Feedbacks between climate, erosion and tectonics influence the rates of chemical weathering reactions, which can consume atmospheric CO2 and modulate global climate. However, quantitative predictions for the coupling of these feedbacks are limited because the specific mechanisms by which climate controls erosion are poorly understood. Here we show that climate-dependent chemical weathering controls the erodibility of bedrock-floored rivers across a rainfall gradient on the Big Island of Hawai'i. Field data demonstrate that the physical strength of bedrock in streambeds varies with the degree of chemical weathering, which increases systematically with local rainfall rate. We find that incorporating the quantified relationships between local rainfall and erodibility into a commonly used river incision model is necessary to predict the rates and patterns of downcutting of these rivers. In contrast to using only precipitation-dependent river discharge to explain the climatic control of bedrock river incision, the mechanism of chemical weathering can explain strong coupling between local climate and river incision.

72Zero-order drainage basins, and their constituent hillslopes, are the fundamental geomorphic unit 73 comprising much of Earth's uplands. The convergent topography of these landscapes generates 74 spatially variable substrate and moisture content, facilitating biological diversity and influencing 75 how the landscape filters precipitation and sequesters atmospheric carbon dioxide. In light of 76 these significant ecosystem services, refining our understanding of how these functions are 77 affected by landscape evolution, weather variability, and long-term climate change is imperative. 78 In this paper we introduce the Landscape Evolution Observatory (LEO): a large-scale 79 controllable infrastructure consisting of three replicated artificial landscapes (each 330 m 2 80 surface area) within the climate-controlled Biosphere 2 facility in Arizona, USA. At LEO, 81 experimental manipulation of rainfall, air temperature, relative humidity, and wind speed are 82 possible at unprecedented scale. The Landscape Evolution Observatory was designed as a 83 community resource to advance understanding of how topography, physical and chemical 84 properties of soil, and biological communities coevolve, and how this coevolution affects water, 85 carbon, and energy cycles at multiple spatial scales. With well-defined boundary conditions and 86 an extensive network of sensors and samplers, LEO enables an iterative scientific approach that 87 includes numerical model development and virtual experimentation, physical experimentation, 88 data analysis, and model refinement. We plan to engage the broader scientific community 89 through public dissemination of data from LEO, collaborative experimental design, and 90 community-based model development. 91 coevolution 93 94 95 1. Introduction 96Hillslopes and their adjacent hollows (i.e., zero-order drainage basins, or ZOBs) 97 constitute a large fraction of upland areas over Earth's surface and provide critical ecosystem 98 services. Within ZOBs there is exchange of water, carbon dioxide, and energy with the 99 atmosphere and transport of soil, water, and solutes into fluvial drainage networks-processes 100 that link ZOBs with the climate system and downstream water quantity and quality. The time-101 varying rates of these exchange and transport processes are integrated responses to many 102 physical and biological phenomena that occur from below the base of the soil profile to the 103 vertical extent of the atmospheric boundary layer (e.g., see discussion by Chorover et al., 2011). 104 Zero-order basins evolve as climate varies, soils form and erode, and biological 105 communities establish, compete, and change in response to environmental stimuli. Across 106 spatial and topographic gradients, these interacting processes may result in consistently 107 observable correlations between temperature and precipitation dynamics, soil depth and hillslope 108 length, and plant biomass accumulation (e.g., Rasmussen et al., 2011; Pelletier et al., 2013). 109 Coupled soil-production and ...

Motivated by convex-concave bedrock river profiles developed across a climate gradient on the wetside of the Kohala Peninsula of the Big Island of Hawai'i, we numerically model how rainfall gradients may influence longitudinal fluvial incision patterns. First, we model transient profile adjustment with two tectonic boundary conditions: subsidence and uplift. In this generalized analysis, we assume that rainfall gradients only influence incision by modifying the relation between upstream drainage area and local discharge. Using a detachment-limited model, downstream increases in rainfall lead to profile convexities during transient adjustment in both tectonic settings, and this is the opposite of the predicted increase in profile concavity that would develop in a steady state uplifting profile. A transport-limited erosion model develops only concave channel profiles without clear signatures of the rainfall pattern. Second, we model the development of convex-concave transient profiles and incision patterns on Kohala using a detachment-limited model. If rainfall gradients only influence incision through the local discharge, reasonable rainfall gradients can only develop channel convexities that are much smaller than those observed. Instead, we hypothesize that local bedrock erodibility increases with the degree of rainfall-dependent chemical weathering. When local erodibility is assumed to scale with local rainfall rate, the model can produce convex-concave profiles similar to those observed in Kohala. Our results suggest that changes in local bedrock erodibility due to local climate-dependent weathering may be an important mechanism by which climate influences landscape form and rates of evolution. This hypothesis requires further testing in this study area and beyond.

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