Presentation on theme: "Soil Carbonates as a Key to the Past Ca +2 + H 2 O + CO 2 = CaCO 3 + 2H + Calcium derived from weathering or atmospheric inputs C in carbonate from soil."— Presentation transcript:
Soil Carbonates as a Key to the Past Ca +2 + H 2 O + CO 2 = CaCO 3 + 2H + Calcium derived from weathering or atmospheric inputs C in carbonate from soil CO 2, which is derived from root respiration or humus decomposition Virtually all O in carbonate is from water in soil (which is derived from precipitation)
Grizzley Peak Miocene Paleosols: buried in basalt flows
Contact somewhat mixed Weathering seems to decrease with depth
Carbonate and paleosols Depth related to water balance C isotopes reflect: –Grass type –Atmopheric CO 2 concentration O isotopes reflect temperature and circulation
Depth of carbonate and precipitation Jenny and others (here from Retallack, 1994) have shown a relationship between depth of Bk and precipitation…..
Application to paleosols: Badlands of SD Base of Badlands highly weathered Eocene paleosol (Oxisols) Most of Badlands have Bk horizons
Depth of carbonate vs. time Eocene soils have no Bk (expected of Oxisols) Depth to Bk decreases with time (up section) indicating increasing aridity…..
Isotopes and carbonates 13 C = 1.11% 12 C= 98.89% 18 O=~0.20% 16 O=~99.8 % 13 C or 18 O = (R sample /R std -1)1000 Where R sample = 13 C/ 12 C or 18 O/ 16 O and R std = ratio in marine carbonate (C ) or ocean water (O) + values more heavy isotope than std - values less heavy isotope than std
Carbon isotopes in carbonate indicate plant type CO 2 atm plant CO 2 soil CaCO 3 Therefore, 13 C CaCO3 - 14 o/oo = 13 C plant CO 2 =-7 o/oo plant = -27 or -14 o/oo Soil C = plant Soil CO 2 = plant CaCO 3 = soil CO2 + 14 o/oo
Carbon Isotopes and Plants (grasses) Two photosynthetic pathways in grass: C 3 = -27 o/oo (cool season grasses) C 4 = -14 o/oo (warm season - tropical -grasses)
O Isotopes in carbonate Cloud Water Rain Water (-9 o/oo) soil water (same as rain or positive) carbonate (+~28 o/oo temperature dependent) Therefore: 18 O carbonate - 28 o/oo = 18 O soil water (~ rain water) H 2 O CaCO 3
What do O isotopes in rain tell us? O isotopes related to regional temperature O isotopes also vary with circulation patterns (water sources) Ocean = 0 o/oo Cloud = -9 o/oo Rain and remaining clouds become more depleted in 18 O Rain 9 o/oo more enriched than cloud
Oxygen isotopes in rain loosely correlated to MAT Basis for interpretation of ice cores and our estimates of earth temperature during Quaternary Caveats: –Correlation is weak in modern world (and doesn’t even work some places –No guarantee that past can be explained by these modern relationships –However, O isotope shifts mean some significant environmental changes to hydrological cycle
Oxygen Isotopes and storm paths in USA O isotopes in USA precipition due to combined effect of storm source (Pacific vs. Gulf) and distance traveled (which is related to MAT).
C and O isotope summary C isotopes in carbonate tell us about photosynthetic pathway of plants (and proportion of C 4 grasses) Later - we will talk about how changes in atmospheric CO2 can exert a secondary impact on C isotopes O isotopes tell us about some combination of temperature and circulation changes
Paleosol Example 2: Paleosol carbonate isotope record in miocene Pakistan soil carbonates Work of Jay Quade and Thure Cerling Nature.1989. 342:163 Nature. 1997. 389:153 A variety of soils buried by floodplain deposits on rivers draining Himalayas –Entisols –Aridisols –Mollisols –Alfisols
Modern floodplain environments in Pakistan Dominated by summer precipitation of south Asian monsoon Dominated (where not cultivated) by C 4 grasses (and snakes) Soil types include Entisols, Inceptisols, and Alfisols
Carbon Isotopes of carbonate (and organic matter where present) show large shift to C 4 flora about 6 Ma Shift from pure C3 to pure C4 (the modern situation) Why?
Oxygen isotopes show simulataneous shift toward more positive (18O rich) values Why? Temperature got warmer? Circulation shifted toward summer monsoonal precipitation (vs. winter precipitation –Consistent with shift to C4 grasses (they are summer grasses) –Summer is warmer and generally has more 18O –Winter cooler and less 18O –What could cause a circulation change?
Role of Himalayas on South Asian Monsoon Heating of Tibetan Plateau creates a summer low pressure that drives summer onshore flow of wind and moisture in south Asia. Without the Himalayas and related highlands, the circulation and precipitation patterns are modeled to be very different…..
With much lower Himalayas (500 m)…. Summer wind patterns change (monsoon is reduced) Summer rain is significantly reduced (2 to 4 mm/day) So, uplift of Himalayas could (1) shift precipitation seasonality and (2) shift to summer vegetation (grass)
Present state of the cause of C4 grassland expansion Two view: 1. Cerling and colleagues now support idea that C4 grasses expanded because global atmospheric CO2 levels dropped (favoring C4 vs. C3 photosynthesis) 2. Most others argue against it: a. C4 grassland did not appear globally at same time b. C4 grassland only exist where summer precip dominates c. Evidence suggests CO2 wasn’t very low d. Thus, paleosols provide environmental conditions but reasons still disputed
Carbon isotopes in carbonates as a guide to atmospheric CO2 concentrations 13 C soil CO2 = 13 C biological CO2 + 13 C atm CO2 -in modern world, atm CO2 unimportant below 50 cm -In past world with high CO2, atm signal will be more pronounced…….. Atmospheric CO2 = ~ -7 o/oo Biological CO2 = -27 o/oo Diffusion of soil CO2 into atmosphere
Carbon isotope CO2 barometer 13 C soil CO2 = (1-F) 13 C biological CO2 + (F) 13 C atm CO2 Where F = function of atmospheric CO2 pressure (concentration) As atmospheric CO2 increases, the C isotope composiiton of soil carbonate becomes more positive (larger atm signal)……..