ALBERT

Description: 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 main stream is smaller than the basin-wide one. Despite this imbalance, denudation rates calculated from 10 Be/ 9 Be ratios from bedload, suspended sediment, and water samples from Amazon Rivers agree within a factor of ca. 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 bedload. 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 ca. 40% of the total denudation from the Andes across the lowlands to the Amazon mouth. Therefore, weathering in the Amazon floodplain is not detected.