Erosion rates and sediment flux within the Potomac River basin quantified over millennial timescales using beryllium isotopes

Beryllium isotopes measured in detrital river sediment are often used to estimate rates of landscape change at a basin scale, but results from different beryllium isotope systems have rarely been compared. Here, we report measurements of in situ and meteoric 10Be (10Bei and 10Bem, respectively) along with measurements of reactive and mineral phases of 9Be (9Bereac and 9Bemin, respectively) to infer long-term rates of landscape change in the Potomac River basin, North America. Using these data, we directly compare results from the two different 10Be isotope systems and contextualize modern sediment flux from the Potomac River basin to Chesapeake Bay. Sixty-two measurements of 10Bei in river sand show that the Potomac River basin is eroding on average at 29.6 ± 14.1 Mg km–2 yr–1 (11 ± 5.2 m m.y.–1 assuming a rock density of 2700 kg m–3)—a rate consistent with other estimates in the mid-Atlantic region. 10Bei erosion rates correlate with basin latitude, suggesting that periglacial weathering increased with proximity to the former Laurentide Ice Sheet margin. Considering the 10Bei-derived erosion rate as a sediment flux over millennia, rates of sediment delivery from the Potomac River to Chesapeake Bay are up to ∼5× lower than contemporary sediment yields implying modern land-use practices have accelerated erosion and sediment transport over background rates. However, 10Bei erosion rate data suggest that regulatory benchmark levels used to manage sediment export from the Potomac River basin to Chesapeake Bay are set appropriately to reduce sedimentation and restore the Bay’s ecological health. The mean of 56 10Bem/9Bereac-derived denudation rates (40.0 ± 21.7 Mg km–2 yr–1) is higher than, but statistically indistinguishable from, the mean 10Bei erosion rate (29.6 ± 14.1 Mg km–2 yr–1; p = 0.003). However, when considered basin by basin, 10Bem/9Bereac-determined denudation rates are only weakly correlated (R2 = 0.208; p < 0.001) with sediment fluxes determined from the well-established and widely used 10Bei technique. This suggests that the 10Bem/9Bereac technique may not reflect the same geomorphic processes as 10Bei technique, or that the 10Bem/9Bereac technique operates over different time and/or depth scales. Erosion indices (EIs, sensu Brown et al., 1988) derived from 10Bem measurements and contemporary sediment yield data range from 0.07 to 1.24; 75% of basins sampled have EIs that are >1, suggesting that 10Bem is being retained and sediment is being stored within the Potomac River basin. The Appalachian Plateau is the only physiographic province where sediment export dominates, likely as the result of ongoing relief growth in catchments draining the Appalachian Mountain divide. 10Bem concentrations measured in the 150 k.y. Hybla Valley sediment core, taken from the lower Potomac River basin, suggest that 10Bem and sediment are preferentially stored in the catchment when vegetation proxies for climate suggest warmer conditions prevailed. 10Bem and sediment are exported when vegetation proxies for climate suggest conditions are colder, perhaps a reflection of periglacial activity or changes in storm frequency and/or magnitude over glacial-interglacial cycles. GeoRef Subject alluvium Appalachians Central Appalachians Eastern U.S. alkaline earth metals clastic sediments Be-10/Be-9 beryllium North America Potomac River basin geomorphology isotopes isotope ratios metals sediments radioactive isotopes sand stable isotopes United States