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 supportMatt Meadows, Jason Smith, Ryan Lucas and R. Hoang (Sierra Nevada Research Institute, UC Merced)

Carolyn Hunsaker (USDA Forest Service)

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 km2

1.323 km2

0.487 km2

Southern Sierra Critical Zone ObservatoryKings River Experimental Watersheds (KREW)

P300

soil waterstream waterground water

sampling locations

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

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!!

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

Testing the model to predict steady state and quantify Mg export by chemical weathering in the Sierra Nevada weathering zone

WM

g = Q

∙ [M

g]di

ss. s

tream

EMg =

E ⋅[

Mg]

sedi

men

t

• d26Mg 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 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

Independent E/W data agrees with model results from Mg isotopes