Natural versus anthropogenic erosion in Central Brazil, a confrontation of time and space scales

Abstract

<p>Land degradation resulting from increased continental surface erosion is a worldwide and systemic phenomenon due to human activities. Already fragile, the intertropical zone is likely to be further affected by climate change and increased aridity, an aggravating factor of soil erosion and land degradation. A major challenge is thus to provide the necessary knowledge to not only deepen our understanding of the Earth system and its critical thresholds but also to help achieving sustainable development goals. Understanding the factors that control the properties and processes of the critical zone, and especially what will be its responses to ongoing climate and land use changes, requires multidisciplinary efforts to tackle time scales that are compatible with morphogenesis and soil development as well as environmental disturbances of anthropogenic origin. Due to its prominent ecological importance, the Brazilian Cerrado biome is an ideal natural laboratory were to gauge the consequences of recent and strong human activities on continental surface erosion. We focused on the region of Brasília where our approach allows confronting the temporal scales of long-lived and stable cosmogenic nuclides with that of short-lived radioactive isotopes, through a comparison of natural and anthropogenically disturbed land surfaces. Our results indicate that long-term, background denudation rates are lower than 10 mm kyr<sup>-1</sup>whereas recent erosion rates due to human activities may reach rates at least 160 times higher, exceeding by far the sustainability rates of the soil resource.</p>

Natural Denudation Versus Anthropogenically Accelerated Erosion in Central Brazil: A Confrontation of Time and Space Scales

“…In 1975and 1985, anthropogenic activities such as the excavation of surface material for the opening of new roads significantly affected soil losses through water erosion. This is particularly the case for the Acampamento Basin, on the edges of BR-020 road, for the Barrigudo Catchment, along DF-001 road, and along the northern margin of the Torto Watershed (Figure S3.2 in Siame et al, 2023). et al, 1991, 2011)-A = R × K × LS × CP-where A is the calculated soil loss (t ha −1 yr −1 ), R is the yearly rainfall runoff erosivity factor (MJ mm ha −1 h −1 ), K is a soil erodibility factor (t ha MJ −1 mm −1 ), L is the slope length factor (m), S is the slope steepness factor, C is a cover-management factor, and P is a supporting practice factor.…”

“…In situ 10 Be-derived denudation values are given in mm kyr −1 . See Siame et al (2023) for supplementary material and analytical results. (c) Geomorphic map showing the regional geomorphic units and drainage networks with main regional watersheds.…”

“…In the region of Brasília, the quartz derived from Neo-Proterozoic basement rocks (0.5-1.0 Ga) and might thus be contaminated by a significant amount of nucleogenic 21 Ne, which could partly explain the observed difference in the production ratio. In our duricrust sample, the quartz grains may have had experienced an initial previous exposure episode several millions of years ago, then followed by a burial episode, before they reached progressively the surface, with a denudation rate of about 0.6 ± 0.1 mm kyr −1 (e.g., Siame et al, 2023). Within this framework, the 21 Ne concentration most probably represents a minimum cumulated exposure duration, experienced by the quartz grains now embedded in the duricrust, that is between 5.9 ± 0.9 and 13.0 ± 3.0 Myr (9.5 ± 1.7 Myr) whether (a) using a cosmogenic production scaling scheme accounting for temporal geomagnetic variations or not (e.g., Lifton et al, 2015;Stone, 2000), and (b) correcting for nucleogenic 21 Ne content or not (e.g., Siame et al, 2023).…”

“…It can thus happen that pixels with very high soil losses occur, due a high K-factor, flux convergence (high LS-factor) or a more exposed soil (high CP-factor), although it is masked by its pixel class. See Siame et al (2023) for supplementary material information about the RUSLE parameter values. (b) Sankey diagram showing the fluxes of land areas (km 2 ) between "natural" (green) and "anthropogenic" (red) states during the periods encompassed by the RUSLE models (1975-1985 and 1985-2018).…”

“…The linear tendencies are given with 95% confidence intervals (dotted lines) for the regional basins only (red) and for combined surrounding catchments and watersheds of the plateau (green). (c) Be-ratio values ( 10 Be m / 9 Be aut ) reac against in situ 10 Be-derived denudation rates ( 𝐴𝐴 𝐷𝐷 in mm kyr −1 ) obtained independently from the same samples (seeSiame et al, 2023). The contour lines showing a variety of 𝐴𝐴 𝐴𝐴…”