Reconstruction of Past Climates

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Reconstruction of Past Climates

• Eugene F Kelly
• Dept. of Soil and Crop Sciences

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Why should we care about climate change ?

• We may be witnessing one of the most profound
  climatic changes in the Earth's history.
• Larger changes in global climate have occurred in
  the past, but over much longer time periods.
• The danger facing society today is that
  anthropogenic global warming may be too fast to
  allow humans, and other species, to adapt to its
  detrimental impacts.

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Paleoclimatology

• Is the study of climate prior to the widespread
  availability of records of temperature,
  precipitation and other instrumental data.
• Useful in establishing the range of natural
  climatic variability in a period prior to
  global-scale human influence.
• We are particularly interested in the last few
  thousand years because this is the best dated and
  most sampled part of the ancient climatic
  record.
• Examine climate change going back hundreds and
  thousands of years using paleoclimatic records
  derived from environmental and climatic proxies.

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Empirical Study 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

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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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Timescales of Paleoclimatic Studies

• Long term - Hundreds of millions of years
• Medium term - One million years
• Short term 1,000 to 160,000 years
• Modern period - Hundreds of years

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Geologic or Deep Time
99.95 of Earth History
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Conjectured climate supercycles.
Continental drift, Orogeny
High CO2
Pangea
Low CO2

• Controls
• -CO2 levels
• Position of continents
• Cycles of weathering,
• Tectonics

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Positions of Continents
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Cenozoic Orogeny
(Raymo-Ruddiman Hypothesis)

• After 50 m.y. ago,
• climate began to cool.
• India began crashing into
• Asia around 40 m.y. ago,
• raising up the Himalayas
• and the Tibetan Plateau .
• Rapid weathering of the
• Himalayas (aided by the
• Indian monsoon rains)
• drew down atmospheric
• CO2, making the climate
• grow colder

60 Ma
40 Ma
20 Ma
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Himalayas and Tibetan Plateau
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Ocean and Ice Core Proxies

• Ocean sediments have been used to reconstruct
  palaeoclimate changes over a range of time
  scales.
• Snow and ice accumulate on polar and alpine
  systems and record the environmental conditions
  at the time of formation

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How do we know how warm it was millions of years
ago?

• Ice cores bubbles contain samples of the
  atmosphere that existed when the ice formed.
  (ancient pCO2)
• Stable isotopes oxygen isotopes in carbonate
  sediments from the deep ocean preserve a record
  of temperature.
• The records indicate that glaciations advanced
  and retreated and that they did so frequently and
  in regular cycles.

CSU
Go CAL
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Oxygen Isotopes
Precipitation favors H218O
Snow and ice are depleted in H218O relative to
H216O.
Evaporation favors H216O
H218O
H218O
Ice
Land
H216O, H218O
Ocean
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Oxygen isotopes and Ocean Sediments

• As climate cools, marine carbonates record an
  increase in d18O.

Warming yields a decrease in d18O of marine
carbonates.
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Marine Microfossils
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Oxygen Isotopes in Marine Microfossils
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Ocean temperature from d18O data
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Terrestrial and Biological Proxies

• Tree rings (width, density, isotope analysis)
• Pollen (species, abundances)
• Paleosols (types of plants and animals in ancient
  soils)

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Dendroclimatology

• Offers a high resolution (annual) form of
  paleoclimate reconstruction for most of the
  Holocene (8,000 to 12,000 years).
• Densitometric parameters (Schweingruber et al.,
  1978).
• Chemical or isotopic variables such as Oxygen and
  Hydrogen Isotopes (e.g. Epstein et al., 1976).

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• The mean width of the tree ring is a function of
  many variables
• tree species,
• tree age,
• soil nutrient availability
• other climatic factors

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Time can be quantified by C-14 dating and even
counting annual rings
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Dendroclimatic reconstruction

• Collect (sample) data from a set of trees (within
  a tree population) which have been selected on
  the basis that climate (e.g. temperature,
  moisture) should be a limiting factor
• Build up a network of site chronologies for a
  region
• Use these relationships to reconstruct climatic
  information from the earlier period covered by
  the tree-ring data.

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Extending a chronology way back
                                               
                                                  
                                                  
                                                  
                                 
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Palynology

• Pollen and spores from plants have accumulated
  over time, a record of the past vegetation of an
  area may be preserved.
• Changes in the vegetation of an area may be due
  to changes of climate.
• Interpreting past vegetation through pollen
  analysis may provide information about past
  climatic conditions

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Pollen from Lake Sediments
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Intervals can be C-14 dated much like trees
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Pollen Profiles
(Brubaker, unpublished)
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Paleosols

• Soils that formed in the past and have been
  buried.
• Contain plant remains and minerals that can be
  used to assess past vegetation and climatic
  conditions
• Wide geographic distribution (soil is everywhere)
  !

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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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Goals of Great Plains Research

• Reconstruct temperature and precipitation changes
  during last 15ka
• Stable C, O isotopes of plant opal phytoliths,
  CaCO3
• Reconstruct vegetation changes, especially at LGM
  Holocene transition and during Holocene
• Plant opal phytoliths
• Thin section micromorphology
• Faunal fabrics - cicadas versus earthworms

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Properties of Phytoliths

• Chemically Simple SiO2 x nH20
• Contain 1-3 C (C-14 dates, 13C, 18O content
  obtainable)
• 1-10 of the soil mass
• Stable in soils
• Different density than soils
• Morphologically unique

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Paleoecology of Eastern Colorado
C4 Vegetation
Warmer/dry C4 grasses
Mid Holocene thermal max ?
Cooler/wet C3 shrubs and grasses ?
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Use Other Evidence

• Brady
• Soil

Bignell Loess
Peoria Loess
Cicada burrowsas indicators ofshrub-steppepaleo
vegetation
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Holocene Thermal Maximum
(Compiled from several regional Proxies)
1 or 2C warmer than today
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Soils 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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Benefit of Paleoclimatic Studies?

• Understanding how climate has changed on
  inter-annual to inter-decadal time scales in the
  past can help us understand how climate may
  change in the future.
• These data are used as a foundation for
  scientists by providing crucial information such
  as rates of past climate change and how
  vegetation and animal populations responded to
  the change.
• The challenge for scientists is to better
  understand the climate system, and ultimately
  evaluate potential ramifications of global
  climate.