Stable isotopes in charophyte incrustations: relationships with climate and water chemistry

Abstract Age gradients of stable C and O isotopes were found in modern Chara hispida calcite incrustations allowing seasonal changes in water chemistry to be observed. Gradients resulted from apical calcification in growing charophytes. In a regional study of 12 species and varieties of charophytes the carbonate δ 13 C differed significantly with ‘low value’ and ‘high value’ species such as Chara globularis and C. baltica , respectively. Samples from lakes and other stagnant waters had significantly higher δ 13 C values than those sampled from flowing water. The differences were related mainly to photosynthesis. Charophyte δ 18 O (carbonate) was not significantly different between species but water chemistry was important for brackish water Lamprothamnium papulosum . The overall δ 18 O (carbonate) was positively correlated with groundwater δ 18 O and it is suggested that more reliable environmental information is available from δ 18 O than δ 13 C in charophyte marls. The mean charophyte carbonate δ 13 C and δ 18 O was −5.7‰ and −6.4‰ VPDB, respectively. Environmental temperature estimates suggested that at some sites the carbonate was deposited close to isotopic equilibrium, but in others a slight to moderate disequilibrium was indicated.