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

Age gradients of stable C and O isotopes were found in modem Chara hispida calcite incrustations allowing seasonal changes in water chemistry to be ob served. Gradients resulted from apical calcification in growing charophytes . In a regional study of 12 species and varieties of charophytes the carbon ate delta C-13 differed significantly with 'low value' and 'high value' spe cies such as Chara globularis and C baltica, respectively. Samples from lak es and other stagnant waters had significantly higher delta C-13 values tha n those sampled from flowing water. The differences were related mainly to photosynthesis. Charophyte delta O-18 (carbonate) was not significantly different between s pecies but water chemistry was important for brackish water Lamprothamnium papulosum. The overall delta O-18 (carbonate) was positively correlated wit h groundwater delta O-18 and it is suggested that more reliable environment al information is available from delta O-18 than delta O-18 in charophyte m arls. The mean charophyte carbonate delta C-13 and delta O-18 was -5.7 part s per thousand and -6.4 parts per thousand VPDB, respectively. Environmenta l temperature estimates suggested that at some sites the carbonate was depo sited close to isotopic equilibrium, but in others a slight to moderate dis equilibrium was indicated. (C) 2001 Elsevier Science B.V. All rights reserv ed.