Biogeochemical Cycles

Ecological Drivers and Environmental Impact

Katerina Dontsova, Zsuzsanna Balogh-Brunstad, Gaël Le Roux, Katerina Dontsova, Zsuzsanna Balogh-Brunstad, Gaël Le Roux

• Publisher: American Geophysical Union
• Year: 2020
• ISBN:
  9781119413318
• Edition: 1
• Topic: Sciences physiques
• Subtopic: Minéralogie

Elements move through Earth's critical zone along interconnected pathways that are strongly influenced by fluctuations in water and energy. The biogeochemical cycling of elements is inextricably linked to changes in climate and ecological disturbances, both natural and man-made.

Biogeochemical Cycles: Ecological Drivers and Environmental Impact examines the influences and effects of biogeochemical elemental cycles in different ecosystems in the critical zone.

Volume highlights include:

• Impact of global change on the biogeochemical functioning of diverse ecosystems
• Biological drivers of soil, rock, and mineral weathering
• Natural elemental sources for improving sustainability of ecosystems
• Links between natural ecosystems and managed agricultural systems
• Non-carbon elemental cycles affected by climate change
• Subsystems particularly vulnerable to global change

The American Geophysical Union promotes discovery in Earth and space science for the benefit of humanity. Its publications disseminate scientific knowledge and provide resources for researchers, students, and professionals. Find out more about this book from this Q&A with the Author. Book Review: http://www.elementsmagazine.org/archives/e16_6/e16_6_dep_bookreview.pdf

Part I
Biological Weathering

1
Biological Weathering in the Terrestrial System: An Evolutionary Perspective

Dragos G. Zaharescu¹, Carmen I. Burghelea², Katerina Dontsova^2,3, Christopher T. Reinhard¹, Jon Chorover³, and Rebecca Lybrand⁴

¹ School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA

² Biosphere 2, University of Arizona, Tucson, Arizona, USA

³ Department of Environmental Science, University of Arizona, Tucson, Arizona, USA

⁴ Oregon State University, Corvallis, Oregon, USA

ABSTRACT

Weathering is the process by which a solid breaks up into its building blocks when in thermodynamic disequilibrium with the surrounding environment. Weathering plays an important role in the formation of environments that can support life, including human life. It provides long‐term control on nutrient availability in natural and agricultural ecosystems through release of lithogenic elements and formation of secondary minerals that allow storage of nutrients in soils. Life itself, however, has a profound effect on weathering processes. Absence of oxidants characterized the weathering environment on early Earth (4.6–2.4 Ga), when CO₂ released during volcanic activity was the principal driver of weathering processes. The advent of photosynthesis in the Archean and resulting biogenic flux of O₂ to the atmosphere, ultimately shifted weathering towards oxidation, influencing the mineral landscape and the cycles of nutrients that supported an evolving biosphere. Land colonization by vascular plants in the early Phanerozoic and evolution of mycorrhizal symbiosis enhanced weathering by selectively mining minerals and redistributing nutrients across plant and fungi in the ecosystem. Development of complex human societies and the ever‐increasing influence people exert on the environment further impact weathering and nutrient cycling, both directly and indirectly.

1.1. INTRODUCTION

Modern‐day silicate weathering is strongly influenced by abundant organic and inorganic forms of carbon linked to biological activity. Dissolution of the rock releases nutrients and creates ecological niches for microorganisms and plants, while microorganisms and plant roots in symbiosis with mycorrhizal fungi create hot spots where intense gradients in carbon and water affect mineral dissolution and chemical denudation, influencing soil formation, soil fertility, landscape evolution, and long‐term productivity of terrestrial ecosystems. The fine balance between abiotic and biotic factors driving rock weathering is modulated by both planetary‐scale forces (solar radiation, gravity, plate tectonics) and molecular‐scale interactions, and is fundamental to the evolution of the terrestrial critical zone and its capacity for supporting life.

1.2. WEATHERING

Weathering is the process of physical and chemical breaking up of a solid, such as rock, into its elementary building blocks due to the thermodynamic disequilibrium with the surrounding environment (Figure 1.1). This simple but ubiquitous process in nature is a direct consequence of the universal Second Law of Thermodynamics, which connects energy and work (e.g., heat, chemical, mechanical) along the dimension of time. The law postulates that in an isolated physical system, entropy (a thermodynamic measure of unavailable energy) increases irreversibly over time (e.g., energy dissipates) when the system is out of equilibrium, or it remains constant when the system is at equilibrium (Bailyn, 1994). Open, out of equilibrium systems, such as natural environments, spontaneously evolve to reach a thermodynamic equilibrium with the outside environment, dissipating the available free energy to maintain existing gradients, unless electromagnetic radiation, kinetic/chemical, and gravitational sources of external energy are introduced. As a result, comets disintegrate over time, oceans mix, and exposed rock weathers irreversibly.

Figure 1.1 The principle of entropy in a theoretical, closed system, and how it applies to open‐system natural processes, such as weathering. Initial conditions are characterized by low entropy (e.g., ordered mineral structures, water crystals) and high potential energy. As electromagnetic energy is applied over time, a portion of the initial potential energy irreversibly changes the system to a new, higher entropic state, e.g., breaking of mineral structures and binding of elements with liquid water molecules. Removal of destabilizing energy causes the system to move to a new configuration state, different from the initial one.

Thermodynamics is a unifying principle in Earth sciences, and can predict energy and mass transfer processes among Earth’s various solid, fluid, and gaseous reservoirs, from weather, to crustal renewal and weathering. These processes can be quantified in terms of mass and energy balance between input and output components. For instance, in the present‐day terrestrial environment, rock weathering can be expressed as the sum of its products (equation 1.1) (Zaharescu et al., 2017):

(1.1)

1.3. THE EARLY ANOXIC EARTH

Earth is subject to one of the largest thermodynamic disequilibria in the inner solar system, with large fractions of matter and energy mixing in surface and subsurface portions of global cycles (Kleidon, 2010a). Despite a considerable decrease in the available energy from its formation, but with an evolving biosphere, Earth surface processes have maintained strong environmental gradients counteracting entropy. One important gradient is th...