1. Modeling Novel Stable Isotope Ratios in the Weathering Zone ----- Application to Mg Isotopes as Weathering Tracer in the Sierra Nevada, California J AN A. S CHUESSLER, J ULIEN B OUCHEZ, F RIEDHELM VON B LANCKENBURG, D AVID U HLIG, J EAN L. D IXON 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 sampling: bedrock soil/saprolite depth profiles vegetation sediments soil water and stream water sampling time series (ongoing thanks to Matt) 0.992 km 2 1.323 km2 0.487 km 2 Southern Sierra Critical Zone Observatory Kings River Experimental Watersheds (KREW) P300 soil water stream water ground water sampling locations

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

4. Mg isotope composition in the Sierra Nevada weathering zone constant  26 Mg 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. Bedrock Regolith (soil and saprolite) Vegetation Water 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 Model allows quantitative prediction of erosion and weathering fluxes from stable isotope ratios

6. Testing the model to predict steady state and quantify Mg export by chemical weathering in the Sierra Nevada weathering zone W Mg = Q ∙ [Mg] diss. stream E Mg = E ⋅ [Mg] sediment  26 Mg data is consistent with measured Mg export from the weathering zone by erosion (from 10Be) and weathering (discharge) … suggesting steady state steady state 1:1 fraction of Mg export by chemical weathering derived from E Mg ( 10 Be)/W Mg (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 Independent E/W data agrees with model results from Mg isotopes