Kimberlite eruptive frequency and major low latitude glaciation through the Phanerozoic

An examination of the temporal distribution of kimberlites throughout the Phanerozoic shows a strong anti- correlation between kimberlite eruption frequency and major glaciation. A cumulative frequency plot of kimberlite eruption ages shows a kimberlite-free window in the Permo-Carboniferous and no significant kimberlite eruption since c. 48 Ma. Both of these intervals coincide with geochemically modelled episodes of low atmospheric CO2 concentration (<600 ppmv). A dip in kimberlite eruptive frequency is also seen in the Late Ordovician-Early Silurian. Earlier kimberlite hiatuses are hinted at for the Neoproterozoic although the dataset is incomplete for this time interval and low confidence can be placed in these. The period around 45 Ma is recognised as a time of global plate reorganisation, and has recently been identified as a time of cooling in the equatorial Pacific and Antarctic. Although kimberlite eruptions are volumetrically small, their frequency during kimberlitic periods of the Phanerozoic is high (at least 8000 are known) and the magma likely possessed a high carbon dioxide content (up to 20 % by volume). The temporal relationships suggest that kimberlites are either a proxy for an unrecognized mantle source of CO2 or are themselves directly responsible for buffering of atmospheric CO2 levels against the long-term steady draw-down by tectonic and depositional processes, maintaining them at >600-1000ppmv. A conceptual model is presented suggesting that a progressive drop in atmospheric pCO2, following the loss of this mantle-derived CO2 buffer in the early Cenozoic, early Pennsylvanian, and possibly Late Ordovician, made the global climate system more sensitive to other perturbations resulting in prolonged (> 2 million years) global glaciation.