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

在地球历史的最初40亿年里，陆地表面没有生物活动。没有植物，没有森林，没有树根，几乎没有土壤。然后，在地质学的一眨眼之间，大约4亿年前，一系列的进化进步在陆地上开始了。植物进化出更坚硬、更坚硬的细胞结构，使它们能够到达邻近植物的上方以捕捉阳光，并通过根部向下捕捉更多能量并形成稳定的结构。这些早期的树根风化了地壳，形成了地球上最早的土壤；然而，植物面积分布稀疏，早期土壤很少保留。最终，在泥盆纪晚期（大约3.3亿年前），一个由一种古老的蕨类树木（被称为始祖树）组成的森林生态系统出现了，从那时起，地球表面就再也没有失去过森林覆盖或土壤。土壤的发展从根本上改变了风化和侵蚀发生的方式，因为新进化的植物酸增强了岩石形成沉积物的化学风化作用。地球表面这种巨大变化的后果已经从几个方面进行了探索，包括由风化脉冲引起的海洋大灭绝和陆地上磷的释放引起的施肥效应，类似于肥料径流导致的藻类过度生长和现代墨西哥湾死区。但是海洋中由磷引起的大灭绝的情景从来没有从源头得到证实？即使用陆地记录来查看土壤发育是否真的导致了景观中磷的急剧流失，如果是这样，那么这些磷的数量是否足以在海洋中形成一个全球性的“死亡区”。研究人员将通过检查保存在靠近风化源的古湖泊沉积物中的营养地球化学记录来探索这一关键区间。这项工作将涉及国际合作，并将培养地球化学、地球生物学和地球历史方面的研究生和本科生。此外，研究人员将为小学和初中学生开发学习练习，通过观察过去富营养化的例子，探索墨西哥湾死亡地带的现代环境问题。研究人员将量化各种以根为基础的生态系统的增殖和减少对土壤风化和全球关键养分磷的陆地循环的影响。他们还将研究碳/氮/磷、碳和氮同位素组成变化之间的关系，以及在这些陆地进化步骤中风化强度的地球化学指标，以探索现代土壤系统的出现如何影响土壤总养分和碳平衡。它们将限制陆地磷的质量平衡，以提供向海洋输出磷的定量估计，这是检验现存泥盆纪中晚期幕式海洋缺氧模型的关键证据。他们的研究结果将与南安普顿大学的同事们在孢粉学和同位素地球化学方面所做的补充工作相结合。最重要的假设是，土壤磷转化的时间记录可以从泥盆纪中晚期湖泊系统的沉积记录中得到解决。这项工作的智力价值在于：(1)参考现存的古植物记录，以地层的方式约束泥盆纪晚期陆地营养物质的演化（古土壤作为时间整合的记录，不能独立地这样做）；(2)在地球历史上陆地风化和侵蚀条件最动态的转变之一中，探索营养物质限制在推动进化和灭绝中的潜在作用；(3)开发关键输入数据，以约束海洋重建。更广泛的影响包括：(1)对陆地营养动态有了更深入的了解，从而为更广泛的地球生物界提供了信息；(2)对生物地球化学和地球历史博士研究生进行了高级培训；(3)促进了IUPUI和南安普顿大学之间的研究合作，并为一些教育工作提供了信息，包括有针对性的课堂内容应用和科学推广计划。九年级学生无法在学校获得足够的STEM资源。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.