By Charlie Self
This issue of Cave and Karst Science contains four short reports and one highly technical paper which is so unusually long it could almost be called a monograph.
A brief description is given of a limestone rock bridge on Antigua, one of the Leeward Islands. Though largely created by marine action, karstic features on the upper surface of the bridge and on a nearby rock platform suggest it may have a more complex origin. Scientific descriptions of natural rock bridges are relatively rare, despite their popularity as tourist attractions. The Antiguan karst, like that of many of the smaller Caribbean islands, still awaits a detailed scientific study.
Boreham Cave is a complex, partly-submerged cave system in the eastern part of the Yorkshire Dales. Studies of cave sediments, passage sizes, wall scallop sizes and their orientation have revealed a complicated history of cave development. The oldest parts of the cave are dry high-level passages accessed from beyond Sump 5; these originally drained south-eastwards into Littondale. A large phreatic loop seen in the dry passages between Sumps 1 and 2 is also old and once took water to a high-level resurgence close to the present valley side. As the valley deepened, the passages of Sumps 1 to 5 developed, with the water resurging from the current cave entrance. Vadose downcutting in the high-level passages and stream capture within the cave followed, with breakdown deposits forming in some high-level passages. The present-day hydrology is a complete reversal; the entrance passages are part of a perched "water table" while the active flow heads north-westwards into the hillside. In Far Downstream Passage, the active flow is lost within Sump 6 to an (as yet) undiscovered lower-level or parallel passage.
Similarities between the development history of Boreham and other Yorkshire Dales cave systems may help with the geomorphological interpretation of this little-studied area.
Don't be fooled by the friendly-sounding title! The words all make sense, but in the hands of Dr Faulkner they quickly morph into abbreviations (symbols) which are then able to combine into equations. There are 42 of these symbols, listed for convenience at the beginning of the paper where they occupy almost a whole page. Meanwhile, the first page is devoted to just the title and abstract. This is a monumental work to which a short précis like this cannot do justice. If you work in the field of hydrology, you need to read this paper for yourself. If you work in other areas of cave science, you can probably afford to wait until a condensed version appears in the next major geomorphology textbook; it should be in there. Meanwhile, I will do my best to provide a layman's abstract with no equations.
Speleogenesis is the origin of cave formation; in its initial stage, a flow path is established allowing water to travel through an outcrop of soluble rock. In most studies (including this one) the calculations are based on the solubility of calcite and the host rock is assumed to be limestone. The conduit is modelled either as a planar fissure or as a cylindrical tube. At this initial stage, the water travels extremely slowly and is almost (but not quite) saturated with respect to calcite. Solutional widening begins; it is slow but significant - and with widening comes an increasing flow rate. Thus dissolution rates increase along the whole flow path. Eventually the flow rate becomes high enough for the water to be significantly undersaturated as it resurges. This is known as "chemical breakthrough" and the conduit now rapidly increases in size. In typical hydrological situations, this switch from slow (high order) to fast (first order) kinetics approximately coincides with a change from laminar to turbulent flow and the start of sediment transport.
However, the conduit size for the onset of turbulence depends on the hydraulic gradient (head / path length) whereas the onset of fast kinetics depends on the hydraulic ratio (head / path length squared). Also, at breakthrough there is no typical Reynold's number (a dimensionless number used in hydrology equations) as this too is proportional to path length. Two more variables need to be considered, temperature and a friction factor, before calculations can be made to determine the conduit length at which breakthrough kinetics and the onset of turbulence coincide for any hydraulic gradient and its related conduit size. For a conduit shorter than this "coincidental" length, breakthrough occurs first; longer and onset of turbulence occurs first. Short and steep conduits can be sub-millimetre size at breakthrough; for long but shallow-angled conduits, the size may need to be over a metre. Clearly, this has implications for speleogenesis because if transitional turbulence occurs first, this reduces flow velocity and so delays the start of fast first order kinetics. Chemical breakthrough can then only be achieved at a larger aperture size, which in turn may reduce the extent of karstification.
Under hydraulic (constant head) control, the conduit is fully water-filled and the supply is sufficient for the carrying capacity of the conduit. Speleogenesis progresses through the stages of laminar flow, transitional turbulent flow (before or after breakthrough) to fully turbulent flow. Full turbulence allows the development of scallops on the cave walls, whose size depends on flow velocity. As the conduit continues to enlarge, flow velocity increases and scallop size decreases until, at about 1 cm length, they disappear entirely as mechanical erosion becomes dominant over chemical dissolution.
At the next evolutionary stage, the conduit capacity keeps growing until it exceeds the available recharge. This is known as constant recharge or catchment control. Conduits become vadose except in the vicinity of upward trending sections (which become phreatic loops). This applies to many mature karst systems fed by surface runoff (allogenic recharge). Climate change or (upstream) stream captures can result in a change to reducing recharge conditions, with even less water passing through the conduit. In both constant and reducing recharge conditions, there is a reduction in flow velocity within phreatic loops. This causes wall scallops (which formed originally in constant head conditions) to increase in size. Another consequence of flow velocity reduction is that the conduits start to reverse back through their previous flow regimes towards transitional turbulent and (ultimately) laminar flow. The author then devotes three pages to "practical applications" in both natural and artificial karst hydrological situations, where hydraulic ratios can be considered over 17 orders of magnitude.
These then are the main qualitative aspects of this paper. For an explanation of the quantitiative aspects, I am sorry but you need someone with a better head for equations than myself.
Histoplasmosis is a very common but usually benign medical condition caused by the fungus Histoplasma. Infection is normally a result of inhaling fungal spores (conidia), typically disseminating into the atmosphere from the guano of chicken coops and bird or bat roosts. The acute form of the illness may give the patient a fever, an unproductive cough and general feelings of malaise. Provided the patient is fit and healthy, recovery occurs naturally within a few weeks.
Caves and mines are known repositories for guano deposits and histoplasmosis is also known as "cave fever". Dr Craven provides a very useful world list of caves where people have been infected or where the fungus itself has been detected. The survey is based mainly on English language references, so is necessarily incomplete, but it does confirm that the illness is largely (but not exclusively) confined to the tropics and sub-tropics.
Floor samples and wall scrapings were taken from two small lava tubes in a mountain tundra region of the Russian Far East. The samples were then cultivated in a laboratory before examination by light microscopy. 3 cyanobacteria and 13 algal species were identified in total, though there were some differences between the two lava tubes in terms of species and species abundance. The relative paucity of species compared with caves elsewhere in the world reflects the high latitude of the Kamchatka peninsular.