Cosmogenic 10Be/9Be in speleothem as a proxy for paleo-weathering rate

Meteoric 10Be is introduced to the ground surface as fallout from cosmic-ray production in the atmosphere, while 9Be is a trace element released by mineral weathering.  The 10Be/9Be ratio in soil and sediment is therefore used to infer the rate of primary mineral weathering over the timescale of 10Be accumulation, typically 103-105 y.  Although beryllium is retentive in soils, a small fraction is released to groundwater.

In carbonate landscapes, groundwater beryllium accumulates in cave speleothem, and can be used to track changes in soil 10Be/9Be over time.  Independently dated speleothem offer a novel way to monitor weathering rate at a single site over glacial-interglacial timescales.  Here, we present a 10Be/9Be record from Soreq Cave, Israel, which we interpret as a weathering history of beryllium-bearing silicates (primarily feldspar) in the overlying terra rossa soil derived from loess.  Results show that silicate weathering rates vary 2-fold with temperature over the past ~175 ky, from a low of 2-3 t km-2 y-1 during cool glacials to a high of 5-6 t km-2 y-1‑during warm interglacials.  Considered as an Arrhenius relationship, the weathering rate indicates an activation energy of 97 ± 16 kJ/mol, similar to other field-based estimates for feldspar weathering.

Although the beryllium isotopic record in speleothem is likely to be complicated by factors such as changing vegetation and soil depth, our initial work shows promise as a proxy for local soil weathering rates through time, offering a new method for quantifying relationships between climate, soils, and landscape evolution.