Hydrologic Regulation of Global Geochemical Cycles

Abstract:

Earth’s temperature is thought to be regulated by a negative feedback between atmospheric CO₂ levels and chemical weathering of silicate rocks. However, direct evidence for the operation of this feedback over million-year timescales is difficult to obtain. For example, weathering fluxes over the last 20 million years of the Cenozoic Era, calculated using marine isotopic proxies (i.e. ⁸⁷Sr/⁸⁶Sr, δ⁷Li, and ¹⁸⁷Os/¹⁸⁸Os), appear inconsistent with past atmospheric CO₂ levels and carbon mass balance. Similarly, observations from modern catchments suggest that chemical weathering fluxes are strongly correlated with erosion rates and only weakly correlated with temperature.

As an alternative approach to evaluating the operation of a negative feedback, we use the major surface reservoirs of carbon to determine the imbalance in the geologic carbon cycle and the required silicate weathering flux over the Cenozoic. A miniscule (0.5-1%) increase in silicate weathering is necessary to explain the long-term decline in CO₂ levels over the Cenozoic, providing evidence for a strong negative feedback between silicate weathering and climate. Rather than an appreciable increase in the silicate weathering flux, the long-term decrease in CO₂levels may be due to an increase in the strength of the silicate weathering feedback. To explain the observed variations in the strength of the weathering feedback during the Cenozoic, we present a model for silicate weathering where hydrologic processes regulate climatic and tectonic forcings due to the presence of a thermodynamic limit to weathering fluxes. Climate regulation by silicate weathering is thus strongest when global topography is elevated, similar to today, and lowest when global topography is more subdued, allowing planetary temperatures to vary depending on the global distribution of topography and mountain belts.

These results also motivate several key outstanding challenges in earth surface processes, including the need to understand the processes controlling marine isotopic weathering processes, the need to integrate solute and solid transport into Earth system models, and the need to understand the role of extreme physical and temporal heterogeneities in moderating geochemical fluxes.