Climate Variability on Millennial Time Scales

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Climate Variability on Millennial Time Scales

• Introduction
• Dansgaard-Oeschger events
• Heinrich events
• Younger Dryas event
• Deglacial meltwater
• Meridional overturning circulation

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Introduction

• Some proxy records provide evidence of
  substantial climate variability at time scales
  that are considerable shorter than those of
  orbital forcing.
• Typical time scales are O(103) years
• Only certain natural archives have sufficient
  temporal resolution to record such variability.

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Ice Core Paleoclimatology

• As snow falls on very cold glaciers or ice sheets
  and gradually is converted to ice, air is trapped
  in bubbles.
• This fossil air can be chemically analyzed to
  determine past atmospheric composition.
• Other paleoclimatic proxies (isotopes, dust,
  acidity) can also be determined from the ice,
  providing information about temperature, sulfate
  aerosols, precipitation.

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GISP2 Drilling Project
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Extracting An Ice Core
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Annual Layers In Ice Core
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Dansgaard-Oeschger Events

• Analysis of rapidly accumulating ice cores in
  Greenland yielded evidence of rapid shifts in
  isotopic composition.
• Source Dansgaard et al. (1982)

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Glacials, Interglacials, Stadials and
Interstadials

• Glacials Cold phases of 100-kyr cycles
• Interglacials Warm phases of 100-kyr cycles.
• Stadials Relatively cold periods during
  glacials.
• Interstadials Relatively warm periods during
  glacials.

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Glacials, Interglacials, Stadials and
Interstadials
Interglacial
Glacial
Interglacial
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Methane Variations DuringD-O Events

• Methane is regarded as an index of tropical
  wetland variations.
• Methane covaries with isotopes in Greenland ice
  cores.
• Source Brook et al. (2000)

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Global Extent?

• Millennial-scale climate variations have been
  found in a number of records.
• Most are in or near the North Atlantic region,
  but there is some evidence elsewhere.

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Heinrich Events

• Ice-rafted material appears in marine sediments
  in North Atlantic every several thousand years.
• Events appear to be correlated with D-O events
  in Greenland ice cores.
• Source Bond and Lotti (1995)

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A Heinrich Event Sediment Core

• In this image, a Heinrich event is represented by
  the light-colored sediment in the bottom half of
  this core segment.
• The black patches within the light-colored
  section is probably due to bioturbation, the
  mixing of sediments by living organisms such as
  deep sea worms.

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Evidence of Heinrich Events
Site withice-rafteddebris
Site withoutice-rafteddebris
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Heinrich Event Chronology
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• Possible effects of Heinrich events have been
  found outside of the region of ice-rafted debris.
  (Bard et al. 2000)

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Heinrich Events in Florida?
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The relative magnitude of Heinrichand
Dansgaard-Oeschger eventsvaries with
location. Only Heinrich events are evidentin
Fe/Ca (continental runoff proxy)from Brazilian
Margin.
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A Pervasive 1500-Year Climate Cycle?
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Source Bond et al. (1997)
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Source deMenocal et al. (2000)
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Solar Forcing of 1500-Year Cycle?
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The Younger Dryas

• During the last deglaciation, a dramatic climate
  flip-flop occurred in which the deglacial
  warming was interrupted by a return to
  near-glacial conditions.
• This flip-flop in known as the Younger Dryas,
  deriving its name from a cold-loving plant
  species whose pollen reappeared during this
  interval.

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Schematic Deglaciation History
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Younger Dryas Climate Records
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Greenland Accumulation Rates
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Younger Dryas Climate Records
Proxy for NorthAtlantic DeepWater formation
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Cariaco Basin (Venezuela)
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Cariaco Basin Bathymetry

• Water exchange with the open Caribbean Sea is
  restricted
• Intense seasonal productivity and high
  sedimentation rate
• Anoxic below 300 m
• Limited bioturbation (post-depositional mixing
  of sediments by marine life)

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The Younger Dryas in the Cariaco Basin
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The Younger Dryasin the AmazonBasin
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Deglacial Meltwater

• As the Pleistocene ice sheets melted, meltwater
  collected in large postglacial lakes, such as
  Glacial Lake Agassiz.
• As crustal rebound occurred, these lakes
  discahrged into the ocean.

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Meltwater Discharge Paths
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Barbados Sea Level Changes
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Schematic of Meridional Overturning Circulation
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Meridional Overturning Streamfunction
Units of ? are Sverdrups 1 Sv 106 m3s-1
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Temperature-Salinity Diagram

• At low temperatures, salinity has a large effect
  on the density of sea water.
• Higher salinity ? more dense.
• Lower salinity ? less dense.

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Meltwater Discharge Paths
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Younger Dryas Climate Records
Meltwater RoutingHudson R./St. Lawrence
R.Hudson Strait
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Meltwater Routing OscillatorClark et al. (2001)