Jennings’ Karst (1971,MIT Press) and current critical zone science

I Introduction

Karst, as defined by Jennings (1971), is terrain with distinctive relief and drainage characteristics arising primarily from a higher degree of rock solubility in natural water than is found elsewhere. The most important characteristic of karst is the high solubility of the rock and, thus, the active interaction between rock and water. Rock is consumed by water and, hence, water creates the rock. To report the origin, evolution and characterization of karst geomorphology systematically, Jennings described the influence of rock formation, geology, climate and history on geomorphology. His book provided a clear and comprehensive review of the origins of karst geomorphology studies. Using the book as a basis, it is possible to assess current studies and look towards the next steps in critical zone science.

II A close look at karst geomorphology

Jennings’ (Figure 1) book was published in 1971 as an advanced textbook. It consists of 12 chapters and takes a close look at the formation of karst geomorphology from micro to macro scale, from the tertiary period to the present.

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Figure 1.

J.N. ‘Joe’ Jennings.

In the first chapter, the author provided a clear definition of the object of study, ‘karst’, pointing out that the key to this special geomorphology is the high solubility of rocks in natural water. The resulting capacity of the rock to transmit large amounts of water rapidly is responsible for the further development of underground geomorphology. Karst geomorphology can be split into several types, the simplest division being bare and covered karst, according to the position of the bedrock. Thus, the rock may be covered or not, and this determines whether biological features will take part in the karst formation processes, hence making a difference to the geomorphology. Although karst phenomena had caught Greek and Roman people’s eyes, scientific study began in earnest only after cave exploration techniques were improved in the 20th century.

In the second chapter, multiple kinds of karst rocks were described, that is, limestone, reek faces, dolomite and evaporates. Limestone, which is mostly composed of CaCO₃, is the most widespread type of rock in karst areas. Most limestone is of marine origin – detrital, chemical and organic materials are deposited and turned into rock. Most dolomites appear to be secondary in origin through the replacement of limestone, and they are composed mainly of CaMg(CO₃)₂ instead of CaCO₃. Rock porosity and weakness, correlated with rock permeability, mechanical strength and purity, decide the response of rocks to geomorphological processes. Significant karst development requires at least 60% CaCO₃, whereas full karst development requires 90% CaCO₃. Older carbonate rock is more likely to be non-porous, thus more resistant to geomorphological processes. However, to a great extent, the inhomogeneity of rock weakness makes the behaviour of rock variable and unpredictable.

The third chapter introduced karst rock dissolution processes. Geomorphological processes lead to the formation of karst in the same way as processes that occur in other landscapes create different geomorphological features; however, the action of water plays a more dominant role in the formation of karst because of the high solubility of the rocks. Carbonic, organic and sulphuric acids that dissolve in the water make it a much more powerful solvent than pure water. Through a series of reversible reactions and ionic dissociations within the carbon dioxide–water–calcium carbonate system, a large amount of limestone is constantly recycled into water and air. The dissolution rate is controlled mainly by the temperature of the water, its speed and turbulence, and the contact between rock, water and the atmosphere. Dolomite rock is usually less soluble than limestone under normal atmospheric and water interface conditions because of the different characteristic behaviours of mineral forms. Under constant dissolution, eluviation and subsidence, water transforms the rock, and the resulting piping, subsidence and collapse makes the underground geomorphology of karst complex and variable.

The fourth chapter introduced sculptures made on rock surfaces by minor dissolution, and three categories of factors are responsible for these: passive factors, active factors and position. Passive factors include the intrinsic characteristics of rock, and active factors include the many kinds of environmental influences on it such as rainfall, soil, vegetation and even glaciation. All effects are time-dependent. Generally speaking, the most important factor responsible for sculpture is position, that is, whether the rock is covered. Bare karst that has water moving freely over it on exposed surfaces can form sculptures such as rainpits, dissolution rippling, dissolution flutes and bevels, dissolution runnels and grikes, whereas covered karst with restricted water movement will form grikes (dissolution-widened joints or cleavage planes) and vertical dissolution pipes more easily.

The fifth chapter described drainage. Underground drainage plays an important role in karst hydrological systems. On the contrary, surface drainage is more likely to be intermittent, disrupted, widely spaced or absent in karst landscapes. The rate of infiltration of the surface water flow is adjusted by the permeability of the planes and infiltration will enlarge the permeability further. Karst cover, for example, soil, may either accelerate infiltration rates by moderating lateral water movement or decrease them by hindering percolation and blocking joints in the underlying rock. The net efficiency of cover depends on soil texture and the influence of vegetation depends mainly on water loss by transpiration. All the above-mentioned aspects of drainage make the relative importance of infiltration and runoff variable in different places. In large karst areas, however, vulnerable spots with drainage into the underground hydrological system will exist, leading to the sinking of rivers. By and large, pure karst rocks tend to convey most of the water to the ocean by underground paths. Surface rivers may indicate less pure karst with plenty of precipitation, although their drainage density would still be less than other rocks under the same conditions. Underground drainage cuts off the linkage between water and the atmosphere, renders the regional hydrological system more efficient and increases the influence rainfall can have on geomorphology. In addition, plentiful voids in the rock in karst regions increase water storage capacity and can affect river regimes and river erosion significantly. Such underground reservoirs can moderate flood peaks and raise base flows, thus minimizing physical river erosion during floods. However, a slower flow of water will not decrease chemical dissolution capacity, because minor turbulence is sufficient for dissolution. Thus, corrosion is much more important in the formation of karst than corrasion. The principles of karst hydrology are basic and essential to the study of karst landforms, and there are two dominant but conflicting theories: the first, which is called single aquifer, claims that water moves through intergranular pores, whereas the second, which is called multiple aquifers, maintains that water moves through narrow fissures and large caves. Whatever the support for the two arguments, it is an undeniable fact that in springs and caves, the different systems of surface and underground drainage are closely linked, forming the integrated karst hydrology system.

In the sixth chapter, the author introduced several typical surface landforms in karst landscape caused by the action of water: gorges, meander caves, natural bridges, tufa, dolines, uvalas, semiblind, blind and dry valleys, poljes and karst margin plains were all described specifically.

In the seventh and eighth chapters, the author introduced karst caves and their deposits. Caves formed by karst processes of chemical dissolution and mechanical action are often numerous, large and complex, their eventual appearance being decided by the chemical purity and mechanical strength of the rocks. In general, chemical dissolution and mechanical actions work together, although their relative roles vary with the lithological and hydrodynamic circumstances in different caves. Deposits are important records for both cave history and deducing the paleoclimate. In particular, speleothems growing from chemical precipitates are mainly caused by calcite precipitation brought about by the diffusion of CO₂ from water to air. These stalagmites are made up of many concentric circles, by which scientists can reconstruct historical climate change. As well as speleothems, there are other kinds of karst cave deposits such as cave ice, biogenic deposits, clastic sediments and entrance facies.

The next two chapters clarified the two main factors of karst geomorphology, that is, climate and geological structure. Basically, liquid water is needed for karst development, which, therefore, is inhibited by arid and cold climates. Additionally, extreme arid and cold environments impede vegetation and soil microbial activities that would provide extra CO₂ to make the water more aggressive for karst development. On the contrary, tropical humid climates will accelerate karst formation processes in what could be called a ‘botanic hothouse’. Many remarkable and fully developed karst landscapes that are the products of rapid and vigorous dissolution have been formed in tropical humid areas, for example, the tower karst discovered in Cuba, China and Vietnam. Along with external climatic factors, internal geological structural characteristics including lithology, and the tectonic features of rocks also have a powerful and pervasive influence on karst formation processes.

In the last two chapters, the author discussed the history of karst and the current state and value of karst geomorphology. The simplest and most universal scheme of erosion in karst is summarized as the ‘karst cycle’. The process starts when the land is raised above the water table. Dolines develop at points that favour dissolution. In time, these increase in number and size and the original surface is formed into separated ridges and, finally, cone-shaped hills. The karst cycle ends when the depression floors reach the water table. However, climate change and tectonic movements make the history of karst formation much more complicated. The changing climate moves the water table, changes the activity of soil microbes and vegetation and, finally, alters the denudation rate of rock. Tectonic events have the power to change the base level in quite a short time, varying the relative location of rock and water table, and suddenly changing the phase within the karst cycle. A mixture of climatic, geological and historical factors contributes to the complex evolution of karst geomorphology.

Referring to the current state of the study of karst geomorphology at the time, Jennings mentioned several aspects that needed to be improved in further studies, the most important of which were better quantitative analyses and the use of multiple research methods. Investigations into karst were seen to have considerable practical advantages because karst geomorphology is closely linked to many social and economic activities. For example, karst groundwater is an important part of the supply of drinking water, karst springs have been used to generate mechanical power, karst caves contain phosphate and nitrates resources and karst landscapes are wonderful tourist attractions. Jennings concluded that karst geomorphology will have an enduring interest and application and will always hold a fascination for scientists, explorers and the public.

III From karst geomorphology to critical zone science

Karst provides a systematic review of the advances in karst geomorphology, its processes and causes before 1970. It is a classic textbook for early career researchers in this field and also for any educated member of the public who is interested in karst geomorphology, because it is comprehensive, systematic and easy to understand. Moreover, in its advocacy for a systematic approach it led to the advancement of critical zone science.

As noted by Jennings, quantitative analyses and multiple research methods were lacking in many areas of karst studies in 1971. Complicated underground processes were a mystery because they were unreachable and invisible, making them difficult to describe. Technical developments in recent years have facilitated the study of karst geomorphology by solving some of the methodological problems. For example, isotope tracer techniques can help locate the source of elements (Yuan et al., 2016) and also extract high-resolution paleoenvironmental information from karst records to reconstruct the process of environmental change (Yuan et al., 2004). This has resulted in progress being made in relation to quantitative analyses of invisible underground karst hydrology processes. In addition, regional karst dynamic observation systems have been built in many places around the world, which is a representative achievement of the use of multiple research methods. Guided by earth system science, the observation systems concentrate on the material and energy transmission taking place at the interface of lithosphere, hydrosphere, atmosphere and biosphere. Through monitoring, the basic laws of carbon, water, calcium and other life elements are understood as they are applied to the karst dynamic system under different geological and ecological conditions. Consequently, a model that covers all the processes of material and energy transmission has been established (Yuan et al., 2006).

As Jennings explained in 1971, climate, vegetation and water flow shaped karst geomorphology. Now, however, it is time to think about how karst geomorphology shapes regional vegetation growth, water and carbon cycles and even climate change. It is difficult for soil to form in karst areas and also easy to lose it because of the low quantity of acidic insoluble matter and the high porosity of carbonate rocks (Liu et al., 2021; Wang et al., 2011). A thin layer of soil reduces the amount of water that vegetation can utilize in karst areas. Thus, vegetation lying on karst areas is observed to have a generally low biomass accumulation (Jiang et al., 2020; Liu et al., 2019) and a high resistance to drought (Editorial Committee of Guizhou Forest, 1992; Liu et al., 2021). The classical climate–vegetation relationships become unsuitable for karst vegetation, and low productivity of vegetation cannot simply be explained as the result of low precipitation. Water stored in rock can also support the needs of vegetation when rainfall is limited (Zhu et al., 2021). The high permeability of karst ensures that precipitation is guided into the complicated underground hydraulic system in a short time, reducing evaporation to a great extent. In the light of this, more extreme precipitation patterns caused by climate change might benefit groundwater recharge in karst regions (Cardella Dammeyer et al., 2016). Karst formation processes absorb 17.74 million tC from the atmosphere every year in China and 608 million tC/a globally, which is one-third of the missing CO₂ sink in global carbon cycle modelling (Yuan, 1998, 2002). The high solubility of the rocks makes the connection between rock, soil, vegetation, water and atmosphere much tighter and more active in karst areas than in other regions. Research shows that the link is never unidirectional.

The theory of the earth’s critical zone provides a meaningful platform from which to describe the sophisticated linkage between lithosphere, hydrosphere, biosphere and atmosphere. The earth’s critical zone includes all the features that shape the earth’s surface and support its life systems (National Research Council, 2001), from the vegetation canopy to the aquifer bottom (Lin, 2010). The critical zone is characterized by material exchange on the interfaces between rock, soil, vegetation and the atmosphere. Given the vertical depth of the interactions, karst is an exemplar of the critical zone concept. Based on this, scientists have conducted some preliminary research in karst regions, and have already made significant advances in knowledge with regard to the interactions between rock, soil, vegetation and the atmosphere, and provided scientific guidance for vegetation restoration and agricultural activities. For example, the importance of rock in the composition and dynamics of karst vegetation has been clarified (Liu et al., 2019; Zhu et al., 2021), and the classic climate–vegetation relationships have been updated. Furthermore, it has been proved that vegetation restoration activities do not necessarily reduce soil erosion in karst regions because climatic drought events threaten the sustainability of planting and its ability to conserve soil (Feng et al., 2021). Crop yield from an upper slope position would be greatly reduced by soil erosion (Liang et al., 2021), and this type of area may be one where forest planting activities such as those carried out by the Grain for Green project should first be applied. So far, however, the critical zone is only a statement of ‘concept’ rather than a systematic ‘science’, although 20 years have passed since this notion was first put forward in the book Basic Research Opportunities in Earth Science (An et al., 2016; National Research Council, 2001). The lack of a unified research paradigm makes the current study of the karst critical zone neither systematic nor comparable. In most instances, only correlation is used as the proxy of interactions between rock, soil, vegetation and the atmosphere. This is still far from appropriate, especially in such a complicated system as a karst region.

IV Summary

In this article, we revisited the book Karst, which was written by Jennings in 1971 and laid a comprehensive foundation of karst geomorphology and its formation. By standing on the shoulders of giants it is possible to see the whole picture of karst geomorphology and its processes. Future research however, needs to understand karst with more quantitative analyses, more use of multiple research methods and the theories of the critical zone, which are concerned about the interactions between the lithosphere and other spheres rather than the unidirectional influences of water on rocks.