iv Tripati, Roberts and Eagle( 2009)
Fig. 4. Boron / Calcium pCO 2 estimates plotted against oxygen isotopes modified from Tripati et al.( 2010). Red line added
This record spans the period 5-20 Ma and is based on measurements of the B / Ca ratio in marine carbonates( as opposed to the boron isotope ratio). It shows two estimates > 400 ppm in the early and middle Miocene(~ 15 and ~ 18 Ma). It should be noted however that the method used has been subject to criticism and the understanding of the proxy, as detailed in Tripati et al.( 2010) has been challenged( Allen and Hönisch 2012).
v. Pagani et al.( 2010)
Fig. 5. Alkenone pCO 2 records from Pagani et al.( 2010). Red lines added
This paper produced new alkenone records from six different sites extending back to about 5 Ma( covering the Pliocene and Pleistocene epochs). Most of the sites seem to show a trend for declining