Phanerozoic atmospheric CO 2 change: evaluating geochemical and paleobiological approaches

Abstract

The theory and use of geochemical modeling of the long-term carbon cycle and four paleo-PCO 2 proxies are reviewed and discussed in order to discern the best applications for each method. Geochemical models provide PCO 2 predictions 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 δ 13C of pedogenic carbonates provide the best proxy-based PCO 2 estimates for the pre-Tertiary, with error estimates ranging from ±500–1000 ppmV. Pre-Devonian estimates should be treated cautiously. Error estimates for Tertiary reconstructions via this proxy are higher than other proxies and models (±400–500 ppmV), and should not be solely relied upon. We also show the importance of measuring the δ 13C of coexisting organic matter instead of inferring its value from marine carbonates. The δ 13C 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) PCO 2 estimates that can be near continuous for most of the Tertiary. Its high temporal resolution and availability of continuous sequences is advantageous for studies aiming to discern short-term excursions. This method, however, must correct for changes in growth rate and oxygen level. At elevated PCO 2 (∼750–1250 ppmV), this proxy loses its sensitivity and is not useful. The stomatal density and stomatal index of land plants also provide high temporal resolution (<10 2 year), high precision (±10–40 ppmV for the Tertiary and possibly Cretaceous) PCO 2 estimates, and so also is ideal for discerning short-term excursions. Unfortunately, this proxy also loses sensitivity at some level of PCO 2 above 350 ppmV (which, currently, is largely undetermined). Our analysis of the recently developed δ 11B technique shows that it currently is not yet well constrained. Most importantly, it requires the assumption that the boron isotopic composition of the ocean remains nearly constant through time. In addition, it assumes that there are no biological or temperature effects and that diagenetic alteration of the boron isotopic composition does not occur. A fifth CO 2 proxy, based on the redox chemistry of marine cerium, has several fundamental flaws and is not discussed in detail here.