HR: 1340h
AN: OS33A-0983    [Abstracts]
TI: Tracing Chemical Weathering Fluxes in the Fly River, Papua New Guinea
AU: * Kurtz, A C
EM: kurtz@bu.edu
AF: Boston University, Department of Earth Sciences, Boston, MA 02215, United States
AU: Moore, E A
EM: emoore2@bu.edu
AF: Boston University, Department of Earth Sciences, Boston, MA 02215, United States
AB: The mountainous rivers of Papua New Guinea have the highest basin-area normalized Si and alkalinity fluxes of all major world rivers. Our work seeks to understand the controls on these anomalously high weathering fluxes. In January 2007, with the assistance of Ok Tedi Mining, LTD., we sampled the Fly River system extensively from upland tributaries to the floodplain confluence of the Middle Fly and Strickland Rivers. The highest alkalinities (up to 3400 μeq/L) come from upland streams draining Mesozoic limestones near the drainage divide. The highest Si concentrations (up to 314 μmol/L) are found in major upland tributaries draining Mesozoic clastic sediments outcropping to the south of the high peaks. These observations suggest a decoupling between the production of alkalinity and silicate weathering, which has implications for the system's impact on the long- term carbon cycle. Despite lower overall alkalinity and Si concentrations in the main-stem Middle Fly River, we observed a significant downstream increase in the fluxes of both alkalinity (3.1 fold) and silica (2.3 fold) carried by the Middle Fly as it crosses the lowland floodplain. The increased flux reflects a combination of new water (and solutes) from floodplain tributaries and return flow of floodwaters to the channel. This observation may indicate that weathering reactions in the lowland floodplain contribute significantly to the river's dissolved load. We are currently applying several tracers to help resolve some of the ambiguities that arise from the major solute data. We are using the Sr isotopic contrast between Miocene carbonates (⁸⁷Sr/⁸⁶Sr ~ 0.708) and Jurrasic clastic sediments (⁸⁷Sr/⁸⁶Sr ~ 0.715) to model the relative contributions of carbonate vs. silicate weathering to the river's alkalinity flux. Sediment fluxes in the Fly River system are profoundly impacted by mining activity at Ok Tedi. The impact on the river's dissolved load is less significant, but detectable. High germanium concentrations in mine sulfides allow streamwater Ge/Si ratios to serve as a tracer. The Ok Tedi signal is observed as anomalously high Ge/Si ratios (10 μmol/mol) in upland tributaries receiving mine influence. This mine-derived signal decreases downstream due to input of silica from pristine upland tributaries and floodplain weathering.
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 1806 Chemistry of fresh water
DE: 1886 Weathering (0790, 1625)
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting