Using environmental tracers to evaluate the preservation of palaeoclimate signals in aquifers of the London Basin, UK

The concept of aquifer basins as palaeoclimate archives has existed for some decades, yet few detailed studies comparing types have been carried out. To assess the potential a particular an archive, its hydrogeochemical characteristics must be thoroughly investigated, ideally in comparison to adjacent which can shown substantially preserve ice-age endowment at depth. London Basin (UK) presents such opportunity, containing two main aquifers contrasting type: Chalk, fractured microporous limestone, and Lower Greensand, porous sandstone. Despite intensive exploitation both, evidence Devensian (late-glacial) water remains depth, though this differs between type. understand reasons this, suite environmental tracers applied. In addition hydrochemistry, stable isotopes (δ18O, δ2H), carbon (δ13C-DIC, 14C-DIC) noble gases (He, Ne, Ar, Kr Xe), new basin (CFCs 14C-DOC) used. effect Greensand appears ‘reference aquifer’, preserving recharge from prior Last Glacial Maximum (LGM), while Chalk contains mixed water, with no remaining trace undiluted pre-LGM end member even depth remote parts confined basin. Whereas both had past given maximum 14C-DIC model ages ≥ 30 kyr (the effective limit that method), present study use 14C-DOC reduced 23.4 (Lower Greensand) 17.2 (Chalk). Similar contrasts isotope depletions (−8.2 ‰ −7.8 δ18O) noble-gas-derived temperature minima (2.6° 4.1 °C) were also observed. CFCs found all sites, traces detectable 40 km outcrop, so climate signal degradation inevitable throughout A correlation 14C activity excess 4He suggests deep saline could 50 old. key understanding how reliable these individual are palaeoarchives.