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 =-7 o/oo
plant =
-27 or
Soil CO2 = plant
-14 o/oo
CaCO3 = soil
CO2 + 14 o/oo
Soil C = plant
CO2 atm  plant  CO2 soil  CaCO3
Therefore,
13CCaCO3 - 14 o/oo = 13Cplant
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
H2O CaCO3
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)
What do O isotopes in rain tell us?
Cloud =
-9 o/oo
Rain 9 o/oo more
enriched than
cloud
Ocean = 0
o/oo
Rain and remaining clouds become more depleted in 18O
• O isotopes related to regional
temperature
•O isotopes also vary with
circulation patterns (water
sources)
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
Atmospheric CO2 =
~ -7 o/oo
Diffusion of soil CO2
into atmosphere
Biological CO2
= -27 o/oo
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……..
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)……..
Today: ~ 370 ppm and
rising
General decline during
late Cenozoic
Biological draw down due
to coal burial