Transient features of the erosion of shales in the Mackenzie basin (Canada), evidences from boron isotopes

Abstract

In order to refine our knowledge of the mechanisms that drive the weathering reactions and the rate at which elements are exported out of a shale basin, we have analyzed the boron concentrations and isotopic compositions of 24 rivers contributing to the Mackenzie basin (Canada). Boron has been chosen to conduct this study because of the relatively high sensitivity of its isotopic system to water/rock interactions. After examination of the chemical composition of the Mackenzie River samples, boron appears to be regulated, in the largest part, by reactions involving silicate rocks. The other contributions of rainwaters and the dissolution of carbonate and evaporite rocks can locally control the B budget but they remain of second importance at the regional to continental scales. It also appears that dissolved B in rivers is regulated by input of groundwaters. The comparison between the isotopic composition of the B being exported by rivers (soluble and particulates) and of its bulk source indicates that it is behaving out of steady state in most of the Mackenzie tributaries and particularly in rivers draining the plain. A model of transport/reaction is proposed in this study, which demonstrates that B is controlled by rapid ion exchanges at the mineral/water interface and by long-term dissolution of shales. The responses of the model to tested conditions reveal that the B isotopic ratio of river waters is highly sensitive to hydrological conditions and to changes of the weathering rate of shales, of its partition coefficient with adsorbing surface, and of the water velocity. The departure of the B geochemical cycle from a steady state is interpreted as the record of changes in the weathering regime in the Mackenzie basin. The temporal evolution of the B cycle modeled here implies an increase of the weathering rate of shales in the Mackenzie basin and/or an increase of the partition coefficient between soluble B and adsorbing surfaces. It also appears that these changes are consistent with timescales of the recent deglaciation of the basin. This study demonstrates that the present B fluxes are actually responding to past changes of the water/rock interactions and that the weathering history is a key parameter to interpret the modern geochemical fluxes. This conclusion is valid for any other element that has similar chemical properties with solid surfaces.