Increased land surface reactivity as the driver of Neogene cooling

Long-term cooling, pCO₂ decline, and the establishment of permanent, polar ice sheets in the Neogenehas frequently been attributed to increased uplift and erosion of mountains and consequent increases in silicate weathering, which removes atmospheric CO₂. However, an increasing weathering flux is incompatible with a balanced atmospheric CO₂budget [1]. For example, a weathering increase scaled to frequently invoked erosional increase [2] would have removed nearly all carbon from the atmosphere. Further, the marine ¹⁰Be/⁹Be proxy indicates constant silicate weathering fluxes over the past 10 Ma [3].

Rather, as volcanic CO₂ emissions have been largely constant yet atmospheric CO₂ decreased, as indicated by the marine ¹¹B/¹⁰B proxy, an increase in “land surface reactivity” has likely driven global cooling [4]. Land surface reactivity quantifies the likelihood of weathering zone material to react with carbon derived from atmospheric CO₂ and represents the degree of coupling between weathering and climate. That surface reactivity has increased during the Neogene is confirmed by the stable ⁷Li/⁶Li seawater proxy, which increases during the Neogene. The question we now need to address is thus: what has caused the increase in land surface reactivity? What is needed is an increased availability of Ca and Mg-rich primary minerals in the global critical zone. This could have come about by 1) an increased exposure of mafic volcanic rock; 2) supply of fresh glacial debris; 3) widespread rejuvenation of the continental land surface by faulting; 4) more efficient mineral dissolution by biota; or 5) an increase in erosion rate with or without mountain uplift. Only explanation 1) can be discounted as this hypothesis fails to satisfy the marine Sr and Os radiogenic isotope records. Explanations 2 – 5 remain. In all of these the role of erosion is to remove weathered material. Indeed, parsimonious geochemical models are roughly compatible with a doubling in global erosional mass flux since 10 Ma [1].

(1) Caves Rugenstein, J.K., D.E. Ibarra, and F. von Blanckenburg, Neogene cooling driven by land surface reactivity rather than increased weathering fluxes. Nature, 2019.

(2) Molnar, P., Late Cenozoic increase in accumulation rates of terrestrial sediment: how might climate change have affected erosion rates? Ann. Rev. Earth Planet. Sc., 2004.

(3) Willenbring, J.K. and F. von Blanckenburg, Long-term stability of global erosion rates and weathering during late-Cenozoic cooling. Nature, 2010.

(4) Kump, L.R. and M.A. Arthur, Global chemical erosion during the Cenozoic: Weatherability balances the budgets, in Tectonic Uplift and Climate Change. 1997.