Paleosols and Paleoclimate

• Soils are everywhere and can be used in conjunction with other proxies that are limited geographically

• Maintain “Geochemical Fingerprints” of plant and animals

• Are now considered a critical link to ocean biogeochemistry

Paleosols

Empirical Studies of Climate Change

• Instrumental DataClimate Elements

-Temperature - Rainfall - Humidity - Wind

• Proxy DataIce Cores -Stable Isotopes - Radiometric Dating Dendroclimatology Ocean/Lake Sediments - Biogenic Material - Terrigenous Matter - Pollen Analysis Terrestrial Sediments- Glaciers

- SOILS

Geologic or Deep Time

99.95% of Earth History

Biotic Fingerprints in soil

Soil Organic MatterCarbonates and Silicates

What is a good proxy ?

• The proxy (i.e. tree-ring width, stable isotope composition of ice) is sensitive to changes in environmental conditions (temperature, precipitation, productivity, or other).

• A good proxy can be calibrated (i.e. establish studies that provide calibration of the proxy in contemporary settings or across environmental gradients).

• A good proxy “records or finger prints” climatic or biological information and preserves it for long periods of time (microbial life, ice, minerals, organic matter)

C4 vs. C3 grass in Great Plains

Carbon isotopes and paleoclimate

• Carbon has two stable isotopes: 13C and 12C .

• 13C is heavier than 12C .

• The amount of 13C compared to 12C is expressed using delta notation:

• Fractionation: Natural processes tend to preferentially take up the lighter isotope, and preferentially leave behind the heavier isotope.

δ13C ‰ = 13C / 12C of sample - 13C/ 12C of standard13C / 12C of standard 1000

Carbon Isotopes and Plants (grasses)

Two photosynthetic pathways in grass:

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

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

13C varies in terrestrial systems

Controls on Pedogenic Carbonates

• Form when low productivity and/or arid and/or high calcium or bicarbonate

• Soil is open system (CO2 produced many times faster than CaCO3 precipitates)

• So isotopic equilibrium reaction (gas-soln-solid)

Ca2+ + 2HCO3- ped-CaCO3 + CO2 +

H2O

Pedogenic carbonates

13C of CaCO3 controlled by d13C of soil CO2

13C of soil CO2 controlled by:– Proportion of C3 and C4 veg – Diffusion– Productivity and CO2 production rate

- Heavier isotope accumulates in the solid phase

Ca2+ + 2HCO3- ped-CaCO3 + CO2 +

H2O

How does Ca13CO3 form?

• TWO fractionation steps:

δ 13Cped = δ 13Csom + ∆CO2 diffus + ∆CO2-CaCO3

• ∆CO2 diffus accounts for slower diffusion by heavier molecule, ~4.4‰

• ∆CO2-CaCO3 accounts for equilibrium fractionation during phase changes– Temperature sensitive in temperature range of soils:

103lnaCO2-CaCO3= -2.988(106T-2)+7.6663(103T-1)-2.4612

• 10.3‰ at 20°C

δSOM δ soil-respired CO2

δ 13C of CaCO3 controlled by SOM

δ 1

3C

SO

M

δ 13C soil carbonate

O°C

35°C

Site productivity

Higher production means less influence of atmosphere

δ SOM not a constant, so neither is δ CO2 or δ carbonate

C4 veg

C3 veg

Pedogenic carbonates

• So in a pure C3 community pedogenic carbonates should be near ____ ‰ vs. _____ in a pure C4 community

• Natural range of δ 13C in CaCO3 –12‰ to +4‰

-12.6+ 1.2

Oxygen isotopes and paleoclimate

• Oxygen isotopes are fractionated during evaporation and precipitation of H2O– H2

16O evaporates more readily than H218O

– H218O precipitates more readily than H2

16O

• Oxygen isotopes are also fractionated by marine organisms that secrete CaCO3 shells. The organisms preferentially take up more 16O as temperature increases.

Oxygen isotopes and paleoclimate

• Oxygen has three stable isotopes: 16O, 17O, and 18O. (We only care about 16O and 18O.)

• 18O is heavier than 16O.

• The amount of 18O compared to 16O is expressed using delta notation:

• Fractionation: Natural processes tend to preferentially take up the lighter isotope, and preferentially leave behind the heavier isotope.

d18O ‰ = 18O/16O of sample -18O/16O of standard18O/16O of standard

1000

Oxygen Isotopes

OceanH216O, H2

18O

Evaporation favorsH2

16O H218O

Precipitation favorsH2

18O

H218O

Snow and ice are depleted in H2

18O relative to H2

16O.

Land

Ice

Carbonate sediments in equilibriumwith ocean water record a δ18O signal which reflects the δ18O of seawater and the reaction of marine CaCO3

producers to temperature.

CaCO3

δ 18O of CaCO3 controlled by meteoric water (rain +

snow)

R^2 = 0.84

δ 1

8O

wate

r

δ 18O carbonate

Fractionation during formation

Cerling and Quade 1993; Kelly et al

1991

10+4.4

More C4 when warmer & drier, but that is not the

whole story

Amount of evaporation

& temperature

Var in 13C due to atmos

contribution

Properties of Phytoliths

• Chemically Simple SiO2 x nH20• Contain 1-3% C (C-14 dates and C13

content obtainable)• 1-10% of the soil mass• Stable in soils• Different density than soils• Morphologically unique

C4 vs. C3 grass in Great Plains

Phytolith Distribution and Age

Stable C isotope composition vs C-

14 date

Paleosols in eastern Colorado

0-10 ka Bignell Loess

10-13 ka Brady Soil

13-23 ka Peoria Loess

Grassland Evolution and Expansion

Quade and Cerling, 1995

Kelly et al 1998 Stromberg, 2004

Cerling et al, 1997

Cerling et al. 2010

Relationships between the fraction of C4 biomass from paleosols δ13C values of SOC in modern ecosystems, ecosystem classification, and % woody canopy cover

Goals of Great Plains Research

• Reconstruct temperature and precipitation changes during last 15ka– Stable C, O isotopes of plant opal phytoliths

• Reconstruct vegetation changes, especially at LGM-Holocene transition and during Holocene– Plant opal phytoliths– Thin section micromorphology– Faunal fabrics - cicadas versus earthworms– Preliminary assessment of Si and C

sequestration in loess

Paleosols in eastern Colorado

0-10 ka Bignell Loess

10-13 ka Brady Soil

13-23 ka Peoria Loess

Research Sites-Great Plains

1. Beecher Island CO

2. Wauneta NE

3. Moran Canyon NE1

2

3

Similar distancesfrom sand sources;increasing precip.

gradient

Beecher Island CO

Peoria Loess

Bignell LoessBrady Soil

Cicada burrowsas indicators ofshrub-steppe

paleovegetation

Old Wauneta NE

0-10 ka Bignell Loess

10-13 ka Brady Soil

13-23 ka Peoria Loess

13-10 ka Brady Soil at Old

Wauneta NE

A

Bk

BC

0-4 ka Buried Soils

at Old Wauneta NESamples for thin

section microscopy

Beecher Island CO

Warmer/dry

C4 grasses

Cooler/wet

C3 shrubs and

grasses ?

Mid Holocene thermal max ?

% C4 Vegetation

Kelly et al, unpublished

Paleosol carbonate δ13C and δ18O values along the east-west transect of

paleosols

East West

Kelly et al, unpublished

Increasing C4

Increasing temps

Key results to date• Peoria loess is older than the Bignell was

deposited in western Nebraska between about 25,000 and 14,000 OSL yr BP.

• Bignell loess, the youngest recognized in this region, is Holocene in age and was deposited starting approximately 11,000 to 9,000 yr BP and deposition continued episodically until less than 1000 yr BP.

• Bignell loess is thickest downwind of inactive dune fields, strongly suggesting that Bignell loess resulted from climatically driven episodes of Holocene dune activity.

Key results to date

• Isotopic shifts suggest dramatic changes in biota and climatic conditions over time.

• Oxygen isotope signatures suggest a time transgressive change in temperature (much like today)

• The range and variability is comparable to current regional variations in temperature

• The complexity of temperature relationships with regard to oxygen isotope technique are still be reconciled.