PALAEOPROTEROZOIC SEDIMENTATION AND STROMATOLITE GROWTH IN AN ADVANCED INTRACONTINENTAL RIFT: IMPLICATIONS FOR THE POSITIVE d13C-CARB EXCURSION

All major positive isotopic excursions of 13C/12C in sedimentary carbonates, including those that occurred in the Palaeoproterozoic and during the Neoproterozoic/Cambrian transition, have been ascribed to enhanced accumulation of organic matter. This very logical and simple scenario has only one deficiency, albeit a crucial one, which is the absence of a geological record of such enhanced accumulations of organic material prior to or synchronous with the isotopic events. What has been preserved in the rocks is, in fact, far too little to balance the positive shifts of d13Ccarb by +15‰ in the Neoproterozoic, and up to +28‰ in the Palaeoproterozoic. Perhaps this major inconsistency in mass balance of the carbon cycle during the Palaeoproterozoic and Neoproterozoic positive isotopic shifts can be understood when considering the local settings of the investigated rocks. C-isotopic modification by evaporation and stromatolite-forming bacteria could be significant in partially closed and restricted depositional sites. An appropriate example of the interplay between open, partially closed and restricted depositional sites is provided by a Palaeoproterozoic rock sequence, which is located at the northern end of the Bothnian Bay of the Fennoscandian Shield. The sequence comprises a markedly variable succession of carbonate, siliciclastic, volcaniclastic and volcanic rocks. Well-preserved sedimentary structures suggest that deposition took place in a shallow, restricted (evaporitic), though tidally influenced environment. The sedimentation was accompanied by subaerial mafic volcanism and syndepositional tectonics, thus indicating an advanced intracontinental rift site connected to an open sea. A partially restricted environment should be taken into account when interpreting the carbon isotopic composition of the carbonate rocks.