Roots, Weathering, and the Terrestrial Phosphorus Cycle of the Late Devonian

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For the first 4 billion years of Earth history, the land surface was devoid of biological activity. There were no plants, no forests, no roots, and little to no soil. Then, in a geologic blink of an eye, a whole series of evolutionary advances began on land about 400 million years ago. Plants evolved to make harder, more rigid cell structures, allowing them to reach above their neighbors to catch sunlight and downward with roots to both capture more energy and to develop stabilizing structures. These early roots weathered the crust and formed the first soils on Earth; however, the areal distribution of plants was sparse and early soils were rarely retained. Eventually, in the late Devonian period (roughly 330 million years ago), a forest ecosystem of an ancient fern-like tree, known as Archaeopteris, emerged, and the Earth's surface never lost its forest cover or soils since that time. The development of soil fundamentally changed the way weathering and erosion occurs, as the newly evolved plant acids enhanced the chemical weathering of rock into sediment. The ramifications of this massive transformation of the earth surface have been explored from several fronts, including mass extinctions in the oceans by a weathering pulse and release of phosphorus from land inducing a fertilization effect, similar to the excessive algal growth from fertilizer runoff and the modern Gulf of Mexico Dead Zone. But the scenario of a phosphorus-driven mass extinction in the ocean has never been corroborated at the source?namely using land-based records to see if soil development really did result in a dramatic loss of phosphorus from the landscape, and if so, whether this amount of phosphorus was adequate to drive a global "Dead Zone" in the ocean. Investigators will explore this critical interval by examining nutrient geochemical records stored in ancient lake sediments nearer the weathering sources. This work will involve an international collaboration and will train graduate and undergraduate students in geochemistry, geobiology, and earth history. Additionally, researchers will develop learning exercises for elementary and junior high students to explore the modern environmental issues of the Gulf of Mexico Dead Zone by looking through the lens of past examples of eutrophication. Investigators will quantify what impacts the proliferation and declines of various root-based ecosystems had on soil weathering and terrestrial cycling of the key global nutrient phosphorus. They will additionally examine relationships between carbon/nitrogen/phosphorus, carbon and nitrogen isotopic compositional changes, and geochemical proxies of weathering intensity during these terrestrial evolutionary steps to explore how the emergence of the modern soil systems impacted total soil nutrient and carbon balances. They will constrain terrestrial phosphorus mass balances to provide quantitative estimates of phosphorus export to the oceans, key evidence by which to test extant models of mid-late Devonian episodic oceanic anoxia. Their results will be coupled with complementary work done on palynology and isotope geochemistry by colleagues at the University of Southampton. The overarching hypothesis is that a temporal record of soil phosphorus transformations can be resolved from the sedimentary record of lacustrine systems in the mid-late Devonian. The intellectual merits of this work are to: (1) constrain terrestrial nutrient evolution during the late Devonian in reference to the extant paleobotanical record in a stratigraphic manner (which paleosols, as time-integrated records, do not independently do), (2) explore the potential role of nutrient limitation in driving evolution and extinction during one of the most dynamic transitions in terrestrial weathering and erosion conditions in Earth history, and (3) develop critical input data to constrain ocean reconstructions. Broader impacts include: (1) a significantly greater understanding of terrestrial nutrient dynamics, thus informing the broader geobiological community, (2) advanced training for a PhD student in biogeochemistry and Earth history, (3) facilitating research collaborations between IUPUI and Southampton, and informing several educational endeavors, including targeted class content applications and a science outreach program for 3rd ? 9th grade students with inadequate access to STEM resources at their schools.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.