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Abstract:

The Marble Arch cave is a high energy cave located in County Fermanagh, Northern Ireland. The first 500 metres of the system has been opened as a Showcave for approximately 13 years and has been visited by over half a million visitors since opening. Microclimatic investigations over a six month sampling period (31.7.96-10.1.97) found that the cave has a variable microclimate in response to, the surface climate, the caves hydrology and influences within the Showcave.
High positive correlation coefficient's were attained between the cave and surface air temperature. As distance increased into the cave correlation coefficients' decreased indicating a time lag. Air temperatures varied on both a temporal and spatial scale. Summer air temperature ranges of 2.1 °C were noted between sites within the cave, which increased to 7.1 °C during the winter season. The cave air temperature changed progressively from the entrance to the interior, decreasing in the summer and increasing in the winter months. During the summer months the mean surface air temperature (17.4°C) was greater than the cave mean air temperature (10.8°C). In the winter months the mean surface air temperature (1.9°C) was less than the cave mean air temperature (7.2°C). Site variability was generally greater during the summer months indicated by higher Coefficient of Variations.
The 'chimney effect' is noticeable in some parts of the cave as a result of surface and cave air temperature differences. Airflow within the cave changed direction in response to seasonal air temperature variations, flowing out of the cave during summer months and into the cave in the winter. Other processes such as the 'Entrainment Effect' were also evident. Airflow throughout the cave was generally weak, although in some parts of the cave was noticeable as a 'wind'.
The main river flowing through the cave was responsive to the seasonal cave air temperatures. During the summer months the water temperature once entering the cave decreases by approximately 2°C and increases by up to 7°C during the winter months. A relatively constant seasonal water temperature is maintained whilst travelling through the cave.
The largest air temperature variations occurred within the Showcave in which the presence of tourists and electrical lighting are believed to be partially responsible, the latter of which being the greater contributor. Results show that a tour of 18 people on average increased the surrounding air temperature by up to 1.3°C. The effect was reduced when a tour was moving past a point rather than remaining stationary in the same place.
Electronic lighting increased the overall air temperature throughout the Showcave. Each type of light used within the Showcave influenced the air temperature up to a metre away from the light source. Air temperature increases around the lighting was a result of the type of light used rather than the bulb wattage installed. In some cases the air temperature remained 2°C higher than the mean cave air temperature 1 metre away from the light source.