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 Data Climate Elements -Temperature - Rainfall - Humidity - Wind Proxy Data Ice 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 Matter Carbonates 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: 13 C and 12 C. 13 C is heavier than 12 C. The amount of 13 C compared to 12 C is expressed using delta notation: Fractionation: Natural processes tend to preferentially take up the lighter isotope, and preferentially leave behind the heavier isotope. δ 13 C ‰ = 13 C / 12 C of sample - 13 C/ 12 C of standard 13 C / 12 C of standard  1000

Carbon Isotopes and Plants (grasses) Two photosynthetic pathways in grass: C 3 = -27 o/oo (cool season grasses and trees) C 4 = -14 o/oo (warm season - tropical -grasses)

13 C 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 (CO 2 produced many times faster than CaCO 3 precipitates) So isotopic equilibrium reaction (gas- soln-solid) Ca HCO 3 -  ped-CaCO 3 + CO 2 + H 2 O

Pedogenic carbonates  13 C of CaCO 3 controlled by d 13 C of soil CO 2  13 C of soil CO 2 controlled by: –Proportion of C3 and C4 veg –Diffusion –Productivity and CO 2 production rate - Heavier isotope accumulates in the solid phase Ca HCO 3 -  ped-CaCO 3 + CO 2 + H 2 O

How does Ca 13 CO 3 form? TWO fractionation steps: δ 13 C ped = δ 13 C som + ∆ 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: 10 3 lna CO2-CaCO3 = (10 6 T -2 ) (10 3 T -1 ) ‰ at 20°C

δ SOM  δ soil-respired CO 2

δ 13 C of CaCO 3 controlled by SOM δ 13C SOM 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 CO 2 or δ carbonate C4 veg C3 veg

Pedogenic carbonates So in a pure C 3 community pedogenic carbonates should be near ____ ‰ vs. _____ in a pure C 4 community Natural range of δ 13 C in CaCO 3 –12‰ to +4‰

Oxygen isotopes and paleoclimate Oxygen isotopes are fractionated during evaporation and precipitation of H 2 O –H 2 16 O evaporates more readily than H 2 18 O –H 2 18 O precipitates more readily than H 2 16 O Oxygen isotopes are also fractionated by marine organisms that secrete CaCO 3 shells. The organisms preferentially take up more 16 O as temperature increases.

Oxygen isotopes and paleoclimate Oxygen has three stable isotopes: 16 O, 17 O, and 18 O. (We only care about 16 O and 18 O.) 18 O is heavier than 16 O. The amount of 18 O compared to 16 O is expressed using delta notation: Fractionation: Natural processes tend to preferentially take up the lighter isotope, and preferentially leave behind the heavier isotope. d 18 O ‰ = 18 O/ 16 O of sample - 18 O/ 16 O of standard 18 O/ 16 O of standard  1000

Oxygen Isotopes Ocean H 2 16 O, H 2 18 O Evaporation favors H 2 16 O H 2 18 O Precipitation favors H 2 18 O Snow and ice are depleted in H 2 18 O relative to H 2 16 O. Land Ice Carbonate sediments in equilibrium with ocean water record a δ 18 O signal which reflects the δ 18 O of seawater and the reaction of marine CaCO 3 producers to temperature. CaCO 3

δ 18 O of CaCO 3 controlled by meteoric water (rain + snow) R^2 = 0.84 δ 18O water δ 18O carbonate

Fractionation during formation Cerling and Quade 1993; Kelly et al 1991 

More C 4 when warmer & drier, but that is not the whole story Amount of evaporation & temperature Var in 13 C due to atmos contribution

Properties of Phytoliths Chemically Simple SiO 2 x nH 2 0 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 ka Brady Soil ka Peoria Loess

Grassland Evolution and Expansion Quade and Cerling, 1995 Kelly et al 1998 Stromberg, 2004 Cerling et al, 1997

Cerling et al Relationships between the fraction of C4 biomass from paleosols δ 13 C 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 ka Brady Soil ka Peoria Loess

Research Sites-Great Plains 1.Beecher Island CO 2.Wauneta NE 3.Moran Canyon NE Similar distances from sand sources; increasing precip. gradient

Beecher Island CO Peoria Loess Bignell Loess Brady Soil Cicada burrows as indicators of shrub-steppe paleovegetation

Old Wauneta NE 0-10 ka Bignell Loess ka Brady Soil ka Peoria Loess

13-10 ka Brady Soil at Old Wauneta NE A Bk BC

0-4 ka Buried Soils at Old Wauneta NE Samples 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 δ 13 C and δ 18 O 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.