Chemical Weathering, Part II: Weathering & River Chemistry

1. Chemical Weathering, Part II: Weathering & River Chemistry

2. Weathering Basics: Review & Structure • dissolution of ionic salts • carbonation of carbonates • oxidation of reduced minerals • silicate hydrolysis Types of weathering processes Typical chemical reactions associated with each process e.g. simple dissolution for halite; more complicated for pyrite oxidation (we’ll cover mechanism of silicate hydrolysis next week) Typical mechanisms for each reaction to take place Why each process is important in Earth’s environment e.g. global cycles of C & O; heavy metal release; formation of soils & other features like karst

3. What’s the interesting science here? What controls weathering processes? Why do they happen at specific times & places, and which reactions and mechanisms dominate? How quickly do they happen & what determines this? Can we quantitatively predict these things, in the past, present, and future? What do basic chemical principles tell us should be the case? principle of equilibrium; kinetic theory - next week What do we observe in the natural world & how does this relate to our theoretical predictions? field & laboratory studies

4. Why rivers? • arteries of the continents - integrate global terrestrial processes • major input to the oceans - regulate ocean chemistry & global geochemical cycles • surface water supply - critical natural resource for human civilization • How would you measure the “chemistry” of a river? • collect a sample in the field & bring it back while preserving it and without contaminating it • analyze it in the lab - but how do you determine the chemical composition of something like a river water?

5. Ion Chromatography Pass liquid sample through a ion-exchange resin Elute with a solution that displaces retained ions Detector Measure conductivity of output solution over time

6. Plasma Spectrometry Inject liquid sample into high-T plasma (Argon plasma) to excite ions • “Mass Spectrometry” • use high energy to force ions through a flight tube in vacuum • deflect ions of different mass with high-strength magnetic field • count all the ions of a given mass from a sample • “Optical Emmision Spectrometry” (OES) • excited ions emit light of characteristic frequency • emission intensity related to amount of each ion • measure emitted light of given frequency with optical detector

7. Plasma Spectrometry, cont. Element2 Mass Spectrometer, upstairs in this department: Fast scanning across a wide mass range

8. Standardization Chromatography & spectrometry give a measure of intensity for a specific chemical species Requires comparison to the intensity derived from a substance of known composition to determine “real” concentration in an unknown sample Inferred concentration Measured sample Pure halite Standard (NaCl)

9. What does river chemistry look like? Rivers Si Ca Different from rain - increase in Si; much more concentrated (>5 times more total ions) Na K Mg Rain Na Mg Must be explained by input from weathering to rivers Ca K Rivers also very different from rocks (e.g. extremely low Al, Fe) Rocks Al Ca Si Fe Must be explained by incongruent weathering processes K Mg Na

10. Explaining the general character of river solutions: The “inverse” of soil • resistant minerals (eg, quartz) don’t dissolve • aluminosilicates alter to clays • soluble elements removed in waters • some precipitate lower in soil or in sediments (Fe-oxides, carbonate) O-Layer: Organic Debris What’s left behind? “regolith” = accumulation of fine rock material “soil” = regolith plus organic matter - often vertically stratified A-Layer: Organics Ion-Depleted Clays Resistant Minerals B-Layers: Primary & Secondary Minerals C-Layer: Actively Weathering Rock (“Saprolite”) Fresh Rock

11. Variability of River Chemistry: Kalix River, Sweden Difference between filtering methods, more significant for Al Difference between seasonal pattern of Al&Mg [µM] 3 Al Bulk 2 1 <0.22µm <10kDa 0 Jan Feb Mar Apr May June [µM] 60 Mg Bulk 50 40 30 <0.22µm 20 <10kDa 10 0 Jan Feb Mar Apr May June

13. Geochemical Contributions to the Aquatic System • different processes control sediments & solutes • systems related, but can’t be treated together • problem of timescale

14. Malemchi Khola In-depth study of weathering rates across a climate gradient in central Nepal (West et al., Geology 2002) Himalayan Small Catchment Solutes Silicate cation weathering rates, dissolved, kmols.ha-1.yr-1: High Mountains: 1-2 Middle Hills: 3-4 Higher weathering rate attributed to higher temperature, despite lower runoff & erosion rates Langtang Lirung

15. Sediment Chemistry • no signal in High Himalayas - erosion too fast • more intense weathering in Middle Hills - as expected • consistent with soil chemistry

16. Quantitative Treatment of Sediment Chemistry Total Denudation = Physical Erosion (ie, sediments) + Chemical Weathering (Louvat, Gaillardet @ Paris) Total Denudation = Soil Formation + Chemical Weathering (Riebe @ UC Berkeley) “Steady State” = weathering determined from sediment/soil chemistry is equivalent to weathering determined from dissolved chemistry

17. 2 + 2  4 for Weathering Fluxes !! • series of possible explanations - both natural & analytical • dissolved & particulate loads not a single system, at least as measured • can we better understand the sediment signal? Same observed from soil data …

18. The Potential of U/Th • define time scale of sediment reaction - independent measure of particulate behavior • U & Th fractionate during weathering • return to “equilibrium” governed by decay constants, providing time information Model U/Th fractionation & compare to observation from sediments to determine “weathering time” (Vigier et al., 2001)

19. Applying the U/Th Model “Sediment Transfer Time” in Large Himalayan Rivers Chabaux et al., 2006 Andes System, Dosetto et al., 2006 Do U/Th “Times” Link to Sediment Chemistry? This is a new & unexplored research question . . . Important across Environmental Geochemistry

20. most samples very close to equilibrium • significant differences in initial U/Th ratios U/Th of Himalayan Range Soils & Sediments Not possible to quantify timescale of early-stage erosion!! • no trend through soil profiles with depth • sediments show more disequilibrium than soils

21. A Discrepancy… • soils & sediments have similar depletion of weatherable elements - yet sediments have greater U fractionation • soils & sediments both significantly weathered despite overall little U/Th disequilibrium

22. U-Series Timescales After Dosetto et al., 2006 Little U/Th fractionation during early stages of chemical weathering - U-series may operate differently from other geochemical processes What does the U-series ‘weathering (or transport) timescale’ really represent?