Other proposed proxy methods include analyzing the carbon isotope ratio of soil carbonate, B / Ca ratio of marine calcite, or the phases of evaporate minerals which are CO 2 dependent. This later method probably involves the fewest assumptions of them all but unfortunately can only be applied to very rare geological deposits( so far just 3 in the last 50 million years; Lowenstein and Demicco, 2006). It does, however, confirm the predictions from boron isotopes and alkenones that pCO 2 was much higher than modern in at least some portion of the Paleogene period.
v Cautionary remarks
From the above discussion it can be seen that all the proxy methods are somewhat experimental in their application, especially over millions of years. Various laboratories are working hard to develop them and better constrain the assumptions. This means that the methods themselves are changing as they evolve, often making old pCO 2 estimates non- comparable with newer ones because different assumptions are made and as new knowledge is acquired. All the methods produce‘ noise’ in the records which arises not just from analytical uncertainty but also from the secondary assumptions( like undetected variations in temperature for example). Potentially more worryingly, the proxies might involve systematic biases of one sort or another. Hence to understand the record in any given publication it is necessary to examine the assumptions carefully. There is no space to do that here- we shall just examine the pCO 2 reconstructions themselves.
The good news is that the three proxies listed all work under controlled experimental conditions and also work fairly well at reconstructing the glacial-interglacial cycles in pCO 2 seen in the ice core records. It so happens that for different reasons, all the methods work less well at higher pCO 2. The stomatal method is the most problematic at levels of CO 2 higher than modern because the calibration data sets are small and variable and non-linear( the effect seems to flatten out at high CO 2). Moreover the biological relations of fossil leaves to their modern counterparts become increasingly distant with geological time. For those reasons I will focus mainly on alkenone and boron estimates.
3. The records i. Pearson and Palmer( 2000)