科研之友
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摘要
We present an extensive investigation of a new erosion and weathering proxy derived from the Be-10(meteoric)/Be-9(stable) ratio in the Amazon River basin. This new proxy combines a radioactive atmospheric flux tracer, meteoric cosmogenic Be-10, with Be-9, a trace metal released by weathering. Results show that meteoric Be-10 concentrations ([Be-10]) and Be-10/Be-9 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 Be-10 shows a deficit with respect to the atmospheric depositional Be-10 flux. Most likely, the actual area from which the Be-10 flux is being delivered into the mainstream is smaller than the basin-wide one. Despite this imbalance, denudation rates calculated from Be-10/Be-9 ratios from bed load, suspended sediment, and water samples from Amazon Rivers agree within a factor of 2 with published in situ Be-10 denudation rates. Erosion rates calculated from meteoric [Be-10], measured from depth-integrated suspended sediment samples, agree with denudation rates, suggesting that grain size-induced variations in [Be-10] 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 Be-9 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.
- 出版日期2015-12
