Abstract:

Drips in the forward part of the Murray Cave between 5 and 50m below the surface were sampled about once a month for 2 years, carbon dioxide in the soil above and in the cave air being measured also. Mean soil CO2 content was fifteen times atmospheric, summer yeilding higher values than winter though the dry 1972-3 summer had low values. Greater depths in the soil had more CO2 than shallower ones. Cave air had on the average little more CO2 than the atmosphere but river flooding of the cave was followed by large CO2 fluctuations. There was a slight tendency for drips to be warmer and to vary less in temperature inwards. Drip pH was greater in summer than winter because of high CO2 production. The (Ca+Mg)/(Na+K) ratio of the drips was nearly ten times that of the Blue Waterholes, showing that igneous rock weathering around the Plain supplies more of the Na and K in the spring output than was envisaged before. The drip Mg/Ca ratio lies close to that of the Blue Waterholes, underlining the dominance of the limestone in the output hydrochemistry. The mean total hardness of 141 mg.L-1, not significantly different from earlier Murray cave drip measurements, sustains the previous estimate that the superficial zone provides about 2/3 of the limestone solution. The summer value (149 mg.L-1) is significantly greater than the winter mean (132 mg.L-1), including high values in the dry 1972-3 summer when CO2 values were low. Lagged correlation on a weekly and three weekly basis of individual drip hardness on air temperature and precipitation yielded few significant results. Only a weak case for dominance of hardness by temperature through rhizosphere CO2 was evident but neither was the conflicting hypothesis of hardness in such contradictory ways that more detailed observations over equally long time periods are necessary to elucidate their influence.