Variations in the global carbon cycle during the Cretaceous related to climate, volcanism, and changes in atmospheric CO2

Abstract

The stratigraphic record from both deep-sea and shallow-water depositional environments indicates that during late Aptian through Cenomanian time (1) global climates were considerably warmer than at present; (2) latitudinal gradients of atmospheric and oceanic temperatures were considerably less than at present; (3) rates of accumulation of organic matter of both marine and terrestrial origin were as high as or higher than during any other interval in the Mesozoic or Cenozoic; (4) the rate and volume of accumulation of CaCO 3 in the deep sea were reduced in response to a marked shoaling of the carbonate compensation depth; (5) seafloor spreading rates were somewhat more rapid than at any other time in the Cretaceous or Cenozoic; (6) off-ridge volcanism was intense and widespread, particularly in the ancestral Pacific Ocean basin; and (7) sea level was relatively high, forming widespread areas of shallow shelf seas. A marked increase in the rate of CO 2 outgassing due to volcanic activity between about 110 and 70 m.y. ago may have resulted in a buildup of atmospheric CO 2. A significant fraction of this atmospheric CO 2 may have been reduced by an increase in the production and burial of terrestrial organic carbon. Some excess CO 2may have been consumed by marine algal photosynthesis, but marine productivity apparently was low during the Aptian-Albian relative to terrestrial productivity. Terrestrial productivity also may have been stimulated by increased rainfall that resulted 504 from a warm global climate and increased marine transgression as well as by the higher CO 2.