CALL FOR PROPOSALS:

ORGANIZERS

  • Harvey Thorleifson, Chair
    Minnesota Geological Survey
  • Carrie Jennings, Vice Chair
    Minnesota Geological Survey
  • David Bush, Technical Program Chair
    University of West Georgia
  • Jim Miller, Field Trip Chair
    University of Minnesota Duluth
  • Curtis M. Hudak, Sponsorship Chair
    Foth Infrastructure & Environment, LLC

 

Paper No. 4
Presentation Time: 8:50 AM

LITHOLOGIC CONTROL ON RIVER CHEMISTRY IN CENTRAL PANAMA


HARMON, Russell S.1, LYONS, W. Berry2, PRIBIL, Michael J.3, OGDEN, Fred L.4, WELCH, Susan A.5, WELCH, Kathleen A.6, WEGNER, Wencke7 and WORNER, Gerhard7, (1)Dept. of Marine, Earth, & Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, (2)Byrd Polar Research Center and School of Earth Sciences, The Ohio State University, Columbus, OH 43210, (3)USGS, Denver Federal Center, MS 973, Denver, CO 80225-0046, (4)Department of Civil and Architectural Engineering, University of Wyoming, 1000 E. Univ. Ave, Laramie, WY 82071, (5)School of Earth Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, OH 43210-1398, (6)Byrd Polar and Climate Research Center, The Ohio State University, 1090 Carmack Rd, 108 Scott Hall, Columbus, OH 43210-1002, (7)Geoscience Center Gottingen, University of Gottingen, Goldschmidtstrasse 1, Gottingen, Germany, rsharmon@ncsu.edu

Chemical weathering of geological materials is a primary process affecting landscape development. Hydrochemical examination of 97 rivers and streams along a ~400 km transect across Panama from the Bayano region in the east to the Costa Rica border in the west documents large variations, e.g. SPC = 38-2924 mS/cm, Si = 0.005-3.02 mM & Ca/Mg = 0.68-5.32 for large streams and rivers compared to SPC = 32-1736 mS/cm, Si = 0.011-1.601 mM, & Ca/Mg = 0.66-33.7 for soil seeps and small streams. Watershed lithology exerts the major control on riverine chemistry, as illustrated by two adjacent watersheds in central Panama. The Rio Pacora watershed (374 km2) is predominantly composed of mafic bedrocks consisting of Late Cretaceous gabbro and diorite, as compared to the Rio Chagres watershed (580 km2) which is developed on more a compositionally and lithologically varied bedrock that ranges from hydrothermally altered Late Cretaceous igneous lithologies (gabbro to granite, basalt to dacite) and Mid-Tertiary volcanic rocks. In order to better determine the rocks/minerals undergoing weathering in each watershed, 20 samples from soil seeps and low-order headwater streams to the main stem rivers were analyzed for their 87Sr/86Sr signatures and compared to bedrock compositions. Data for the two watersheds form a linear trend on a 87Sr/86Sr versus 1/Sr plot, with no difference observed between the soil seeps, headwater streams, and main channel river waters. Rio Chagres waters are enriched in 87Sr and depleted in Sr and also have lower TDS levels and Ca/Mg ratios compared to Rio Pacora waters, clearly documenting lithologic control on water chemistry throughout the two watersheds. Less-radiogenic Sr-isotope water ratios are equivalent to values measured for the least-altered rocks in the two watersheds, whereas the more radiogenic values suggest an input from weathering of minerals produced via early post-deposition alteration of volcanic lithologies by seawater.
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