Paleo-CO2 variation trends and the Cretaceous greenhouse climate (original) (raw)

Introduction

As one of the primary greenhouse gases, atmospheric CO2 affects the surface temperature of the Earth, and is thought to be a major factor in global warming (Boucot and Gray, 2001, Royer, 2006, Fletcher et al., 2008). During the geological history of the last 450

Ma, global temperatures have co-varied with atmospheric CO2 and it is considered the primary driver of Phanerozoic climate (Royer et al., 2004, Royer, 2008, Royer, 2010). Understanding paleo-CO2 variations during the Earth's ancient greenhouse episodes are essential for predicting the response of climate to future elevated atmospheric CO2 levels (Breecker et al., 2010).

The Cretaceous (145–65

Ma) represents one of the best examples of greenhouse climates in Earth history. As the greenhouse climate reached its summit in the mid-Cretaceous, the Earth was characterized by equably distributed warmth with mean annual polar temperatures exceeding 14

°C (Tarduno et al., 1998). There were no permanent polar ice sheets (Frakes et al., 1992), and sea levels were 100–200

m higher than those of today (Haq et al., 1987). During this time atmospheric CO2 levels are estimated to have been 4 to 10 times higher than those prior to the Industrial Revolution (Cojan et al., 2000, Berner and Kothavala, 2001, Bice and Norris, 2002, Huber et al., 2002) and the Cretaceous Oceanic Red Bed (CORBS) are globally distributed (Wang and Hu, 2005, Wang et al., 2011).

More precise quantitative reconstructions of atmospheric CO2 levels using terrestrial and marine records are critical for a better understanding of the “greenhouse” conditions of the Cretaceous, but proxy data for several of the stages of this period remain quite limited. In the past decade, an increasing number of paleo-CO2 data have been reported for the Cretaceous. In this paper, we summarize the major approaches for paleo-CO2 reconstructions for the Cretaceous, and review the progresses of paleo-CO2 data collection from various parts of the world, covering different episodes during the Cretaceous. Using our comprehensive data compilation, we analyze the variations in atmospheric CO2 during the Cretaceous, concentrating on critical intervals representing geological and biological events.

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Major paleo-CO2 reconstruction methods

For deep time, paleo-CO2 can be inferred from either proxies or by the modeling of the long-term carbon cycle. Berner (1994) established the first model (GEOCARB) specifically designed to describe the evolution of atmospheric CO2 levels during the entire Phanerozoic. Since then geochemical models have been refined (Ekart et al., 1999, Tajika, 1999, Berner and Kothavala, 2001, Wallmann, 2001, Hansen and Wallmann, 2003). During the last two decades a variety of proxies have been developed to

Paleo-CO2 variations in the Cretaceous

Estimates of Cretaceous atmospheric paleo-CO2 levels have been published for a variety of localities in North and South America, Europe, and Asia, based on either stomata or isotope analysis. In addition, several geochemical models also estimate the trend of Cretaceous CO2 levels (e.g., Tajika, 1999, Wallmann, 2001, Berner and Kothavala, 2001, Berner, 2006). The data cover the geological intervals from Berriasian–Valanginian and Hauterivian–Albian of the Early Cretaceous, and

Cretaceous paleo-CO2 and major geological events

The Cretaceous represents one of the most remarkable periods of geologic history. It was not only a long greenhouse episode, but several unusual geological phenomena occurred: the Oceanic Anoxic Events (OAEs) (Schlanger and Jenkyns, 1976, Wang et al., 2005) and the K–T boundary catastrophe (KTB) (Alvarez et al., 1980). These triggered rapid environmental and climate changes and had profound impacts on the biosphere (McElwain et al., 2005, Davis et al., 2009, Royer et al., 2007). They have also

Paleo-CO2 and the variability of Cretaceous greenhouse climates

The Cretaceous represents a classic greenhouse climate period in Earth history. During this time, the warm climate was equably distributed around the world. The polar regions were much warmer than today. There were no polar ice sheets. Instead, thermophilic floras and faunas spread to high latitudes. The latitudinal temperature gradient was much lower than today. This general picture of a Cretaceous climate is depicted by Frakes et al. (1992) and Francis and Frakes (1993). However, the trends

Summary and prospects

This paper synthesizes the CO2 variations throughout the Cretaceous to contribute to understanding its contribution to global warming in deep time. The results indicate that CO2 concentrations remained at a relatively high level throughout the Cretaceous, but at lower levels in the early Cretaceous, higher in the mid-Cretaceous and with a gentle decline during the late Cretaceous. These trends were punctuated at several intervals by rapid paleo-CO2 changes often associated with critical events,

Acknowledgments

This is one of the contributions for the “Cretaceous Oceanic Red Beds (CORBs)”collection. We thank Dr. Timothy Horscroft for the kind invitation and encouragement of the work. Prof. Chengshan Wang (China University of Geosciences, Beijing), Prof. Xiumian Hu and Prof. Xianghui Li (Nanjing University) are acknowledged for their kind support and helpful comments on the manuscript. Prof. William W. Hay is appreciated for his helpful linguistic corrections and suggestions of the manuscript revision.