Human topographic signatures and derived geomorphic processes across landscapes

Highlights

• Human topographic signatures and derived processes across landscapes

• Humans as a geological agent in shaping the landscapes

• Anthropogenic geomorphologies case studies: mines, road networks, agricultural practices

• High-resolution topography: an effective tool for the analysis of the anthropogenic signature

• New frontiers and future challenges

Abstract

The Earth's surface morphology, in an abiotic context, is a consequence of major forcings such as tectonic uplift, erosion, sediment transport, and climate. Recently, however, it has become essential for the geomorphological community to also take into account biota as a geomorphological agent that has a role in shaping the landscape, even if at a different scale and magnitude from that of geology. Although the modern literature is flourishing on the impacts of vegetation on geomorphic processes, the study of anthropogenic pressures on geomorphology is still in its early stages. Topography emerges as a result of natural driving forces, but some human activities (such as mining, agricultural practices and the construction of road networks) directly or indirectly move large quantities of soil, which leave clear topographic signatures embedded on the Earth's morphology. These signatures can cause drastic changes to the geomorphological organization of the landscape, with direct consequences on Earth surface processes. This review provides an overview of the recent literature on the role of humans as a geological agent in shaping the morphology of the landscape. We explore different contexts that are significantly characterized by anthropogenic topographic signatures: landscapes affected by mining activities, road networks and agricultural practices. We underline the main characteristics of those landscapes and the implications of human impacts on Earth surface processes. The final section considers future challenges wherein we explore recent novelties and trials in the concept of anthropogenic geomorphology. Herein, we focus on the role of high-resolution topographic and remote-sensing technologies. The reconstruction or identification of artificial or anthropogenic topographies provides a mechanism for quantifying anthropogenic changes to landscape systems. This study may allow an improved understanding and targeted mitigation of the processes driving geomorphic changes during anthropogenic development and help guide future research directions for development-based watershed studies. Human society is deeply affecting the environment with consequences on the landscape. Therefore, establishing improved management measures that consider the Earth's rapidly changing systems is fundamental.

Introduction

Landscapes are characterized by distinctive morphologies that are mainly caused by major forcings such as tectonic uplift, erosion, sediment transport, and climate that have shaped the Earth's surface and left characteristic topographic signatures. In addition to these processes, biota forcing might play a role in shaping the landscape, but, of course, at a different scale and magnitude than geological forcing. In biotic landscapes, vegetation through its roots influences soil formation and surface erosion. Biota also influence climate and, as a consequence, the mechanisms that control erosion rates and the evolution of the landscape. Dietrich and Perron (2006) compared the hypothetical frequency of occurrence of landform properties for the present Earth and an abiotic Earth. They suggested that the frequency distributions of measurable landform properties (such as mountain height, steepness, and curvature) could greatly differ, although all observed landform types would be found in both biotic and abiotic worlds (Fig. 1a). The question is, if we can suppose that there is evidence of biota forcing, what is the role of humans? Fig. 1b shows a real case study of the relative likelihoods that landform slopes will take a given value (probability density functions, PDFs) in a natural and a terraced landscape (the slope was calculated using high-resolution topography derived from LiDAR data). The result is interesting as the distributions seem to confirm the hypothesis of Dietrich and Perron (2006). Even if all slope values are found in the natural and in the anthropogenic environments, humans actively alter the frequency of occurrence of some specific slope values. In the case presented in Fig. 1b, the natural morphology presents a bell-shaped distribution of slopes, with a peak around the average slope (~ 35°). On the other hand, the anthropogenic slope PDF presents two peaks: one around the average value and one around low slope values. These two peaks are caused by the walls (having high values of slopes) and by the benches (low values of slopes) of the terraces.

Society developed significantly during the Holocene (from hunting–gathering, to farming, to complex societies and metropolises), and the increase in population was related to a progressive increase in intensive agriculture, raw material demand and urbanization. As a consequence, human activities are leaving a significant signature on the Earth by altering its morphology, processes, and ecosystems (Ellis, 2011). Anthropogenic landscapes cover as great an extent of the Earth's land surface as many other globally important ecosystems (Foley et al., 2005). In anthropogenic landscapes, human alteration is reflected by artificial Earth surface features (e.g., artificial channels, road networks, agricultural practices and mining activities) that may affect natural processes (Tarolli, 2014). Indeed, the scientific community is now debating the fact that we are living in a new geological epoch, the Anthropocene (Zalasiewicz et al., 2008, Zalasiewicz et al., 2011a, Zalasiewicz et al., 2011b, Zalasiewicz et al., 2014, Monastersky, 2015), where human activities leave a significant, if not dominant, signature on the Earth. Whether this is true for the whole of mankind or for only some civilizations (Crist, 2013) and whether human bioturbation (Zalasiewicz et al., 2014) constitutes a negligible (Visconti, 2014) or substantial fraction of long-term Earth geological activities are still up for debate. Some authors have highlighted how human impacts are often difficult to separate from naturally driven activities (Fuller et al., 2015), and others have argued that it might be too soon to determine the human impact on geological records (Lewin and Macklin, 2014, Visconti, 2014, Smil, 2015). Aside from this recent ongoing debate, geomorphologists have long investigated the impacts of human societies on Earth surface processes and landform records, without needing the motivation of creating a new geological time interval (Brown et al., 2013a, Brown et al., 2013b).

Defining humans as a geologic agent might sound a little strange or even a provocation. Further, of course, if we compare human-induced changes to geologic forcing through the millennia, we are looking at a different scale and magnitude. However, humans have the potential to amplify geomorphic processes (Wolf et al., 2014). Bioturbation by humans (‘anthroturbation’) is a phenomenon without precedent in Earth history and is orders of magnitude greater in scale than any preceding nonhuman type of bioturbation (Zalasiewicz et al., 2014). Humans have become the dominant element in many Earth surface processes at different scales (Steffen et al., 2007, Wohl, 2013), to the point that human activities can be considered distinct from, but comparable to, the effects of climatic or tectonic transformations (Macklin et al., 2014). According to Wilkinson (2005), humans move increasingly large amounts of rock and sediment during various construction activities, and therefore become a geological agent. They modify the spatial distribution and rates of hydraulic and geomorphic processes (Fryirs and Brierley, 2012), which might result in land degradation, and geomorphic changes (Doolittle, 2006). Furthermore, anthropogenic transformations coupled with the natural system can cause geomorphological processes to continue in a ‘genetically modified’ form (Lewin, 2013), and the results of this combined forcing might be preserved in long-term geological records, at least in some environments (Brown et al., 2013a, Brown et al., 2013b, Jefferson et al., 2013).

Unlike natural geomorphic processes, the work done by humans is focused in specific locations with well-defined intent (Guthrie, 2015), and as a consequence, specific anthropogenic topographic signatures emerge. Looking at the different signatures of humans, we can mention mining activities, road networks and agricultural practices such as terracing and land reclamation. These signatures are very connected to current and future societal changes. In recent years, there has been a rising demand for base metals and industrial minerals, which is also connected to research into new forms of energy production (Vidal et al., 2013). Populations' needs for food will more than likely push the expansion of arable lands (D'Odorico and Rulli, 2013). Irrigated lands are also expected to expand (Foley et al., 2011). Terraced areas are greatly threatened by abandonment (Tarolli et al., 2014) or, on the other hand, by the intensification and specialization of agriculture (Cots-Folch et al., 2009). In addition, in the next few decades there may be a progressive increase in transportation networks (Sidle and Ziegler, 2012).

These signatures can cause drastic changes to the geomorphic organization of the landscape, with direct consequences on Earth surface processes (e.g., Mossa and James, 2013, Sofia et al., 2014c, Tarolli et al., 2014, Tarolli et al., 2015). Thus, this review provides an overview of the recent challenging theme of the role of humans as a geological agent, specifically as connected to those elements that leave a clear topographic signature on the Earth's surface. The goal is to present the state of the science of the analysis of human bioturbation that can serve as a basis to improve environmental planning, mitigate the consequences of anthropogenic alteration, and provide useful guidelines in the future challenges section. Identifying signals connected to anthropogenic topographic signatures within Earth surface processes might help provide a better understanding of current settings, the future and past steps in the evolution of our planet, and to schedule appropriate environmental planning for sustainable development, to mitigate the consequences of anthropogenic alteration.