By Charlie Self
This issue of Cave and Karst Science contains three normal papers and one very large text that was originally composed as a chapter for a planned BCRA book.
Environmental monitoring studies in caves are often rather limited in scope and only of local interest. Not so this report from Japan. The location of the cave places it at the northern limit of the Asian Summer Monsoon (ASM), in a volcanic region where limestone is rare and caves with dateable deposits are of particular importance. In this study, air temperature measurements were taken at 9 stations every 30 minutes for 2½ years. This gives an authoritative picture of the diurnal (daily) and seasonal variations near the cave's two entrances, and also demonstrates the stability of environmental conditions in the inner parts of the cave. This is crucial for the selection of a speleothem for future stable isotope studies. In turn, such research could provide a record of palaeoclimate variations in the ASM during the growth period of the speleothem.
This large paper was originally written as a chapter for a book. However, a change in the terms of reference for what became Caves and Karst of the Yorkshire Dales Volume 1 meant a major reduction in both size and subject matter in the published text. This is the original draft Chapter 8.
This paper is a geology-centred view of cave and karst development, based on examples from the Yorkshire Dales. It is also in part a modified version of the Inception Horizon Hypothesis (IHH), first proposed by the lead author more than twenty years ago. The IHH was mainly based on evidence from the Yorkshire Dales, but the same rock types and processes occur worldwide. This revised interpretation is therefore of international importance. However, the text was written with a (book) audience in mind who were already familiar with the area and the place names therein. International readers should bear this in mind when the text mentions local topographic or geological features without further elaboration.
The IHH is a radical departure from the traditional story of cave development, as found in school geography textbooks. The original view of karstic drainage is that rain or river water enters a limestone rock that is essentially homogeneous. It then travels via cave passages developed along natural features in the rock (such as bedding planes and joints), selected apparently at random, before resurging at the topographic base of the limestone. By contrast, the IHH restricts inception (the earliest stage of cave development) to a limited number of pathways that have an ancient origin.
The IHH takes as its starting place the obvious fact that limestone is not homogeneous. The rock contains depositional cycles whereby coarse-grained material is followed by finer and finer sizes; some cycles continue with the deposition of mud (ancestor of shale) and finally by plant remains (ancestor of coal). What happens next is either erosion of some of the deposited sediment, or a new cycle of deposition begins. New cycles always start with coarser-grained material. This means that between each cycle of deposition there is a coarse- over fine-grained boundary. As the sediment pile builds up, so does the pressure. The (connate) sea water trapped between the lime particles wants to escape, so it looks for planes of weakness; the obvious ones are the boundaries between cycles. Some boundaries, particularly those with coarse lime particles over mud, are more effective at transmitting water than others and these become embryonic inception horizons. All this occurs before the lime particles are properly consolidated into limestone. In the case of the Yorkshire Dales, this happened more than 300 million years ago!
A lot can happen in 300 million years. The sediment pile becomes rock strata. Regional earth movements tilt and fold these rocks and also cause small fractures (joints) and large fractures with vertical or lateral displacement (faults) to form. Uplift and erosion creates a new landscape of limestone hills and valleys; in the Yorkshire Dales, many of the hills have impermeable non-limestone caprocks while impermeable basement rocks outcrop in some valleys. This is a textbook karst landscape, but how does its development start? Limestone rock has a low porosity and is relatively impervious to groundwater. However, extensive ancient low-efficiency bedding plane hydraulic pathways are already in place. These embryonic inception horizons are no longer separated by impervious rock, but are linked by younger joints and faults (inception links) to create a three-dimensional system of possible routes. All these routes develop simultaneously, but some are more efficient and carry more of the flow. These routes therefore are more likely to grow into caves of explorable size.
However, the landscape itself changes through time. This combination of geological structures and a changing landscape allows other inception horizons of previously lower efficiency to become major hydraulic conduits, and therefore new cave passages. Older passages either change from phreatic to vadose, or become dry and abandoned. In the middle of the paper, there is a distinct shift from theoretical to detailed local examples of this interaction between ancient inception pathways and more recent geological structures. This is the most interesting part of the paper for cavers who frequently visit the Yorkshire Dales. For international readers, the integration of familiar geological influences on cave development with the IHH provides a blueprint for the study of other karst areas.
Central Asia has some spectacular high mountain limestone scenery. Russian cavers began exploring the mountains of southern Uzbekistan in the 1980s and found caves that were both deep and extensive. The cave Boi-Bulok (on Surkhan-tau mountain) reaches a depth of 1415 metres. On the nearby Baisun-tau massif, the discovery of the Festival'naya-Ledopadnaya system marked the beginning of regular expeditions to this mountain. International cavers joined the Russian teams for the expeditions of 1989-1991; Festival'naya was explored to a depth of 625 metres and a length of 12.5 km. On a different part of the mountain, a British team in 1990 found a cave they named Dark Star and surveyed 2 km of icy passages.
Political difficulties following the break-up of the Soviet Union meant that during the period 1992-2010 only one Russian expedition took place. After a reconnaissance expedition in 2010, international cavers re-joined the Russian expeditions and major extensions have been found in Festival'naya (now 16 km long) and in Dark Star. The 2011 expedition explored beyond the British end point in Dark Star and found a lower entrance; this turned out to be a cave already partially explored by a Russian team. This new entrance gave easier access to a complex series of passages leading deeper into the mountain (described in detail in this report). Re-named the Central Karst System of Hodja-Gur-Gur-Ata and with six entrances, the cave is now 858 metres deep and 9.5 km long. Exploration continues, as does environmental research and the scientific study of bacteriological samples. The splendid photos accompanying this paper (in colour on the back cover of the journal) must surely rekindle British interest in this remote but fascinating region.
Environmental monitoring in this popular show cave has tracked the transition from summer to winter conditions. During the summer months a clear diurnal (daily) cycle can be seen, led by temperature which rises soon after the cave lights are switched on. Suspended cave aerosols and (to a lesser extent) carbon dioxide concentrations also increase as visitors trek through the cave. At night, cave ventilation allows a return to minimum values; gravitational forces speed the deposition of aerosols. During the progression into winter, an overall fall in cave temperature and carbon dioxide levels indicates an increase in cave ventilation and exchange with outside air. Counter-intuitively, the fall in visitor numbers in winter is not matched by a fall in the aerosol count. Instead, when the cave air temperature starts to become warmer than the air outside, aerosol minimum values increase as natural ventilation becomes dominant over anthropogenic (human) influences.
Aerosol daily minimum values are a very sensitive tool for monitoring cave environments. It would be nice to see a similar experiment run in other show caves, as would an extended study involving the transition from winter back into summer conditions.