SEA FLOOR SPREADING RATE AND ATMOSPHERIC CO2 OVER THE PAST 150 MA

The effects on the level of atmospheric CO2 of different formulations of global degassing over the past 150 million years have been examined using the GEOCARB III carbon cycle model which assumes that global degassing is linearly proportional to seafloor spreading rate. Results show similar Mesozoic CO2 values for the Engebretson and Gaina spreading rate formulations and lower values for the that of Rowley. However, in all three cases there is a general decline of CO2 towards the present. Assuming no change with time in spreading rate and/or no change in the carbonate content of subducting crust, one obtains very different calculated Mesozoic CO2 values from the standard case assumed in GEOCARB modeling, but in all cases there is a decline of CO2 towards the present. Use of the relative abundance of volcanic rocks over the past 150 Ma as an alternative indicator of degassing, results in CO2 values that are closer to those derived from the Engebretson and Gaina formulations than that of Rowley.

Use of spreading rate as a guide to Phanerozoic CO2 evolution has several problems. Degassing due to metamorphic and diagenetic processes, not directly connected to spreading rate, may be major sources of CO2. Also, sensitivity analysis shows that over both the past 150 million years and the entire Phanerozic, changes in global degassing may be of lesser importance than changes in silicate weathering as major controls on atmospheric CO2. This is especially true of the relatively poorly defined quantitative effects on weathering of plant evolution.