Understanding rapid climate change: clues from the sedimentary record

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Understanding rapid climate change clues from
the sedimentary record

• Thomas Stevens
• tstevens_at_alumni.jesus.ox.ac.uk

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Climate change

• Sedimentary record evidence of changing climate
• Sedimentology central role
• Palaeoclimatology interdisciplinary, infancy,
  explosion of research since Shackleton and Opdyke
  1973 V28-238
• Focus shifted to the study of sediments for
  climates sake

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Unpleasant surprises?

• 1980-90s increasing evidence for dramatic and
  abrupt shifts in climate
• Abrupt climate change
• when the climate system is forced to cross some
  threshold, triggering a transition to a new state
  at a rate determined by the climate system itself
  and faster than the cause (Alley et al., 2003).
• An unpleasant surprise in store? Broecker (1987)
• IPCC models not able to model rapid changes well
• Focus shifts to sediments

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Heinrich events

• Abrupt collapse of NH ice sheets - cooling
• Last hundreds of years, onset in years
• 6 identified in late Quaternary more earlier?

Bond et al., 1992

• Lithics in N Atlantic ocean cores ice bergs
• Other evidence
• 2.3x106 km3 freshwater
• 7 kyr cyclicity
• Cause internal or external?
• Focused on N. Atlantic but do they propagate to
  other locations?

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Dansgaard-Oeschger events
Greenland Ice Core Data

• Rapid climate changes of period 1.470.5 ka

• Identified strongly in glacials, weakly in
  Holocene
• Identified from Greenland (5-8 C warming) ice
  core records
• NH rapid warming (decades or less), slow
  cooling
• SH slow warming, less intense precede? So
  called AIM events.
• Causes solar (Bond et al. 2001)/ internal
  (THC/ice sheets/wind field/harmonic)

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H and D-O events recent picture

• Pervasive D-O events seen in pre-Late
  Quaternary records (loess, Antarctic ice
  650-800 ka)
• Near global extent Asia, Africa, Americas,
  Antarctica
• Many rapid proxy changes are seen as regional
  manifestations

• Typified by the loess record in China (e.g.
  Porter and An, 1995 and more recently Guan et
  al., in press).
• Some recent caveats

Porter and An, 1995, Nature
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Chinese loess

• Chinese loess wind blown silt
• Quartz, micas, feldspars, massive
• Up to 400m Chinese Loess Plateau
• Miocene - Holocene
• Chinese loess considered key monsoon archive

• Terrestrial equivalent of ocean sediments

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Caveats - 1

• Synchronicity of events?
• Changes linked across world
• New evidence suggests this is problematic
• Age models

Porter and An, 1995, Nature
Stevens et al., 2008, Geology
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Caveats - 2

• Lateral sedimentary facies differences
• V. small changes used to infer abrupt shifts
  noise?

Porter and An, 1995, Nature
Stevens and Lu, in press, Sedimentology
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Caveats - 3

• Complex and site specific sedimentary processes
• Independently calculated SRs
• Many sites needed

Stevens and Lu in press, Sedimentology
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Caveats - 4

• D-O events restricted to the Greenland??
• claims, however, of D-O presence in most remote
  locations cannot be distinguished from the
  hypothesis that many regions are just
  experiencing temporal variability in climate
  proxies with approximately similar frequency
  content Wunsch (Quaternary Research, 2006)

Greenland Ice Core Data
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More uncertainties

• Pre Quaternary evidence pervasive?
• Carbonate beds should be evidence
• Tucker in ISC 2006
• Spectral analysis problematic?
• Spurious cycles depositional complexities
• Lu et al., 2004.
• GCMs
• Poor at simulating apparent distribution of rapid
  climate change

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Evidence for H events in loess record?

• Heinrich Events?
• Some examples of match

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Evidence for H events in loess record?

• Heinrich Events?
• Some examples of match
• Some examples of no change

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Evidence for H events in loess record?

• Heinrich Events?
• Some examples of match
• Some examples of no change
• Some examples of change and no H event (or close
  but not quite)

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Evidence for H events in loess record?

• Heinrich Events?
• Some examples of match
• Some examples of no change
• Some examples of change and no H event (or close
  but not quite)
• Complex interaction of monsoon with site
  specifics
• No firm evidence?

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Challenges in light of this

• What is the geographic distribution?
• What is the relative timing?
• Terrestrial evidence?
• Pre-Quaternary evidence?

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New approach

• Marrying sedimentology into multi-discipline
  palaeoclimatology
• Palaeoclimatologists seldom sedimentologists
• Sedimentary evidence of rapid sediment
  emplacement (gas escape structures, giant
  ripples, rip up clasts, abrupt grain-changes,
  micromorphological analyses etc)
• Geochronology timing and duration
• Relative timing and distribution elucidates the
  causes
• CO2 driven and/or propagated?

P. Hoffman
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Thanks for your attention