Cosmogenic nuclide analysis of river sediment provides insight into catchment‐wide erosion rates and dynamics. Here, we investigate spatial patterns and controls on10Be‐inferred erosion rates in Madagascar, a moderately seismically active microcontinent surrounded by passive margins with locally steep topography and a climate that varies from humid tropical to semiarid. We use a compiled dataset of 99 detrital10Be measurements, 63 of which are new, covering more than 30% of the country and a wide range of topographic, bioclimatic and geologic characteristics. Overall,10Be erosion rates are low (2.4–51.1 mm kyr−1), with clear differences between regions. The lowest rates are measured on the central highlands ( 8 mm kyr−1), in the Alaotra–Ankay graben ( 11 mm kyr−1) and in the large north‐central catchments ( 11 mm kyr−1). Higher rates are found on the steep eastern escarpment ( 20 mm kyr−1), in the northwest ( 31 mm kyr−1) and in the southwest ( 29 mm kyr−1). A stepwise linear regression model identified elevation as the main factor associated with variations in10Be erosion rates (lower rates for higher catchments). Random within‐between statistical models (REWB), on the other hand, indicated that the differences between different regions can be explained by differences in river concavity, seismic events and gully (lavaka) densities, whereas additional variation within regions is only linked to seismicity. We find no correlation between catchment or river steepness and10Be‐inferred erosion rates. Our results indicate that in Madagascar, long‐term erosion rates are overall low and that simple topography‐based models do not explain variations in rates of landscape change inferred from10Be concentrations in river sediment. We demonstrate that identifying different regions aids in interpreting spatial patterns of erosion rates and that REWB models can be a powerful tool in deciphering environmental controls on10Be erosion rates.
The connection between topography and erosion rate is central to understanding landscape evolution and sediment hazards. However, investigation of this relationship in steep landscapes has been limited due to expectations of: (a) decoupling between erosion rate and “threshold” hillslope morphology; and (b) bias in detrital cosmogenic nuclide erosion rates due to deep‐seated landslides. Here we compile 120 new and published10Be erosion rates from catchments in the San Gabriel Mountains, California, and show that hillslope morphology and erosion rate are coupled for slopes approaching 50° due to progressive exposure of bare bedrock with increasing erosion rate. We find no evidence for drainage area dependence in10Be erosion rates in catchments as small as 0.09 km2, and we show that landslide deposits influence erosion rate estimates mainly by adding scatter. Our results highlight the potential and importance of sampling small catchments to better understand steep hillslope processes.
more » « less- NSF-PAR ID:
- 10449718
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 50
- Issue:
- 16
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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