ALBERT

Description: Life has significantly altered the Earth's atmosphere, oceans and crust. To what extent has it also affected interior geological processes? To address this question, three models of geological processes are formulated: mantle convection, continental crust uplift and erosion and oceanic crust recycling. These processes are characterised as non-equilibrium thermodynamic systems. Their states of disequilibrium are maintained by the power generated from the dissipation of energy from the interior of the Earth. Altering the thickness of continental crust via weathering and erosion affects the upper mantle temperature which leads to changes in rates of oceanic crust recycling and consequently rates of outgassing of carbon dioxide into the atmosphere. Estimates for the power generated by various elements in the Earth system are shown. This includes, inter alia, surface life generation of 264 TW of power, much greater than those of geological processes such as mantle convection at 12 TW. This high power results from life's ability to harvest energy directly from the sun. Life need only utilise a small fraction of the generated free chemical energy for geochemical transformations at the surface, such as affecting rates of weathering and erosion of continental rocks, in order to affect interior, geological processes. Consequently when assessing the effects of life on Earth, and potentially any planet with a significant biosphere, dynamical models may be required that better capture the coupled nature of biologically-mediated surface and interior processes.

Description: Vernadsky described life as the geologic force, while Lovelock noted the role of life in driving the Earth's atmospheric composition to a unique state of thermodynamic disequilibrium. Here, we use these notions in conjunction with thermodynamics to quantify biotic activity as a driving force for geologic processes. Specifically, we explore the hypothesis that biologically-mediated processes operating on the surface of the Earth, such as the biotic enhancement of weathering of continental crust, affect interior processes such as mantle convection and have therefore shaped the evolution of the whole Earth system beyond its surface and atmosphere. We set up three simple models of mantle convection, oceanic crust recycling and continental crust recycling. We describe these models in terms of non-equilibrium thermodynamics in which the generation and dissipation of gradients is central to driving their dynamics and that such dynamics can be affected by their boundary conditions. We use these models to quantify the maximum power that is involved in these processes. The assumption that these processes, given a set of boundary conditions, operate at maximum levels of generation and dissipation of free energy lead to reasonable predictions of core temperature, seafloor spreading rates, and continental crust thickness. With a set of sensitivity simulations we then show how these models interact through the boundary conditions at the mantle-crust and oceanic-continental crust interfaces. These simulations hence support our hypothesis that the depletion of continental crust at the land surface can affect rates of oceanic crust recycling and mantle convection deep within the Earth's interior. We situate this hypothesis within a broader assessment of surface-interior interactions by setting up a work budget of the Earth's interior to compare the maximum power estimates that drive interior processes to the power that is associated with biotic activity. We estimate that the maximum power involved in mantle convection is 12 TW, oceanic crust cycling is 28 TW, and continental uplift is less than 1 TW. By directly utilizing the low entropy nature of solar radiation, photosynthesis generates 215 TW of chemical free energy. This high power associated with life results from the fact that photochemistry is not limited by the low energy that is available from the heating gradients that drive geophysical processes in the interior. We conclude that by utilizing only a small fraction of the generated free chemical energy for geochemical transformations at the surface, life has the potential to substantially affect interior processes, and so the whole Earth system. Consequently, when understanding Earth system processes we may need to adopt a dynamical model schema in which previously fixed boundary conditions become components of a co-evolutionary system.

Description: Given the potential for elements of the Earth system to undergo rapid, hard to reverse changes in state, there is a pressing need to establish robust methods to produce early warning signals of such events. Here we present a conceptual ecosystem model in which a diversity of stable states emerge, along with rapid changes, referred to as critical transitions, as a consequence of external driving and non-linear ecological dynamics. We are able to produce robust early warning signals that precede critical transitions. However, we show that there is no correlation between the magnitude of the signal and magnitude or reversibility of any individual critical transition. We discuss these findings in the context of ecosystem management prior to and post critical transitions. We argue that an understanding of the dynamics of the systems is necessary both for management prior and post critical transitions and the effective interpretation of any early warning signal that may be produced for that system.

Description: The local climate represents the primary selection pressure acting on vegetation, but competitive interactions between plant strategies determine their composition. We link growth and reproduction characteristics from different plant strategies, that emerge from climatic constraints, to their competitive abilities and calculate explicitly their spatial dynamics. DIVE (Dynamics and Interactions of VEgetation), a simple generic model is built, that calculates population dynamics in the presence of perturbations, seed and resource competition. To understand the impacts of competition and perturbations on the population dynamics, a range of sensitivity experiments are conducted. DIVE simulations feature successional dynamics from fast-growing towards slow-growing plant strategies and as such corresponds to widely observed characteristics of terrestrial vegetation. Perturbations, seed and resource competition were found to affect succession and diversity, with the community composition at steady state ranging from competitive exclusion to coexistence and total extinction. We conclude that linking ecophysiological characteristics of vegetation to competition is a valid approach to determine population dynamics. Furthermore, incorporating mechanisms of perturbations and competition may be essential in order to effectively predict the response of community dynamics to changing environmental conditions.

Description: While the regional climate is the primary selection pressure for whether a plant strategy can survive, however, competitive interactions strongly affect the relative abundances of plant strategies within communities. Here, we investigate the relative importance of competition and perturbations on the development of vegetation community structure. To do so, we develop DIVE (Dynamics and Interactions of VEgetation), a simple general model that links plant strategies to their competitive dynamics, using growth and reproduction characteristics that emerge from climatic constraints. The model calculates population dynamics based on establishment, mortality, invasion and exclusion in the presence of different strengths of perturbations, seed and resource competition. The highest levels of diversity were found in simulations without competition as long as mortality is not too high. However, reasonable successional dynamics were only achieved when resource competition is considered. Under high levels of competition, intermediate levels of perturbations were required to obtain coexistence. Since succession and coexistence are observed in plant communities, we conclude that the DIVE model with competition and intermediate levels of perturbation represents an adequate way to model population dynamics. Because of the simplicity and generality of DIVE, it could be used to understand vegetation structure and functioning at the global scale and the response of vegetation to global change.