Evolution of carbon cycle over the past 100 million years

Abstract

It is generally accepted that progressive cooling of global climate since the Late Cretaceous results from decreasing partial pressure of atmospheric CO2 (pCO2). However, details on how and why the carbon cycle evolved and how it would affect pCO2 have not been fully resolved. While the long-term decline of pCO2 might be caused by the decrease of volcanic degassing through the negative feedback between pCO2 and silicate weathering, seafloor spreading, the major control of CO2 degassing, seems to have remained relatively constant. Alternative explanation, known as ‘uplift driven climate change’ hypothesis, proposes that tectonic uplift may have enhanced the sink of atmospheric CO2 by silicate weathering, and thus produced the decline of pCO2. However, increasing weathering sink of CO2 could deplete atmosphere all of its CO2 within several million years while holding volcanic outgassing constant. In this work, major fluxes of long-term carbon cycle are calculated based on a reverse model constrained by marine C, Sr and Os isotopic records and the spreading rate of sea floor. Weathering of island basalt and continental silicate rocks are separated in the new model. The results indicate a long-term decline of island basalt weathering in consistent with the global cooling trend over the past 100 million years. Dramatic changes of the CO2 fluxes associated continental silicate weathering, reverse weathering, volcanic degassing and the growth of organic carbon reservoir have been observed. Disturbance of atmospheric CO2 cycle by these fluxes seems to be maintained by the concomitant adjustments of island basalt weathering that were sensitive to the pCO2 controlled environment factors such as temperature and runoff. The negative feedbacks between pCO2 and weathering of island basalt might have played a significant role in stabilizing the long-term carbon cycle.