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According to conventional thinking, plant life first took hold on Earth after oceans and rivers formed; the soil produced by liquid water breaking down bare rock provided an ideal medium for plants to grow in. It certainly sounds logical, but a new study is challenging that view – the theory is that vascular plants, those containing a transport system for water and nutrients, actually created a cycle of glaciation and melting, conditions which led to the formation of rivers and mud which allowed forests and farmland to later develop. In short, they helped actually create the landscapes we see today.
The evidence was just published in two articles in a special edition of Nature Geoscience.
In the first article, analysis of the data proposes that vascular plants began to absorb the carbon dioxide in the atmosphere about 450 million years ago. This led to a cooling of temperatures on a global scale, resulting in widespread glaciation. As the glaciers later started to melt, they ground up the Earth’s surface, forming the kind of soils we see today.
The second article goes further, stating that today’s rivers were also created by vascular plants – the vegetation broke the rocks down into mud and minerals and then also held the mud in place. This caused river banks to start forming, acting as channels for water, which up until then had tended to flow over the surface much more randomly. As the water was channeled into more specific routes, rivers formed. This led to periodic flooding; sediments were deposited over large areas which created rich soil. As trees were able to take root in this new soil, debris from the trees fell into the rivers, creating logjams. This had the effect of creating new rivers and causing more flooding. These larger fertile areas were then able to support the growth of larger lush forests and farmland.
According to Martin Gibling, a professor of Earth science at Dalhousie University, “Sedimentary rocks, before plants, contained almost no mud. But after plants developed, the mud content increased dramatically. Muddy landscapes expanded greatly. A new kind of eco-space was created that wasn’t there before.”
The new theory also leads to the possibility that any exoplanets that happen to have vegetation would look different from Earth; varying circumstances would create a surface unique to each world. Any truly Earth-like exoplanets might be very similar in general, but the way that their surfaces have been modified might be rather different.
It’s an interesting scenario, but it also raises other questions. What about the ancient river channels on Mars? Some appear to have been formed by brief catastrophic floods, but others seem more similar to long-lived rivers here on Earth, especially if there actually was a northern hemisphere ocean as well. How did they form? Does this mean that rivers could form in a variety of ways, with or without plant life being involved? Could Mars have once had something equivalent to vascular plant life as well? Or could the new theory just be wrong? Then there’s Titan, which has numerous rivers still flowing today. Albeit they are liquid methane/ethane instead of water, but what exactly led to their formation?
From the editorial in Nature Geoscience:
Without the workings of life, the Earth would not be the planet it is today. Even if there are a number of planets that could support tectonics, running water and the chemical cycles that are essential for life as we know it, it seems unlikely that any of them would look like Earth. Even if evolution follows a predictable path, filling all available niches in a reproducible and consistent way, the niches on any Earth analogue could be different if the composition of its surface and atmosphere are not identical to those of Earth. And if evolution is random, the differences would be expected to be even larger. Either way, a glimpse of the surface of an exoplanet — if we ever get one — may give us a whole new perspective on biogeochemical cycling and geomorphology.
Just as the many exoplanets now being found are of a previously unknown and amazingly wide variety, and all uniquely alien, even the ones that (may) support life are likely to be just as diverse from each other as they are from Earth itself. Earth’s “twin” may be out there, but in terms of outward appearance, it may be somewhat more of a fraternal twin than an exact replica.
Rivers could follow fault lines, down aretes, or meander aimlessly. I don’t think it takes a tree to make a river. And I don’t see any benefit for the tree if it did. Our hydologic cycle had been active for millions of years before life became as ubiquitous as we know it today.
The basic idea is old geology 101, the presence of a biosphere modifies the atmosphere, the soil chemistry, weathering and the associated CHONSP cycles. CHONSP is a list of the main elements that goes into a cell (C for carbon et cetera) in roughly that order of mass.
The crustal carbon (C), nitrogenous (N), sulfurous (S) and phosphorous (P) cycles are dominated by the biosphere. The oxygen atmosphere and its precipitated rusting (O) is a product of the biosphere. The hydrological (H) cycle is heavily controlled on cloud formation around organic (algae, forests) and inorganic (soil) condensation nuclei and on water retention by soil, plants and modification of the drainage basins and runoff properties.
It is the latter that these articles points to. The conceptual leap is in the editorial articles, where it is hypothesized that these characteristics would be observably different in different biospheres. But this is neither what the research articles looked at nor it is offered measures for these characteristics or their likeness on different planets.
Obviously the hydrological cycle has changed under Earth’s history, dominantly influenced by the mature biosphere. But I don’t think these changes have been much elaborated as of yet. Perhaps this science can start there, before we speculate too much in differences between planets. But as a first guess, the basin & runoff properties (number and size of basins, rivers and lakes, as well as drainage rates and soil water table) are visually but minutely affected by plant types.
As an example, I live in a country that has been soil plowed and then lake seeded by recent land ice, as late as ~ 10 k years ago. The result is many small lakes and marshy or dried out lake remnants as the soil distribution changes and the still rapid land rise after ice melting affects runoff possibilities. Then you can start to discuss how global glaciation cycles are affected by the biosphere et cetera. But the point is that recent geographical changes dominates the plant effects, whether they heavily modify the basins or not.