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Speleothem initial uranium isotope ratios ((²³⁴U/²³⁸U)_i) can be influenced by processes along the seepage water flow‐path including alpha‐recoil into porewater during²³⁸U decay and hostrock weathering, the balance of which can reflect the infiltration rate. Thus, speleothem (²³⁴U/²³⁸U)_imay provide information about past changes in rainfall amounts. However, the utility of (²³⁴U/²³⁸U)_ias a paleo‐infiltration proxy has only been explored in a limited set of rainfall regimes. We present a speleothem (²³⁴U/²³⁸U)_irecord from Mawmluh Cave in northeast India, an area influenced by the Indian Summer Monsoon, covering 1964–2012 CE. Speleothem (²³⁴U/²³⁸U)_iwas relatively constant from 1964 to 1984 but then linearly increased by 0.05 over ∼15 years, a trend that does not correspond with observed rainfall changes. To evaluate potential drivers of (²³⁴U/²³⁸U)_ivariability, we model the evolution of water (²³⁴U/²³⁸U) in a simple karst system using an advection‐reaction model parameterized by Mawmluh Cave variables. Although varying infiltration influences modeled water (²³⁴U/²³⁸U), the larger, sustained change observed in the speleothem record can only be modeled by varying the U concentration and (²³⁴U/²³⁸U) of the weathering hostrock. This suggests that larger shifts in speleothem (²³⁴U/²³⁸U)_imay result from flow path changes, bringing waters in contact with hostrock of variable U characteristics. Consideration of published Mawmluh Cave records suggests that these mechanisms may also explain variability in stalagmite (²³⁴U/²³⁸U)_ion precessional timescales. Further examination of speleothems (²³⁴U/²³⁸U)_ifrom climates characterized by high rainfall and extensive weathering is warranted to better constrain the controls on (²³⁴U/²³⁸U)_iin these dynamic environments.

Climate change is causing increasingly widespread, frequent, and intense wildfires across the western United States. Many geomorphic effects of wildfire are relatively well studied, yet sediment transport models remain unable to account for the rapid transport of sediment released from behind incinerated vegetation, which can fuel catastrophic debris flows. This oversight reflects the fundamental inability of local, continuum-based models to capture the long-distance particle motions characteristic of steeplands. Probabilistic, particle-based nonlocal models may address this deficiency, but empirical data are needed to constrain their representation of particle motion in real landscapes. Here we present data from field experiments validating a generalized Lomax model for particle travel distance distributions. The model parameters provide a physically intuitive mathematical framework for describing the transition from light- to heavy-tailed distributions along a continuum of behavior as particle size increases and slopes get steeper and/or smoother. We show that burned slopes are measurably smoother than vegetated slopes, leading to 1) lower rates of experimental particle disentrainment and 2) runaway motion that produces the heavy-tailed travel distances often associated with nonlocal transport. Our results reveal that surface roughness is a key control on steepland sediment transport, particularly after wildfire when smoother surfaces may result in the preferential delivery of coarse material to channel networks that initiate debris flows. By providing a first-order framework relating the statistics of particle motion to measurable surface characteristics, the Lomax model both advances the development of nonlocal sediment transport theory and reveals insights on hillslope transport mechanics.

Soil mixing over long (>10²y) timescales enhances nutrient fluxes that support soil ecology, contributes to dispersion of sediment and contaminated material, and modulates fluxes of carbon through Earth’s largest terrestrial carbon reservoir. Despite its foundational importance, we lack robust understanding of the rates and patterns of soil mixing, largely due to a lack of long-timescale data. Here we demonstrate that luminescence, a light-sensitive property of minerals used for geologic dating, can be used as a long-timescale sediment tracer in soils to reveal the structure of soil mixing. We develop a probabilistic model of transport and mixing of tracer particles and associated luminescence in soils and compare with a global compilation of luminescence versus depth in various locations. The model–data comparison reveals that soil mixing rate varies over the soil depth, with this depth dependency persisting across climate and ecological zones. The depth dependency is consistent with a model in which mixing intensity decreases linearly or exponentially with depth, although our data do not resolve between these cases. Our findings support the long-suspected idea that depth-dependent mixing is a spatially and temporally persistent feature of soils. Evidence for a climate control on the patterns and intensities of soil mixing with depth remains elusive and requires the further study of soil mixing processes.

The response of eroding topography to changes in base level is driven by patchy and intermittent mass flux, both on hillslopes and in channels. However, most models of soil‐mantled landscape evolution use continuously differentiable equations that average over these patchy transport processes. Because of the limited time and space resolution of field observations, the relationship between zeroth‐order landscape evolution (i.e., over long space and time scales) and first‐order fluctuations due to patchy, intermittent transport (e.g., tree throw and landsliding) remains unclear. Here, we use five physical experiments of an eroding experimental landscape to examine how the signature of first‐order transport, as described by autocorrelation functions of local elevation time series, varies as a function of the vigor of hillslope transport relative to channel incision. Our results show that experiments with higher hillslope transport efficacy have higher autocorrelation coefficients, suggesting that differences in zeroth‐order transport coefficients may be driven by differences in patchy, first‐order transport processes. These higher autocorrelation coefficients also imply that in landscapes where hillslope transport dominates, landscape dynamism is reduced and patterns of elevation change are more persistent over time. These findings suggest that the balance between channelized and hillslope transport processes is fundamental to landscape response to perturbation and may control landscape susceptibility to unsteady processes like large‐scale reorganization.

Across arid landscapes, desert shrubs affect where and how sediment is transported by various physical processes such as overland flow, wind, and rain splash. Simplistic biological models such as logistic growth curves offer important first steps towards representing and linking life to landscape dynamics. More sophisticated descriptions of desert shrub dynamics on scales commensurate with downslope sediment transport, however, are essential for more rigorously understanding how complex shrub‐sediment interactions may be affecting hillslope geomorphology. Here we present such a model that features a strong biophysical foundation by including, for example, basic aspects of desert soil‐water hydrology and population ecology such as recruitment, growth, and mortality. Model input parameters can also be modified to account for the influence of different environmental conditions and stressors (e.g. precipitation, soil types, droughts, grazing, fires, and climate change). Model behaviors mimic well documented aspects of how desert shrub populations respond to changes in precipitation, for example, productivity decreases with increasingly arid conditions and density declines during prolonged periods of drought. Model output (position and size of shrubs occupying a hillslope in a given year) represents the basic biological input variables necessary for calculating, for example, how rain‐splash induced mound building by individual shrubs may be affecting downslope sediment fluxes. Future research aimed at coupling this biological model with existing sediment transport models can therefore help advance our understanding for how desert shrub populations affect hillslope erosion across a broad range of scenarios. © 2018 John Wiley & Sons, Ltd.

Over 3.7 billion years of Earth history, life has evolved complex adaptations to help navigate and interact with the fluid environment. Consequently, fluid dynamics has become a powerful tool for studying ancient fossils, providing insights into the palaeobiology and palaeoecology of extinct organisms from across the tree of life. In recent years, this approach has been extended to the Ediacara biota, an enigmatic assemblage of Neoproterozoic soft‐bodied organisms that represent the first major radiation of macroscopic eukaryotes. Reconstructing the ways in which Ediacaran organisms interacted with the fluids provides new insights into how these organisms fed, moved, and interacted within communities. Here, we provide an in‐depth review of fluid physics aimed at palaeobiologists, in which we dispel misconceptions related to the Reynolds number and associated flow conditions, and specify the governing equations of fluid dynamics. We then review recent advances in Ediacaran palaeobiology resulting from the application of computational fluid dynamics (CFD). We provide a worked example and account of best practice in CFD analyses of fossils, including the first large eddy simulation (LES) experiment performed on extinct organisms. Lastly, we identify key questions, barriers, and emerging techniques in fluid dynamics, which will not only allow us to understand the earliest animal ecosystems better, but will also help to develop new palaeobiological tools for studying ancient life.

Three mathematical models of hillslope sediment transport are common: linear diffusion, nonlinear diffusion, and nonlocal transport. Each of these is supported by a different theory, but each contains land‐surface slope as a central ingredient. As such, land‐surface evolution by all three of these models is largely similar in that topographic highs degrade and lows fill in. However, details of land‐surface form reveal diagnostic clues to linear or nonlinear behavior of the land surface. We cast land‐surface evolution into wavenumber (Fourier) domain, which effectively separates signals into coarse‐ and fine‐scale elements of land‐surface form, such as hillslope‐valley sequences and pit‐mound features, respectively. In wavenumber domain linear diffusion results in vertical spectral decay, which is associated with landform straightening and smoothing of sharp concavities. Nonlinear diffusion results in spectral compression toward low wavenumbers, which is associated with landform lengthening and is similar to slope replacement. Nonlocal processes share elements of linearity or nonlinearity but are modified by the particular form of the distribution of particle travel distance. Ultimately, all processes tend toward zero topographic variance, but by distinctly different styles as revealed in wavenumber domain. Spectral compression by nonlinear processes can result in temporary spectral growth over certain spectral bands and is interpreted as a signature of nonlinear processes for certain landforms. The signatures come from the evolution of topographic details and landforms with sharp concavities highlight this behavior, whereas landforms with low concavities obscure these diagnostic behaviors.