A test of the cosmogenic¹⁰Be(meteoric)/⁹Be proxy for simultaneously determining basin-wide erosion rates, denudation rates, and the degree of weathering in the Amazon basin

Abstract

We present an extensive investigation of a new erosion and weathering proxy derived from the 10 Be(meteoric)/ 9 Be(stable) ratio in the Amazon River basin. This new proxy combines a radioactive atmospheric flux tracer, meteoric cosmogenic 10 Be, with 9 Be, a trace metal released by weathering. Results show that meteoric 10 Be concentrations ([ 10 Be]) and 10 Be/ 9 Be ratios increase by >30% from the Andes to the lowlands. We can calculate floodplain transfer times of 2-30 kyr from this increase. Intriguingly however, the riverine exported flux of meteoric 10 Be shows a deficit with respect to the atmospheric depositional 10 Be flux. Most likely, the actual area from which the 10 Be flux is being delivered into the mainstream is smaller than the basin-wide one. Despite this imbalance, denudation rates calculated from 10 Be/ 9 Be ratios from bed load, suspended sediment, and water samples from Amazon Rivers agree within a factor of 2 with published in situ 10 Be denudation rates. Erosion rates calculated from meteoric [ 10 Be], measured from depth-integrated suspended sediment samples, agree with denudation rates, suggesting that grain size-induced variations in [ 10 Be] are minimized when using such sampling material instead of bed load. In addition, the agreement between erosion and denudation rates implies minor chemical weathering intensity in most Amazon tributaries. Indeed, the Be-specific weathering intensity, calculated from mobilized 9 Be comprising reactive and dissolved fractions that are released during weathering, is constant at approximately 40% of the total denudation from the Andes across the lowlands to the Amazon mouth. Therefore, weathering in the Amazon floodplain is not detected. (2015), A test of the cosmogenic 10 Be(meteoric)/ 9 Be proxy for simultaneously determining basin-wide erosion rates, denudation rates, and the degree of weathering in the Amazon basin,

Model predictions of long-lived storage of organic carbon in river deposits

“…3b using T cut values selected such that the MTT for each distribution shape varied between 10 3 and 10 6 years. This approach is consistent with field estimates of MTTs, which range from 10 3 to 5×10 5 years when measured using sediment budgeting (Blöthe and Korup, 2013) or radionuclide approaches (Dosseto et al, 2006;Granet et al, 2010;Wittmann et al, 2015Wittmann et al, , 2016Li et al, 2016).…”

The geological significance of cosmogenic nuclides in large lowland river basins

“…This bias may then also affect denudation rates calculated from equation 1011 14 where Be data from 1014 suspended sediment depth profiles; this data was corrected for grain size bias during 1015 riverine sorting following the approach shown in Fig. 12C and is thus considered 1016 representative for the Amazon River where the majority of sediment flux is 1017 transported by suspended load (Wittmann et al, 2015). The f f values 1018 are ca.…”

“…Another way to obtain a representative 10 Be concentration from river 910 sediment is to measure the Al/Si ratio in suspended sediment too, and correct the 911 meteoric 10 Be concentration by using depth-integrated Al/Si ratios (Fig. 12C) (Wittmann et al, 2015) Be inventory of the basin is at steady state, J / J equals 1 (Brown et 936 al., 1988). If this ratio <1, the sediment storage duration can be calculated by solving 937 equation 12 for t. We illustrate this case for the riverine 10 Be flux deficit observed in 938 the Amazon basin in Figure 13A, i.e.…”

“…9,10). Be proxy indicate similar weathering 712 intensities in the Andes and in the lowlands, respectively (Wittmann et al, 2015). 713…”

“…This reactive Be is separated from 877 samples by sequential chemical extractions (Tessier et al, 1979;Bourlès et al, 1989;878 Wittmann et al, 2012), the mineral-bound 9 Be retained in undissolved minerals is 879 excluded, and the concentration is reported as 10 Be reac and 9 Be reac (in at/g (solid) or 880 g/g (solid) ), respectively. 881 approach to obtain that is most likely valid for large basin-wide studies 887 (Wittmann et al, 2015) is by using a general atmospheric circulation model (GCM) 888 coupled to an aerosol module (e.g. Heikkilä et al (2013)) that also incorporates a 889 physical model simulating processes of cosmic particle production and transport 890 (Masarik and Beer, 1999) and variations in solar modulation and magnetic field 891 strength.…”