Secular Change in Carbonate Facies From the Precambrian to the Phanerozoic

Kennedy, Martin J.¹ (1) University of California, Riverside, Riverside, CA

One of the fundamental changes to the Earth System that occurred from the Precambrian to the Phanerozoic was the advent of skeletal precipitation. This had at least two important effects on the Phanerozoic carbonate record; establishing a means of deep sea compensation through pelagic sedimentation, and focusing carbonate deposition to low latitude oligotrophic reef systems. The combination of these mechanisms act as an effective buffer of carbonate concentration within the oceans with periods of high atmospheric CO2 resulting in a rise in the lysocline and dissolution of deep-sea, pelagic carbonate. By contrast, carbonate deposition in the Precambrian was dominantly involuntary, facilitated by localized microbial removal of CO2. Carbonate precipitation was focused on shelves and directly influenced by hypsographically induced changes in shelf area following sea level change. Broader shelves during highstand provided a greater area for carbonate production and lower carbonate concentration within the oceans when compared to lowstand. Precambrian oceans were thus less well buffered than Phanerozoic oceans, resulting in greater changes in carbonate concentration and dependent atmospheric CO2. The Precambrian carbonate system was characterized by feedbacks enhancing climate change by contrast to the Phanerozoic system that buffered it. The presence of thin (<10m) widespread carbonate deposits that occupy transgressive portions of Precambrian successions are a physical record of the changing saturation state of the ocean brought about by shelfal flooding. The best known of these deposits sharply overlie Neoproterozoic glacial deposits and are known as ‘cap carbonates' which are Precambrian analogs to accelerated (post glacial) Holocene reef growth.