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

Jeffery Valenza博士获得了NSF EAR博士后奖学金，在加州大学圣巴巴拉分校的Vamsi Ganti博士和伦敦帝国理工学院的Alexander Whittaker博士的指导下进行研究和教育活动。本研究旨在了解河流在没有加强河岸植被的情况下如何发挥作用。虽然几乎所有的现代河流环境都受到植被的影响，但这种情况只发生在地球历史的最近10%左右。在陆生植物进化之前，河流使大陆干涸，没有根系植物的稳定作用，很像火星和土卫六上的外星环境。该研究将通过物理实验和露头分析来描述无植被河流随时间变化的方式。实验的重点是发现产生无植被的蜿蜒河流的条件，并测量结果的特征。这些数据将用于描述和解释苏格兰西北部砂岩暴露的古代环境条件。连接实验室和地质尺度将提供新的测量方法，了解未被植被覆盖的河流是如何演变的，并适用于其他行星体，如火星，在没有植物的环境中保存河流特征。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。对本科生的指导将通过加州少数民族参与联盟项目来促进，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项目来完成。古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进化相吻合。这种地层变化被假设为标志着从无植被景观中浅的、多线河流的优势到由于植被稳定河岸而发展的低梯度曲流河流的广泛发生的转变。然而，最近的观察表明，单线河流可以很容易地在贫瘠的沙漠景观中形成，它们的存在越来越多地在地球表面和大陆绿化之前的沉积物中得到承认。这些观察结果挑战了河流地层的志留纪过渡是由于河面的几何位移而发生的范式。通过在没有植物的情况下进行单线河流演化的缩小规模的物理实验，以及对苏格兰西北部元古代河流沉积物的实地观察，我将检验志留纪时代地层转移是由于河流演化速率的运动变化而发生的假设，而河流演化速率与植物演化是同时发生的。具体来说，我将检验以下假设：(1)植被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前的单线河流大一个数量级；(2)相对于河流崩解速率而言，较高的横向迁移速率导致河漫滩沉积物和细粒沉积物从河域被冲进河漫滩，形成垂直截短、保存较差、横向粗放的坝包和保存较少的冲积泥岩。从这项研究中产生的新的实验和现场数据将使我们能够对河流地层神秘变化的原因和后果进行批判性评估，这些变化记录了我们星球上生命和地表过程的共同进化。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。

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.