Rock properties and sediment caliber govern bedrock river morphology across the Taiwan Central Range

Feedbacks between surface and deep Earth processes in collisional mountain belts depend on how erosion and topographic relief vary in space and time. One outstanding unknown lies in how rock strength influences bedrock river morphology and thus mountain relief. Here, we quantify boulder cover and channel morphology using uncrewed aerial vehicle surveys along 30 kilometers of bedrock-bound river corridors throughout the Taiwan Central Range where regional gradients in rock properties relate to tectonic history. We find that boulder size systematically increases with increasing metamorphic grade and depth of exhumation. Boulder size correlates with reach-scale channel steepness but does not explain observations of highly variable channel width. Transport thresholds indicate that rivers are adjusted to mobilize boulders and are well in excess of the threshold to transport gravel and cobbles, as previously assumed. The linkage between metamorphic history, boulder size, and channel steepness reveals how rock properties can influence feedbacks between tectonics and topography throughout the life span of a mountain range.

Climate controls on erosion in tectonically active landscapes

The ongoing debate about the nature of coupling between climate and tectonics in mountain ranges derives, in part, from an imperfect understanding of how topography, climate, erosion, and rock uplift are interrelated. Here, we demonstrate that erosion rate is nonlinearly related to fluvial relief with a proportionality set by mean annual rainfall. These relationships can be quantified for tectonically active landscapes, and calculations based on them enable estimation of erosion where observations are lacking. Tests of the predictive power of this relationship in the Himalaya, where erosion is well constrained, affirm the value of our approach. Our model allows estimation of erosion rates in fluvial landscapes using readily available datasets, and the underlying relationship between erosion and rainfall offers the promise of a deeper understanding of how climate and tectonic evolution affect erosion and topography in space and time and of the potential influence of climate on tectonics.

Exoplanet Biosignatures: Future Directions

We introduce a Bayesian method for guiding future directions for detection of life on exoplanets. We describe empirical and theoretical work necessary to place constraints on the relevant likelihoods, including those emerging from better understanding stellar environment, planetary climate and geophysics, geochemical cycling, the universalities of physics and chemistry, the contingencies of evolutionary history, the properties of life as an emergent complex system, and the mechanisms driving the emergence of life. We provide examples for how the Bayesian formalism could guide future search strategies, including determining observations to prioritize or deciding between targeted searches or larger lower resolution surveys to generate ensemble statistics and address how a Bayesian methodology could constrain the prior probability of life with or without a positive detection. Key Words: Exoplanets-Biosignatures-Life detection-Bayesian analysis. Astrobiology 18, 779-824.

Equilibrium in the balance? Implications for landscape evolution from dryland environments

Equilibrium is a central concept in geomorphology. Despite the widespread use of the term, there is a great deal of variability in the ways equilibrium is portrayed and informs practice. Thus, there is confusion concerning the precise meanings and usage of the concept. In this chapter we draw on examples from dryland environments to investigate the practical implications of applying and testing the concept of equilibrium. Issues that we cover include the importance of scale and spatial variability, time, the assumption of constant environmental feedbacks and nonlinearities. The evaluation demonstrates that there are a range of problems inherent with using ideas of geomorphological equilibrium explicitly or implicitly to structure research in drylands. Many of these problems also apply to other environments.

Time scales of tectonic landscapes and their sediment routing systems

In regions undergoing active tectonics, the coupling between the tectonic displacement field, the overlying landscape and the redistribution of mass at the Earth's surface in the form of sediment routing systems, is particularly marked and variable. Coupling between deformation and surface processes takes place at a range of scales, from the whole orogen to individual extensional fault blocks or contractional anticlines. At the large scale, the attainment of a steady-state between the overlying topography and the prevailing tectonic conditions in active contractional orogens requires an efficient erosional system, with a time scale dependent on the vigour of the erosional system, generally in the range 106–107 years. The catchment–fan systems associated with extensional fault blocks and basins of the western USA are valuable natural examples to study the coupling between tectonic deformation, landscape and sediment routing systems. Even relatively simple coupled systems such as an extensional fault block and its associated basin margin fans have a range of time scales in response to a tectonic perturbation. These response times originate from the development of uniform (steady-state) relief during the accumulation of displacement on a normal fault (c. 106 years), the upstream propagation of a bedrock knickpoint in transverse catchments following a change in tectonic uplift rate (c. 106 years), or the relaxation times of the integrated catchment–fan system in response to changes in climatic and tectonic boundary conditions (105–106 years). The presence of extensive bedrock or alluvial piedmonts increases response times significantly. The sediment efflux of a mountain catchment is a boundary condition for far-field fluvial transport, but the fluvial system is much more than a simple transmitter of the sediment supply signal to a neighbouring depocentre. Fluvial systems appear to act as buffers to incoming sediment supply signals, with a diffusive time scale (c. 105–106 years) dependent on the length of the system and the extent of its floodplains, stream channels and proximal gravel fans. The vocabulary for explaining landscapes would benefit from a greater recognition of the importance of the repeat time and magnitude of perturbations in relation to the response and relaxation times of the landscape and its sediment routing systems. Landscapes are best differentiated as ‘buffered’ or ‘reactive’ depending on the ratio of the response time to the repeat time of the perturbation. Furthermore, landscapes may be regarded as ‘steady’ or ‘transient’ depending on the ratio of the response time to the time elapsed since the most recent change in boundary conditions. The response of tectonically and climatically perturbed landscapes has profound implications for the interpretation of stratigraphic architecture.

The search for a topographic signature of life

Landscapes are shaped by the uplift, deformation and breakdown of bedrock and the erosion, transport and deposition of sediment. Life is important in all of these processes. Over short timescales, the impact of life is quite apparent: rock weathering, soil formation and erosion, slope stability and river dynamics are directly influenced by biotic processes that mediate chemical reactions, dilate soil, disrupt the ground surface and add strength with a weave of roots. Over geologic time, biotic effects are less obvious but equally important: biota affect climate, and climatic conditions dictate the mechanisms and rates of erosion that control topographic evolution. Apart from the obvious influence of humans, does the resulting landscape bear an unmistakable stamp of life? The influence of life on topography is a topic that has remained largely unexplored. Erosion laws that explicitly include biotic effects are needed to explore how intrinsically small-scale biotic processes can influence the form of entire landscapes, and to determine whether these processes create a distinctive topography.