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Hydrological Implications from 14 C Profiling of UK Tufa

1980; Cambridge University Press; Volume: 22; Issue: 3 Linguagem: Inglês

10.1017/s0033822200010304

1945-5755

P M Thorpe, R L Otlet, M. M. Sweeting,

Isotope Analysis in Ecology

Tufa is a superficial, secondary deposit of calcium carbonate which accumulates on precipitation from emergent spring waters. It occurs as discrete, localized masses in regions of calcareous country rock. In the United Kingdom, deposits vary widely in structure and thickness but little is known of the rate of deposition. Some deposits contain laminae assumed to represent annual growth increments and which should contain a 14 C and stable isotope record related to the original water from which it was precipitated. Investigations are reported on tufa from, three areas of different limestones in the United Kingdom (Northwest Yorkshire, South Derbyshire and North Oxfordshire). Hydrologically, the dating of tufa by 14 C involves the same problems as the dating of groundwater. In the case of actively forming tufa, however, it is possible to derive a sequence of measurements, beginning with present day deposition, which clearly demonstrates the applications of age corrections. At Gordale Scar, Northwest Yorkshire, a profiling study of laminated tufa appears to show bomb trial 14 C to a depth of 18mm below the surface, with almost constant values around 50 percent modern (raw data) from 18 to 48mm below the surface. The 14 C content of surface tufa lies within the seasonal range of 14 C measurements from the parent stream waters. Results of 50 percent modern in the sequence are consistent with the simplest correction procedures based on δ 13 C balance and the observed δ 13 C change on tufa precipitation is a practical demonstration of the fractionation factor ∊ 13 . However, the application of corrections to active, surface tufa and parent waters collected monthly over a period of study (14 months) from all three sites, produce results higher than would be expected from published world 14 C levels.