Complex patterns of schist tor exposure and surface uplift, Otago (New Zealand)

Landscapes are subjected to surface denudation during their complex and non-linear evolution. In order to quantify the in situ surface lowering and, thus, denudation or soil erosion rates, new, multi-millennia archives are needed and must be rigorously tested. Large residual rocks, tors, are the basis for the Tor Exhumation Approach. Here we present novel results on meta-sedimentary (schist) rock tors using this approach, which previously has only been applied in granitic terrains. The exhumation patterns of eight schist tors in three landscape locations (valley, ridge, distal) of Otago, New Zealand, were studied using cosmogenic dating. The in situ 10Be ages have high variability along individual vertical tor profiles. Average surface age is 122 ± 12 ka and ranges from 836 ± 89 ka to 19 ± 2 ka. The majority of investigated tors have surfaced during the MIS 5 which was one of the wettest and warmest climate periods. The resulting surface denudation trend of the three locations differs. The valley commenced denudation no earlier than ~200 ka with rates of ~0.22 [m kyr−1] to ~0.02 [m kyr−1]. In contrast, exposure started at the ridge position around 230 ka at ~0.03 [m kyr−1]. An age inversion found in the valley is considered to be the result of mushroom-like exposure by undercutting and repeated rock breakoffs. The distal site tor has been exhumed continuously for ~120 ka at a rate of ~0.2 to ~0.05 [m kyr−1]. We identified a mix of surface emergence patterns of the tors such as continuous-, mushroom-, tafoni- and structural-like. The comparison to modern erosion rates indicates that surface erosion has increased up to a factor of ten during the last few decades. To determine the actual surface uplift, we linked the tor derived surface denudation rates with rock uplift data. The data indicates that the surface uplift rates started to decrease during the Middle Pleistocene (0.04–0.09 [m kyr−1]), remained relatively low during the Late Pleistocene (~0.01 [m kyr−1]) and started to increase again during the Holocene (c. 0.21–0.64 [m kyr−1]). In summary, the emergence pattern of local tors enabled reconstruction of the evolution of Pleistocene-Holocene surfaces in East Otago

Soil weathering dynamics and erosion in a dry oceanic area of the southern hemisphere (Otago, New Zealand)

Landscape evolution is driven by tectonics, climate and surface denudation. In New Zealand, tectonics and steep climatic gradients cause a dynamic landscape with intense chemical weathering, rapid soil formation, and high soil losses. In this study, soil, and elemental redistribution along two adjacent hillslopes in East Otago, New Zealand, having different landscape settings (ridge versus valley) are compared to identify soil weathering and erosion dynamics. Fallout radionuclides ($^{239+240}$Pu) show that over the last ~ 60 years, average soil erosion rates in the valley (~ 260 [t km$^{−2}$ year$^{−1}$]) are low compared to the ridge (~ 990 [t km$^{−2}$ year$^{−1}$]). The ridge yields up to 26% lower soil weathering intensity than the topographical-protected valley. The lowest soil weathering intensity is found at both hilltop positions, where tors (residual rocks) are present and partially disintegrate. The soil weathering intensity increases with distance from tors, suggesting that tors rejuvenate the chemical weathering signature at the hilltop positions with fresh material. The inversed and decreasing weathering degree with all soil depth indicates that the fresh mineral contribution must be higher at the soil surface than at the bedrock weathering front. Higher erosion rates at the exposed ridge may be partially attributed to wind, consistent with rock abrasion of tors, and low local river sediment yields (56 [t km$^{−2}$ year$^{−1}$]). Thus, the East Otago spatial patterns of soil chemistry and erosion are governed by tor degradation and topographic exposure

Denudation variability of the Sila Massif upland (Italy) from decades to millennia using 10Be and 239+240Pu

Landscapes and soils evolve in non‐linear ways over millennia. Current knowledge is incomplete as only average denudation (or erosion) rates are normally estimated, neglecting the temporal discontinuities of these processes. The determination of regressive and progressive phases of soil evolution is important to our understanding of how soils and landscapes respond to environmental changes. The Sila Massif (Italy) provides a well‐defined geomorphological and geological setting to unravel temporal variations in soil redistribution rates. We used a combination of in situ cosmogenic radionuclide measurements (10Be) along tor (residual rock) height profiles, coupled with fallout radionuclides (239+240Pu) in soils, to model soil denudation rates over the last 100 ka. We measured rates prior to the Last Glacial Maximum (LGM) of ≤30 t km−2 yr−1 (~0.036 mm yr−1). Following the LGM, during the transition from the Pleistocene to the Holocene, these rates increased to ~150–200 t km−2 yr−1 and appeared to be above soil production rates, causing regressive soil evolution. For the last ~50 years, we even describe erosion rates of ≥1,000 t km−2 yr−1 (~1.23 mm yr−1) and consider human impact as the decisive factor for this development. Consequently, the natural soil production rates cannot cope with the current erosion rates. Thus, a distinct regressive phase of soil formation exists, which will give rise to shallowing of soils over time. Overall, our multimethod approach traced denudation and erosion histories over geologic and human timescales and made a new archive to soil science and geomorphology accessible

Ricostruzioni climatiche, ambientali e geoarcheologiche attraverso lo studio di suoli olocenici in Calabria

Scuola di Dottorato" Archimede" in Scienze, Comunicazione e Tecnologie, Dottorato di Ricerca in Scienze della Terra, XXIV Ciclo, a.a. 2008-2011Università della Calabri

Climate and relief‐induced controls on the temporal variability of denudation rates in a granitic upland

How soil erosion rates evolved over the last about 100 ka and how they relate to environmental and climate variability is largely unknown. This is due to a lack of suitable archives that help to trace this evolution. We determined in situ cosmogenic beryllium‐10 (10Be) along vertical landforms (tors, boulders and scarps) on the Sila Massif to unravel their local exhumation patterns to develop a surface denudation model over millennia. Due to the physical resistance of tors, their rate of exhumation may be used to derive surface and, thus, soil denudation rates over time. We derived soil denudation rates that varied in the range 0–0.40 mm yr‐1. The investigated boulders, however, appear to have experienced repositioning processes about ~20–25 ka bp and were therefore a less reliable archive. The scarps of the Sila upland showed a rapid bedrock exposure within the last 8–15 ka. Overall, the denudation rates increased steadily after 75 ka bp but remained low until about 17 ka bp. The exhumation rates indicate a denudation pulse that occurred about 17–5 ka bp. Since then the rates have continuously decreased. We identify three key factors for these developments – climate, topography and vegetation. Between 75 and 17 ka bp, climate was colder and drier than today. The rapid changes towards warmer and humid conditions at the Pleistocene–Holocene transition apparently increased denudation rates. A denser vegetation cover with time counteracted denudation. Topography also determined the extent of denudation rates in the upland regime. On slopes, denudation rates were generally higher than on planar surfaces. By determining the exhumation rates of tors and scarps, soil erosion rates could be determined over long timescales and be related to topography and particularly to climate. This is key for understanding geomorphic dynamics under current environmental settings and future climate change