EARTH IS (MOSTLY) FLAT: APPORTIONMENT OF THE RATES OF CONTINENTAL MASS REMOVAL OVER MILLENNIAL TIME SCALES. A REAPPRAISAL

Steep, high-relief topography only occupies a small fraction of the Earth’s surface and understanding natural rates of erosion in the low and more gently sloping terrains, making up around 90% of Earth’s continental land mass, is critical for understanding how such areas have been transformed by human activities. In expansive, flat areas, even modest rates of chemical weathering lead to positive, gross denudation taking place even in zones of sediment accumulation [1]. We cite previously published denudation rates from cosmogenic nuclides [2], which measure mass loss by chemical weathering and physical erosion. Such measurements circumvent some possible problems with noise inherent in other short-term measurements, but introduce other issues related to extrapolating results outside the measured ranges. The cosmogenic nuclide-derived rates allow us to calculate the apportionment and the sum of all sediment produced on the Earth by extrapolation of a statistically significant correlation between denudation rates and basin slopes to watersheds without denudation rate data. We will present updates to our past results[1] using a new topographic dataset, which illustrates the critical impact of grid size on slope determinations. The total sediment flux from GMTED dataset is ~24 Gt/yr. We find that non-mountainous areas contribute a significant proportion of the total fluxes though the total fluxes are split almost evenly between the steepest 10% of topography and the rest of the continental land mass. Though these areas are especially important in light of greatly accelerated soil-erosion rates from human land use, few long-term denudation rate measurements in ‘flat’ areas exist and new cosmogenic nuclide techniques developed to overcome methodological challenges in these areas [3] have not yet been attempted.

[1] Bouchez, J., et al., 2012, Floodplains of large rivers: Weathering reactors or simple silos? Chemical Geology, v. 332-333, p. 166–184.

[2] Willenbring, J.K., et al., 2013, Earth is (mostly) flat: Apportionment of the flux of continental sediment over millennial time scales: Geology, v. 41, p. 343-346.

[3] Lauer, J.W., & Willenbring, J.K. 2010. Steady state reach-scale theory for radioactive tracer concentration in a simple channel/floodplain system. Journal of Geophysical Research 115 (F4), F04018.