T6 soil base cation weathering rates

Transcription

1 T6 soil base cation weathering rates julian aherne :: trent university FORFLUX :: biogeochemistry of irish forests [RSF 07510] Advisory Group Meeting [5 6 December 2011]

2 objective (a) to determine the release of base cations (calcium, magnesium, potassium and sodium) from weathering at the Level IIi plots, and (b) to develop a simplified regional approach to estimate weathering rates. the promise (a) new measurements of soil mineralogy, total analysis and particle size analysis, and (b) test inference of soil mineralogy from total analysis at the Level IIi plots. the reality (a) analysed all mineral biosoil samples for total oxides [n = 85], (b) filled quantitative mineralogy gaps at Level IIi plots, (c) carried out direct estimates of soil surface area and particle size at Level IIi plots, and (d) applied a regional site specific approach to determine weathering rate for all biosoil plots weathering total oxide analysis and mineralogy the sites

3 Level I and II Forflux Level II intensive

4 weathering is the physical and chemical breakdown (weathering) of rock and soil minerals and release of soil base cation (Ca 2+, Mg 2+, K + and Na + ) factors geology (mineralogy) of parent material, acidity of soil solution, temperature and surface reaction rates

5 the weathering rate top nine 1. skokloster assignment 2. soil type texture approximation 3. zirconium method 4. total base cation content correlation 5. PROFILE model 6. steady state water chemistry model 7. catchment mass balance 8. isotopic approaches 9. dynamic models

6 PROFILE multi layer steady state soil geochemical model widely used for the determination of weathering rates (Warfvinge and Sverdrup 1992). Key input data are quantitative soil mineralogy, soil surface area, bulk density and moisture (weathering reactions occur on the wetted mineral surface) mineralogy quantitative soil mineralogy is expensive [$$$], as such, data are available for only ~25 sites (including Level IIi [ballinastoe, brackloon, roundwood and ballyhooly]). However, normative procedures can be used to estimate idealised quantitative soil mineralogy based on the whole rock geochemistry of the soil (i.e., total analysis, which is cheap [$])

7 analysis to mineralogy (A2M) a computer program to determine, for given total chemical analyses, all possible mineral modes from an arbitrary but pre specified set of minerals (Posch and Kurz, 2007) total analysis for biosoils (analysis for all mineral soil layers). biosoil data [particle size, bulk density, etc] A2M weathering of base cations quantitative mineralogy PROFILE

8 step one send off soil samples for total oxide analysis (n ~ 85 samples, plots ~ 33, 1 4 mineral horizons per site) SiO2 Al2O3 Fe2O3 MnO MgO CaO K2O Na2O TiO2 Ballinastoe Brackloon Dromahaire Kilworth Mountrath Taughboy Thomastown step two determine the typical mineralogy for irish forest soils, based on available quantitative soil mineralogy (n ~25): quartz, muscovite (paragonite, Illite), chlorite, plagioclase (albite), k feldspar, rutile, hematite, kaolinite, calcite, dolomite

9 !FORFLUX minerals: Si Ti Al Fe2 Mn Ca Mg Na K P qtz NA quartz kfs K-Feldspar K-feldspar or K-Feldspar anorthoclase ab Plagioclase albite an Plagioclase anorthite lab Plagioclase labradorite ms Muscovite muscovite par Muscovite paragonite ill Muscovite illite ill Muscovite illite-2 cli Fe-Chlorite clinochlore cha Fe-Chlorite chamosite pen Fe-Chlorite pennantite ap Apatite apatite dol Calcite dolomite cal Calcite calcite ant NA anatase klt NA kaolinite gbs NA gibbsite hem NA hematite step three tell A2M which minerals exist in each total oxide sample and let it solve for mineralogy

10 Site: Ballinastoe ( g): grams Si Al Fe2 Mn Mg Ca K Na K-Feldspar Muscovite Plagioclase Fe-Chlorite Site: Dromahaire ( g): grams Si Al Fe2 Mn Mg Ca K Na K-Feldspar Muscovite Plagioclase Fe-Chlorite Site: Kilworth ( g): grams Si Al Fe2 Mn Mg Ca K Na K-Feldspar Muscovite Plagioclase Fe-Chlorite Site: Taughboy ( g): grams Si Al Fe2 Mn Mg Ca K Na K-Feldspar Muscovite Plagioclase Fe-Chlorite Site: Thomastown ( g): 4 Groups grams Si Al Fe2 Mn Mg Ca K Na K-Feldspar Muscovite Plagioclase Fe-Chlorite how well does it work? Level IIi sites have both total oxides and mineralogy, also available for a few other sites (n = 8). Comparison of A2M mineralogy with quantitative is good (eyeball statistic)

11 step four apply PROFILE with site specific mineralogy, mineral stiochemistry, bulk density, surface area, etc base cation weathering rates (mol c ha 1 year 1 ) site base cations calcium magnesium potassium sodium tauboy dromhaire ballinastoe thomastown kilworth

12 conclusions (a) base cation weathering rates determined for biosoil plots using PROFILE in conjunction with A2M mineralogy, (b) procedure can be streamlined to one oxide sample per soil profile, and (c) it is a relatively simple approach to estimate regional weathering rates. deliverables (a) database of total oxides and mineralogy, and (b) data source to input output budget publication