Phanerozoic atmospheric CO2 change: evaluating geochemical and paleobiological approaches

The theory and use of geochemical modeling of the long-term carbon cycle an d four paleo-PCO2 proxies are reviewed and discussed in order to discern th e best applications for each method. Geochemical models provide PCO2 predic tions for the entire Phanerozoic, but most existing models present 5-10 m.y . means, and so often do not resolve short-term excursions. Error estimates based on sensitivity analyses range from +/- 75-200 ppmV for the Tertiary to as much as +/- 3000 ppmV during the early Paleozoic. The delta C-13 of pedogenic carbonates provide the best proxy-based PCO2 es timates for the pre-Tertiary, with error estimates ranging from +/- 500-100 0 ppmV. Pre-Devonian estimates should be treated cautiously. Error estimate s for Tertiary reconstructions via this proxy are higher than other proxies and models (+/- 400-500 ppmV), and should not be solely relied upon. We al so show the importance of measuring the delta C-13 of coexisting organic ma tter instead of inferring its value from marine carbonates. The delta C-13 of the organic remains of phytoplankton from sediment cores provide high temporal resolution (up to 10(3)-10(4) year), high precision ( +/- 25-100 ppmV for the Tertiary to +/- 150-200 ppmV for the Cretaceous) PC O2 estimates that can be near continuous for most of the Tertiary. Its high temporal resolution and availability of continuous sequences is advantageo us for studies aiming to discern short-term excursions. This method, howeve r, must correct for changes in growth rate and oxygen level. At elevated PC O2 (similar to 750-1250 ppmV), this proxy loses its sensitivity and is not useful. The stomatal density and stomatal index of land plants also provide high te mporal resolution (< 10(2) year), high precision (+/- 10-40 ppmV for the Te rtiary and possibly Cretaceous) PCO2 estimates, and so also is ideal for di scerning short-term excursions. Unfortunately, this proxy also loses sensit ivity at some level of PCO2 above 350 ppmV (which, currently, is largely un determined). Our analysis of the recently developed delta B-11 technique shows that it c urrently is not yet well constrained. Most importantly, it requires the ass umption that the boron isotopic composition of the ocean remains nearly con stant through time. In addition, it assumes that there are no biological or temperature effects and that diagenetic alteration of the boron isotopic c omposition does not occur. A fifth CO2 proxy, based on the redox chemistry of marine cerium, has sever al fundamental flaws and is not discussed in detail here. (C) 2001 Elsevier Science B.V. All rights reserved.