Carbon Isotopes and Microbial Sediments

Abstract

Microbial sediments of the biolaminated type, generated by the matting behavior of preferentially prokaryotic microben-thos, commonly carry the isotopic signatures of both the primary microbial biomass and of the carbonate of the surrounding sedi-ment matrix. This is true for present -day stromatolites as well as for their fossil counterparts, which can preserve these signatures with a minor diagenetic overprint for billions of years. While the isoto-pic composition of the organic (kerogenous) carbon fraction may reflect the intrinsic fractionations of the microbial primary pro-ducers as well as several other parameters (productivity, tempera-ture, salinity), the I)"C and I)"0 labels, specifically of sub-Recent laminated stromatolitic carbonates, have encoded a wealth of pala-eohydrological and palaeotemperature information which makes them important stores of palaeoclimatological data. Altogether, the stromatolitic carbon isotope record constitutes an exuberant ar-chive of biogeochemical and palaeoenvironmental evolution that still awaits further evaluation.