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

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.