Chlorine-36 in groundwater as a palaeoclimatic indicator

Abstract

The 36Cl contents of groundwaters from the East Midlands Triassic sandstone have been used to deduce rainfall, its chloride content and evapotranspiration during the Late Pleistocene. At this time, the regional precipitation and evapotranspiration would have been reduced relative to the modern and Holocene climate. It is estimated that the infiltration was reduced by about 40 mm · a-1. The method for evaluation of these palaeoclimatic parameters is dependent upon the major Cl- source being a marine aerosol fall-out. This is the general case, but groundwater east of the exploited zone of the aquifer contains chloride which is derived from saline formation water, with a marine Cl- Br- ratio, and in equilibrium with the in situ neutron flux. The fall-out of cosmogenic 36Cl at the geomagnetic latitude 56.2°N is evaluated as 30.6 atoms m-2s-1 and is significantly greater than that estimated from the theoretical fall-out curve of Bentley et al. [1]. The 'bomb peak' of 36Cl remains entirely within the unconfined zone of the aquifer. The assessment of groundwater residence time from the 36Cl chronometer is shown to be impossible for this aquifer. Show more