Neogene cooling driven by land surface reactivity rather than increased weathering fluxes.

The long-term cooling, decline in the partial pressure of carbon dioxide, and the establishment of permanent polar ice sheets during the Neogene period have frequently been attributed to increased uplift and erosion of mountains and consequent increases in silicate weathering, which removes atmospheric carbon dioxide. However, geological records of erosion rates are potentially subject to averaging biases, and the magnitude of the increase in weathering fluxes-and even its existence-remain debated. Moreover, an increase in weathering scaled to the proposed erosional increase would have removed nearly all carbon from the atmosphere, which has led to suggestions of compensatory carbon fluxes in order to preserve mass balance in the carbon cycle. Alternatively, an increase in land surface reactivity-resulting from greater fresh-mineral surface area or an increase in the supply of reactive minerals-rather than an increase in the weathering flux, has been proposed to reconcile these disparate views. Here we use a parsimonious carbon cycle model that tracks two weathering-sensitive isotopic tracers (stable Li/Li and cosmogenic Be/Be) to show that an increase in land surface reactivity is necessary to simultaneously decrease atmospheric carbon dioxide, increase seawater Li/Li and retain constant seawater Be/Be over the past 16 million years. We find that the global silicate weathering flux remained constant, even as the global silicate weathering intensity-the fraction of the total denudation flux that is derived from silicate weathering-decreased, sustained by an increase in erosion. Long-term cooling during the Neogene thus reflects a change in the partitioning of denudation into weathering and erosion. Variable partitioning of denudation and consequent changes in silicate weathering intensity reconcile marine isotope and erosion records with the need to maintain mass balance in the carbon cycle and without requiring increases in the silicate weathering flux.