Stable carbon and oxygen isotopic compositions of recent charophyte oosporangia and water from Malham Tarn, U.K.: palaeontological implications

Abstract

Charophyte oosporangia and water samples from a highly calcareous lake were measured for stable carbon and oxygen isotopic composition. The time period over which the oosporangia calcify is short, thus any biochemical relationship between the water and oosporangia"s calcite represents only one ‘time window’ (late Summer in Malham Tam). This important temporal restraint must also apply to interpretations of all fossil material measured. The δ18Oc of the charophyte oosporangia is deduced to be in equilibrium with the δ18Oω of the water for a given temperature. The δ13 Cc of the charophyte oosporangia was approximately 2.5 per mil lower than the δ13CDIC in the water we measured. With the release Of CO2 with phosphoric acid from the charophyte oosporangia, there was no significant difference in the δ18Oc values obtained, regardless of whether or not the carbonate was separated from the organic center, however δ13Cc values were marginally lower for carbonate plus organic center measurements. Our results indicate that fossil charophyte gyrogonites can be used to elucidate the geochemistry of the ancient water body in which they lived.