Newswise — Boulder, Colo., USA: 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 major marine extinctions, including one of the five largest mass extinctions ever has happened. Additionally, it was during the Devonian that trees and complex land plants, similar to what we know today, first developed and spread across the landscape. This evolutionary progress involved the development of substantial and complex root systems capable of influencing soil biogeochemistry on a scale the ancient Earth had not yet experienced.
It has been theorized that these two seemingly separate events, the marine extinctions and the evolution and expansion of plants, were intricately linked in the Devonian. In particular, it has been suggested that plant evolution and root development occurred so rapidly and on such a massive scale that the export of nutrients from the land to the ancient oceans would have increased dramatically. This scenario is seen in modern systems where anthropogenic nutrient extraction has greatly increased the nutrient load in areas such as the Gulf of Mexico and the Great Lakes, leading to large-scale algal blooms that eventually deplete oxygen in the water column. Magnified on a global scale, this phenomenon, known as eutrophication, would have been devastating 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 that is elevated above background levels, linking that nutrient flux to either indirect or direct indications of the presence of land plants deeply rooted and ultimately show that this phenomenon occurred in many locations and times.
This study, the first of its kind, was able to do just that using geochemical records from ancient lake deposits in Greenland, northern Scotland and Orkney. Using lake records, elevated nutrient phosphorus values were detected at five distinct sites during the peak of plant evolution and expansion in the Devonian. In any case, the increased values of nutrient input coincide with evidence for the presence of early trees in the form of fossilized spores and, in some cases, fossilized stems of the earliest deep-rooted tree, Archaeopteris. In two cases, this evidence coincided with a Devonian marine extinction event, including the most important Devonian mass extinction, the Frasnian–Famenian extinction (also known as the Late Devonian mass extinction).
Furthermore, this study, published yesterday in Geological Society of America Bulletin, linked the periodic wet/dry climate cycles known to exist in the region during the Devonian to specific episodes of plant colonization. While increased nutrient exports occurred during both wet and dry climates, the most significant export events occurred during wet cycles, suggesting that plant expansion was episodic and linked to climate cyclicity.
The episodic nature of plant expansion could explain why there are at least six major marine extinctions in the Devonian. While the scope of this study was limited to a single geographic area, it is likely that these events occurred across Devon. The colonization of different types of land plants in different areas 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 during the Middle to Late Devonian.
Enhanced terrestrial nutrient release during Devonian forest emergence and expansion: Evidence from lacustrine phosphorus and geochemical records
Matthew Smart; Gabriel Filippelli; William Gilhooly; John Marshall? Jessica Whiteside
Contact: Matthew Smart, [email protected]Indiana University–Purdue University Indianapolis, Earth Sciences, Indianapolis, Indiana
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