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Title: Be-7/pb-210 ratio as an indicator of suspended sediment age or fraction new sediment in suspension

Author
item MATISOFF, GERALD - CASE WESTERN UNIVERSITY
item Wilson, Christopher
item WHITING, PETER - CASE WESTERN UNIVERSITY

Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/17/2005
Publication Date: 8/1/2005
Citation: Matisoff, G., Wilson, C.G., Whiting, P.J. 2005. Be-7/pb-210 ratio as an indicator of suspended sediment age or fraction new sediment in suspension. Earth Surface Processes and Landforms. 30(9):1191-1201.

Interpretive Summary: The ratio of two naturally-produced radio-isotopes, 7Be/210Pbxs, can be used to determine the age of sediment suspended in a stream or the time since the sediment was removed from the land surface. The ratio can also be used to determine the amount of the suspended sediment that is recently eroded from the landscape. Both radio-isotopes are delivered with rainfall in a constant proportion or ratio. This ratio will change as the radio-isotopes are transferred to soil on the landscape. In addition, the ratio will change as the soil is eroded from the landscape surface, is carried by the stream, and is deposited on the bottom of lakes and estuaries. The ratio changes over time because the two elements decay away at very different rates. As time passes or the sediment grows older, the ratio rapidly decreases. The ratio also changes because older sediment (with a low ratio) mixes with newer sediment as the sediment particles move downstream. The changing ratio tells the amount of older sediment that has mixed with the newer sediment. Both the age of the suspended sediment and amount of freshly-eroded sediment from the land surface that is in the stream flow was determined at three places: Old Woman Creek, Ohio; Weeks Bay, Alabama; and South Slough, Oregon. The results at the three places were similar. The age of the sediment increases as it travels from the land surface to the stream to the lake or estuary. The amount of new sediment will decrease as the sediment follows the same path as older sediment mixes with it.

Technical Abstract: Reported 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 watersheds decrease in the following manner: precipitation (~16) > suspended sediments in rivers (6-7) > suspended sediments in estuaries (4-6) > sediment collected in sediment traps in the estuary (~1) > surface sediment of the estuary (~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 watersheds - 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 percent new sediment decreases from 100% in newly-tagged material to about 35-50 % in rivers to 25-35% in the estuary to less than 10% in the sediment traps to 1-4% on the surface of the bottom sediments.