Earth’s critical zone, which spans from the treetops to the base of groundwater, is where soils and minerals, water and air, and plants and animals all interact and influence one another [Brantley et al., 2007]. These interactions shape the physical landscapes we live on, sustain the waterways and soils that nourish us, and influence climate and the atmosphere we breathe.
The linkages between the critical zone and human well-being have motivated studies of this layer of Earth’s surface that since the mid-2000s, have often been coordinated through numerous place-based Critical Zone Observatories (CZOs) and, more recently, in theme-based Critical Zone Networks (CZNs). These coordinated CZO/CZN efforts have achieved major advances in understanding critical zone characteristics such as soil development, porosity and permeability evolution, and water distribution by addressing common problems using shared infrastructure and data [Sullivan et al., 2017].
However, established CZOs cover only a fraction of Earth’s surface and disproportionately represent silicate mineral–dominated landscapes as opposed to those composed of mainly carbonate minerals. The efforts largely neglect the nearly 15% of Earth’s ice-free surface that is underlain almost entirely by carbonate bedrock [Goldscheider et al., 2020] and the larger fraction of the planet with mixed silicate and carbonate bedrock.
Bedrock mineralogy exerts fundamental controls on many aspects of the critical zone, from water availability to soil composition to the many interactions that occur between life and rocks. The overwhelming focus on silicate landscapes in critical zone science has thus left large gaps in our knowledge of Earth’s surface and how it affects humans. Now scientists are looking to address these gaps by refocusing attention on neglected carbonate landscapes through the lens of critical zone science.
Carbonates Versus Silicates
Carbonate minerals such as calcite and aragonite differ in many ways from silicate minerals such as quartz and clays. Whereas silicate minerals are created predominantly through inorganic, or abiotic, means, carbonate minerals form mostly through biotic processes in organisms such as corals and algae. Especially in marine environments, these minerals can form thick deposits of nearly pure carbonate sediments. Carbonate sediments can lithify into dense rock and dissolve to form caves, shaping the critical zone where carbonate minerals dominate.
The physical and chemical breakdown, or weathering, of bedrock minerals, whether silicate or carbonate, influences many characteristics of Earth’s critical zone, such as soil types and distribution; land surface morphology; and water quality, retention, and drainage [National Research Council, 2001]. These characteristics combine to provide services, including reducing pollution and providing plants with nutrients, that sustain ecosystems and societies. Human activities, especially modifications to drainages and application of excess nutrients, also affect how life and rock interact in the critical zone.
With their distinctive properties, carbonate and silicate minerals weather differently. Silicate minerals weather incongruently to produce new solid materials (e.g., alteration minerals) as well as dissolved species. In contrast, carbonate minerals weather congruently—they dissolve completely, leaving behind large voids in the landscape (e.g., caves and sinkholes). Thus, critical zone characteristics, for example, the architecture of physical properties, water and gas flow, and distribution of substrates where biology and geology intersect, will vary depending on the primary mineral content of bedrock. In recognition of the importance of mineralogy, we propose that Earth’s critical zone should be considered a gradient between two compositional end-members: the silicate critical zone and carbonate critical zone.
Neglected No More
The Carbonate Critical Zone Research Coordination Network (CCZ-RCN) was established in 2019, in part to further transdisciplinary and collaborative research into the critical zone amid landscapes composed of carbonate or mixed carbonate-silicate mineralogies (see sidebar).
The CCZ-RCN convened its first workshop in fall 2020, and the 70 participants reached consensus on 22 important research questions that define unknowns about the carbonate critical zone [Martin and CCZ-RCN Participants, 2021]. The questions align with five key research areas that focus on specific carbonate critical zone characteristics, differences between the carbonate and silicate critical zone, and the effects of varying carbonate and silicate mineral contents and that provide directions for future critical zone studies. These areas involve research into the following: (1) the boundaries and scales of critical zone environments; (2) the biological, chemical, and physical processes at work; (3) the rates and time frames of these processes; (4) carbon dynamics in the critical zone; and (5) the critical zone and society.
Below, we describe the processes and background relevant to each of the five research areas and highlight important questions to be addressed in future work.
Boundaries and Scales
An important difference between the effects of congruent and incongruent weathering is varying permeability, which can be orders of magnitude greater in the carbonate critical zone than in the silicate critical zone. Unlike in silicate terrains, permeability in carbonate terrains tends to scale with the distance over which it is measured, from wellbore to basin scales. The configuration of permeable and impermeable materials influences the movement of water, solutes, and gases into, through, and out of the critical zone, leading to questions of how permeability distribution alters carbonate critical zone architecture. Large voids also allow human access below the surface, which can enrich critical zone studies.
A key question about the critical zone is how the lower boundary should be defined [Sullivan et al., 2017]. This question is complicated by differences between the carbonate and silicate critical zone lower boundaries. The lower boundary of the carbonate critical zone varies with the location and morphology of voids, which can form a nonplanar surface hundreds to thousands of meters below the land surface. In contrast, the silicate critical zone lower boundary is typically a few meters to tens of meters below the land surface and roughly follows the surface topography.
Biological, Chemical, and Physical Processes
Feedbacks occur among many biological, chemical, and physical processes within the critical zone and with external drivers that force internal processes. For example, physical weathering and fracturing of rock caused by tectonic forces increase surface area and hydrologic connectivity, thus enhancing mineral dissolution. This feedback can produce heterogeneous porosity and permeability distributions that are hallmarks of the carbonate critical zone, in contrast to the regular decrease in porosity and permeability with depth in the silicate critical zone.
Additional feedbacks exist between flow rates and magnitudes, chemical compositions of water and gases, and biological activity in the critical zone. Recharge locations shift with external forcing, such as when floods raise stream elevations above the groundwater table and recharge aquifers through spring vents [Brown et al., 2014]. These shifts alter the structure and activity of biological communities and disrupt gradients in pH and reduction-oxidation conditions that develop from metabolic processes.
These and other changes in reduction-oxidation conditions are linked to production and consumption of the three primary greenhouse gases: carbon dioxide, methane, and nitrous oxide. However, the impacts of these shifts on atmospheric compositions of the gases are unknown and represent an example of more general questions about how linked hydrological and biological processes within the carbonate critical zone change magnitudes and fluxes of reaction products.
Rates and Time
Timescales for many processes are shortened in the carbonate critical zone compared with the silicate critical zone because of faster reaction rates of carbonate compared to silicate minerals and elevated flow rates through high-permeability zones. These characteristics create sensitivities to rare and short-lived extreme events that may alter equilibrium states within the carbonate critical zone. For example, flooding of caves with organic carbon–rich surface waters is known to cause mass mortality of troglobitic species that live entirely in underground habitats.
The rapid responses in the carbonate critical zone may provide a bellwether for wider climate change impacts on critical zone processes. Improving our understanding of the rates and timescales of processes, such as the effects of changing flood, drought, and fire frequencies, in the carbonate critical zone will provide vital information for comparison with the slower response of the silicate critical zone, where change may occur at timescales longer than common observational periods. Observations of rapid change in the carbonate critical zone should thus aid in the development of models that predict overall responses of Earth’s critical zone to climate change.
Carbonate minerals represent the largest global store of carbon, making research into carbon dynamics in the carbonate critical zone particularly important. Through numerous reactions and interactions, the inorganic carbon store is linked to organic carbon production, remineralization, and production of various natural acids. Carbonate mineral dissolution by carbonic acid consumes carbon dioxide, contributing to short-term drawing down of atmospheric carbon dioxide levels. However, carbonate mineral dissolution by other acids has the opposite effect of producing carbon dioxide, coupling the carbonate critical zone and climate [Martin, 2017].
Although equilibrium is often assumed between soil carbon dioxide and groundwater, disequilibrium may result from heterogeneous distributions of recharge, flow paths, and respiration often seen in the carbonate critical zone. Understanding the controls of this disequilibrium, which drives carbon dioxide dissolution or evasion and alters pH, weathering reactions, and carbonate mineral dissolution or precipitation, is critical in linking the carbonate critical zone to the global climate system.
Organic carbon cycling is coupled to ecosystem metabolism (the ways that plants, animals, and microorganisms process carbon) through fixation of inorganic carbon to organic matter and remineralization of organic matter to carbon dioxide and nutrients like nitrogen and phosphorous. This nutrient generation supports ecosystems above and below the land surface, although excess anthropogenic nutrients can alter aquatic ecosystems common in clear-water streams of carbonate terrains. Links between dissolved and gaseous carbon dioxide distributions, organic carbon fixation, and mineral weathering make finding answers to questions about carbon dynamics key to understanding many carbonate critical zone processes.
The Critical Zone and Society
The carbonate critical zone, especially where karst landscapes form, has important ecological, social, cultural, economic, and aesthetic values. From the water resources it harbors for up to 25% of the world’s population to common geohazards (e.g., sinkholes), the carbonate critical zone is key to the sustenance and resilience of local communities that rely on its services.
However, distinct carbonate critical zone characteristics also increase its vulnerability to human activities, from dispersed infiltration of pollutants to soil erosion and rocky desertification. Local communities’ reliance on and impacts on carbonate critical zone services suggest that those communities should play an important role in the management and maintenance of carbonate critical zone services. In exchange, local communities should receive equitable distribution of the critical zone services, such as access to reliable water supplies, waste handling, and fertile soils. This equity is especially important where these services are limited.
Inputs from local stakeholder groups, through coproduction of scientific research based on experiential and local knowledge, would not only enrich our understanding but also ensure research outcomes reach and benefit the local communities. This approach requires an equitable exchange of rewards and values stemming from research participation by local stakeholders and mainstream scientists [e.g., Harris et al., 2021].
A More Holistic View of the Critical Zone
Carbonate and silicate minerals are end-members of a spectrum of critical zone bedrock compositions, and fundamental differences in their physical and chemical properties create distinct characteristics in Earth’s critical zone. Studies of these two end-members, as well as of regions of mixed mineralogical compositions, can provide a better understanding of the critical zone in its entirety.
To date, however, critical zone research has predominantly emphasized silicate landscapes, leaving us well short of such a holistic understanding. With increased focus on the neglected carbonate critical zone—particularly on the research directions and questions outlined here—we can fill important knowledge gaps about a part of Earth upon which we humans depend so closely.
This material is based upon work supported by the National Science Foundation (NSF) under grant EAR-1905259. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of NSF. We gratefully acknowledge valuable contributions of all participants in the workshop as listed in the workshop report.
Diverse Research Needs Diverse Researchers
A subset of participants at the fall 2020 CCZ-RCN workshop developed an action plan with the goal of increasing diversity in the RCN by implementing inclusive RCN activities and creating a space to nurture equitable and accessible participation by scientists with differing backgrounds and identities. Although the plan is just a small step within a small group, it may provide a template of ways to enhance and improve diversity, equity, and inclusion (DEI) throughout the geosciences. Increased diversity will enrich talents and skills, experiences and interests, worldviews, frameworks, and approaches in the study and understanding of the carbonate critical zone.
The first RCN workshop was planned to enhance collaboration among people with different identities, backgrounds, and experiences. Meeting organizers created small working groups based on demographic information from a preworkshop survey. They promoted equitable and inclusive participation of all attendees using the Technology of Participation facilitation method that fosters authentic participation and meaningful collaboration.
In the working group and plenary sessions, participants reflected on the causes and consequences of limited diversity in the geosciences. They discussed ways to enhance DEI within the RCN. Invited speakers described successful DEI programs in the geosciences, including Sparks for Change and the Louis Stokes Alliances for Minority Participation. A DEI interest group was formed to support the development and implementation of the DEI action.
Future RCN activities will include webinars, training sessions, and networking opportunities at conferences and workshops. Travel grants have been established to support students from underrepresented demographics. The RCN will recruit students from minority-serving colleges and universities, particularly at the graduate level, to attend the next workshop (April 2022), training activities, and field trips. The RCN aims to demonstrate how changing the designs of workshops and other activities can enhance inclusion, diversity, equity, and accessibility and further transdisciplinary, collaborative science to improve understanding of the carbonate critical zone.
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Jonathan B. Martin (firstname.lastname@example.org), Department of Geological Sciences, University of Florida, Gainesville; Paloma Carton De Grammont, Water Institute, University of Florida, Gainesville; Matthew D. Covington, Department of Geosciences, University of Arkansas, Fayetteville; and Laura Toran, Department of Earth and Environmental Science, Temple University, Philadelphia, Pa.
Martin, J. B., P. C. De Grammont, M. D. Covington, and L. Toran (2021), A new focus on the neglected carbonate critical zone, Eos, 102, https://doi.org/10.1029/2021EO163388. Published on 20 September 2021.
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