Aging of basalt volcanic systems and decreasing COsub2/sub consumption by weathering

摘要

Basalt weathering is one of many relevant processes balancing the global carbon cycle via land–ocean alkalinity fluxes. The COsub2/sub consumption by weathering can be calculated using alkalinity and is often scaled with runoff and/or temperature. Here, it is tested if the surface age distribution of a volcanic system derived by geological maps is a useful proxy for changes in alkalinity production with time. A linear relationship between temperature normalized alkalinity fluxes and the Holocene area fraction of a volcanic field was identified using information from 33 basalt volcanic fields, with an rsup2/sup=0.93 . This relationship is interpreted as an aging function and suggests that fluxes from Holocene areas are ～10 times higher than those from old inactive volcanic fields. However, the cause for the decrease with time is probably a combination of effects, including a decrease in alkalinity production from material in the shallow critical zone as well as a decline in hydrothermal activity and magmatic COsub2/sub contribution. The addition of fresh reactive material on top of the critical zone has an effect in young active volcanic settings which should be accounted for,?too. A comparison with global models suggests that global alkalinity fluxes considering Holocene basalt areas are ～60 % higher than the average from these models imply. The contribution of Holocene areas to the global basalt alkalinity fluxes is today however only ～5 %, because identified, mapped Holocene basalt areas cover only ～1 % of the existing basalt areas. The large trap basalt proportion on the global basalt areas today reduces the relevance of the aging effect. However, the aging effect might be a relevant process during periods of globally intensive volcanic activity, which remains to be tested.