An isotopic and geochemical study of carbonate-clay mineralization in basaltic caves:: abiotic versus microbial processes

Stable isotope and geochemical data are used here to differentiate between contemporaneous abiotic and microbial processes leading to formation of modern carbonate- (calcite, aragonite and magnesite) and silicate-rich (kerolite) mineralization in basaltic sea caves on the island of Kauai, Hawaii. Strontium isotope and Ca/Sr ratios in meteoric water and cave carbonates suggest that the majority of Sr and Ca are derived from rock-water interaction within the host basalts situated above the caves. Oxygen and hydrogen isotope ratios and chemical compositions of cave and surface waters indicate that evaporation does not control cave-water composition. However, evaporation of drops and thin films of water in microenvironments can lead to precipitation of some phases. This behaviour is suggested by the covariance in delta O-18 and delta C-13 values of some carbonates, especially magnesite, which is considered to be a late-stage evaporative precipitate. Modelling of water evolution suggests that evaporation can be a cause of supersaturation for magnesite, kerolite and some Ca carbonates. However, the highly elevated delta C-13 values (up to +8.2) of some Ca carbonates, compared to average dissolved inorganic carbon delta C-13 values (similar to-12), are best explained as the product of microbial photosynthesis, in particular by cyanobacteria, present in the upper layers of active microbial mats on cave surfaces. The preferential uptake of C-12 by cyanobacteria is recorded in the low delta C-13 values (-29.1 to -22.6) of organic matter in mats and mineralized microbialites. The resulting C-13-enrichment of dissolved inorganic carbon is recorded in the elevated delta C-13 values of these Ca carbonates. A positive correlation exists between the delta C-13 values of the carbonates and coexisting organic matter. The large enrichment in C-13 of carbonate minerals, relative to dissolved inorganic carbon, and its covariance with the delta C-13 values of coexisting organic matter are useful for identification of carbonate-rich mineralization resulting from autotrophic microbial activity.