The 8.2 kyr event

1
The 8.2 kyr event

• The breaching of Lake Agassiz

2
Introduction

• Sharp and abrupt cooling (1.5ºC) event lasting
  from 8.21 to 8.14 ky
• Climate anomaly centred on Greenland and the N.
  Atlantic but climate perturbations felt worldwide
• This event preceded by (8.47 0.3 kyr) the
  outburst flood from proglacial Lake Agassiz of
  retreating Laurentide ice sheet

3
What caused it?

• Alley et al. (1997) first hypothesised that the
  climate anomaly could be due shutdown of the
  Meridonal Overturn Circulation (MOC)
• Freshwater too buoyant to form NADW ?
  slowdown in circulation and draw of heat up to
  the Arctic

4
Kleiven et al. (2008) Eirik Drift Core

• Eirik Drift Core
• Large shift in all measurements
• Magnetite grains- reduction in size number
• X-ray fluorescence K Si ? 60 CaCO3 17 to
  0.6 - inc sed rates

5
Kleiven et al. (2008) Deep-water changes

• Deep-water changes
• Large and abrupt change in deep-water density,
  flow sed composition
• Pronounced decrease of LNADW influence _at_ site
• Shoaling of boundary between LNADW and southern
  water source across the site
• For 100yrs none of the Nordic seas produced
  low-nutrient deep waters dense enough to reach
  down to 3440m depth
• Slowdown of MOC

6
Kleiven et al. (2008) surface ocean response

• Offset between climate and deep ventilation
  changes time delay of 20yrs
• 100 year perturbation in currents and climate
• Event superimposed on a longer scale cooling
  event

7
Winter-Summer difference

• Norwegian Sea Sediment cores show differences in
  the anomalies found in N. pachyderma
• Winter sinistral-coiling sharp anomaly
• Summer dextral-coiling broad anomaly 8.5-7.9kyr
• Also seen in other proxies world-wide, indicates
  the effect of the slowed overturn was seasonally
  constrained

8
Broad underlying climate anomaly Rohling
Pälike (2005)

• d18O and chloride anomalies show sharp anomalies
  but other proxies show anomalies over a longer
  time
• Greenland dust supply (K-series ECM)
  8.65-8.00kyr
• Cariaco basin aridity due to northward migration
  of ITCZ during summer 8.4-7.75kyr
• African summer-monsoon fed lakes- broad reduction
  in intensity

9
Broad underlying climate anomaly Rohling
Pälike (2005)

• Climate models of flood-related cold event show
  it should last a maximum of 200 yrs and predict
  the associated anomalies but many anomalies last
  over twice the predicted duration and some start
  before the flood
• Recurrent cycle of climatic deteriorations whose
  impact was amplified at 8.2 kyr
• 14C and 10Be indicate an associated reduction in
  solar output

10
Comparison with MIS-3 events

• Switch from modern mode of NADW formation to a
  glacial mode where Antarctic DW extends up to the
  high latitudes, reducing heat transport north
• However, much shorter duration of the event due
  to the short length of the pulse indicates that
  the circulatory system restarted quickly
• Temperature anomalies small in magnitude compared
  to the high oscillations of the MIS-3