Isotope mass-balance constraints preclude that mafic weathering drove Neogene cooling

Park et al. (1) use a global biogeochemical model (GEOCLIM) to suggest that weathering associated with emergence of mafic islands in Southeast Asia resulted in a ∼350-ppm decrease in atmospheric CO2 during the last 15 Ma. However, only matching p CO2 yields nonunique solutions that cannot be distinguished from previous hypotheses, such as increasing erosion and ensuing increased land-surface reactivity (2, 3), even when using the same model (4). Critically, based on only sparse data, Park et al. (1) impose high mafic weathering fluxes from Southeast Asia, but these fluxes fail to satisfy fundamental mass-balance constraints from global weathering proxies. In particular, both the direction and magnitude of change in marine 87Sr/86Sr and 187Os/188Os (5, 6) since 15 Ma cannot be reconciled with an increase in mafic weathering. Both strontium and osmium isotopes are sensitive to the lithologies being … [↵][1]1To whom correspondence may be addressed. Email: jeremy.rugenstein{at}colostate.edu. [1]: #xref-corresp-1-1