Abstract

Small catchments are an ideal tool for deciphering the geochemical signature of continental rock chemical weathering processes. Distinguishing between the effect of biological mineralogical and hydrological processes is still a challenge but recent advancements in non-traditional isotopes have proven useful in this endeavor. Among the different isotopic systems of the weathering toolbox, boron isotopic ratios provide an interesting perspective since boron isotopes in the Critical Zone are fractionated both by biological processes (boron is an essential nutrient to the vegetation) and by re-incorporation in secondary minerals.

We analyzed boron isotopes (δ¹¹B) on a variety of hydrologic, biologic, and geologic materials across the Susquehanna Shale Hills Critical Zone Observatory. Boron isotopic ratios were obtained by MC-ICP-MS Neptune following the Louvat et al. (2010) procedure.  Preliminary results indicate that δ¹¹B of the stream water at Shale Hills is slightly lighter at the outlet of the basin (15.28‰ compared to the upper stream values of 16.43‰ and 17.35‰) and is enriched by as much as 20 ‰ when compared to the stream sediments. The groundwater display a contrasting behavior between the upper (15.62‰) and the lower part (5.67‰) of the catchment. Assuming that boron isotopes are not affected by silicate rocks dissolution, the ¹¹B stream water enrichment can be attributed either to water/rock interaction (with the input of isotopically lighter groundwater); ¹⁰B being preferentially incorporated into secondary phases, or to the contribution of the vegetation, whose isotopic composition is highly enriched in ¹¹B (values as high as 43‰ have been reported in throughfalls by Cividini et al. (2010) in a granitic critical observatory of the Strengbach basin, France).  These clearly distinct isotopic compositions allow us to constrain the processes that control the stream water composition.

Louvat et al. (2010) Geostand and Geoanal Res 35, 75-88
Cividini et al. (2010) Geochim et Cosmochim Acta 74, 3143-3163

Citation

Noireaux, J., Sullivan, P.L., Louvat, P., Gaillardet, J., Brantley, S.L. (2013): Boron isotopes at the Shale Hills Critical Zone Observatory. Abstract H51B-1190 presented at 2013 Fall Meeting, AGU, San Francisco, CA, 9-13 Dec..

This Paper/Book acknowledges NSF CZO grant support.