|Fossilized tree roots in limestone blocks|
A series of mass extinctions that rocked the Earth’s oceans during the Devonian Period over 300 million years ago may have been triggered by the evolution of tree roots.
The Devonian Period, 419 to 358 million years ago, was one of the most turbulent times in Earth’s past and was marked by at least six significant marine extinctions, including one of the five largest mass extinctions ever to have occurred.
Additionally, it was during the Devonian that trees and complex land plants similar to those we know today first evolved and spread across the landscape. This evolutionary advancement included the development of significant and complex root systems capable of affecting soil biogeochemistry on a scale the ancient Earth had yet to experience.
It has been theorized that these two seemingly separate events, marine extinctions and plant evolution and expansion, were intricately linked in the Devonian. Specifically, it has been proposed that plant evolution and root development occurred so rapidly and on such a massive scale that nutrient export from the land to the ancient oceans would have drastically increased.
This scenario is seen in modern systems where anthropogenically sourced nutrient export has vastly increased the nutrient load into areas such as the Gulf of Mexico and the Great Lakes, leading to large-scale algal blooms that ultimately deplete the oxygen in the water column. This effect, known as eutrophication, magnified on a global scale, would have been catastrophic to ancient oceans, fueling algal blooms that would have depleted most of the ocean’s oxygen.
The key to linking mass extinctions and the expansion and radiation of land plants lies in identifying a nutrient flux elevated above background levels, linking that nutrient flux to either indirect or direct evidence of the presence of deeply rooting land plants and finally showing that this phenomenon occurred in multiple locations and times.
This study, the first of its kind, was able to do precisely that by utilizing geochemical records from ancient lake deposits in Greenland, northern Scotland, and Orkney. Utilizing lake records, elevated values of the nutrient phosphorus were detected in five distinct locations during the height of plant evolution and expansion in the Devonian. In each case, elevated values of nutrient input were coincident with evidence of the presence of early trees in the form of fossilized spores and, in some cases, fossilized stems of the earliest deeply rooting tree, Archaeopteris. In two cases, that evidence coincided with a Devonian marine extinction event, including the most significant Devonian mass extinction, the Frasnian–Famennian extinction (also known as the Late Devonian mass extinction).
Additionally, this study, published in the Geological Society of America Bulletin, linked the periodic wet/dry climate cycles known to exist in the region during the Devonian with specific episodes of plant colonization. While elevated nutrient export was noted during both wet and dry climate cycles, the most significant export events occurred during wet cycles, suggesting that plant expansion was episodic and tied to climate cyclicity.
The episodic nature of plant expansion could help explain why there are at least six significant marine extinctions in the Devonian. While the scope of this study was limited to a single geographic region, it is likely that these events occurred throughout the Devonian Earth.
The colonization of different types of land plants in different regions and at different times would have resulted in episodic nutrient pulses significant enough to sustain eutrophication and cause (or at least contribute) to the numerous marine extinction events throughout the mid- to Late Devonian.
The above story is based on Materials provided by Indiana University–Purdue University Indianapolis (IUPUI).