FALLOUT RADIONUCLIDES AS SEDIMENT TRACERS IN SOUTHERN ALPS CHEMICAL AND PHYSICAL WEATHERING STUDIES

Sediment flux estimates suggest that rivers draining high-standing oceanic islands between Australia and Asia contribute much of the total sediment flux to the oceans. Short lengths, steep gradients and flashy hydrographs of these streams enables them to act as chutes, transporting water and sediments quickly from the mountains to the ocean. In our studies of the coupling between physical erosion and chemical weathering, we determined depositional and erosional processes in New Zealand's tectonically active, rapidly uplifting Southern Alps, focused on the Hokitika River watershed, an area of extreme orographic precipitation (7–12 m annually) and high landslide frequency, but with modest topography due to the rapid erosion. In March 2004 sample collection at 7 locations in the Hokitika River watershed included soil cores, sediment cores, water and suspended sediment samples. We used the atmospherically-delivered radionuclides of ⁷Be, ¹³⁷Cs and ²¹⁰Pb _excess to determine the relative magnitude of particle residence time in the high elevation Cropp and Whitcombe subwatersheds and the rates of sedimentation. Our one- and two-box modeling with ⁷Be and ²¹⁰Pb_excess determined soil and sediment residence times. Residence time of fine suspended particles is short and particles can travel the length of the river during a single storm, probably due to the short duration, high-intensity rainfalls which produce rapidly moving, steep flood waves. The readily detected peak of ¹³⁷Cs activity in Cropp terrace and Hokitika gorge soils yielded sedimentation rates of 0.06–0.12 cm yr^-1. At the Cropp terrace, inventory models of ²¹⁰Pb_excess yield soil accumulation rates significantly less than those determined using the ¹³⁷Cs activity peak. We attribute the differences to overestimation of ²¹⁰Pb_excess in surface soils and to contrasting fallout fluxes, geochemical behavior and radionuclide contents of sedimenting materials. Total inventories of ²¹⁰Pb_excess in soils greatly exceed the expected direct atmospheric deposition, suggesting that lateral transport of this nuclide occurs within the watershed. At the Hokitika gorge, all nuclides studied yielded similar sedimentation rates, confirming the potential of ²¹⁰Pb_excess for determining sedimentation rates in New Zealand watersheds with very low ¹³⁷Cs inventories.