<p>Long-term cooling, pCO<sub>2</sub> decline, and the establishment of permanent, polar ice sheets in the Neogene<sup></sup>has frequently been attributed to increased uplift and erosion of mountains and consequent increases in silicate weathering, which removes atmospheric CO<sub>2</sub>. However, an increasing weathering flux is incompatible with a balanced atmospheric CO<sub>2</sub>budget [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 <sup>10</sup>Be/<sup>9</sup>Be proxy indicates constant silicate weathering fluxes over the past 10 Ma [3].</p><p>Rather, as volcanic CO<sub>2</sub> emissions have been largely constant yet atmospheric CO<sub>2</sub> decreased, as indicated by the marine <sup>11</sup>B/<sup>10</sup>B proxy, an increase in &#8220;land surface reactivity&#8221; has likely driven global cooling [4]. Land surface reactivity quantifies the likelihood of weathering zone material to react with carbon derived from atmospheric CO<sub>2</sub> and represents the degree of coupling between weathering and climate. That surface reactivity has increased during the Neogene is confirmed by the stable <sup>7</sup>Li/<sup>6</sup>Li 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 &#8211; 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].</p><p>(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.</p><p>(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.</p><p>(3) Willenbring, J.K. and F. von Blanckenburg, Long-term stability of global erosion rates and weathering during late-Cenozoic cooling. Nature, 2010.</p><p>(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.</p>
Weak impact of microorganisms on Ca, Mg-bearing silicate weathering
