The d13C of marine carbonate (reflected in the oxidized form of carbon CO3=) changes through geologic time. During times when d13C marine carbon is more positive (i.e., the Carboniferous), what is the cause of this +d13C excursion? a. Extensive burial of Organic (reduced) carbon in sediments b. Extensive precipitation and accumulation of limestones c. Stratification of the Oceans with decreased surface productivity d. Release of organic carbon (i.e., methane clathrates) due to global warming