Abstract
AbstractReported here is a technique to use the 7Be/210Pbxs 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 7Be and 210Pbxs are delivered seasonally and stochastically to the landscape by precipitation, the ratio of the two radionuclides varies substantially less. The 7Be/210Pbxs 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 7Be/210Pbxs ratio in suspended sediments can be interpreted to be the result of increased age of the sediment, since 7Be decays faster than 210Pb. Alternatively, a decrease in the 7Be/210Pbxs ratio in suspended sediments can be interpreted to be the result of dilution of newly tagged 7Be‐rich sediment by 7Be‐dead sediment, for example, by erosion of soil below the 7Be‐enriched surface layer or by resuspension of 7Be‐dead bottom sediment. Presented here is a model which uses the 7Be/210Pbxs 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.
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