Soil Carbonates as a Key to the Past

Ca+2 + H2O + CO2 = CaCO3 + 2H+

• Calcium derived from weathering or atmospheric inputs

•C in carbonate from soil CO2, 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

Closeup of paleosol/basalt contact

Carbonate and paleosols

• Depth related to water balance

• C isotopes reflect:

–Grass type

–Atmopheric CO2 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

13C = 1.11%12C= 98.89%

18O=~0.20%16O=~99.8 %

13C or 18O = (Rsample/Rstd -1)1000

Where Rsample = 13C/12C or 18O/16O and

Rstd= 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

CO2 atm plant CO2 soil CaCO3

Therefore,

13CCaCO3 - 14 o/oo = 13Cplant

CO2 =-7 o/oo

plant =

-27 or

-14 o/oo

Soil C = plant

Soil CO2 = plant

CaCO3 = soil CO2 + 14 o/oo

Carbon Isotopes and Plants (grasses)

Two photosynthetic pathways in grass:

C3 = -27 o/oo (cool season grasses)

C4 = -14 o/oo (warm season - tropical -grasses)

C4 vs. C3 grass in Great Plains

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:

18O carbonate - 28 o/oo = 18O soil water (~ rain water)

H2O CaCO3

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 18O

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 C4 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 C4 grasses (and snakes)

• Soil types include Entisols, Inceptisols, and Alfisols

Carbon Isotopes of carbonate (and organic matter where present) show large shift to C4 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

13C soil CO2 = 13C biological CO2 + 13C 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

13C soil CO2 = (1-F) 13C biological CO2 + (F)13C 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)……..

Biological draw down due to coal burial

General decline during late Cenozoic

Today: ~ 370 ppm and rising