Biogenic feedbacks in the carbonate-silicate geochemical cycle and the global climate

Abstract

The carbonate-silicate geochemical cycle is believed to act as a long-term global thermostat by controlling the atmospheric CO2 concentration. We investigated the role of the biota in this system using a dynamic simulation model. The model (a modification of the BLAG'83 model) describes five geochemical reservoirs: atmospheric CO2, lithospheric CaSiO3 (silicate) and CaCO3 (carbonate), oceanic Ca2+, and HCO-3 (whereby Mg2+ was treated as if it were Ca2+). The fluxes between the reservoirs are due to CaSiO3- and CaCO3-weathering, CaCO3-precipitation, and metamorphic-magmatic decarbonation of CaCO3. We modeled the role of the biota as ideal optimum responses of the rates of weathering and CaCO3-precipitation to temperature change. These responses are superimposed on a physico-chemical temperature response. The temperature was calculated from a 0-dimensional radiation balance climate model, which takes the greenhouse gases CO2 and H2O into account. It is shown that, depending on the parameter values, the introduction of optimum responses can either increase or decrease the stability of the simulated global climate with respect to changes in solar luminosity. We achieved enhanced stability by introducing a biological response of weathering that had an optimum temperature higher than the steady-state temperature at the start of a simulation, whereas biological CaCO3-precipitation led to reduced stability. An increase in solar luminosity was offset by a decrease in CO2 concentration. However, above a critical level of solar luminosity, certain values for the optimum curve parameters led to a stable pattern of oscillations. Under near-critical conditions, the model showed frequency-dependent amplification of periodic perturbations in solar luminosity (Milankovitch-type forcing). We speculate that biogenic feedbacks in the carbonate-silicate geochemical cycle played a key role in the major glacial-interglacial cycles of the Pleistocene.