Abstract
Abstract. The quantification of soil bulk density (ρB) is a cumbersome and time-consuming task when traditional soil density sampling techniques are applied. However, it can be important for terrestrial cosmogenic nuclide (TCN) production rate scaling when deriving ages or surface process rates from buried samples, in particular when short-lived TCNs such as in situ 14C are applied. Here, we show that soil density determinations can be made using structure-from-motion multi-view stereo (SfM-MVS) photogrammetry-based volume reconstructions of sampling pits. Accuracy and precision tests as found in the literature and as conducted in this study clearly indicate that photographs taken from both a consumer-grade digital single-lens mirrorless (DSLM) and a smartphone camera are of sufficient quality to produce accurate and precise modelling results, i.e. to regularly reproduce the “true” volume and/or density by >95 %. This finding holds also if a freeware-based computing workflow is applied. The technique has been used to measure ρB along three small-scale (<1 km) N–S transects located in the semi-arid to arid Altos de Talinay, northern central Chile (∼30.5∘ S, ∼71.7∘ W), during a TCN sampling campaign. Here, long-term differences in microclimatic conditions between south-facing and north-facing slopes (SFSs and NFSs, respectively) explain a sharp contrast in vegetation cover, slope gradient and general soil condition patterns. These contrasts are also reflected by the soil density data, generally coinciding with lower densities on SFSs. The largest differences between NFSs and SFSs are evident in the lower portion of the respective slopes, close to the thalwegs. In general, field-state soil bulk densities were found to vary by about 0.6 g cm−3 over a few tens of metres along the same slope. As such, the dataset that was mainly generated to derive more accurate TCN-based process rates and ages can be used to characterise the present-day condition of soils in the study area, which in turn can give insight into the long-term soil formation and prevailing environmental conditions. This implies that the method tested in this study may also being applied in other fields of research and work, such as soil science, agriculture or the construction sector.
Highlights
Soil density determination is a time-consuming task when sampling for a terrestrial cosmogenic nuclide (TCN)-based analysis, but it is an important parameter if process rates and/or ages are inferred from the final nuclide concentration (e.g. Rodés and Evans, 2020; Rodés et al, 2011)
Default processing settings had to be adjusted in the majority of cases to generate the surfaces. This predominantly applied to the point cloud densification step of the pits in Regard3D, which was mostly carried out using the multi-view environment (MVE) method (Fuhrmann et al, 2014) instead of the default patch-based and clustering multi-view stereo algorithms (CMVS/PMVS; Furukawa and Ponce, 2009)
For several fields of research, including the inferences of process rates and ages based on the quantification of concentrations from TCNs, accurate knowledge of soil bulk densities can be of great importance
Summary
Soil density determination is a time-consuming task when sampling for a terrestrial cosmogenic nuclide (TCN)-based analysis, but it is an important parameter if process rates and/or ages are inferred from the final nuclide concentration (e.g. Rodés and Evans, 2020; Rodés et al, 2011). The imperative to obtain accurate knowledge of present-day soil densities increases if short-lived TCNs, such as in situ 14C, are analysed In such a case, soil conditions are more likely to have prevailed over the time span during which the nuclide concentration has built up. Comparing the low-density soil to a highly compacted soil, for example with ρ = 2.3 g cm−3 (Schaetzl and Thompson, 2015), would yield a doubling (97 % increase) of the 10Be production at 100 cm depth (in situ 14C: ∼ 58 %) These numbers exceed the analytical uncertainties, which are < 10 % in most cases for measurements of medium to high 10Be and in situ 14C concentrations Accurate knowledge of local regolith densities is important for in situ TCN-based process rate determinations, underlining the need for a reliable, adaptable and easy-to-apply method to determine excavation densities
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