https://doi.org/10.5194/gchron-4-713-2022

Authors

Nathan Vandermaelen, Koen Beerten, François Clapuyt, Marcus Christl, and Veerle Vanacker

Citation

Vandermaelen, N., Beerten, K., Clapuyt, F., Christl, M., and Vanacker, V.: Constraining the aggradation mode of Pleistocene river deposits based on cosmogenic radionuclide depth profiling and numerical modelling, Geochronology, 4, 713–730, https://doi.org/10.5194/gchron-4-713-2022, 2022.

Abstract

Pleistocene braided-river deposits commonly represent long periods of non-deposition or erosion that are interrupted by rapid and short aggradation phases. When dating these sedimentary sequences with in situ-produced cosmic radionuclides (CRNs), simple concentration depth profiling approaches often fall short, as they assume that the alluvial sedimentary sequence has been deposited with a constant and rapid aggradation rate and been exposed to cosmic radiations afterwards. Numerical modelling of the evolution of CRNs in alluvial sequences permits one to account for aggradation, non-deposition and erosion phases and can simulate which scenarios of aggradation and preservation most likely represent the river dynamics. In this study, such a model was developed and applied to a Middle Pleistocene gravel sheet (Zutendaal gravels) exposed in NE Belgium.

The model parameters were optimised to the observed ¹⁰Be and ²⁶Al concentrations of 17 sediment samples taken over a depth interval of 7 m that constitutes the top of a gravel sheet up to 20 m thick. In the studied sedimentary sequence, (at least) three individual aggradation phases that were interrupted by non-deposition or erosion can be distinguished, each interruption lasting ∼ 40 kyr. The age for the onset of aggradation of the upper 7 m of the gravel sheet was further constrained to  ka. This age, within error limits, does not invalidate previous correlations of the gravel sheet with the Cromerian Glacial B and Marine Isotope Stage (MIS) 16. The deposition of the entire gravel sheet likely represents more than one climatic cycle and demonstrates the importance of accounting for the depositional modes of braided rivers when applying in situ cosmogenic radionuclide techniques.

Summary

Cosmogenic radionuclides accumulate at the Earth’ surface as a function of time and depth. They can be used to constrain the age and the deposition mode of fluvial deposits.

Occupied by the braided channels of the Meuse during the Middle Pleistocene, the (eastern) Campine plateau resisted the overall erosion that affected the Campine area, and nowadays stands out of its environment.

In this study, we applied cosmogenic radionuclides on the eastern Campine plateau to constrain its depositional and post-depositional history. Main results show that the deposition occurred in different phases, interrupted by thousands of years long hiatuses that correspond to climatic fluctuations.

These first numerical ages of the eastern Campine plateau could be used in the future to better constrain the geomorphological evolution of the Campine region and the European Lowlands in general.

The 9th of November, Beatriz will publicly defend her PhD thesis, entitled “Monitoring forest degradation patterns using multi-resolution spatial approaches”.

Summary

The subtropical dry forests of South-America have been impacted by deforestation and degradation over the last two centuries. The forests still provide important ecosystem services but overexploitation of the forested ecosystems may lead to irreversible loss of provision and regulation services. Remote sensing techniques enable us to monitor changes in forest cover at regional scale. Field-based forest inventories remain essential to measure more specific forest attributes and functions. However, the scale at which such field-based forest inventories can be organized is limited for the detection of slow and subtle changes in the forest at regional scale. Therefore, this thesis analyzed the potential of 3D vegetation models based on low-cost stereo-photos taken from unpiloted aerial vehicles to map forest degradation in dry forests. The ecosystem of the Dry Chaco in Argentina, which is considered as one of the major hotspots of deforestation worldwide, was taken as a case study. Therefore, new data on surface and vegetation height were collected during UAV-flights. Flights were realized at 44 locations in contrasting ecological settings of the Dry Chaco. The results of this research show that structure-from-motion algorithms are capable of reconstructing high resolution 3D surface models of the forest cover and this at a much lower cost than LIDAR derived products. Furthermore, this thesis proposes a set of relevant forest attributes that can be derived from these 3D surface models and compares these with indicators from traditional field-based forest inventories. In a next step, the correspondence between the forest structural indicators and degradation states was examined. Finally, the indicators were compared with high resolution laser data from a recently launched spaceborne sensor, which facilitates the assessment of forest degradation patterns at sub-continental scale.

To reconstruct multitemporal land cover change from existing land cover products, a multisource data integration procedure was followed to reduce imprecision and inconsistencies that may result from comparing heterogeneous datasets. The conversion of native forests and grasslands into agricultural land was the principal land cover change in the non-protected area. In 1961, about 94% of the non-protected area was still covered by native vegetation, which decreased to only 7% in 2018. Most of agricultural expansion occurred in the 1960s and 1970s, creating an anthropogenic landscape where 67% of the area is covered by agricultural land and 26% by invasive species. Early clearance of native vegetation occurred in the more accessible, less rugged areas with deeper-than-average and well-drained soils. The first wave of settlement consisted of large and isolated farmsteads, with 19% of the farms larger than 100 ha and specializing in dairy and meat production.

Over the period of 1961–1987, the number of farms doubled from less than 100 to more than 200, while the average farm size decreased from 90 to 60 ha/farmstead. Due to labour constraints in the agricultural sector, these farms opted for less labour-intensive activities such as livestock farming. New farms (popping up in the 1990s and 2000s) are generally small, with <5 ha per farmstead, and settled in areas with less favourable biophysical conditions and lower accessibility to markets. From the 1990s onwards, the surge of alternative income opportunities in the tourism and travel-related sector reduced pressure on the natural resources in the non-protected area.

More information in https://www.mdpi.com/1711014

Alomía Herrera, I.; Paque, R.; Maertens, M.; Vanacker, V. History of Land Cover Change on Santa Cruz Island, Galapagos. Land 2022, 11, 1017. https://doi.org/10.3390/land11071017