paleosols and paleoclimate soils are everywhere and can be used in conjunction with other proxies...
TRANSCRIPT
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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
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Paleosols
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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
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Geologic or Deep Time
99.95% of Earth History
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Biotic Fingerprints in soil
Soil Organic MatterCarbonates and Silicates
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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)
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C4 vs. C3 grass in Great Plains
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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
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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)
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13C varies in terrestrial systems
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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
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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
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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
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δSOM δ soil-respired CO2
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δ 13C of CaCO3 controlled by SOM
δ 1
3C
SO
M
δ 13C soil carbonate
O°C
35°C
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Site productivity
Higher production means less influence of atmosphere
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δ SOM not a constant, so neither is δ CO2 or δ carbonate
C4 veg
C3 veg
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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
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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.
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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
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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
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δ 18O of CaCO3 controlled by meteoric water (rain +
snow)
R^2 = 0.84
δ 1
8O
wate
r
δ 18O carbonate
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Fractionation during formation
Cerling and Quade 1993; Kelly et al
1991
10+4.4
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More C4 when warmer & drier, but that is not the
whole story
Amount of evaporation
& temperature
Var in 13C due to atmos
contribution
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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
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C4 vs. C3 grass in Great Plains
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Phytolith Distribution and Age
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Stable C isotope composition vs C-
14 date
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Paleosols in eastern Colorado
0-10 ka Bignell Loess
10-13 ka Brady Soil
13-23 ka Peoria Loess
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Grassland Evolution and Expansion
Quade and Cerling, 1995
Kelly et al 1998 Stromberg, 2004
Cerling et al, 1997
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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
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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
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Paleosols in eastern Colorado
0-10 ka Bignell Loess
10-13 ka Brady Soil
13-23 ka Peoria Loess
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Research Sites-Great Plains
1. Beecher Island CO
2. Wauneta NE
3. Moran Canyon NE1
2
3
Similar distancesfrom sand sources;increasing precip.
gradient
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Beecher Island CO
Peoria Loess
Bignell LoessBrady Soil
Cicada burrowsas indicators ofshrub-steppe
paleovegetation
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Old Wauneta NE
0-10 ka Bignell Loess
10-13 ka Brady Soil
13-23 ka Peoria Loess
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13-10 ka Brady Soil at Old
Wauneta NE
A
Bk
BC
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0-4 ka Buried Soils
at Old Wauneta NESamples for thin
section microscopy
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Beecher Island CO
Warmer/dry
C4 grasses
Cooler/wet
C3 shrubs and
grasses ?
Mid Holocene thermal max ?
% C4 Vegetation
Kelly et al, unpublished
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Paleosol carbonate δ13C and δ18O values along the east-west transect of
paleosols
East West
Kelly et al, unpublished
Increasing C4
Increasing temps
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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.
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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.