Carbon cycle implications of terrestrial weathering changes since the last glacial maximum

Abstract

We examine the importance of the rock weathering feedback mechanism during the last deglacial period (∼16 000-4000 BCE) using an Earth system model of intermediate complexity (the University of Victoria Earth System Climate Model (UVic ESCM)) with four box-model parameterizations of terrestrial weathering. The deglacial climate change is driven by changes in orbital parameters, ice core reconstructions of atmospheric CO 2 variability, and prescribed removal of continental ice sheets. Over the course of the 12 000 year simulation period, increases in weathering provide a mechanism that slowly removes CO 2 from the atmosphere, in opposition to the observed atmospheric CO 2 increase during this period. These processes transfer both carbon and alkalinity to the ocean, the combination of which results in as much as a 1000 Pg C increase in total ocean carbon, relative to a control simulation with constant weathering. However, the rapid expansion of northern hemisphere vegetation introduces a significant uncertainty among the weathering parameterizations. Further experiments to test the individual impacts of weathering dissolved inorganic carbon and alkalinity fluxes on ocean biogeochemistry suggest that the worldwide distribution of rock types and the ratio of carbonate to silicate weathering may be crucially important in obtaining an accurate estimate of changes in global weathering rates.

Oceanic CO<sub>2</sub> outgassing and biological production hotspots induced by pre-industrial river loads of nutrients and carbon in a global modelling approach

“…For one, this could help tackle questions of the past, since weathering rates are dependent on climate variables (Berner, 1991). For instance, strong differences in weathering for the last glacial maximum have been suggested (Brault et al, 2017). The impacts of future climate change on weathering rates and landsea fluxes could be addressed, as Gislason et al (2009) and Beaulieu et al (2012) suggest major changes in weathering due to changing climatic conditions on a decadal timescale.…”

Author response to Reviewer Comments #1

“…the atmosphere by silicate weathering, to reflect the long-term offset of volcanic emissions by silicate weathering (Walker and Kasting, 1992;Archer et al, 1998, Toggweiler, 2008Zeebe, 2012, Colbourn et al, 2013Brault et al, 2017).…”

“…As described in Walker and Kasting (1992), Toggweiler (2008), Zeebe (2012) Brault et al (2017, Colbourn et al (2013Colbourn et al ( , 2015 and Lord et al (2016), in steady state the silicate weathering flux feedback for CO2 matches the volcanic CO2 emissions, which we have set in SCP-M. Note, for anthropogenic scenarios we separate volcanic emissions from weathering flux, because the silicate weathering feedback under the forcing of atmospheric CO2, is expected to increase at a greater rate than volcanic emissions (volcanic emissions do not respond to anthropogenic emissions of CO2).…”

“…This flux subtracted from the atmosphere negates the effects on atmospheric CO2 of the units of C added to the ocean by the silicate weathering flux of C. The effect of the more alkaline ocean (alk:C is 2:1 in the silicate weathering flux) is to draw down the volcanic emissions of CO2. Volcanic CO2 emissions are set equal to the amount of CO2 taken directly from the atmosphere by silicate weathering, to reflect the long-term offset of volcanic emissions by silicate weathering (Walker and Kasting, 1992;Archer et al, 1998, Toggweiler, 2008Zeebe, 2012, Brault et al, 2017. In Walker and Kasting, 1992;Toggweiler, 2008;Zeebe, 2012;Brault et al, 2007, volcanic emissions are also set to the silicate weathering drawdown of CO2.…”