SECULAR RISE OF BIOTIC SOILS AND DETRITAL CLAY MINERALS AS A CONTROL OF ORGANIC CARBON BURIAL AND THE RISE OF ATMOSPHERIC OXYGEN IN THE LATE PRECAMBRIAN

A late Neoproterozoic rise in oxygen is often attributed to a general model of tectonically enhanced sedimentation rates and organic carbon burial associated with the rifting of Rodinia. Recent findings have identified a more specific mechanism of carbon burial based on a first order spatial association of organic matter with the detrital clay mineral fraction of marine sediments present on the continental shelf; the dominant location of organic matter (OM) deposition. Clay associated burial of OM accounts for >90 % of organic matter in these sediments, as well as ancient organic-rich black shales. Mineral surfaces provide a means of physical sheltering and preservation of molecular scale organic compounds adsorbed to their surfaces. The global flux of detrital clay minerals to the oceans is dominated by clay minerals formed in organically-stabalized soils. The organic component of the soil promotes clay mineral production by concentrating ions and increasing the residence time of the water within regolith. There is thus good reason to believe that clay production should increase with the development of biotic soils as terrestrial environments were colonized by fungi, lichen, microbial mats and possibly eukaryotic multicellular plants towards the end of the Precambrian. The dominance of mechanically generated silt lithologies of Precambrain mudstones and shales compared to the clay dominated Phanerozoic equivalents suggests a fundamental change in detrital clay production. Enhanced detrital clay deposition would influence rates of organic matter burial and preservation and, more importantly, the related oxygen release coupled with organic carbon burial. Continuing studies of the clay composition of Precambrian successions in Australia, China, Baltica, and North America support a secular trend in the abundance of detrital clay minerals; a prediction of the hypothesis that biotically influenced enhancement of terrestrial weathering led to clay production during the late Precambrian, and drove an increase in organic matter burial/preservation in marine sediments. The ultimate effect was a rise in atmospheric oxygen capable of supporting multicellular animals and plants first appearing after the Marinoan glaciation (~620 mys).