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

在地球历史的头40亿年里，陆地表面没有任何生物活动。那里没有植物，没有森林，没有根，几乎没有土壤。然后，在地质上一眨眼的功夫，大约4亿年前的陆地上开始了一系列的进化进步。植物进化成更坚硬、更坚硬的细胞结构，使它们能够高于邻居捕捉阳光，并向下伸展根部，以获取更多能量并形成稳定的结构。这些早期的根风化了地壳，形成了地球上第一批土壤；然而，植物的面积分布稀少，早期的土壤很少保留下来。最终，在泥盆纪晚期(大约3.3亿年前)，一种被称为始祖鸟的古老蕨类树木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。土壤的发育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加强了岩石变成沉积物的化学风化。人们从几个方面探索了地球表面这种大规模变化的后果，包括风化脉冲导致的海洋大规模灭绝，以及陆地磷的释放引发的肥沃效应，类似于化肥径流和现代墨西哥湾死亡区造成的藻类过度生长。但是，磷导致海洋大规模灭绝的情景从未在源头上得到证实--即使用陆地记录来确定土壤开发是否真的导致了地貌中磷的戏剧性损失，如果是的话，那么这些磷是否足以在海洋中形成全球“死区”。研究人员将通过检查储存在更靠近风化源的古代湖泊沉积物中的营养地球化学记录来探索这一关键间隔。这项工作将涉及国际合作，并将培训地球化学、地球生物学和地球历史方面的研究生和本科生。此外，研究人员将为小学生和初中生开发学习练习，通过观察过去富营养化的例子来探索墨西哥湾死亡区的现代环境问题。研究人员将量化各种根基生态系统的增殖和衰退对土壤风化和关键的全球营养磷的陆地循环的影响。他们还将研究这些陆地进化过程中碳/氮/磷、碳和氮同位素组成变化以及风化强度的地球化学指标之间的关系，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。它们将限制陆地磷质量平衡，以提供向海洋输出磷的定量估计，这是检验中、晚泥盆世幕式海洋缺氧现有模型的关键证据。他们的研究结果将与南安普顿大学同事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中晚泥盆世湖泊系统的沉积记录中分辨出来。这项工作的学术价值是：(1)参照现存的古植物学记录，以地层学的方式限制晚泥盆世陆地营养物质的演变(古土壤作为时间综合记录，不是独立做的)，(2)探索在地球历史上陆地风化和侵蚀条件最动态的转变之一期间，营养限制在驱动进化和灭绝方面的潜在作用，以及(3)开发关键的输入数据，以限制海洋重建。更广泛的影响包括：(1)更深入地了解陆地营养动态，从而为更广泛的地球生物学界提供信息；(2)为博士生提供生物地球化学和地球历史方面的高级培训；(3)促进IUPUI和南安普顿之间的研究合作；并为几项教育努力提供信息，包括有针对性的课堂内容应用和针对学校STEM资源不足的3？9年级学生的科学推广计划。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.