EAR-PF: Unraveling Fluvial Kinematics before the Rise of Land Plants: Bridging Laboratory and Geologic Scales

杰弗里·瓦伦扎博士被授予NSF EAR博士后奖学金，在Vamsi Ganti博士的指导下在加州大学圣巴巴拉分校开展研究和教育活动，并在Alexander Whittaker博士的指导下在伦敦帝国理工学院开展研究和教育活动。这项研究旨在了解河流在没有固岸植被的情况下是如何发挥作用的。虽然几乎所有现代河流环境都受到植被的影响，但这只是地球历史上最近10%的情况。在陆地植物进化之前，河流在没有扎根植物的稳定作用的情况下排干了大陆，这很像火星和土卫六上的外星环境。这项研究将利用物理实验和露头分析来表征无植被河流随时间变化的方式。实验将集中在发现产生一条没有植被的蜿蜒河流的条件，并将测量由此产生的特征。这些数据将被用来描述和解释苏格兰西北部砂岩暴露的古代环境条件。桥接实验室和地质尺度将为植被形成前的河流如何演变提供新的衡量标准，并适用于其他行星体，如火星，在没有植物的环境中保留河流特征。该项目还包括通过教育示范和科学培训突出实验室实验的外联和指导活动。对本科生的指导将由加州少数族裔参与联盟计划促进，对高中生和科学教师的指导将通过加州大学圣巴巴拉分校的学徒研究人员计划完成。古代河流的岩石记录发生了不可逆转的转变，这与大约4.5亿年前志留纪时期陆地植物的演化相吻合。这一地层变化被认为标志着从没有植被的地貌中以浅水、多线程河流为主，到由于植被的稳定作用而发展起来的低坡度曲流河的广泛出现。然而，最近的观察表明，单线河流很容易在贫瘠的沙漠地貌中形成，它们在地外表面和大陆变绿之前的沉积物中的存在越来越被认识到。这些观察结果挑战了河流相地层中志留系的转变是由于河流平面上的几何位移而发生的范式。通过在没有植物的情况下进行的单线河流演变的缩小规模的物理实验和对苏格兰西北部元古界河流沉积的实地观察相结合，我将检验这样一个假设，即志留纪时代的地层转变是由于河流演变速度的运动学变化造成的，而河流演变速度与植物演化是相同的。具体地说，我将检验如下假设：(1)植被形成前的单线河流由细粒粘性河岸沉积物维持，但其横向河道迁移速率比植被覆盖的河流高一个数量级；(2)高横向迁移速率--相对于河流撕裂速率--导致泛滥平原沉积和来自河流冲积区的细粒沉积物迅速改建，导致纵向截断、保存欠佳和横向广泛的沙洲堆积物以及稀少保存下来的冲积泥岩。这项研究产生的新的实验和野外数据将使我们能够对河流地层中记录了地球上生命和地表过程的共同进化的神秘变化的原因和后果进行关键评估。这一奖项反映了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。

Dr. Jeffery Valenza has been awarded an NSF EAR Postdoctoral Fellowship to carry out research and education activities at the University of California Santa Barbara under the mentorship of Dr. Vamsi Ganti, and at Imperial College London under Dr. Alexander Whittaker. This study aims to understand how rivers function in the absence of bank-strengthening vegetation. While nearly all modern river environments are impacted by vegetation, this has only been the case for the most recent ~10% of earth’s history. Prior to the evolution of terrestrial plants, rivers drained the continents without the stabilizing effects of rooted plants, much like extraterrestrial environments on Mars and Titan. The research will characterize the ways that vegetation-free rivers change through time using physical experiments and outcrop analysis. Experiments will focus on discovering conditions that produce a vegetation-free, winding river and will measure the resulting features. These data will be used to characterize and interpret ancient environmental conditions using sandstone exposures in NW Scotland. Bridging laboratory and geologic scales will provide new measures of how pre-vegetation rivers evolved and is applicable to other planetary bodies such as Mars, with preserved river features in an environment with no plants. The project also included outreach and mentoring activities that highlight the laboratory experiments through educational demonstrations and scientific training. Mentoring of undergraduates will be facilitated by the California Alliance for Minority Participation program and mentoring of high-school students and science teachers will be accomplished through the Apprentice Researchers program at University of California Santa Barbara. An irreversible shift occurred in the rock record of ancient rivers, which coincides with the evolution of land plants during the Silurian period approximately 450 million years ago. This stratigraphic shift was hypothesized to mark the transition from the predominance of shallow, multi-threaded rivers in un-vegetated landscapes to the widespread occurrence of low-gradient meandering rivers that developed due to the bank-stabilizing effect of vegetation. However, recent observations indicate that single-threaded rivers can readily form in barren desert landscapes, and their occurrence is increasingly recognized on extraterrestrial surfaces and from sediments pre-dating the greening of the continents. These observations challenge the paradigm that the Silurian transition in fluvial strata occurred due to a geometric shift in river planform. Through a combination of reduced-scaled physical experiments of single-threaded river evolution in the absence of plants and field observations of Proterozoic fluvial deposits in NW Scotland, I will test the hypothesis that the Silurian-age stratigraphic shift occurred due to a kinematic shift in the rates of river evolution that was coeval with plant evolution. Specifically, I will test the hypotheses that: (1) pre-vegetation single-threaded rivers were sustained by fine-grained cohesive bank sediment but their lateral channel migration rates were an order-of-magnitude greater than their vegetated counterparts; (2) high lateral migration rates—relative to river avulsion rates—caused the rapid reworking of floodplain deposits and flushed fine-grained sediment from the fluvial realm, resulting in vertically-truncated, poorly preserved, laterally extensive bar packages and a paucity of preserved alluvial mudrock. New experimental and field data generated from this study will enable the critical evaluation of the causes and consequences of an enigmatic shift in fluvial strata that records the co-evolution of life and surface processes on our planet.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.