Increased land surface reactivity as the driver of Neogene cooling
Authors: von Blanckenburg Friedhelm , Caves-Rugenstein Jeremy K. , Ibarra Daniel E. 
Published: March 2021

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