022
Paleobotanist David Greenwood has been gathering fossil plants from Australia for many years, and some of these are so well preserved that it is difficult to believe that they are millions of years old.
Arid today, Australia was once covered by lush forests, according to new research. Image Credit: Adobe Stock.
Such fossils hold information regarding the ancient world in which they flourished, and Greenwood and a research team including climate modeler and researcher David Hutchinson, from the University of New South Wales, and UConn Department of Geosciences paleobotanist Tammo Reichgelt, have started the process of piecing together the evidence in an attempt to learn more from the collection.
The outcomes of the study have been reported in the Paleoceanography & Paleoclimatology journal.
The fossils date back to 55 to 40 million years ago, at the time of the Eocene epoch. During that time, the world was much warmer and wetter, and these hothouse conditions implied there were palms at the North and South Pole and chiefly arid landmasses like Australia were lush and green.
Reichgelt and co-authors were looking for proof of variations in precipitation and plant productivity between the past and the present.
Since different plants tend to flourish under particular conditions, plant fossils have the potential to indicate what types of surroundings those plants lived in.
Through concentrating on the morphology and taxonomic features of 12 various floras, the scientists came up with a more comprehensive view of what the climate and productivity was like in the old hothouse world of the Eocene epoch.
Reichgelt implies that the morphological technique depends on the fact that the leaves of angiosperms—flowering plants—in general, have a process for reacting to climate.
For example, if a plant has large leaves and it is left out in the sun and doesn’t get enough water, it starts to shrivel up and die because of excess evaporation. Plants with large leaves also lose heat to its surroundings.
Tammo Reichgelt, Paleobotanist, Department of Geosciences, University of Connecticut
Reichgelt continued, “Finding a large fossil leaf therefore means that most likely this plant was not growing in an environment that was too dry or too cold for excess evaporation or sensible heat loss to happen.”
“These and other morphological features can be linked to the environment that we can quantify. We can compare fossils to modern floras around the world and find the closest analogy,” added Reichgelt.
The second approach was taxonomic. “If you travel up a mountain, the taxonomic composition of the flora changes. Low on the mountain, there may be a deciduous forest that is dominated by maples and beeches and as you go further up the mountain, you see more spruce and fir forest.”
Reichgelt stated, “Finding fossils of beech and maple therefore likely means a warmer climate then if we find fossils of spruce and fir.”
These climatic preferences of plant groups could be utilized to quantitatively rebuild the ancient climate in which growth was found in a group of plants in a fossil assemblage.
The findings indicate that the Eocene climate would have been very distinct from today’s climate in Australia. The continent needed a steady supply of precipitation to maintain its lush green landscape. Warmer weather means more evaporation, which means more rain was available to move into Australia’s continental interior.
Greater levels of carbon dioxide in the air at the time, 1500 to 2000 parts per million, also added up to the lushness through a process known as carbon fertilization. Reichgelt describes that with the pure abundance of CO2, plants were essentially induldging.
Southern Australia seems to have been largely forested, with primary productivity similar to seasonal forests, not unlike those here in New England today.
Tammo Reichgelt, Paleobotanist, Department of Geosciences, University of Connecticut
Reichgelt added, “In the Northern Hemisphere summer today, there is a big change in the carbon cycle, because lots of carbon dioxide gets drawn down due to primary productivity in the enormous expanse of forests that exists in a large belt around 40 to 60 degrees north. In the Southern Hemisphere, no such landmass exists at those same latitudes today.”
Reichgelt continued, “But Australia during the Eocene occupied 40 degrees to 60 degrees south. And as a result, there would be a highly productive large landmass during the Southern Hemisphere summer, drawing down carbon, more so than what Australia is doing today since it is largely arid.”
Hutchinson stated that the geological proof indicates that the climate is highly sensitive to CO2 and that this effect might be bigger compared to what the climate models forecast.
The data also suggests that polar amplification of warming was very strong, and our climate models also tend to under-represent this effect. So, if we can improve our models of the high-CO2 Eocene world, we might improve our predictions of the future.
Tammo Reichgelt, Paleobotanist, Department of Geosciences, University of Connecticut
Future projects will expand the data set to areas outside of Australia to ask what global productivity does during a hothouse climate on a global scale.
“We have large datasets of plant fossils that have been collected around the world, so we can apply the same methods that we use here to ask what happens to global biosphere productivity,” stated Reichgelt.
With carbon emissions that are on the continuous increase, more research is going into studying what occurs in the biosphere with high water use efficiency and high photosynthetic activity in plants.
Reichgelt describes that modern plants did not have the time to evolve to alter CO2 conditions. But taking a look at the past can help gather some of that data.
Reichgelt stated, “It obviously will take a long time for plants to adapt to changing CO2 levels, but fossil floras allow us to peek into the biosphere of ancient hothouse worlds.”
Journal Reference:
Reichgelt, T., et al. (2022) Plant Proxy Evidence for High Rainfall and Productivity in the Eocene of Australia. Paleoceanography and Paleoclimatology. doi.org/10.1029/2022PA004418.