Tectonic Forcing of Climate and Some Mysteries of the Cenozoic Carbon Cycle

Abstract:

The collision of the Indian subcontinent with Asia in the early Cenozoic is widely believed to be the root cause of the transition from a “greenhouse” world to an “icehouse” world over the last 50 Myr. My colleagues and I proposed that this collision resulted in a globally significant increase in chemical weathering rates that led to a drawdown of atmospheric CO₂ and the eventual build-up of ice sheets at both poles. Geologic and field data provide support for this hypothesis, including: a) the timing of collision and cooling, b) the observation that chemical weathering rates increase with physical erosion and denudation, and c) proxy evidence for changes in ocean chemistry consistent with increased chemical weathering over Cenozoic. However, a significant problem continues to plague this seemingly straightforward interpretation--namely, where does the CO₂ needed to support the inferred increase in chemical weathering over the Cenozoic come from? The ocean-atmosphere reservoir of carbon is so small that it would be depleted after million years or so. A negative feedback within the carbon cycle is needed to prevent atmospheric CO₂ levels being drawn down to snowball Earth levels. Classically, a surface temperature-weathering feedback, first described by Walker in 1981, is invoked--but, if chemical weathering has been higher over the last 40-50 Myr relative to the previous period then this feedback could only work if there has been a similar increase in mantle CO₂ degassing rates uncoupled to seafloor spreading rates (which are currently believed to have remained constant). Alternatively, possible negative-feedbacks within the carbon cycle may exist in the organic carbon cycle, in the seafloor basalt weathering/reverse weathering realm, and/or possibly be disguised by a large changes in the amount of terrestrial silicate weathering happening in regions without a continental signature (e.g., island arcs). Ultimately, the carbosphere, which extends from the mantle to the atmosphere and upon which Earth’s climate and life depends, is influenced by numerous interconnected geologic processes we are only just beginning to understand.