WEATHERING, PLANTS, AND THE LONG-TERM CARBON-CYCLE

The chemical weathering of Ca-Mg silicate rocks is the principal process whereby CO2 is removed from the atmosphere on long or multimillion-year time scales. At present vascular land plants exert an important influence on weathering by recycling water and by bringing about the secretion of acids and the buildup of high levels of CO2 in soils. Before the spread of vascular plants on the continents during the mid-Paleozoic, chemical weathering must have been achieved by higher levels of atmospheric CO2 and/or by the intercession of primitive land biota such as lichens and cyanobacteria. From examination of several pre-vascular weathering scenarios, it is concluded that in order to prohibit unreasonably high levels of atmospheric CO2, weathering during the Precambrian and early Paleozoic must have taken place under essentially closed-system abiotic conditions (linear feedback) or via strong regulation by primitive biota. Preliminary examination of modern weathering by lichens over tens to thousands of years militates against the latter possibility. However, much more work needs to be done on the quantitative effect of both primitive biota and vascular plants on the rate of weathering (and their response to changes in atmospheric CO2) before anything more definitive can be said.