Linking Mass Extinctions to the Expansion and Radiation of Land Plants
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 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 yesterday 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.
FEATURED ARTICLE
Enhanced terrestrial nutrient release during the Devonian emergence and
expansion of forests: Evidence from lacustrine phosphorus and
geochemical records
Matthew Smart; Gabriel Filippelli; William Gilhooly; John Marshall; Jessica
Whiteside
Contact: Matthew Smart, smartm@iu.edu, Indiana University–Purdue University
Indianapolis, Earth Sciences, Indianapolis, Indiana
URL:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36384.1/618814/Enhanced-terrestrial-nutrient-release-during-the
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