The carbon and nitrogen atoms circulating in  biogeochemical cycles are same atoms that were in

The Mesozoic Era is an interval of geological time from about 252 to 66 million years ago. It is also referred to as the age of reptiles. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the Cenozoic era. The atmosphere present in mesozoic era contains nitrogen and carbon atoms. The climate of the Cretaceous is less certain and more widely disputed. Higher levels of carbon dioxide in the atmosphere are thought to have caused the world temperature gradient from north to south to become almost flat: temperatures were about the same across the planet. Average temperatures were also higher than today by about 10${}^o$C. In fact, by the middle Cretaceous, equatorial ocean waters (perhaps as warm as 20${}^o$C in the deep ocean) may have been too warm for sea life, and land areas near the equator may have been deserts despite their proximity to water. The circulation of oxygen to the deep ocean may also have been disrupted. For this reason, large volumes of organic matter that was unable to decompose accumulated, eventually being deposited as "black shale".