Estimating Surface/Subsurface Sediment Mixing in Karst Settings Using 7Be Isotopes

Abstract

AbstractThis study shows that the cosmogenic radionuclide beryllium‐7 can be used to track sediment movement through caves. The activities of beryllium‐7 and cesium‐137 were measured in two different karst settings at both surface and subsurface sites before and after storm runoff events. At one site, 7Be‐enriched sediment was detected up to 1.5 km along a stream conduit after a moderate storm event; however, the activity of 137Cs was too variable to show a meaningful pattern. The percentages of surface sediment that was found ranged from 0 to 52% along the entire 3 km cave stream and from 33 to 52% along the upper 1.5 km. At the other site, as much as 96% of the sediment initially discharged at the spring during a storm event was fresh surface material that had traveled into and through the cave stream. Moreover, during the 4 day runoff event, approximately 23% of the total suspended sediment flux was estimated to originate from surface erosion with 78% being reworked sediment from within the cave. The data in this study show that cosmogenic radionuclides with multiyear half‐lives are too long‐lived to track sediment origins in the caves; whereas, 7Be with a 53.2 day half‐life, can be used to track movement of sediment along cave streams.