Stratigraphy of climate change

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Stratigraphy of climate change

• Lecture 19

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The predominant power in this spectrum is at
about 100,000, 41,000 and 19-23,000 years
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from Alley, 2000
The Milankovitch hypothesis climate change
results from changes in Earths orbital
parameters
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Barbados
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Today well look at examples of climate changes
seen in the stratigraphic record that are NOT
controlled by orbital parameters
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Overview

• Introduction to Pleistocene Climate
• Sources of climatic data
• Pleistocene Climate Cycles
• Glacial Cycles
• Heinrich Events
• Dansgaard-Oeschger Events
• Possible causes of rapid climate shifts

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Factors that influence climate

• Atmosphere 1-10 years
• Solar Activity 1-100 years
• Oceans 10-1000 years
• Orbital Forcing 10,000-100,000 years
• Tectonics Millions to hundreds of millions of
  years

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Marine Sediment Cores

• Available in widespread locations, including low
  latitudes
• Record lots of information about both biological
  and non-biological variables
• Requirements
• High Sedimentation (Bermuda Rise)
• Low Bioturbation (Anoxic conditions)

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Ice Sheet Cores

• Record yearly snowfall annual resolution for
  last 10-15 kyr, but good data for several hundred
  kyrs
• Include many useful climate proxies, mostly
  related to atmospheric circulation
• Only available in certain places (Greenland,
  Antarctica)

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Orbital Forcing Ice Cores

• Ice core ?18O records temperature
• Orbital frequencies are clearly dominant, but
  higher frequencies are present

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Heinrich Events

• Discovered in 1988 in marine sediment cores
• Recognized as distinct layers with significant
  increase in lithic fragments, and large clasts in
  some areas

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Heinrich Layer Isopachs

• Double maxima in isopachs
• Layers thicken to NW into Labrador Sea

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Source of Heinrich events

• Black areas are regions with large carbonate
  deposits
• Sediment must have been ice-rafted

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Heinrich Layer 4 (40,000 years BP)
?18O in polar planktonic foraminifera Modeling
shows about 250-year duration and 2-m rise in sea
level
Nature, Roche et al., 2004
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Modeling shows about 250-year duration and 2-m
rise in sea level
Nature, Roche et al., 2004
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Heinrich Events in context

• Occur at times of coldest weather in N. Atlantic
• Followed by a sharp warming
• No clear periodicity

Bond 1993 figure
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Heinrich Events in California?

• Phillips, 1996
• Sierran Glacial advances seem to correspond to
  Heinrich events 1,2,3,5 well

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Summary of Heinrich events

• Effects are global signature of Heinrich events
  has been found around the world
• Massive discharge of ice into N. Atlantic from
  the Laurentide ice sheet is well established
• No clear explanation for such dynamics in the ice
  sheet

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Dansgaard-Oeschger Events

• Characterized by rapid warming in the N.
  Atlantic, followed by slower cooling
• Quasi-Periodic, with a timescale of 1400 years
• Recorded by diverse climate proxies
• Evidence for global climatic effects

(Data From ftp//ftp.ngdc.noaa.gov/paleo/icecore/g
reenland/summit/grip/isotopes/gripd18o.txt)
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Greenland and D-O events

• Within the Greenland ice cores, several
  independent variables all show D-O events
  prominently
• ?18O Temperature
• Ca/Dust concentrations varying weather in Asia?
• Na/Cl concentrations increased storminess in N.
  Atlantic
• Etc

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Other Evidence of D-O events

• Sediment cores from the Santa Barbara Basin
  (Hendy and Kennet, 1999)

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Other Evidence of D-O events

• Stalagmites from Eastern China (Wang et al., 2001)

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Global Map of D-O records
http//www2.ocean.washington.edu/oc540/lec01-31/
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Theories for rapid climate change

• Heinrich, D-O periods are too rapid for orbital
  frequencies
• Some combination of the ocean/atmosphere/cryospher
  e must be responsible
• Need a source with enough power to affect global
  climate

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Atlantic Circulation
Deep water formation

• Deep Water is formed at the Northern and Southern
  extents of the Atlantic Ocean
• This deep circulation has an overturning
  timescale of 103 years
• Surface currents strongly influence climate in
  many areas, as in the N. Atlantic

Deep water formation
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Stratigraphic Evidence

• Recent work (April 2004) has investigated a proxy
  for Atlantic circulation using a marine sediment
  core from the Bermuda Rise
• Th settles out of water faster than Pa, so the
  ratio between the two can provide information
  about the strength of flow away from source
• Result Atlantic circulation essentially shut
  down during Heinrich events

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Summary

• There is still no clear trigger for Heinrich or
  Dansgaard-Oeschger events, nor an explanation for
  their periods
• However, changes in Atlantic circulation seem to
  account for many of the side effects of both
  processes
• More stratigraphic records more clues

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Why it all matters

• Late Pleistocene was not simply cold it was
  totally chaotic
• Even modern agricultural processes probably
  couldnt overcome such variability

Beginning of Agriculture
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A few references

• Bradley, Raymond S. Paleoclimatology. Harcourt
  Press, 1999.
• Siedov et al., Ed. The Oceans and rapid climate
  change. AGU, 2001.
• Hesse, R. and Khodabakhsh, S. Depositional Facies
  of Late Pleistocene Heinrich Events I nthe
  Labrador Sea. Geology 262 103-106, 1998.
• Dansgaard, W et al. Evidence for general
  instability of past climate from a 250kyr
  ice-core record. Nature 364, 15 July 1993.
• Bond, G. et al. Evidence for massive discharges
  of icebergs into the North Atlantic Ocean during
  the last glacial period. Nature 360, 19 Nov.
  1992.
• Sarnthein, M. et al. Exploring Late Pleistocene
  Climate Variations. Eos. 8151 2000.
• Bond, G. et al. Correlations between climate
  records from North Atlantic sediments and
  Greenland ice. Nature 365, 9 Sept. 1993.
• Bard, E. Climate Shock Abrupt changes over
  Millennial time scales. Physics Today Dec. 2002.
• Hendy and Kennett. Dansgaard-Oeschger cycles and
  the California Current System Planktonic
  foraminiferal response to rapid climate change in
  Santa Barbara Basin, Ocean Drilling Program hole
  893A. Paleoceanography, 151, 2000.
• Phillips, FM. Climatic and hydrologic
  oscillations in the Owens Lake basin and adjacent
  Sierra Nevada, California. Science 2745288,
  1996.
• Wang, YJ. A High-Resolution Absolute-Dated Late
  Pleistocene Monsoon Record from Hulu Cave, China.
  2945550, 2001.