CARBON ISOTOPIC EVIDENCE FOR PHOTOSYNTHESIS IN EARLY CAMBRIAN OCEANS

Were the first metazoan reefs ecologically similar to modern tropical reefs, enabling them to persist under oligotrophic conditions? We test ed the hypothesis of ecological similarity by employing a geochemical approach. Petrography, cathodoluminescence, trace elements, and stable isotope analyses of primary precipitates of the Lower Cambrian Ajar L imestone, South Australia, indicate preservation of original C isotopi c composition. All primary carbonate components exhibit C isotopic val ues similar to the composition of inorganically precipitated fibrous m arine cements, suggesting that archaeocyaths and the calcimicrobe Epip hyton precipitated skeletal carbonate in equilibrium with ambient seaw ater in the absence of vital effects. Such data do not support the con tention that archaeocyaths possessed photosymbionts. However, a +0.5 p arts per thousand shift in delta(13)C occurs in reefs developed under shallower-water conditions relative to deeper reefs. This shift sugges ts the stratification of primary production in Early Cambrian oceans. The pattern is similar to that seen in the modern ocean, whereby signi ficant photosynthesis modulates the C isotopic composition of the phot ic zone.