Abstract
Abstract. The small reservoir of carbon dioxide in the atmosphere (pCO2) that modulates climate through the greenhouse effect reflects a delicate balance between large fluxes of sources and sinks. The major long-term source of CO2 is global outgassing from sea-floor spreading, subduction, hotspot activity, and metamorphism; the ultimate sink is through weathering of continental silicates and deposition of carbonates. Most carbon cycle models are driven by changes in the source flux scaled to variable rates of ocean floor production, but ocean floor production may not be distinguishable from being steady since 180 Ma. We evaluate potential changes in sources and sinks of CO2 for the past 120 Ma in a paleogeographic context. Our new calculations show that decarbonation of pelagic sediments by Tethyan subduction contributed only modestly to generally high pCO2 levels from the Late Cretaceous until the early Eocene, and thus shutdown of this CO2 source with the collision of India and Asia at the early Eocene climate optimum at around 50 Ma was inadequate to account for the large and prolonged decrease in pCO2 that eventually allowed the growth of significant Antarctic ice sheets by around 34 Ma. Instead, variation in area of continental basalt terranes in the equatorial humid belt (5° S–5° N) seems to be a dominant factor controlling how much CO2 is retained in the atmosphere via the silicate weathering feedback. The arrival of the highly weatherable Deccan Traps in the equatorial humid belt at around 50 Ma was decisive in initiating the long-term slide to lower atmospheric pCO2, which was pushed further down by the emplacement of the 30 Ma Ethiopian Traps near the equator and the southerly tectonic extrusion of SE Asia, an arc terrane that presently is estimated to account for 1/4 of CO2 consumption from all basaltic provinces that account for ~1/3 of the total CO2 consumption by continental silicate weathering (Dessert et al., 2003). A negative climate-feedback mechanism that (usually) inhibits the complete collapse of atmospheric pCO2 is the accelerating formation of thick cation-deficient soils that retard chemical weathering of the underlying bedrock. Nevertheless, equatorial climate seems to be relatively insensitive to pCO2 greenhouse forcing and thus with availability of some rejuvenating relief as in arc terranes or thick basaltic provinces, silicate weathering in this venue is not subject to a strong negative feedback, providing an avenue for ice ages. The safety valve that prevents excessive atmospheric pCO2 levels is the triggering of silicate weathering of continental areas and basaltic provinces in the temperate humid belt. Excess organic carbon burial seems to have played a negligible role in atmospheric pCO2 over the Late Cretaceous and Cenozoic.
Highlights
Deep water temperatures determined from the continuous benthic oxygen isotope record (Cramer et al, 2009, 2011; Miller et al, 2005b; Zachos et al, 2001) (Fig. 1a) documentPublished by Copernicus Publications on behalf of the European Geosciences Union.D
It is noteworthy that SE Asia in the equatorial humid belt with a mean annual temperature of 25 ◦C and nearly 140 cm yr−1 runoff has by far the highest CO2 consumption flux (1.033 × 1012 mol yr−1 = 45.5 Mt CO2 yr−1), which constitutes about 25 % of the total CO2 consumption flux estimated for all basaltic provinces and small volcanic islands (4.08 × 1012 mol yr−1 = 180 Mt CO2 yr−1 for total land area of 7.249 Mkm2; Dessert et al, 2003), even though SE Asia represents less than 8 % of their combined surface area
Significant variations in potential sources of CO2 such as oceanic crust production rates and hydrothermal activity cannot be precluded even though they are notoriously difficult to calibrate, the drift-weathering model of varying CO2 sinks arising from the changing latitudinal distribution of land masses, and especially basaltic provinces and island arc terranes, provides a measurable and testable mechanism to account for long-term variations in atmospheric pCO2 levels over the Late Cretaceous and Cenozoic, and further back in Earth’s history
Summary
Deep water temperatures determined from the continuous benthic oxygen isotope record (Cramer et al, 2009, 2011; Miller et al, 2005b; Zachos et al, 2001) (Fig. 1a) document. The equable conditions at the CTM and especially the EECO are associated with warmer global (polar and tropical) sea surface temperatures (Pearson et al, 2001, 2007) that most likely resulted from an enhanced greenhouse effect due to higher atmospheric pCO2 concentrations as inferred from various proxies (Fig. 1c; see review with references in Beerling and Royer, 2011; Royer, 2010) These high pCO2 levels have been conventionally attributed to higher rates of ocean crust production and associated increased CO2 outgassing (Berner et al, 1983), for example, a presumed pulse of increased global sea-floor spreading and the emplacement of the North Atlantic igneous province at the EECO (Miller et al, 2005a; Rea et al, 1990; Thomas and Bralower, 2005; Zachos et al, 2001).
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