OXIDATION OF ORGANIC CARBON AND SILICATE WEATHERING IN RIVER NETWORKS
Oxidation of organic carbon and weathering of silicate minerals during fluvial transit from mountains to depositional basins controls the exchange of carbon dioxide between the atmosphere, biosphere, and rock reservoir and can alter global climate over geologic timescales. Despite this critical role in the global carbon cycle, data documenting organic carbon oxidation and silicate weathering rates within rivers and fluvial deposits are rare, and the mechanisms controlling total silicate weathering and oxidative losses from sediment source to sink are poorly understood. As part of my postdoc, I performed a combination of laboratory flume experiments and targeted field measurements designed to disentangle weathering occurring in active river transport versus during temporary storage in floodplains. These results will elucidate the major mechanistic controls on silicate weathering and organic carbon oxidation in fluvial transit from source to sink, and allow for building process-based models linking sediment transport, organic carbon oxidation, and silicate weathering capable of predicting the influence of changing tectonic and climatic regimes on the global carbon cycle.
FLUVIAL BEDROCK EROSION AT STEEP SLOPES
Fluvial bedrock incision sets the pace of landscape evolution, but little is known about the mechanics of bedrock erosion in steep streams where channels commonly display bedrock steps, pools, and waterfalls, which can retreat upstream. As part of my PhD I performed a controlled laboratory experiment designed specifically to examine bedrock incision processes at steep slopes under constant forcing. Under low sediment supply, a deep inner slot was rapidly incised into the initially planar, 19.5% sloping channel. This inner slot quickly developed into a series of bedrock crests and troughs, which migrated downstream and grew into distinct step-pool topography, and at times, waterfalls. Pools deepened to the point of sediment deposition, after which selective erosion of adjacently exposed bedrock caused upstream propagation of the bedrock step, formation of a new upstream pool, and destruction of the original pool. These experimental results highlight the importance of bedrock step-pool dynamics in setting the rate and style of bedrock incision in steep streams. While waterfalls and bedrock steps are often assumed to form from changes in climatic or tectonic forcing, this experiment suggests waterfalls can be formed from internal system dynamics in bedrock rivers at steep slopes. This work appears in my PhD thesis, and is currently in review for publication.
WATERFALL PLUNGE-POOL SEDIMENT TRANSPORT AND EROSION
In many landscapes, waterfalls are the fastest eroding part of river networks and set the pace of landscape evolution. Despite the integral role of waterfalls in shaping landscapes, current models models cannot accurately predict waterfall erosion and retreat rates in bedrock because they lack physical, process-based mechanisms of erosion, instead relying on general predictors (e.g. channel slope and water discharge).
The bulk of my PhD work focused on a series of flume experiments and field monitoring which allowed the development of process-based models for waterfall plunge-pool erosion via directly linking sediment transport and erosion. This work is published in four separate papers focusing on waterfall plunge-pool sediment transport theory and experiments (Scheingross and Lamb, 2016), controls on plunge-pool sediment filling and evacuation in natural waterfalls (in prep), waterfall plunge-pool bedrock erosion theory (Scheingross and Lamb, 2017) and plunge-pool erosion experiments (Scheingross et al., 2017).
Scheingross, J.S., and M.P. Lamb, 2017, A mechanistic model waterfall plunge-pool erosion into bedrock, JGR – Earth Surface. (PDF)
Scheingross, J.S., Lo, D.Y., and M.P. Lamb, 2017, Self-formed waterfall plunge pools in homogeneous rock, GRL. (PDF)
Scheingross, J.S. and M.P. Lamb, 2016, Sediment transport through self-adjusting, bedrock-walled waterfall plunge pools, JGR – Earth Surface. (PDF)
Mackey, B.H., Scheingross, J.S., Lamb, M.P., and K.A. Farley, 2014, Knickpoint formation, rapid propagation, and landscape response following coastal cliff retreat at last-interglacial sea-level highstand: Kaua‘i, Hawai‘i, GSA Bulletin. (PDF)
SUSPENSION-REGIME BEDROCK EROSION
Suspended sediment can account for over half the total fluvial sediment load, but its influence on bedrock erosion is often ignored. Using abrasion mill experiments similar to Sklar and Dietrich (2001) I led the first set of experiments systematically documenting erosion across the bed load to suspension regime transition. Results from these experiments agree with previously untested theory which allows for erosion by suspended sediment and indicates suspension-regime erosion can dominate total fluvial incision for cases when bed-material is suspended, as commonly occurs during large floods, in steep channels, and in lowland distributary rivers.
This work was performed in collaboration with undergraduate (Daniel Lo, Caltech) and high school (Khadijah Omerdin, Westridge High School) students whom I directly supervised.
[Download some cool, high-speed videos of suspended sediment transport in the abrasion mill experiments]
Scheingross, J.S., Brun, F., Lo, D.Y., Omerdin, K., and M.P. Lamb, 2014, Experimental evidence for fluvial bedrock incision by suspended and bed-load sediment, Geology. (PDF).
INCIPIENT MOTION IN STEEP STREAMS
Small, steep streams are the critical link in moving sediment from hillslopes to large, trunk channels. These streams make up the majority of the river network in terms of channel length, but are often overlooked in favor of study of larger streams. I completed a multi-year monitoring study in headwater streams in Elder Creek, California to examine the conditions for incipient motion of particles. Results from this study showed that mobilization of gravel generally required higher critical Shields stress than expected for lower gradient rivers (τ*c > 0.045), and that these high critical Shields stresses are influenced by slope-dependent hydraulics, grain hiding, and bed patchiness.
Scheingross, J.S., Winchell, E.W., Lamb, M.P., and W.E. Dietrich, 2013, Influence of bed patchiness, slope, grain hiding, and form drag on gravel mobilization in very steep streams, Journal of Geophysical Research Earth Surface. (PDF)
DISTRIBUTION AND MECHANICS OF SLOW-MOVING LANDSLIDES
Slow-moving landslides (earthflows) are common in hilly areas with fine-grained mechanically weak rock. These landslides are an important mechanism of hillslope lowering, can transport large amounts of sediment from hillslopes to river systems, and present a persistent natural hazard. I have undertaken work using airborne InSAR data from NASA’s UAVSAR instrument to analyze the distribution of slow-moving landslides along the central San Andreas Fault, California. In a recent paper, we showed that earthflows cluster near the creeping section of the San Andreas Fault due to fault-induced bedrock fracturing and reduced rock strength. Outside of the creeping section, the occurrence of large-magnitude earthquakes may suppress the development of near-fault earthflows by reducing sediment supply.
Ongoing work involves exploiting airborne InSAR acquisitions from multiple viewing geometries to invert for full 3D earthflow velocity. This data will allow inference of landslide depth, testing of existing landslide deformation theory, and estimates sediment flux to fluvial networks.
Scheingross, J.S., Minchew, B.M., Mackey, B.H., Simons, M., Lamb, M.P., and S. Hensley, 2013, Fault-zone controls on the spatial distribution of slow-moving landslides, GSA Bulletin. (PDF)