The ⁷Be/²¹⁰Pb_xs ratio as an indicator of suspended sediment age or fraction new sediment in suspension

Abstract

Abstract Reported here is a technique to use the 7 Be/ 210 Pb xs ratio as a measure of suspended sediment age or as an indicator of the fraction of the suspended sediment that is recently eroded from the landscape. Although both 7 Be and 210 Pb xs are delivered seasonally and stochastically to the landscape by precipitation, the ratio of the two radionuclides varies substantially less. The 7 Be/ 210 Pb xs ratios measured in three different catchments decrease in the following manner: precipitation ( c. 16) > suspended sediments in rivers (6–7) > suspended sediments in estuaries (4–6) > sediment collected in sediment traps in the estuary ( c. 1) > surface sediment of the estuary ( c. 0·5). Decreases in the 7 Be/ 210 Pb xs ratio in suspended sediments can be interpreted to be the result of increased age of the sediment, since 7 Be decays faster than 210 Pb. Alternatively, a decrease in the 7 Be/ 210 Pb xs ratio in suspended sediments can be interpreted to be the result of dilution of newly tagged 7 Be‐rich sediment by 7 Be‐dead sediment, for example, by erosion of soil below the 7 Be‐enriched surface layer or by resuspension of 7 Be‐dead bottom sediment. Presented here is a model which uses the 7 Be/ 210 Pb xs ratio in suspended sediments to determine the time since the particles were tagged by precipitation‐derived radionuclides (i.e. the age of the suspended sediment). In addition, an alternative model is presented to determine the fraction of the sediment that is ‘newly tagged’. These two models are applied to three catchments – Old Woman Creek, Ohio; Weeks Bay, Alabama; and South Slough, Oregon – and yield similar findings at all three sites. Sediment ages increase from 0 in newly tagged material, to 50–80 days in rivers, to about 80–100 days in the estuaries, to about 200 days in the sediment traps, to about 300 days on surface bottom sediments. Alternatively, the percentage new sediment decreases from 100 per cent in newly tagged material, to about 35–50 per cent in rivers, to 25–35 per cent in the estuary, to less than 10 per cent in the sediment traps, to 1–4 per cent on the surface of the bottom sediments.