Reply on RC2

Eric Portenga

doi:10.5194/gchron-2021-23-ac2

Abstract

Long-term erosion rates in Tasmania, at the southern end of Australia’s Great Dividing Range, are poorly known, yet such knowledge is critical for making informed land-use decisions and improving ecological health of coastal ecosystems. Here, we present the first quantitative, geologically-relevant estimates of erosion rates for the George River basin, in northeast Tasmania, based on in-situ produced 10Be (10Bei) measured from stream sand at two trunk channel sites and seven tributaries (average 10.5 mm kyr−1). These new 10Bei-based erosion rates are strongly related to mean annual precipitation rates and elevation, and we suggest that the current East-West precipitation gradient across George River greatly influences erosion in northeast Tasmania. This stands in contrast to erosion rates along the mainland portions of Australia’s Great Dividing Range, which are more strongly related to basin slope. We also extract and measure meteoric 10Be (10Bem) from sediment grain coatings of the stream sand at each site, which we use to estimate 10Bem-based erosion and denudation rates for George River. 10Bem based erosion and denudation metrics, particularly those from the central and eastern tributaries, are also closely related to elevation and precipitation in the same manner as 10Bei erosion rates. Although 10Bem-based denudation rates replicate 10Bei erosion rates within a factor of two, 10Bem-based erosion rates are systematically 5–6x higher than 10Bei erosion rates. 10Bem erosion and denudation metrics for the westernmost headwater catchments are significantly lower than expected and have likely been affected by intensive and widespread topsoil erosion related to forestry, which delivers large volumes of sediment rich in 10Bem to tributary streams. The 10Bei erosion rates presented in this study may be useful for land managers seeking to restore ecological health of Tasmania’s estuaries by reducing sediment input to levels prior to landscape disturbance.

Highlights

Erosion rates of river basins derived from measurements of the in-situ produced cosmogenic isotope, 10Bei, have been used to elucidate and infer topographic, tectonic, and climate drivers of landscape evolution for thousands of individual river basins 30 (Codilean et al, 2018; Harel et al, 2016; Mishra et al, 2019; Portenga and Bierman, 2011; Wittmann et al, 2020)
10Bei was extracted from quartz from each sample at the University of Vermont following standard methods, during which a known amount of a 9Be carrier (9Becarr) was added to each sample (Kohl and Nishiizumi, 1992; Corbett et al, 2016); no native beryllium was detected in quartz concentrates from any sample, which can otherwise lead to significant overestimates of 10Beibased erosion rates (Portenga et al, 2015). 10Bei/9Becarr ratios were measured by accelerator mass spectrometry at the Lawrence
Ratings of soil erosivity have been derived for Tasmania (Kidd et al, 2014, 2015) and are strongly tied to hillslope angle within George River basin (Fig. 5); comparing erosion and denudation metrics against basin slope metrics provides an adequate assessment of whether models of hillslope erodibility influences erosion in George River. 230 4 Results 4.1 10Bei erosion rates, ε Erosion rates, ε, based on measured concentrations of in situ 10Be (Table 3) range from 4.8 to 24.5 mm kyr-1 (Appendix A), and we find that the average ε from tributaries (13.6 ± 1.0 mm kyr-1; 2σ) is greater than either of the trunk channel samples (TG-1 = 9.6 ± 1.6 mm kyr-1; TG-9 = 8.3 ± 1.4 mm kyr-1; 2σ)

Summary

Introduction

Erosion rates of river basins derived from measurements of the in-situ produced cosmogenic isotope, 10Bei, have been used to elucidate and infer topographic, tectonic, and climate drivers of landscape evolution for thousands of individual river basins 30 (Codilean et al, 2018; Harel et al, 2016; Mishra et al, 2019; Portenga and Bierman, 2011; Wittmann et al, 2020). Delunel et al (2020) find that 10Bei erosion rates across the European Alps are strongly linked to mean basin slope and influenced by uplift and glaciations. A number of 35 north-south latitudinal studies from the South American Andes show that some segments of the range are driven by uplift (Carretier et al, 2015; Starke et al, 2017) and slope (Carretier et al, 2018) but not necessarily rainfall unless one considers the effects of vegetation in driving soil weathering rates (Carretier et al, 2015; Starke et al, 2020)

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