Chemical weathering as a mechanism for the climatic control of bedrock river incision

Abstract. Landscape evolution models can be used to assess the impact of rainfall variability on bedrock river incision over millennial timescales. However, isolating the role of rainfall variability remains difficult in natural environments, in part because environmental controls on river incision such as lithological heterogeneity are poorly constrained. In this study, we explore spatial differences in the rate of bedrock river incision in the Ecuadorian Andes using three different stream power models. A pronounced rainfall gradient due to orographic precipitation and high lithological heterogeneity enable us to explore the relative roles of these controls. First, we use an area-based stream power model to scrutinize the role of lithological heterogeneity in river incision rates. We show that lithological heterogeneity is key to predicting the spatial patterns of incision rates. Accounting for lithological heterogeneity reveals a nonlinear relationship between river steepness, a proxy for river incision, and denudation rates derived from cosmogenic radionuclide (CRNs). Second, we explore this nonlinearity using runoff-based and stochastic-threshold stream power models, combined with a hydrological dataset, to calculate spatial and temporal runoff variability. Statistical modeling suggests that the nonlinear relationship between river steepness and denudation rates can be attributed to a spatial runoff gradient and incision thresholds. Our findings have two main implications for the overall interpretation of CRN-derived denudation rates and the use of river incision models: (i) applying sophisticated stream power models to explain denudation rates at the landscape scale is only relevant when accounting for the confounding role of environmental factors such as lithology, and (ii) spatial patterns in runoff due to orographic precipitation in combination with incision thresholds explain part of the nonlinearity between river steepness and CRN-derived denudation rates. Our methodology can be used as a framework to study the coupling between river incision, lithological heterogeneity and climate at regional to continental scales.

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How rivers incise to survive periodic inputs of immobile landslide-derived boulders

10.5194/egusphere-egu2020-12867 ◽

2020 ◽

Author(s):

Noah Finnegan

Keyword(s):

Drainage Area ◽

Channel Width ◽

Coast Range ◽

Narrow Channels ◽

River Incision ◽

Eel River ◽

Tectonically Active ◽

Consistent Picture ◽

Bedrock River Incision ◽

Periodic Inputs

Bedrock landsliding provides a strong negative feedback on bedrock river incision by causing long-lived burial events and hence hiatuses in downcutting.&#160; Nevertheless, rivers in tectonically active settings carve deep canyons despite being periodically inundated with immobile boulders. How is this possible? In this contribution, we explore the processes through which rivers incise bedrock canyons within the Franciscan m&#233;lange in the actively uplifting California Coast Range. The Franciscan m&#233;lange is well known for its &#8220;melting ice cream topography&#8221; in which slow-moving landslides (&#8220;earthflows&#8221;) festoon the walls of river canyons and deliver car- to house-sized boulders to channels.&#160;&#160;Analysis of valley widths and river long profiles over &#8764;19&#8201; km of Alameda Creek (185&#8201; km2 drainage area) and Arroyo Hondo (200&#8201; km2 drainage area) in central California shows a very consistent picture in which earthflows that intersect these channels deposit immobile boulders that force tens of meters of gravel aggradation for kilometers upstream, leading to apparently long-lived sediment storage and channel burial at these sites. In contrast, over a &#8764;30&#8201; km section of the Eel River (5547&#8201; km2 drainage area), there are no knickpoints or aggradation upstream of locations where earthflows impinge on its channel. Neither boulder supply nor transport capacity explains this difference. Rather, we find that the dramatically different sensitivity of the two locations to landslide blocking is linked to differences in channel width relative to typical seasonal displacements of landslides. The Eel River is &#8764;5 times wider than the largest annual seasonal displacement. In contrast, during wet winters, earthflows are capable of crossing and blocking the entire channel width of Arroyo Hondo and Alameda Creek. Hence, by virtue of having wide valley bottoms, larger rivers are more likely to simply flow around the toes of earthflows.&#160;&#160;For the smaller rivers in our study area that are chronically buried in landslide debris, our field observations provide evidence for two processes that may allow periodic bedrock river incision. Narrow channels in the Franciscan m&#233;lange that are buried in debris can incise epigenetic gorges around the margins of boulder jams during periods of earthflow dormancy when boulders are no longer input into channels.&#160; Alternatively, during periods of earthflow dormancy, abrasion (and hence size reduction) of boulders in place from suspended sediment may ultimately render boulders mobile.&#160;&#160;Without explicit representation of these three processes, modeling the coupling of hillslope and channel evolution in this setting is not possible.&#160;

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Strong tectonic and weak climatic control of long-term chemical weathering rates

Geology ◽

10.1130/0091-7613(2001)029<0511:stawcc>2.0.co;2 ◽

2001 ◽

Vol 29 (6) ◽

pp. 511 ◽

Cited By ~ 222

Author(s):

Clifford S. Riebe ◽

James W. Kirchner ◽

Darryl E. Granger ◽

Robert C. Finkel

Keyword(s):

Chemical Weathering ◽

Weathering Rates ◽

Climatic Control

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Quantifying the competing influences of lithology and throw rate on bedrock river incision

Geological Society of America Bulletin ◽

10.1130/b35783.1 ◽

2020 ◽

Author(s):

E. Kent ◽

A.C. Whittaker ◽

S.J. Boulton ◽

M.C. Alçiçek

Keyword(s):

Rock Type ◽

Active Faults ◽

Sedimentary Rocks ◽

Normal Fault ◽

Metamorphic Rocks ◽

Stream Power ◽

Western Turkey ◽

River Incision ◽

Bedrock River Incision

River incision in upland areas is controlled by prevailing climatic and tectonic regimes, which are increasingly well described, and the nature of the bedrock lithology, which is still poorly constrained. Here, we calculated downstream variations in stream power and bedrock strength for six rivers crossing a normal fault in western Turkey, to derive new constraints on bedrock erodibility as function of rock type. These rivers were selected because they exhibit knick zones representing a transient response to an increase in throw rate, driven by fault linkage. Field measures of rock mass strength showed that the metamorphic units (gneisses and schists) in the catchments are ∼2 times harder than the sedimentary lithologies. Stream power increases downstream in all rivers, reaching a maxima upstream of the fault within the metamorphic bedrock but declining markedly where softer sedimentary rocks are encountered. We demonstrate a positive correlation between throw rate and stream power in the metamorphic rocks, characteristic of rivers obeying a detachment-limited model of erosion. We estimated bedrock erodibility in the metamorphic rocks as kb = 2.2−6.3 × 10−14 ms2 kg−1; in contrast, bedrock erodibility values were 5−30 times larger in the sedimentary units, with kb = 1.2−15 × 10−13 ms2 kg−1. However, in the sedimentary units, stream power does not scale predictably with fault throw rate, and we evaluated the extent to which the friable nature of the outcropping clastic bedrock alters the long-term erosional dynamics of the rivers. This study places new constraints on bedrock erodibilities upstream of active faults and demonstrates that the strength and characteristics of underlying bedrock exert a fundamental influence on river behavior.

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Geochemical analysis for evaluating the paleoweathering, paleoclimate, and depositional environments of the siliciclastic Miocene-Pliocene sequence at Al-Rehaili area, Northern Jeddah, Saudi Arabia

Arabian Journal of Geosciences ◽

10.1007/s12517-021-06538-0 ◽

2021 ◽

Vol 14 (4) ◽

Author(s):

Faisal Alqahtani ◽

Mohammed Khalil

Keyword(s):

Chemical Weathering ◽

Major And Trace Elements ◽

Climatic Conditions ◽

Depositional Environments ◽

Geochemical Data ◽

Geochemical Analysis ◽

Climatic Control ◽

Trace Elements Analysis ◽

Area North ◽

Semi Arid

AbstractGeochemical data and their various approaches are useful to evaluate the climatic control on the depositional environments. This study aims to evaluate the paleoweathering and plaeoclimate condition that have controls on the depositional environments of the Miocene to Pliocene siliciclastic sequence at Al-Rehaili area, north of Jeddah. To achieve this aim, selected sandstone samples were geochemically (major and trace elements analysis) and petrographically examined. The results of these analyses reveal that the examined sandstones were deposited in non-marine (fluvial/alluvial-lacustrine) environments and suffered from weak to intermediate chemical weathering and intense physical induration under semi-arid to semi-humid climatic conditions.

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Supplemental Material: Quantifying the competing influences of lithology and throw rate on bedrock river incision

10.1130/gsab.s.13194737.v1 ◽

2020 ◽

Author(s):

E. Kent ◽

A.C. Whittaker ◽

et al.

Keyword(s):

River Incision ◽

Bedrock River Incision

Methods; Tables S1–S15; and Figures S1–S4.

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A process-based model for production and evolution of sediment particles by physical and chemical weathering in mountain catchments

10.5194/egusphere-egu2020-13008 ◽

2020 ◽

Author(s):

Leonard S. Sklar ◽

Clifford S. Riebe

Keyword(s):

Sierra Nevada ◽

Chemical Weathering ◽

Published Data ◽

Coarse Particles ◽

Initial Particle ◽

Sediment Production ◽

River Incision ◽

Bedding Planes ◽

Sediment Particles ◽

Physical And Chemical

Landscapes evolve through interactions between subsurface processes that move and deform bedrock, and surface processes that redistribute mass through erosion, transport, and deposition of sediment. Sediment is composed of discrete particles that are produced from bedrock and modified during transport by physical and chemical weathering. Sediment particle attributes, including size, angularity, and durability, therefore depend on the climatic, tectonic, and lithologic factors that regulate weathering processes. These attributes, in turn, influence rates and modes of sediment transport, and the tools and cover effects that control rates of river incision into bedrock. Thus the production of sediment helps set the slopes of river channels and the relief structure of landscapes, making it central to the feedbacks between tectonics, climate, and erosion that create topography. Despite their importance, sediment particles are rarely included explicitly in landscape evolution modeling due to gaps in understanding of sediment production on hillslopes, the particle evolution that occurs on hillslopes and in channels, and the implications of sediment attributes for river incision into bedrock. Although these processes have been studied in isolation, they have not been combined together in a comprehensive model of the role of sediment in climate-tectonic-erosion feedbacks.&#160;Here we present results from a new, spatially-explicit model that predicts the evolution of individual particle attributes, including size, angularity, and durability. The model also predicts the resulting distributions of particle attributes as sediment from different sources is mixed, and as particles evolve during transport through catchments. The model has two components. The first predicts the initial particle attributes as sediments are produced from bedrock on hillslopes. The initial particle size distribution depends on the spacing of fractures and sizes of mineral grains in crystalline rocks, and on the spacing of bedding planes and the size of cemented particles in clastic sedimentary rocks. Initial size, as well as particle angularity and durability, are also influenced by chemical weathering, which depends on the fraction of soluble minerals, the local climate (parameterized as mean temperature and precipitation), and the residence time of bedrock as it is exhumed through the hillslope weathering engine.The second model component quantifies how particles change as they are transported across hillslopes and through channel networks. Particle sizes are reduced by abrasion as a function of three factors: the potential energy lost in transport; particle angularity; and particle durability, which depends on initial rock tensile strength and subsequent loss of strength due to chemical weathering. Mass lost from abrasion of coarse particles is converted to sand and silt. Particles become less angular as a function of cumulative mass loss. However, high rates of energy loss on steep slopes cause fragmentation, which creates new coarse particles and resets particle angularity. Model relationships are parameterized using published data as well as newly acquired data from laboratory experiments and field studies in the Sierra Nevada, California. We couple the model with the saltation abrasion/bedrock river incision model to simulate evolution of river longitudinal profiles, and explore potential feedbacks between rock uplift, climate, and sediment production.

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