Combining bulk sediment OSL and meteoric¹⁰Be fingerprinting techniques to identify gully initiation sites and erosion depths

Abstract

Deep erosional gullies dissect landscapes around the world. Existing erosion models focus on predicting where gullies might begin to erode, but identifying where existing gullies were initiated and under what conditions is difficult, especially when historical records are unavailable. Here we outline a new approach for fingerprinting alluvium and tracing it back to its source by combining bulk sediment optically stimulated luminescence (bulk OSL) and meteoric 10Be (10Bem) measurements made on gully‐derived alluvium samples. In doing so, we identify where gully erosion was initiated and infer the conditions under which such erosion occurred. As both 10Bem and bulk OSL data have distinctive depth profiles in different uneroded and depositional settings, we are able to identify the likely incision depths in potential alluvium source areas. We demonstrate our technique at Birchams Creek in the southeastern Australian Tablelands—a well‐studied and recent example of gully incision that exemplifies a regional landscape transition from unchanneled swampy meadow wetlands to gully incision and subsequent wetland burial by post‐European settlement alluvium. We find that such historic alluvium was derived from a shallow erosion of valley fill upstream of former swampy meadows and was deposited down the center of the valley. Incision likely followed catchment deforestation and the introduction of livestock, which overgrazed and congregated in valley bottoms in the early 20th century during a period of drought. As a result, severe gully erosion was likely initiated in localized, compacted, and oversteepened reaches of the valley bottom.

Catchment-wide weathering and erosion rates of mafic, ultramafic, and granitic rock from cosmogenic meteoric 10Be/9Be ratios

“…Its concentration in soil and river particles has been used to e.g. date river terraces (Pavich et al, 1986) and glacial deposits (Ebert et al, 2012;Egli et al, 2010) by using the "inventory method", evaluate the steady state between atmospheric 10 Be delivery and its export in river sediment You et al, 1988), riverine sediment mixing and source allocation (Belmont et al, 2014;Neilson et al, 2017), quantification of soil movement along slopes (Jungers et al, 2009;Mckean et al, 1993;West et al, 2013), general soil and weathering profile characterization (Graly et al, 2010), gully erosion (Portenga et al, 2017;Reusser and Bierman, 2010), or episodic erosion events from lake records (Valette-Silver et al, 1986). The calculation of erosion rates from river sediment, as done routinely using the in situ 10 Be variety in quartz, is rarely possible as meteoric 10 Be concentrations are highly grain-size dependent and are thus biased by hydrodynamic sorting (Wittmann et al, 2015).…”

Erosion rates and sediment flux within the Potomac River basin quantified over millennial timescales using beryllium isotopes

“…Since the 1980s, the use of isotopic tracers, specifically the cosmogenic isotope 10 Be, has greatly increased our knowledge of the rate at which Earth surface processes operate (e.g., Graly et al, 2010;Harel et al, 2016;Portenga and Bierman, 2011;Willenbring and von Blanckenburg, 2010). Both the 10 Be produced by cosmic-ray interactions in the atmosphere (meteoric, 10 Bem) and that created by interactions in mineral grains at Earth's surface (in situ, 10 Bei) have been measured and used to infer process rates and trace sediment across the landscape (e.g., Brown et al, 1998;Helz and Valette-Silver, 1992;Kirchner et al, 2001;Ouimet et al, 2009;Portenga et al, 2017;Reusser and Bierman, 2010;Schaller et al, 2001;You et al, 1988). Recently, the stable isotope of beryllium, 9 Be, has been measured in soil and sediment, and its abundance has been used to normalize measured activities of 10 Bem produced in the atmosphere and incorporated in soil and sediment grain coatings (Dannhaus et al, 2018;Rahaman et al, 2017;von Blanckenburg et al, 2012;Wittmann et al, 2015).…”

Luminescence as a Sediment Tracer and Provenance Tool

“…Their meteoric 10 Be measurements provided further constraints on the likely motions of sediment. Following this correlation, Portenga et al () inferred that erosion must have occurred to certain scour depths to access the sediment with high bulk luminescence. Further research involving coupled fingerprinting/provenance methods promises to be fruitful as the different mechanics of additional methods can add nuance to interpretations.…”

“…Provenance interpretations based on portable luminescence reader data can be strengthened when coupled with additional sediment fingerprinting/provenance methods such as meteoric 10 Be or 137 Cs (Muñoz‐Salinas & Castillo, ). Portenga et al () presented a new method for using coupled bulk OSL and meteoric 10 Be to fingerprint the sources of sediment produced by gully erosion. Gullies are small erosional features that can arise from changes in runoff following changes in vegetation cover (Rengers et al, ).…”