Biocrusts are photosynthetic communities that extensively cover Earth's exposed soil surfaces, occupying the critical interface between the atmosphere and the pedosphere. Here, we present a comprehensive review of the definition, distribution, composition, structure, and function of biocrusts, emphasizing their relevance to the Earth sciences through influences on pedogenesis, landscape evolution, hydrological processes, surface energy balance, and biogeochemical cycles, both in the present and throughout Earth's history. We summarize both the established understanding and existing knowledge gaps regarding the complex interactions between biocrusts and the Earth system. Through this review, we demonstrate that biocrusts are a crucial linkage among various subdisciplines of Earth sciences and adjacent fields, thereby contributing to the refinement and advancement of Earth sciences to keep pace with global changes. As a lever to support vital Earth system processes, biocrust research helps find transdisciplinary solutions to combat climate change, conserve biodiversity, and promote sustainable land use. ▪ Biocrusts are photosynthetic communities inhabiting the interface between atmosphere and pedosphere, covering about 12% of Earth's terrestrial surface. ▪ Biocrusts influence the Earth system by mediating pedogenesis, landscape evolution, surface energy balance, and hydrological and biogeochemical cycles. ▪ Biocrusts form a link among geology, geomorphology, climatology, hydrology, soil science, environmental science, cryosphere science, and geospatial science. ▪ Biocrust research provides potential solutions to combat climate change, conserve biodiversity, and promote sustainable land use.
The search for life beyond Earth has long been a scientific goal, but as this quest expands, astrobiologists must grapple not only with where life may be but also with what life can be. Researchers now realize that our descriptions of biosignatures—the physical and chemical clues used to discern the presence of life—must expand to reflect fundamental features of living systems, not just analog expressions that assume a common heritage with Earth. These new biosignatures are thus agnostic in their relationship to life on Earth. New interdisciplinary endeavors now design generalized biosignatures that first consider the criteria and functions of life in any physical setting. These frameworks challenge the way we both seek and define life. Here we summarize the history of these efforts from their disciplinary roots and provide the conceptual foundations of the pursuit to sufficiently describe life without relying on definitions or familiar features. ▪ Agnostic biosignatures are indications of past or present biology that are not based on molecular or metabolic signs of life on Earth. ▪ The generalized search for life is rooted in both paleobiology and physics, where life is discerned from context and fundamental first principles. ▪ This work summarizes universal features and theories of life that recognize how biological systems differ from abiotic chemical systems. ▪ Proposed frameworks for a generalized search for life are described, as well as systematic needs for effective extraplanetary deployment.
The Famatinian arc provides an extensive cross section through an ancient magmatic arc, elucidating the evolution of subduction-related magmatism and the generation of the continental crust. Although the mantle-crust transition is not exposed, integrated field observations and laboratory analyses allow the reconstruction of a spatiotemporal model for the arc's thermal structure and magmatic evolution. The resulting cross section demonstrates that depth-dependent petrological processes govern igneous differentiation. At greater depths, crystallization-driven fractionation and melt-mush interactions produce variations within mafic-ultramafic complexes. From the middle crust to volcanic levels, crystal fractionation, partial melting, and the reactive assimilation of prearc host crust drive the evolution toward intermediate and silicic compositions. Comparative analysis with other ancient arcs indicates that subduction-related plutonic-volcanic columns are density filtered and organized as vertically interconnected systems. Within these trans-lithospheric magmatic columns, the influx of mantle-derived melts, multi-level crystal-melt-fluid fractionation, and assimilation of preexisting crust occur simultaneously in space and time from source to surface. ▪ The Famatinian arc represents a 30-km-thick segment of Ordovician arc crust that has undergone differential uplift along several hundred kilometers of its paleo-axis. ▪ Mantle melt input, polybaric igneous fractionation, and intracrustal melting controlled lithological and chemical diversification across the magmatic column. ▪ Extensive igneous columns transition from melt-dominated magmatic systems in the middle crust to crystal-rich mushy reservoirs in the upper crust. ▪ Comparable lithological architectures in ancient arc sections allow assessment of how the building blocks of continental crust develop over convergent plate margins.
The mechanism of rupture nucleation, the process by which ruptures are formed, is still not well-understood in the friction, fracture, and earthquake mechanics fields. Rupture propagation is relatively well-understood, as it can be described by the framework of fracture mechanics. Rupture nucleation, which is inherently unpredictable, remains elusive. Nucleation is observed to be a slow process relative to the propagation of the rupture that follows it. Numerous unique features that characterize the nucleation process elude existing models. Here, we first review recent experimental studies that have revealed intriguing characteristic properties of frictional rupture nucleation. We then describe a recently derived theoretical formulation that extends fracture mechanics (utilizing energy balance) to provide a full quantitative description of both extremely slow (aseismic) nucleation dynamics and their consequent transition to rapid (seismic) rupture. This approach highlights the importance of geometry—both interface confinement and the 2D shape of the rupture patch itself—in the rupture process. This framework provides a seamless transition between these seemingly disparate regimes, providing a new and fundamental understanding of both frictional and earthquake nucleation. These ideas may also be relevant for understanding material creep, which generally precedes catastrophic failure. ▪ Nucleation, the slow initial process of rupture formation, is qualitatively different than the rapid propagation of ruptures once formed. ▪ The transition from slow expansion to rapid propagation of frictional ruptures is related to the finite width of a frictional interface. ▪ A new model extends fracture mechanics to provide a mechanical description of slow rupture expansion during nucleation. ▪ This framework provides understanding of earthquake nucleation along natural faults and creep processes preceding catastrophic failure.
Roman architectural and marine concrete structures, caementiciae structurae , have remained intact and functional for two millennia. A mortar fabricated with scoriaceous or pumiceous volcanic tephra aggregate ( harenae fossiciae , pulvis ) binds cobble-sized volcanic or carbonate rock and/or ceramic coarse aggregate ( caementa ). These clasts form a conglomeratic framework that reinforces the concrete. An early pozzolanic cementitious system, in which aggregates react with lime ( calyx ) hydrated with freshwater or seawater, binds the concrete. The components of this system then react with pore fluids to produce post-pozzolanic hydrated silicate phases, mainly strätlingite, Al-tobermorite, and phillipsite crystals, which remodel and toughen the concrete. A heterogeneous permeability structure facilitates these beneficial fluid-concrete interactions. The Ses Llumetes shipwreck provides a window into transport of pulvis pumiceous tephra as aggregate in marine structures. Roman natural scientists recorded accurate empirical observations and hypotheses for these dynamic cementitious processes, which have great relevance to self-sustaining marine concrete infrastructure and alternatives to cement-based concrete. ▪ Advanced analytical methods validate Roman hypotheses for mechanisms of self-sustaining resilience in ancient architectural and marine concrete. ▪ An early pozzolanic system consolidates and strengthens the concrete, mainly through production of C-A-S-H (calcium-aluminum-silicate-hydrate) binding phase in a cementing matrix. ▪ Post-pozzolanic reactions with pore fluids produce silicate mineral cements and remodel the cementing matrix, toughening the concrete. ▪ A heterogeneous permeability structure derived from reactive aggregates, mainly volcanic tephra and cobble-sized caementa , aids these processes. ▪ Roman designs in modern marine concrete could produce beneficial interactions with saltwater, improving structural functionality and resilience.
