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