Earth is (mostly) flat: apportionment of the flux of continental sediment over millennial time scales

We use a new compilation of global denudation estimates from cosmogenic nuclides to calculate the apportionment and the sum of all sediment produced on Earth by extrapolation of a statistically significant correlation between denudation rates and basin slopes to watersheds without denudation rate data. This robust relationship can explain approximately half of the variance in denudation from quartz-bearing topography drained by rivers using only mean slopes as the predictive tool and matches a similar fit for large river basins. At slopes >200 m/km, topography controls denudation rates. Controls on denudation in landscapes where average slopes are 10 mm/k.y. We use global topographic data to show that the vast majority of the Earth’s surface consists of these gently sloping surfaces with modest, but positive, gross denudation rates, and that these areas contribute the most sediment to the oceans. Because of the links between silicate weathering rates and denudation rates, the predominance of low sloping areas on the Earth’s surface compared to areas of steep mountainous topography implies that mountain uplift contributes little to drawdown of CO2 at cosmogenic nuclide time scales of 103–106 yr. The poorly understood environmental controls that set the pace of denudation for the largest portion of Earth’s surface hold the key to understanding the feedbacks between erosion and climate.