Ken L. Ferrier

Climatic control of bedrock river incision

Bedrock river incision drives the development of much of Earth’s surface topography, and thereby shapes the structure of mountain belts and modulates Earth’s habitability through its effects on soil erosion, nutrient fluxes and global climate. Although it has long been expected that river incision rates should depend strongly on precipitation rates, quantifying the effects of precipitation rates on bedrock river incision rates has proved difficult, partly because river incision rates are difficult to measure and partly because non-climatic factors can obscure climatic effects at sites where river incision rates have been measured. Here we present measurements of river incision rates across one of Earth’s steepest rainfall gradients, which show that precipitation rates do indeed influence long-term bedrock river incision rates. We apply a widely used empirical law for bedrock river incision to a series of rivers on the Hawaiian island of Kaua‘i, where mean annual precipitation ranges from 0.5 metres to 9.5 metres (ref. 12)—over 70 per cent of the global range—and river incision rates averaged over millions of years can be inferred from the depth of river canyons and the age of the volcanic bedrock. Both a time-averaged analysis and numerical modelling of transient river incision reveal that the long-term efficiency of bedrock river incision across Kaua‘i is positively correlated with upstream-averaged mean annual precipitation rates. We provide theoretical context for this result by demonstrating that our measurements are consistent with a linear dependence of river incision rates on stream power, the rate of energy expenditure by the flow on the riverbed. These observations provide rare empirical evidence for the long-proposed coupling between climate and river incision, suggesting that previously proposed feedbacks among topography, climate and tectonics may occur.

The root of branching river networks

Branching river networks are one of the most widespread and recognizable features of Earth’s landscapes and have also been discovered elsewhere in the Solar System. But the mechanisms that create these patterns and control their spatial scales are poorly understood. Theories based on probability or optimality have proven useful, but do not explain how river networks develop over time through erosion and sediment transport. Here we show that branching at the uppermost reaches of river networks is rooted in two coupled instabilities: first, valleys widen at the expense of their smaller neighbours, and second, side slopes of the widening valleys become susceptible to channel incision. Each instability occurs at a critical ratio of the characteristic timescales for soil transport and channel incision. Measurements from two field sites demonstrate that our theory correctly predicts the size of the smallest valleys with tributaries. We also show that the dominant control on the scale of landscape dissection in these sites is the strength of channel incision, which correlates with aridity and rock weakness, rather than the strength of soil transport. These results imply that the fine-scale structure of branching river networks is an organized signature of erosional mechanics, not a consequence of random topology.

Covariation of climate and long-term erosion rates across a steep rainfall gradient on the Hawaiian island of Kaua‘i

Erosion of volcanic ocean islands creates dramatic landscapes, modulates Earth’s carbon cycle, and delivers sediment to coasts and reefs. Because many volcanic islands have large climate gradients and minimal variations in lithology and tectonic history, they are excellent natural laboratories for studying climatic effects on the evolution of topography. Despite concerns that modern sediment fl uxes to island coasts may exceed long-term fl uxes, little is known about how erosion rates and processes vary across island interiors, how erosion rates are infl uenced by the strong climate gradients on many islands, and how modern island erosion rates compare to long-term rates. Here, we present new measurements of erosion rates over 5 yr to 5 m.y. timescales on the Hawaiian island of Kaua‘i, across which mean annual precipitation ranges from 0.5 to 9.5 m/yr. Eroded rock volumes from basins across Kaua‘i indicate that million-year-scale erosion rates are correlated with modern mean annual precipitation and range from 8 to 335 t km –2 yr –1 . In

Earth science: Mainly in the plain.

The finding that global mass loss from landscapes is dominated by physical erosion and chemical weathering from flat terrain, rather than from mountains, challenges our understanding of how Earths surface evolves.

Testing for supply‐limited and kinetic‐limited chemical erosion in field measurements of regolith production and chemical depletion

Chemical erosion contributes solutes to oceans, influencing atmospheric CO2 and thus global climate via the greenhouse effect. Quantifying how chemical erosion rates vary with climate and tectonics is therefore vital to understanding feedbacks that have maintained Earth’s environment within a habitable range over geologic time. If chemical erosion rates are strongly influenced by the availability of fresh minerals for dissolution, then there should be strong connections between climate, which is modulated by chemical erosion, and tectonic uplift, which supplies fresh minerals to Earth’s surface. This condition, referred to as supply-limited chemical erosion, implies strong tectonic control of chemical erosion rates. It differs from kinetic-limited chemical erosion, in which dissolution kinetics and thus climatic factors are the dominant regulators of chemical erosion rates. Here we present a statistical method for determining whether chemical erosion of silicate-rich bedrock is supply limited or kinetic limited, as an approach for revealing the relative importance of tectonics and climate in Earth’s silicate weathering thermostat. We applied this method to published data sets of mineral supply rates and regolith chemical depletion and were unable to reject the null hypothesis that chemical erosion is supply limited in 8 of 16 cases. In seven of the remaining eight cases, we found behavior that is closer to supply limited than kinetic limited, suggesting that tectonics may often dominate over climate in regulating chemical erosion rates. However, statistical power analysis shows that new measurements across a wider range of supply rates are needed to help quantify feedbacks between climate and tectonics in Earth’s long-term climatic evolution.

Seasonal Slumps in Juventae Chasma, Mars

Dark topographic slumps several meters wide, tens of meters in length and up to a meter in depth are observed on the slopes of Juventae Chasma (JC), Valles Marineris (VM), Mars. These slumps usually originate near the terminal points of recurring slope lineae (RSL). Near their initiation points, the slumps have topographic depressions due to the removal of materials; near their lowermost reaches, new materials are deposited in lobes. Over the course of three Mars years, ten active slumps have been observed in JC, all of which formed in or near the same season (areocentric longitude: Ls 0°–120°). Mars Color Imager (MARCI) observations show low-altitude atmospheric obscurations confined within the topography of the VM and JC in the seasons when the slumps form. In one instance, data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and MARCI show evidence of H2O ice in the atmospheric obscuration, likely due to the formation of a low-level afternoon cloud above a dust storm, or mixing of condensate clouds with a diffuse dust cloud. The presence of atmospheric obscurations with H2O ice near times when the slumps form is intriguing, but no direct evidence currently exists to support that they aid in slump formation. Further monitoring of this site will help establish if RSL and/or atmospheric events play a role in the creation of contemporary slumps.