Abstract
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.
Highlights
Earth's surfaces have evolved and reshaped over billions of years
The10Be content increases with tor height (Fig. 3a), but the highest content was measured at sample Tor 6–5 below the soil surface
Schist tor samples selected along the structural x- or z-axis generally resulted in higher contents relative to samples selected along the y-axis (Fig. 3b)
Summary
Earth's surfaces have evolved and reshaped over billions of years. Once primary mineral assemblages are exposed, chemical weathering begins to transform them into neogenic phases (e.g. clay minerals, iron oxyhydroxides). The continuous weathering of bedrock (W) and later removal by erosion (E) causes landscape surfaces to be denuded (Dsurface;; [m yr−1]) over time. The spatial and temporal variations of surface forming processes often result in a complex and non-linear landscape evolution history. Modern geomorphology aims at comprehending these past processes to understand present landscapes and predict their future evolution. With the use of geochronological methods (e.g. surface exposure dating, burial dating, isotope decay counting), time fragments of landscape evolution can be deciphered. A major challenge for deciphering past landscape histories is quantification of surface lowering processes at the denudational zone itself. Intra-terrain or terminal depositional zones are used to determine Dsurface rates where the deposited material has originated from Intra-terrain or terminal depositional zones are used to determine Dsurface rates where the deposited material has originated from (e.g. Wittmann et al, 2007)
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