Boron isotopes at the Shale Hills Critical Zone Observatory

Abstract:

The Shale Hills Critical Zone Observatory is a Northern Appalachian catchment site where a series of geochemical tracers have been applied in order to build a multi-isotope integrative model (referred to as “CZ-tope”). The catchment is small (8ha) and relief is about 30 m. It receives about 107 cm of precipitation per year. Mean annual temperature is 10°C. Shales Hills observatory has a relatively simple lithology consisting of organic-poor shales rich in illite and relatively infrequent interbedded carbonates and sandstones. Vegetation consists mainly of deciduous trees. Soil thickness ranges from 0.3 m at the ridgetop to 3 m in the valley floor.

Following the CZ-tope concept, boron isotopes were analysed in the main geochemical reservoirs of the SH catchment (stream, vegetation, soil pore waters, solid phases, groundwaters). Measurements were conducted using MC-ICPMS and a direct injection system after a chemical procedure aiming at isolating boron from geological matrix. Results are expressed as δ¹¹B. Error bars are better than 0.5‰

Boron isotopes in Shale Hills catchment show a large range of variation. While bedrock values are within a narrow range around -5‰, stream waters range between 10‰ and 15‰, and exhibit temporal variations. This very strong ¹¹B enrichment is also observed in the vegetation, groundwater and rainwater reservoirs but with a much larger range of variation. The input of ¹¹B-enriched water by precipitation is contributing to the B budget at the catchment outlet but cannot explain all the ¹¹B enrichment with respect to parent bedrock. The solid phases collected along two different soil profiles and as suspended sediments in the stream are close to the bedrock value or slightly ¹⁰B-enriched. The most important conclusion from boron isotope investigation at Shale Hills CZO is that a simple mass budget is not able to reconcile the strong ¹¹B-enrichment measured in the water phases and vegetation with the isotopic signature of the bedrock, soil and sediments. Boron isotopes, like previously inferred for other isotopic systems at the CZO, are therefore consistent with a missing reservoir, such as fine particles that have not been sampled and analysed or a non-steady state behavior of the critical zone at Shale Hills for boron.