Tracing hillslope sediment production and transport with in situ and meteoric ¹⁰Be

Abstract

We use in situ‐produced and meteoric 10Be, analyzed in soils from 28 pits on four hillcrest‐parallel transects along a 14° hillslope in the Great Smoky Mountains, North Carolina, as tracers of soil production and transport. We rely upon amalgamation both to investigate and smooth spatial variability in 10Be concentrations. Lidar indicates that the hillslope is topographically complex and that soil is moved downslope diffusively until it encounters the ephemeral channel network and is rapidly exported. In situ‐produced 10Be, measured in depth profiles, indicates that over millennial timescales, soils are mixed above the soil‐saprolite boundary. In contrast, meteoric 10Be concentrations increase with depth and are correlated to concurrent increases of dithionite‐extractable Al and pH, observations explained by similar Al and Be mobility in the soil. The concentrations of both meteoric and in situ‐produced 10Be increase downslope proportional to the maximum soil particle path length. The data suggest virtual downslope soil velocities of 1.1–1.7 cm yr−1 in a well‐mixed active transport layer ∼60 cm thick. The thickness of this transport layer is constant downslope and depends on the rooting depth and consequent root wad thickness of downed trees on the slope, both of which reflect depth to the soil/saprolite boundary. Both meteoric and in situ‐produced 10Be suggest that soil production is balanced by surface denudation at rates between 10 and 13 m Myr−1. Soil residence times on the slope range from 21 to 33 kyr based on the meteoric 10Be inventories. Major element geochemical analysis suggests little if any elemental loss during soil transport downslope.

Meteoric 10Be in soil profiles – A global meta-analysis

“…Correlation between meteoric 10 Be concentration and oxyhydroxide abundance is expected because Be can be incorporated as a cation in oxyhydroxide complexes (Barg et al, 1997;Takahashi et al, 1999), and because other meteoric 10 Be phases (such as clays) may be illuviated similarly in podzolized soils (Birkeland, 1999). Al has a charge-ionic radius ratio similar to that of Be and has been suggested as a geochemical analog for meteoric 10 Be (Jungers et al, 2009). While Al d is significantly correlated to meteoric 10 Be in several profiles (Fig.…”

“…This meteoric 10 Be inventory (atoms/cm 2 ) is calculated: In press 1 Waipaoa, New Zealand 38°30 0 S 177°54 0 E Hillslope Decline a GS = grain size distribution, Cl = clay percent, 9 Be = 9 Be in leachable soil phases, Al = dithionite-extracted Al, Fe = dithionite-extracted Fe, OC = percent organic carbon, and CEC = cation exchange capacity. b Some soil properties data is reported elsewhere (Harden, 1987;Markewich et al, 1987;Pavich et al, 1989;Vidic, 1994;Nagatsuka and Maejima, 2001;Jungers et al, 2009). c Monaghan et al, 1983;Maejima et al, 2005;Jungers et al, 2009 have profiles under 100 cm deep the ultimate shapes of which are difficult to assess.…”

“…b Some soil properties data is reported elsewhere (Harden, 1987;Markewich et al, 1987;Pavich et al, 1989;Vidic, 1994;Nagatsuka and Maejima, 2001;Jungers et al, 2009). c Monaghan et al, 1983;Maejima et al, 2005;Jungers et al, 2009 have profiles under 100 cm deep the ultimate shapes of which are difficult to assess.…”

“…Meteoric 10 Be is also used to quantify erosion rates, trace hillslope processes (e.g. McKean et al, 1993), and to estimate rates of soil transport (Jungers et al, 2009). Deposition of meteoric 10 Be in ice (e.g.…”

Earth surface erosion and weathering from the 10Be (meteoric)/9Be ratio

“…Recent improvements in our understanding of the spatial and temporal patterns of delivery of the meteoric fallout nuclide (summarised in Willenbring and von Blanckenburg (2010b)) and overviews of its distribution in soils (Graly et al, 2010) are currently triggering new applications in erosional settings (e.g. Jungers et al, 2009). …”