Confirmation of ¹³⁷Cs dating by algal stratigraphy in Rostherne Mere

Abstract

THE deep sediments of Rostherne Mere, Cheshire (National Grid Reference SJ 745843) offer a promising situation in which to study the relationship between biological stratification and radionuclide dating in the absence of gross bioturbation. The mere is small (46.5 hectare), but relatively deep (30 m maximum), and is reported to be largely devoid of benthic fauna in the persistently deoxygenated deep sediments¹. The sediments of Rostherne Mere contain the remains of a great variety of algae, in addition to diatoms. The preservation of non-siliceous algae is uncommon, although found in the sapropel deposits of the USSR². In Rostherne the empty spores of Ceratium hirundinella O.F. Mull., Anabaena species and Aphanizomenon flos-aquae (L) Ralfs are recognisable, as are colonies of Microcystis species and cells of Staurastrum species. The phytoplankton of Rostherne Mere has been recorded at intervals since 1912 (refs 3–5) and sampled more frequently from 1962 (refs 6–8). The recent records (Table 1) show fluctuations in summer dominance, mainly between Ceratium hirundinella and Microcystis aeruginosa Kütz. emend., but blue–green algae have become increasingly dominant⁹. These changes, combined with occasional maxima of the diatoms Melosira granulata (Ehr.) Ralfs, Cyclotella pseudostelligera Hust. and Stephanodiscus astraea (Ehr.) Grun., can be correlated with algal assemblages in the sediments and permit these to be accurately dated. We now report that such algal remains from the lake sediment have been used to establish a detailed chronology which is shown to confirm the ¹³⁷Cs dating method.