The behavior of particle-reactive tracers in a high turbidity environment: 234Th and 210Pb on the Amazon continental shelf.
Excess 234Th and 210Pb seabed inventories were measured in cores collected from the Amazon continental shelf to examine particle scavenging and seabed dynamics. Typical excess 210Pb inventories range from 100 to 300 dpm cm-2, and the total excess 210Pb inventory for the Amazon shelf was determined to be 2.7 X 1017 dpm. The 210Pb measurements indicate that particle-reactive species are scavenged not only from the Amazon River but also from the lateral advection of offshore water. In order to sustain the 210Pb inventories, the volume of water supplied by the lateral advection from offshore must be approximately five to ten times the water discharge of the Amazon River. This lateral advection supplies about 67% of the total excess 210Pb to the Amazon continental shelf with relatively small contributions from riverine input (31%), atmospheric fallout (2.3%), and in-situ production (0.1%). The 234Th inventories were measured on four cruises, which occurred during periods of differing river discharge, wind stress, and flow rates of the North Brazil Current. The 234Th excess seabed inventories show large spatial and seasonal variability, with a range from 0 to 22 dpm cm-2. Ratios of predicted 234Th inventories over observed 234Th inventories indicate that a large portion of the inventories on the inner and mid shelf occur in the fluid mud layer or the suspended sediments. The 234Th excess seabed inventories are controlled by resuspension, cycling the inventories between the water column and the seabed. 210Pb and 234Th excess seabed inventories are compared directly using corrections for the different time scales and offshore concentrations. This approach indicates that for most of the shelf, the inventories of the shorter-term tracer (234Th) are less than predicted by the inventories of the longer-term tracer (210Pb). There are two explanations for this trend. The first is that a larger portion of the 234Th inventory occurs in the fluid muds or the water column relative to 210Pb. The second is that the supply of offshore water, scavenging efficiency, and/or deposition have been lower over the two year study period relative to the last one hundred years.
Smoak, J.M., DeMaster, D.J., Kuehl, S.A., Pope, R.H. & McKee, B.A. (1996). The behavior of particle-reactive tracers in a high turbidity environment: 234Th and 210Pb on the Amazon continental shelf. Geochimica et Cosmochimica Acta, 60(12), 2123-2137.
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