Testing the effect of reduced weathering as a forcing for Middle Eocene warming: a cGENIE modelling study

Abstract

The Middle Eocene Climatic Optimum (MECO) was a ~500 kyr period of ocean warming and higher pCO2 levels, superimposed on a trend of Eocene cooling and characterized by widespread calcium carbonate (CaCO3) depletion in deep sea sediments. Carbonate dissolution over such timescales during greenhouse warming is remarkable, since the classical carbon cycle theory predicts that global warming enhances continental weathering, transporting more alkaline solutes to the ocean so that CaCO3 preservation is restored again after a few thousand years, i.e. carbonate compensation. In this study I challenge the traditional carbon cycle theory by reversing the traditional weathering feedback and test whether reduced weathering could have been responsible for MECO warming. With an Earth System Model of intermediate complexity, cGENIE, I ran simulations in which weathering acts as a forcing, rather than as a feedback and investigate if warming and long-term CaCO3 dissolution can concur through this reversed feedback. Qualitatively, modelled output is in line with proxy data. A 20% reduction in weathering rates (inferred from 187Os/188Os isotope measurements), yields an increase in pCO2, ocean temperatures and CaCO3 dissolution in deep sea sediments. Hence, this study demonstrates that Middle Eocene warming and CaCO3 dissolution could have been initiated and maintained by diminution of alkalinity fluxes to the surface ocean, for example through reduced (silicate) weathering. Sensitivity experiments with cGENIE suggest that atmospheric CO2 concentrations and changes in ocean water pH alone are less determinative for CaCO3 preservation than the balance between alkalinity input and output on intermediate timescales such as the MECO.