Abstract

New meteoric ¹⁰Be data from 73 samples of bulk regolith collected along north- and south-facing hillslopes at the Susquehanna Shale Hills Critical Zone Observatory (SSHO) provide first-order constraints on the timescales of regolith formation. The SSHO is located in the presently temperate climate zone of central Pennsylvania; however, sustained periglacial climate during the time of maximal extent of the Laurentide ice sheet (~19-21 ka) and deforestation during mid-19th Century charcoal production may have exerted significant influence on regolith production. Here, we quantify soil residence times and corresponding rates of regolith production and erosion on the north- and south-facing slopes at SSHO, using meteoric ¹⁰Be in samples of regolith collected at 25 locations along each hillslope from ridge top to toe slope. Hillslopes within the SSHO are relatively planar, but exhibit a pronounced asymmetry; north-facing slopes are steeper (~20°) than south-facing slopes (~15°).

Meteoric ¹⁰Be concentrations decrease systematically with depth at all 6 profile sites. Meteoric ¹⁰Be inventories are similar at the north and south ridgetop sites (1.89 ± 0.55 at/cm² and 1.63 ± 0.41 at/cm², respectively) and generally increase with position downslope. Assuming that the delivery of meteoric ¹⁰Be to regolith is balanced by its removal via erosion, the total meteoric ¹⁰Be inventories at the north and south ridgetops are consistent with soil ¹⁰Be residence times of 10.5 ± 3 ky and 9.1 ± 2 ky, and with steady lowering rates of ~ 16 m/My and ~ 19 m/My, respectively. Increases in meteoric ¹⁰Be inventories downslope are consistent with relatively slow creep, with transport velocities of 0.45 cm/y and 0.38 cm/y for the north and south hillslopes, respectively. Comparison of our results with previously-published estimates of regolith production rates inferred from U-series disequilibrium reveals that estimates of steady-state erosion calculated using meteoric ¹⁰Be are considerably slower than regolith production
rates. We are currently exploring whether this result implies non-steady erosion in the SSHO, or whether it results from differences inherent in the different isotopic techniques. Overall, the meteoric ¹⁰Be results suggest that most of the regolith on hillslopes within the SSHO watershed formed during the Holocene since the time of periglacial influence. In turn, estimated erosion rates on the ridgetops may reflect average lowering since that time.

Citation

West, N., Kirby, E., Bierman, P.R., Rood, D.H. (2011): Constraints on regolith formation and erosion rates at the Susquehanna Shale Hills Critical Zone Observatory, PA, determined using meteoric 10Be. AGU Annual Fall Conference Proceedings.

This Paper/Book acknowledges NSF CZO grant support.