Characterizing coastal margin fluvial and marine processes using lithogenic and fallout radionuclides: Isotope fractionation, sediment sourcing and transport.

摘要

Sources, transport and deposition of alluvial and suspended sediment over time and space were discerned in the lower Loco Bayou Basin, Texas. Surfaces of upland interfluves and subsoils, sources of alluvium and suspended sediment were distinguished using isotope ratios 226Ra/232Th, 226Ra/230Th and 230Th/232Th. Source contributions were apportioned at three points during bankfull and flood stages. 137Cs and 210Pb_xs were used to determine rates of floodplain sedimentation throughout the basin. Sources of alluvium showed a transition from interfluve surface to subsoil during flood and bankfull stages with distance downstream. Suspended sediments indicate dominance of subsoil sources during bankfull stage and a progressive change to interfluve surface sediment sources at flood stage. Floodplain sedimentation was similar in the upper basin and much less in the lower basin. These results indicate strong coupling between hillslope and channel processes, reflecting land use changes.; Radionuclide activities were compared to particulate organic carbon (POC), percent fines and total concentrations of Al, Fe and Mn in interluve and alluvial samples to further strengthen the conclusions drawn from the modeling of sediment transport with radionuclides. Strong positive correlations were observed between radionuclides and Mn (Th) and % fines (Ra and Th). Isotope ratios effectively mitigate these influences, supporting their use to offset extrinsic variability in terrestrial sediments. Strong association of 210Pb _xs with POC agrees with data from marine settings.; Biological and abiotic processes affecting sediment transport, mixing and deposition in marine settings were assessed by using a suite of radionuclides (210Pb_xs, 234Th and 239,240 Pu). Analyses of four sub cores collected within the Gulf of Mexico (GOM) indicate that: (1) all cores exhibit bioturbation, (2) POC profiles show strong variability in upper 4 cm of all cores; after ≈5 cm, profiles either remain relatively constant or gradually decrease, (3) profiles of 210Pb_xs and 239,240Pu show signs of bioturbation, (4) 234Th_xs is constrained to the upper 2–4 cm of most profiles, with bioturbation rate coefficients (D_b) of ≈6 to 351 cm2/yr., and (5) the range of apparent sedimentation rates at all stations were 0.06–0.24 cm yr. −1, which compare favorably with rates from similar continental margin settings.