Quantifying the magnitude and timing of Holocenesoil erosion events on parent materials of knownage using in-situ cosmogenic C-14 and Be-10depth-profiles

QE Geology

Fülöp, Réka-Hajnalka ; Bishop, Paul

University:University of Glasgow

Department:School of Geographical and Earth Sciences

关键词: soil erosion, cosmogenic nuclide depth-profiles, cosmogenic in situ C-14, carbon-14, beryllium-10;   

Others  :  http://theses.gla.ac.uk/3358/1/2012F%C3%BCl%C3%B6pphd.pdf

来源: University of Glasgow

【 摘 要 】

Conventional methods for the determination of past soil erosion provide only average rates oferosion of the sediment's source areas and are unable to determine the rate of at-a-site soil loss.This study addresses this issue by exploring the extent to which in situ cosmogenic Be-10 and C-14depth-profiles can be used to quantify the magnitude and timing of site-specific soil erosion eventson soils of known age. The study focuses on two sites located on end moraines of the Loch LomondReadvance (LLR) in Scotland: Wester Cameron and Inchie Farm, both near Glasgow. The LLR iswell documented and several LLR moraine radiocarbon ages exist in the literature allowing for theplacement of a first order age constraint on soil/till emplacement. In addition, the site at WesterCameron is in the proximity of Croftamie, a well-studied LLR type-locality. The site near WesterCameron does not show any visible signs of soil disturbance and so this has been selected in orderto test (1) whether a cosmogenic nuclide depth profile in a sediment body of Holocene age can bereconstructed, and (2) whether in situ Be-10, C-14 and Al-26 yield concordant results. Field evidencesuggests that the site at Inchie Farm has undergone soil erosion and so this was selected so as toexplore whether the technique can also be applied to determine the broad timing of soil loss.The results of the cosmogenic Be-10, C-14 and Al-26 analyses in the Wester Cameron site samplesconfirm that the cosmogenic nuclide depth-profile to be expected from a sediment body of Holoceneage can be reconstructed. Moreover, the agreement between the total cosmogenic Be-10 inventoriesin the erratics and the Wester Cameron soil/till samples indicate that there has been no erosion atthe sample site since the deposition of the till/moraine. Further, the Wester Cameron depth profilesshow minimal signs of homogenisation, as a result of bioturbation, and minimal cosmogenic nuclideinheritance from previous exposure periods. The results of the cosmogenic Be-10 and C-14 analysesin the Inchie Farm site samples show a clear departure from the zero-erosion cosmogenic nuclidedepth profiles suggesting that the soil/till at this site has undergone erosion since its stabilisation.The LLR moraine at the Inchie Farm site is characterised by the presence of a sharp break inslope, suggesting that the missing soil material was removed instantaneously by an erosion eventrather than slowly by continuous erosion. The results of a Monte-Carlo type analysis carried outto constrain the magnitude and timing of this erosion event suggest that the event was relativelyrecent and relatively shallow, resulting in the removal of ca. 20-50 cm of soil less than 1500 yearsBP.The results of sensitivity analyses show that the predicted magnitude and timing of the InchieFarm erosion event are highly sensitive to the assumptions that are made about the backgroundrate of continuous soil erosion at the site and also about the stabilisation age of the till. Theresults further indicate that the density of the sedimentary deposit will also affect the magnitudeand timing of the predicted erosion event. All three parameters can be independently determineda priori and so despite the method presented in this study being sensitive to variations in theseparameters, they do not impede future applications of the method to other localities. The resultsof the sensitivity analyses further show that the predicted erosion event magnitude and timing isvery sensitive to the in situ cosmogenic C-14 production rate used and to the assumptions that aremade about the contribution of muons to the total production of this cosmogenic nuclide. Thus,advances in this regard need to be made for the method presented in this thesis to be applicablewith confidence to scenarios similar to the one presented here.