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Jan A. Schuessler, Julien Bouchez , Friedhelm von Blanckenburg , David Uhlig, Jean L. Dixon

Modeling Novel Stable Isotope Ratios in the Weathering Zone ----- Application to Mg Isotopes as Weathering Tracer in the Sierra Nevada, California. Jan A. Schuessler, Julien Bouchez , Friedhelm von Blanckenburg , David Uhlig, Jean L. Dixon

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Jan A. Schuessler, Julien Bouchez , Friedhelm von Blanckenburg , David Uhlig, Jean L. Dixon

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  1. Modeling Novel Stable Isotope Ratios in the Weathering Zone-----Application to Mg Isotopes as Weathering Tracer in the Sierra Nevada, California Jan A. Schuessler, Julien Bouchez, Friedhelm von Blanckenburg, David Uhlig, Jean L. Dixon German Research Centre for Geosciences GFZ, Potsdam, Germany acknowledgements for support Matt Meadows, Jason Smith, Ryan Lucas and R. Hoang (Sierra Nevada Research Institute, UC Merced) Carolyn Hunsaker (USDA Forest Service)

  2. Mg isotope as weathering tracer - study site Southern Sierra Critical Zone Observatory Kings River Experimental Watersheds (KREW) samplinglocations • sampling: • bedrock • soil/saprolitedepth profiles • vegetation • sediments • soil water and stream water sampling time series (ongoing thanks to Matt) soilwater streamwater groundwater 0.992 km2 1.323 km2 P300 0.487 km2

  3. [Mg] and Mg isotope composition in the SN weathering zone

  4. Mg isotope composition in the Sierra Nevada weathering zone • constant d26Mg in soil and saprolite depth profiles; equal to bedrock and river sediments • isotopically light water • isotopically heavy plants (wood), which show Mg isotope fractionation during intra-plant Mg translocation • Plant uptake controls the Mg isotope fractionation!!

  5. Development of a steady-state mass balance model of a weathering system from the scale of a soil column to that of a large river Vegetation Water Regolith (soiland saprolite) Model allows quantitative prediction of erosion and weathering fluxes from stable isotope ratios Bedrock

  6. Testing the model to predict steady state and quantify Mg export by chemical weathering in the Sierra Nevada weathering zone Independent E/W data agrees with model results from Mg isotopes 1:1 steady state EMg= E⋅ [Mg]sediment WMg = Q ∙ [Mg]diss. stream • fraction of Mg export by chemical weathering derived from EMg(10Be)/WMg(discharge) consistent with Mg isotope model • only 5% of the total Mg is exported in dissolved form; 95% is eroded in solids • kinetically limited weathering regime • d26Mg data is consistent with measured Mg export from the weathering zone by erosion (from 10Be) and weathering (discharge) • … suggesting steady state

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