Landscape Evolution Processes Research Articles

Evolution of submarine canyons and hanging-wall fans: insights from geomorphic experiments and morphodynamic models

Abstract. Tectonics play a significant role in shaping the morphology of submarine canyons, which form essential links in source-to-sink (S2S) systems. It is difficult, however, to investigate the resulting morphodynamics over the long term. For this purpose, we propose a novel experimental approach that can generate submarine canyons and hanging-wall fans on continuously evolving active faults. We utilize morphometric analysis and morphodynamic models to understand the response of these systems to fault slip rate (Vr) and inflow discharge (Q). Our research reveals several key findings. Firstly, the fault slip rate controls the merging speed of submarine canyons and hanging-wall fans, which in turn affects their quantity and spacing. Additionally, the long profile shapes of submarine canyons and hanging-wall fans can be decoupled into a gravity-dominated breaching process and an underflow-dominated diffusion process, which can be described using a constant-slope relationship and a morphodynamic diffusion model, respectively. Furthermore, both experimental and simulated submarine canyon–hanging-wall fan long profiles exhibit strong self-similarity, indicating that the long profiles are scale independent. The Hack's scaling relationship established through morphometric analyses serves as an important link between different scales in S2S systems, bridging laboratory-scale data to field-scale data and submarine-to-terrestrial relationships. Lastly, for deep-water sedimentary systems, we propose an empirical formula to estimate fan volume using canyon length, and the data from 26 worldwide S2S systems utilized for comparison show a strong agreement. Our geomorphic experiments provide a novel perspective for better understanding of the influence of tectonics on deep-water sedimentary processes. The scaling relationships and empirical formulas we have established aim to assist in estimating volume information that is difficult to obtain during long-term landscape evolution processes.

Interplay between tectonics and surface processes in the evolution of mountain ranges: Insights from landscape dynamics, uplift, and active deformation of Talesh Mountains (NW Iranian Plateau margin)

Drainage divides are dynamic features of a landscape that migrate over time during the development of river networks. In this study, we focused on the geomorphic response of a landscape to the interaction among tectonics, climate, and lithology in the Talesh Mts. along the northwest of the Iranian Plateau. The footprints of these competing forces have been analyzed by divide stability analysis and applying χ, χQ, and Gilbert metrics. Additionally, we examined the effects of bedrock erodibility on landscape evolution processes by measuring the mechanical rock strength of the lithological units in different sectors of the mountain range. The distribution of the χ values suggests a disequilibrium of the drainage networks across the whole main divide and its tendency to migrate towards the interior of the plateau. In contrast, Gilbert metrics, which focuses on a narrow topographic zone across the divide, suggest that the uppermost divide in the northern and southern Talesh Mts. is stable. Moreover, asymmetry in Gilbert metrics across the central part of the range suggests that the divide might be migrating towards the plateau interior. The possible scenario for the whole divide's behavior is associated with the contrasting erosion rates across the divide. In the northern and southern sectors of the Talesh Mts. range, the erosional wave related to the most recent uplift pulses has not reached the divide yet. Mechanism of topographic rejuvenation is controlled by heterogeneous uplift of strong bedrock units in the northern and southern sectors, and weak bedrock units outcropping in the central sector, initiated topographic rejuvenation largely in the central Talesh Mts. Parallelly, the precipitation has a limited effect in assisting and magnifying the divide mobility, reorganization, and landscape transience. In general, we can infer that divide is in a transient state due to the spatially varying rock uplift and bedrock erodibility.

An ancient river landscape preserved beneath the East Antarctic Ice Sheet

The East Antarctic Ice Sheet (EAIS) has its origins ca. 34 million years ago. Since then, the impact of climate change and past fluctuations in the EAIS margin has been reflected in periods of extensive vs. restricted ice cover and the modification of much of the Antarctic landscape. Resolving processes of landscape evolution is therefore critical for establishing ice sheet history, but it is rare to find unmodified landscapes that record past ice conditions. Here, we discover an extensive relic pre-glacial landscape preserved beneath the central EAIS despite millions of years of ice cover. The landscape was formed by rivers prior to ice sheet build-up but later modified by local glaciation before being dissected by outlet glaciers at the margin of a restricted ice sheet. Preservation of the relic surfaces indicates an absence of significant warm-based ice throughout their history, suggesting any transitions between restricted and expanded ice were rapid.

Dam periodic storage drives the spatial pattern of vegetation towards banded evolution

Emerging and fragile ecosystems such as reservoir riparian zones are of increasing concern with the increasing number of damming projects worldwide. Current knowledge of riparian vegetation in reservoirs focuses on the individual and community level, with little understanding of spatial patterns and ecological processes at the landscape level. Here, we demonstrate the evolution process, evolution direction, and evolution mechanism of vegetation spatial patterns under persistent and periodic inundation disturbances in the reservoir riparian zone, through coupled analyses of landscape evolution and ecological processes, using continuous years of drone observation data in the riparian zones of two major reservoirs, Xiaowan and Nuozhadu of the Lancang River. We found that vegetation landscape evolution is driven by seed dispersal, environmental factors fluctuations, species tolerance differences, and competition for resource availability, showing a clear trend towards banded evolution. Ecological niche processes such as periodic wet-dry environmental filtering and interspecific competition dominated the formation of vegetation banding patches, and neutral processes of random seed dispersal dominated the areas where banding patches were formed. The water requirements of plants during the exposing period lead to a greater influence of environmental factors that are closely related to water, such as net rainfall, sunshine hours, groundwater storage, and soil moisture. Water redistribution between vegetated and bare ground patches is key to their formation and stability in the upper part of the riparian zone, where drought stress is severe, while in the lower part, vegetation is mainly affected by inundation stress during flooding periods. Therefore, riparian vegetation restoration is critical to maintain a certain area of bare ground patches for drought areas in the exposing period, in contrast to the emphasis on vegetation patch coverage. And it is possible to carry out spatial planning based on the development trend of the belt distribution, taking into account the flood and drought tolerance of the species as well as the fluctuating rhythms of the main factors, such as the rainfall, which is worth exploring further.

A novel approach for quantifying similarities between different debris flow sites using field investigations and numerical modelling

AbstractDebris flows are geomorphological processes that affect the landscape evolution process of any region. In this study, an integrated methodology is proposed to identify the chance of further debris flows and quantify the similarities between debris flow locations, materials and rheology, using field and laboratory investigations and remote sensing data. The method was tested for four failure‐triggered debris flow sites in the Western Ghats of India, using dimensionless parametric similarity values ranging from 0 to 1. The maximum parametric similarity was observed as 0.84 when comparing the flow accumulation values of Sites 3 and 4, and the maximum overall site similarity was 0.68. The calibrated rheological parameters of one site were found to be satisfactory in modelling the shape of debris flow at all other sites. The findings can be used to identify similar hotspots in the region and to simulate debris flows for quantitative hazard assessment.

A geospatial platform for the tectonic interpretation of low-temperature thermochronology Big Data

Low-temperature thermochronology is a powerful tool for constraining the thermal evolution of rocks and minerals in relation to a breadth of tectonic, geodynamic, landscape evolution, and natural resource formation processes through deep time. However, complexities inherent to these analytical techniques can make interpreting the significance of results challenging, requiring them to be placed in their geological context in 4-dimensions (3D + time). We present a novel tool for the geospatial archival, analysis and dissemination of fission-track and (U-Th)/He data, built as an extension to the open-access AusGeochem platform (https://ausgeochem.auscope.org.au) and freely accessible to scientists from around the world. To demonstrate the power of the platform, three regional datasets from Kenya, Australia and the Red Sea are placed in their 4D geological, geochemical, and geographic contexts, revealing insights into the tectono-thermal evolutions of these areas. Beyond facilitating data interpretation, the archival of fission track and (U-Th)/He (meta-)data in relational schemas unlocks future potential for greater integration of thermochronology and numerical geoscience techniques. The power of formatting data to interface with external tools is demonstrated through the integration of GPlates Web Service with AusGeochem, enabling thermochronology data to be readily viewed in their paleogeographic context through deep time from within the platform.

Observation of a rapid lake‐drainage event in the Arctic: Set‐up and trigger mechanisms, outburst flood behaviour, and broader fluvial impacts

AbstractLakes set in arctic permafrost landscapes can be susceptible to rapid drainage and downstream flood generation. Of many thousands of lakes in northern Alaska, hundreds have been identified as having high drainage potential directly to river systems and 18 such drainage events have been documented since 1955. In 2018 we began monitoring a large lake with high drainage potential as part of a long‐term hydrological observation network designed to evaluate impacts of land use and climate change. In early June 2022, surface water was observed flowing over a 30‐m wide bluff, with active headward erosion of ice‐rich permafrost soils apparent by late June. This overflow point breached rapidly in early July, draining almost the entire lake within 12 h and generating a 191 m3/s flood to a downstream creek. Water level and turbidity sensors and time‐lapse cameras captured this rapid lake‐drainage event at high resolution. A wind‐driven surface seiche and warming waters following ice‐out helped trigger the initial thermomechanical breach. We estimate at least 600 MT of lake sediment was eroded, mobilized, and transported downstream. A flood wave peaking at 42 m3/s arrived 14 h after the initial breach at a river gauge 9‐km downstream. Comparing this event with three other quantified arctic lake‐drainage floods suggests that lake surface area coupled with drainage gradient height can predict outburst flood magnitude. Using this relationship we estimated future flood hazards from the 146 lakes in the Arctic Coastal Plain of northern Alaska (ACP) with high drainage potential, of which 20% are expected to generate outburst floods exceeding 100 m3/s to downstream rivers. This fortunate and detailed drainage‐event observation adds to a growing body of research on the impact of lakes on arctic hydrology, hazard forecasting in a region with an increasing human footprint, and broader processes of landscape evolution in arctic lowlands.

Physically based numerical model for the landscape evolution of soil-mantled watersheds driven by rainfall and overland flow

Soil and gully erosion is a critical threat to sustainable agricultural land management and productivity. Soil erosion by rain splash, sheet runoff, and concentrated flow are complex problems conditioned by the combined interactions of soil physical properties, hydrology, human activity, landscape topography, and climate. Effective management of erosion processes driven by rainfall and surface runoff requires a combined effort of field observation, physical experimentation, and numerical simulation. A physically-based hydrodynamic numerical model, CCHE2D with updated raindrop erosion effects, is applied to simulate landscape evolution processes as a result of raindrop impact and overland flow. The numerical model simulates a thin layer of runoff and concentrated flow over complex terrains, and predicts sediment transport due to raindrop splash detachment, transport and soil surface erosions. Soil erosion and landform evolution simulations due to rainfall are presented for an experimental landscape and a tilled large scale agricultural field. Photogrammetric field data were collected using an unmanned aerial system (UAS) following planting and subsequent rainstorms. Numerical simulation results agreed well with the observed experimental soil surface topography, sediment yield, and drainage network developments. The simulated soil and gully erosion in the tilled field also agreed well with observations. This research successfully demonstrates how high resolution spatial and temporal topographic measurements in experimental and field landscapes can be simulated and interrogated by 2D physically-based hydrodynamic and morphodynamic models. The accurate predictions of tillage field and gully erosion process in agriculture watersheds with complex tillage conditions also indicated the usefulness of the model for studying soil and landscape degradation.

Estimating the Age of Abandoned Alluvial Surfaces Using Morphologic Dating of Gully Incision

AbstractThe age of abandoned alluvial surfaces is a key component in quantifying landscape evolution processes, tectonic activity, and paleoclimate. However, limited resources, restricted field accessibility, lacking dating material and analytical constraints are often encountered when dating such landforms. To help mitigate these limitations, we propose a new and complementary surface dating approach that is based on calculating the duration of gully incision into a surface since its abandonment using a locally calibrated landscape evolution model. The approach consists of calibration of incision model parameters for abandoned alluvial surfaces with known age and then using the calibrated model to calculate the time required for gully profiles to form on nearby undated alluvial surfaces as a proxy for surface abandonment. The approach was tested on previously dated late Pleistocene (34 and 70 ka) alluvial terraces in the hyper‐arid Negev desert (Israel). Calibrated model parameters were within the range reported for incision models in arid regions worldwide and late Pleistocene surface abandonment ages were recovered to within 10% accuracy. In contrast, modeled durations for gully incision into an older mid‐Pleistocene surface previously dated to 230–549 ka were grossly underestimated at <120 ka. Field observations and approach tests indeed indicated that late Pleistocene model parameters should not be extrapolated as‐is to model gully incision farther back through geologic time. Because the validity of the proposed ages depends on extrapolating the locally calibrated parameters of the incision model, we recommend that such dating be conducted with care and where assumptions will probably be valid.

Effects of sediment properties on the erosion resistance of natural cohesive soils in Taiwan

Soil erosion and sedimentation are major factors in landscape evolution and river morphology. The erosion resistance of sediments plays an important role in landscape evolution processes because it determines the threshold of an acting force at the initiative sediment motion, and the amount of sediment transports. This study examined the sediment properties and their correlations to the erosion resistance of mudstone deposits and laterites, which are two representative natural cohesive soils in hill and terrace topography in Taiwan. Soil physical properties and pore water chemical properties, and the erosion resistance were investigated and determined using laboratory tests and hydraulic flume experiments, respectively. Investigations of sediment properties showed that the mudstone deposits and laterites had significant differences in median particle size, soil texture, level of plasticity, internal structure, and pH value and electrical conductivity of their aqueous slurry. Results from the hydraulic flume experiments showed that the laterites had stronger erosion resistance, indicating by larger values of critical shear stress and the Shields’ parameter, than the mudstone deposits. Through the stepwise multiple regression analysis, a quantitative equation was proposed to estimate the Shields’ parameter using fine sediment properties including water content, clay content, and the plasticity index expressed in decimal fractions, of the two soils. Despite more experiments of the two natural soils with different ranges of the sediment properties are needed to validate the empirical equation for predicting the Shields’ parameter, the proposed methodology provides an essential reference in which erosion resistance is an important parameter.

Formation and evolution of the Asian landscape during the Cenozoic

<p>Asia has a unique set of landscapes, notably characterized by the highest plateau in the world, the strongest monsoon circulation, and an immense arid region. When and how this landscape was formed have long been investigated, but explanations remain elusive. In this paper, through a synthesis of both terrestrial and marginal marine sedimentary records and numerical simulation analyses, we propose that tectonic uplift/deformation and global cooling during the Cenozoic have controlled the evolution of Asian topography and climate, respectively, and, the Asian landscape. The Asian landscape has undergone three main stages of evolution, <i>viz</i>: development of low-relief topography and warm-dry climate during the Eocene (~50-34 Ma); emergence of high relief topography in the southwest, along with a diverse range of landforms elsewhere, associated with a warm monsoon-arid climate coupling system during the Oligocene to early Pliocene (34-3.0 Ma); and development of the typical Asian summer and winter monsoon circulation and present landscape since the late Pliocene, accompanied by significant global cooling. Our analyses reveal that the distribution of river catchments, flora, fauna and soils are controlled by tectonic uplift/deformation and palaeoclimate changes. Stepwise evolution of the Asian landscape since ~50 Ma was driven by tectonic deformation and global temperature changes, each of which may play different role(s) in landscape evolution. For instance, the India-Asia collision played a vital role in the formation of the Asian topographic configuration in the middle Eocene, which generated regional atmospheric circulation. On the other hand, global cooling during the late Pliocene played an important role in generating the modern-like Asian landscape including the rapid accumulation of loess deposits, strengthening of the Asian monsoon circulation, and the evolution of modern topography. To summarize, we broadly propose a three-stage history of landscape evolution in Asia during the Cenozoic. The detailed processes of Asian landscape evolution and the specific roles of tectonism and global temperature in controlling Asian landscapes and palaeoclimate evolution need to be further investigated.</p>

Reply to Comment by Anand et al. on “Groundwater Affects the Geomorphic and Hydrologic Properties of Coevolved Landscapes”

AbstractHere we acknowledge an omission and offer a perspective on the importance of domain size for the simple landscape evolution model that appears in our original manuscript. For completeness, the domain size should have been included in the nondimensionalization. However, it can be shown to be of little consequence in the region of the parameter space that we examined. Furthermore, we show that our choice of nondimensionalization (with landscape evolution process rates rather than domain size) allows for a clearer view of controls on intrinsic features of the results, including valley spacing and channel head contributing areas, which were the focus of our original analysis.

Stabilization of fluvial bed sediments by invasive quagga mussels (Dreissena bugensis)

AbstractRiver gravel mobility is an important control on river behaviour, morphology, and ecosystem processes. Gravel stability is dependent on abiotic flow and sediment properties, alongside less widely acknowledged biotic processes. The quagga mussel (Dreissena bugensis), a highly invasive bivalve, frequently occurs at high population densities in rivers and lakes. Quagga mussels attach to benthic sediments using byssal threads, which might affect sediment stability and thereby broader river geomorphology. We aimed to (1) characterize controls of quagga mussel sediment attachment by conducting a field survey in an invaded river, (2) investigate resultant changes in the critical shear stress needed to entrain fluvial bed materials via an ex situ flume experiment, for measured average (135 m−2) and potential future (270 m−2) mussel densities, and (3) model how this may affect sediment transport rates. From field surveys, mean quagga mussel density was 122 m−2, attaching to an average of 591 g m−2 of mineral bed sediments. Across the survey reach, mussels attached to all grain sizes available, with attachment driven by grain availability, rather than active selection of particular grain sizes. In the ex situ flume experiment, densities of 135 mussels m−2 did not significantly increase the critical shear stress of fluvial bed materials, but a density of 270 mussels m−2 significantly increased critical shear stress by 40%. Estimates of the proportion of time these critical stresses are exceeded at the field site indicated high densities of quagga mussels may reduce the occurrence of a geomorphically active flood event from Q30 to Q2. These results present feasible invasion scenarios, as quagga mussels frequently reach benthic densities orders of magnitude greater than observed here. This study indicates that substantial alterations to bedload sediment transport may occur following quagga mussel invasion. Future geomorphic modelling should include biology to better understand rates and processes of landscape development.

Data-based analysis about the influence on erosion rates of the Tibetan Plateau

Understanding the mechanisms and influential parameters of erosion rates is of great significance to comprehend landscape evolution and surface process. For years, many scientists have dedicated themselves into investigating the controlling factors of erosion rates over different spatial–temporal scales and various erosion mechanisms have been proposed, such as tectonics, climate, lithology, vegetation and etc., solely or coupled. Here, we exert data-based analysis of the catchment-wide 10Be erosion rates around the Tibetan Plateau (TP) (n = 289) to establish the correlation between erosion rates and other geological and topographic indices with bivariate analysis (Pearson and Spearman correlation), polynomial fitting (linear and quadratic) and data-based clustering (K-medoids). The results show that it is better to cluster the whole TP into two parts (the Inner Group (n = 176) and the Peripheral Group (n = 113)) where the quadratic fitting polynomials can reach an acceptable fitting (R2 > 0.6 and RMSE < 0.23) for erosion rates. This relationship is non-linear and multi-factor-coupling, indicating complex mechanisms towards erosion.

How to evaluate the dynamic relevance between landscape pattern and thermal environment on urban agglomeration?

Landscape management and planning is an important way to mitigate climate change. Previous studies have proved landscape pattern (composition and configuration) was correlated to thermal environment based on a single image data in time, but the static results were inconclusive and contradictory. In this study, a framework to investigate the dynamic relevance between landscape pattern change and near surface air temperature (NST) difference was constructed through the Weather Research and Forecasting Model (WRF) and geospatial methods including landscape metrics. The landscape pattern change of Beijing-Tianjin-Hebei (BTH) urban agglomeration from 2000 to 2020 were examined and their impacts on NST were simulated. Results showed it was dominant that cropland, water and grass land urbanized in the landscape evolution process, which led BTH average NST rise by 0.05℃ and 0.177℃ in January and July, respectively. The dynamic relevance (DR) between landscape composition and configuration and NST of all land classifications over the two decades were investigated quantitatively. Results indicated the DR between the change of landscape pattern and NST in different periods was different and it was seasonal various. DR of composition metric was more significantly correlated than non-area related spatial configuration metrics. In this twenty years, the DR of urban land (UBL) was positive, and it was negative for cropland (CPL). The landscape pattern of water (WTR) was positively correlated with NST in winter and negatively in summer. The DR of green space was an opposite correlation with DR of WTR in winter and summer, due to the newly added green space was mainly transformed from WTR. The dynamic perspective of relevance based on WRF and landscape indicators can enhance the understanding the effects of landscape pattern on thermal environment and provide a methodological framework for evaluating the dynamic relationship of landscape indicators and thermal effects.

Uncovering the Controls on Fluvial Bedrock Erodibility and Knickpoint Expression: A High‐Resolution Comparison of Bedrock Properties Between Knickpoints and Non‐Knickpoint Reaches

AbstractBedrock erodibility exerts a fundamental control on fluvial incision, and therefore on the evolution of entire landscapes. However, the roles and relative significance of specific bedrock properties like rock strength and discontinuity spacing in setting erodibility remain poorly understood. As a result, erodibility is often overlooked or oversimplified in numerical and field‐based investigations of landscape evolution processes, leading to misinterpretation of channel profile convexities (i.e., knickpoints) in studies seeking to infer uplift or baselevel histories from longitudinal profile analyses. Here, we investigate the controls on fluvial bedrock erodibility and knickpoint expression by conducting detailed surveys of 21 lithologic knickpoints and non‐knickpoint reaches (representing end‐member bedrock erodibility cases) and corresponding bedrock properties from small channels in the south‐central Appalachian Mountains. We use binary logistic regression of field data to test the relative strength of rock strength and discontinuity spacing as predictors of knickpoint occurrence, and therefore their relative impact on fluvial bedrock erodibility. We find that discontinuity spacing more strongly influences bedrock erodibility in this setting, where both rock strength and discontinuity spacing vary widely, confirming for the first time quantitatively the hypothesis that discontinuities exert a dominant control on fluvial erodibility. We also find that knickpoint expression is unique to a given stratigraphic interval, implying that knickpoint morphology is intimately linked to local conditions and therefore effectively unpredictable without detailed field measurements. Finally, because all 21 of our study knickpoints occur within a single geologic unit, our results illustrate that intra‐unit heterogeneity must be accounted for when considering lithologic influence on channel profile convexities.

Quantification of river network types based on hierarchical structures

The development of river networks is closely related to landscape evolution processes and terrain analysis. Several methods for classifying river networks based on qualitative evaluation or quantitative indices have been developed. However, these methods pay little attention to the hierarchical structures of river networks and omit some generalized and detailed information about basins. In this work, a classification method based on the hierarchical structure of river networks is established. Drainage texture, flow direction and aspect ratios are employed to describe river network characteristics. The specific attributes calculated include the drainage density, the stream frequency of river networks and the maximum frequency, and the distribution uniformity of the flow direction of river reaches on each order. Classification trees are established using these metrics. The method is applied to classify 83 river networks in the Yellow River source zone ranging in size from 17 to 241 km2. A previous classification method using basin invariance attributes was contrasted with our network structure approach, and cross-validation (CV) accuracies were obtained of 66.8% and 82.0% respectively. To verify the applicability of the proposed method, another 45 river networks classified in previous studies are considered. A CV accuracy of 76.7% is achieved with the proposed method. These results demonstrate that the proposed method based on the hierarchical structures of river networks is more accurate than methods that ignore the hierarchical structure.

Literature Review on Farmland Landscape Pattern Change Analysis

Farmland landscape pattern refers to the composition and spatial distribution of landscape elements in a certain spatial range. Farmland landscape pattern formation has natural, historical, cultural and other factors. Since the reform and opening up, China’s rapid socio-economic development, urbanization and a series of social development caused a large number of land occupation, farmland landscape pattern has undergone major changes. Domestic and foreign scholars based on natural geography, landscape ecology, combined with 3S technology, FRAGSTATES landscape index calculation method, from the aspects of landscape evolution process, landscape heterogeneity and diversity and landscape dynamics and driving factors correlation, analyzes the characteristics of regional landscape pattern change, and on this basis, analyzes the influence mechanism and correlation of landscape pattern succession. However, at present, most studies are based on the time axis, focusing on the changes of landscape patterns such as forests, wetland waters and towns. There are relatively few studies on the changes of farmland landscape patterns, and there are relatively few studies on the combination of time and space. Therefore, through the study and analysis of the characteristics of farmland landscape pattern change, the driving factors of landscape pattern change are known, and on this basis, the methods and ways of landscape pattern optimization are studied to provide scientific basis for constructing and optimizing farmland ecosystem, regional ecological network and protecting biodiversity.

The (mis)conception of average Quaternary conditions

AbstractThe concept of Quaternary average conditions has gained popularity over the past few decades, especially with studies of long-term landscape evolution. In this paper, we critically assess this concept by analyzing the marine isotope record (LR04 δ18O stack) relative to the Quaternary. This shows that the frequency and amplitude of climate glacial-interglacial cycles are not constant throughout the Quaternary, with a clear change during the Middle Pleistocene Transition (MPT), and that many minor oscillations exist within each cycle. For this reason, the identification of pre- and post-MPT most-frequent and, cumulatively, longest-lasting (rather than average) conditions is recommended. The most-frequent pre-MPT δ18O value of 3.725 ± 0.025‰ last occurred during 11.31–11.47 ka, while the most-frequent post-MPT δ18O value of 4.475 ± 0.025‰ last occurred during 14.81–15.04 ka. However, many other δ18O values were almost as frequent throughout the Quaternary and we present geomorphological reasons as to why it is unlikely that the present-day landscape reflects Quaternary average or, indeed, most-frequent conditions. Collectively, our results indicate that extreme caution should be taken when attempting to infer long-term landscape evolution processes (including the buzzsaw hypothesis) based on average Quaternary conditions.

Rainfall spatial-heterogeneity accelerates landscape evolution processes

Catchment hydro-morphological response is mainly conditioned on rainfall properties, such as rainfall intensity, storm duration and frequency, and the timing of these events. Rainfall spatial variability is likewise a major determinant affecting streamflow, erosion, and sediment transport, and is explored largely in the context of heavy rain triggering floods and fast morphological changes on hillslopes and in channels. In this study, we examine how the spatial structure of rainfall influences landscape evolution at the catchment scale over hundreds of years. To achieve this, multiple realizations of hourly rainfall fields, each differing only by their spatial distribution but identical in all other respects, were simulated using a weather generator. The impact of storm spatial-heterogeneity on the catchment morphology was then assessed with a landscape evolution model (CAESAR-Lisflood). A virtual “open-book” type catchment was used for this numerical experiment. The mean streamflow and low-flows remained the same while the magnitude of the annual peak streamflow increased by up to 12% in response to higher rainfall spatial heterogeneity. However, the erosion and deposition rates significantly increased (up to 50%) and the net erosion and deposition areas changed (increased by up to 9% and decreased by 13.5%, respectively) when the rain became less uniform in space. Furthermore, new gullies were found to be longer, deeper, and more branched in response to increased rainfall heterogeneity. The results suggest that heterogeneity in rainfall spatial patterns speeds up landscape development, even when rainfall volumes and temporal structures are the same. This implies that the spatial structure of rainfall may have more of an influence on catchment morphology at long time scales than previously thought.