THE BEHAVIOR OF PARTICLE-REACTIVE TRACERS IN A HIGH TURBIDITY ENVIRONMENT - TH-234 AND PB-210 ON THE AMAZON CONTINENTAL-SHELF

Excess Th-234 and Pb-210 Seabed inventories were measured in cores col lected from the Amazon continental shelf to examine particle scavengin g and seabed dynamics. Typical excess Pb-210 inventories range from 10 0 to 300 dpm cm(-2), and the total excess Pb-210 inventory for the Ama zon shelf was determined to be 2.7 x 10(17) dpm. The Pb-210 measuremen ts indicate that particle-reactive species are scavenged not only from the Amazon River but also from the lateral advection of offshore wate r. In order to sustain the Pb-210 inventories, the volume of water sup plied by the lateral advection from offshore must be approximately fiv e to ten times the water discharge of the Amazon River. This lateral a dvection supplies about 67% of the total excess Pb-210 to the Amazon c ontinental shelf with relatively small contributions from riverine inp ut (31%), atmospheric fallout (2.3%), and in-situ production (0.1%). T he Th-234 inventories were measured on four cruises, which occurred du ring periods of differing river discharge, wind stress, and flow rates of the North Brazil Current. The Th-234 excess Seabed inventories sho w large spatial and seasonal variability, with a range from 0 to 22 dp m cm(-2). Ratios of predicted Th-234 inventories over observed Th-234 inventories indicate that a large portion of the inventories on the in ner and mid shelf occur in the fluid mud layer or the suspended sedime nts. The Th-234 excess Seabed inventories are controlled by resuspensi on, cycling the inventories between the water column and the seabed. P b-210 and Th-234 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 (Th-234), less than predicted by the invent ories of the longer-term tracer (Pb-210). There are two explanations f or this trend. The first is that a larger portion of the Th-234 invent ory occurs in the fluid muds or the water column relative to Pb-210. T he second is that the supply of offshore water, scavenging efficiency, and/or deposition have been lower over the two year study period rela tive to the last one hundred years.