The behavior of particle-reactive tracers in a high turbidity environment: {sup 234}Th and {sup 210}Pb on the Amazon continental shelf

Journal Article · Sat Jun 01 04:00:00 EDT 1996 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/0016-7037(96)00092-0· OSTI ID:285088

Smoak, J M; DeMaster, D J; Pope, R H ^[1]; Kuehl, S A ^[2]; McKee, B A ^[3]

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1. North Carolina State Univ., Raleigh, NC (United States)
2. Virginia Inst., of Marine Science, Gloucester Point, VA (United States)
3. Louisiana Marine Consortium, Chauvin, LA (United States)

Excess {sup 234}Th and {sup 210}Pb seabed inventories were measured in cores collected from the Amazon continental shelf to examine particle scavenging and seabed dynamics. Typical excess {sup 210}Pb inventories range from 100 to 300 dpm cm{sup {minus}2}, and the total excess {sup 210}Pb inventory for the Amazon shelf was determined to be 2.7 x 10{sup 17} dpm. The {sup 210}Pb measurements indicate that particle-reactive species are scavenged not only form the Amazon River but also from the lateral advection of offshore water. In order to sustain the {sup 210}Pb 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 {sup 210}Pb to the Amazon continental shelf with relatively small contributions from riverine input (31%), atmospheric fallout (2.3%), and in-situ production (0.1%). The {sup 234}Th 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 {sup 234}Th excess seabed inventories show large spatial and seasonal variability, with a range from 0 to 22 dpm cm{sup {minus}2}. This approach indicates that for most of the shelf, the inventories of the shorter-term tracer ({sup 234}Th) are less than predicted by the inventories of the longer-term tracer ({sup 210}Pb). There are two explanations for this trend. The first is that a larger portion of the {sup 234}Th inventory occurs in the fluid muds or the water column relative to {sup 210}Pb. 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. 38 refs., 7 figs.

• OSTI ID: 285088
• Journal Information: Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: 12 Vol. 60; ISSN GCACAK; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
ALLUVIAL DEPOSITS
AMAZON RIVER
BRAZIL
COASTAL REGIONS
CONTINENTAL SHELF
ECOLOGICAL CONCENTRATION
LEAD 210
THORIUM 234
TRACER TECHNIQUES