The over-arching theme of this study is to better understand what drives long-term climate change, specifically oscillations between greenhouse and icehouse states over timescales of 100 My. These climatic oscillations, integrated over Earth?s history, profoundly influenced the evolution of life and the surface of the Earth. To first order, variations in the CO2 budget of the ocean-atmosphere-biosphere system drive climatic variation over timescales greater than 10 My: because of the greenhouse effect of CO2 in the atmosphere, Earth?s surface temperature warms when atmospheric CO2 is high and cools when CO2 levels are low, all other variables (like albedo) being equal. The C content of the Earth?s exogenic system, over long timescales, is controlled by volcanic inputs from the Earth?s interior and outputs from the exogenic system via sediment burial and subduction. Long-term climate variability is thus intimately linked to whole Earth carbon cycling, that is, the cycling of C between the endogenic and exogenic systems. Exactly how and why these inputs and outputs have changed through time is the question. This study will focus on the most recent greenhouse-icehouse transition. This begins with the Cretaceous to early Cenozoic (150-60 My) greenhouse interval when dinosaurs roamed the Earth, atmospheric CO2 pressure was possibly 4-8 times higher than today, polar ice caps were absent, and much of Earth?s economically viable hydrocarbon source rocks were generated. In contrast, the mid-Cenozoic (~55 My to present) was characterized by cooler surface temperatures, polar ice sheets, lower atmospheric CO2, and the proliferation of mammals. We will evaluate a number of hypotheses for elevated CO2 during the Cretaceous. These include enhanced carbonate subduction and subsequent output of CO2 through arc volcanoes, enhanced oceanic crust production, and an increase in the frequency of episodic flood basalts. In particular, we will also explore a new hypothesis that CO2 inputs into the exogenic system are strongly influenced by secular changes in the nature of subduction zone volcanoes. During periods of enhanced continental arc activity, carbonate sediments stored on the continents over Earth?s history, are magmatically liberated, whereas during periods dominated by island arc activity the CO2 inputs return to baseline levels because of the smaller volumes of carbonates in the oceanic upper plate. The transition from Cretaceous greenhouse to mid-Cenozoic icehouse conditions may have coincided with a decline in the number of continental arc volcanoes, suggesting that there may also be a mechanism driving long term oscillations between the nature of subduction zones. The goal of this study is to evaluate the relative importance of all these potential sources of CO2 so that a more complete model of the whole Earth carbon cycle can be developed. We are specifically interested in how deep Earth dynamics modulates these sources of CO2. To test these hypotheses and place bounds on each of these processes, we have assembled an interdisciplinary team to quantify the stability of carbonates in the shallow crust and in the deep parts of subduction zones, map out how the distribution of arc volcanoes and the extent of magmatic decarbonation has changed through time, quantify global volcanic inputs of CO2, and develop a model for long-term climate evolution coupled to the cycling of C between the deep Earth and the exogenic system.
