1. Rapid Climatic Changes:
Dansgaard-Oeschger Events (DOE)
Heinrich Events (HE)

2. Dansgaard-Oeschger Events
Also called DO interstadials, DO oscillations, etc.

3. LOCATION OF GREENLAND ICE CORES
DO events particularly conspicuous in Greenland ice core records
Denton et al. (2005)

4. Dansgaard-Oeschger Events
Ice d18O Record from GISP2, Central Greenland
d18O (o/oo)
The DO events are the numbered d18O peaks.
They are sometimes referred to as DO interstadials (with stadials between two subsequent DO interstadials).
20 DO events between ca. 20 to 80 kyr BP
The most recent one (event 1) is the part of a complex called the Bolling-Allerod.
No apparent DO events during the Holocene (ca. last 10 kyr).
d18O increased over a few decades, then decreased over a several centuries to few millennia.
Recurrence time between DO events is typically 1-2 millennia.
Number assignment involves some arbitrariness.
Grootes & Stuiver (1997)

5. Precipitation d18O versus Air TemperatureoC
If we use the modern relationship between d18O of precip. and annual mean T, one gets an estimate of (10/7)Dd18O = ca. 7 degC for the DO warming, where Dd18O = 5 permil is a representative value for d18O increase at the onset of DO events.
Dansgaard (1964)
Fig: Broecker (2002)

6. The Polar Firn Broecker (2002)
There is another technique to estimate temperature changes at the onset of DO events.
This is based on the fact that gases can move vertically in the firn (unconsolidated porous layer of snow on top of ice sheet).
During a sudden warming at the surface of the firn, thermal diffusion will cause the heavy molecules (e.g., N-15N-14 or Ar-40) to become more concentrated at the base of the firn than light molecules (e.g., N-14N-14 or Ar-36).
The principle thus relies on thermal diffusion, which changes the isotopic composition of air near the depth in the firn where bubbles of air are closed off and locked in the ice, leaving a trace of an abrupt climate change.
By measuring the nitrogen or argon isotopic composition of trapped in air, one could reconstructe the temperature change …
Broecker (2002)

7. Younger Dryas to Preboral Transition in Greenland
Warming of 10 ± 4 oC
Gas isotope data have been used to constrain amplitudes of warming at the termination of the Younger Dryas.
Estimated warming depends on thermal diffusion constants for the N and Ar isotopes in air.
Two processes affect N and Ar isotope ratios (N-15/N-14 and Ar-40/Ar-36 ratios): changes in temperature and gravitational settling.
The effect of gravitational settling thus needs to be corrected for.
Grachev and Severinghaus have applied three different approaches to constrain the amplitude of warming from gas isotope data from GISP2 ice core (2 based on gas isotope data; one based on ice d18O data).
Note the red line in upper panel is the modeled temperatute change at the lock-in depth.
A model describes vertical diffusion of gas isotopes and heat in the firn.
A 10 degC step-increase in temperature was found to produce a good fit to the data (envelope of 8 to 12 degC).
This estimate of temperature jump was confirmed by two other approaches.
The YD is a cold interval (ca – 11.6 kyr BP) during the last deglaciation, which was characterized by a return to near-glacial conditions (in a sense, this is the most recent stadial).
Grachev & Severinghaus (2005)

8. Greenland Temperature during MIS 3
Huber et al. applied the gas isotope fractionation technique to estimate temperature changes during MIS 3, during which the DO events are particularly pronounced.
The upper panel shows NGRIP d18O
The middle panel shows NGRIP d15N (measured and modeled)
The lower panel shows the estimated T (2-sigma) and measured CH4 from NGRIP and GISP2.
T curve obtained from fit of firn densification and heat diffusion model to d15N data
Estimated T jumps at onset of DO events range from 8 to 15 degC.
The lower panel also shows CH4 concentrations measured on air trapped in Greenland ice cores.
Each warming is associated with an increase of CH4.
CH4 is a greenhouse gas. It is produced from multiple sources and consumed predominantly by oxidation by the OH- radical.
Its residence time in the atmosphere is estimated to ca. 10 years.
During MIS 3, CH4 is thought to have been emitted to the atmosphere mostly from tropical wetlands.
Huber et al. (2006)

9. Spatial Extent of DO-type Variability
“clear” DO-type variations
unclear or absent DO-type
variations
There is evidence that DO events are not strictly Greenland phenomena. In fact, they may have occurred world-wide.
Voelker (2002)

10. Correlation between Greenland & North Atlantic Sediment Records
There is evidence for millennial-scale variability in surface conditions in the North Atlantic.
Plot of GISP2 d18O and N. pachyderma (s.=sinistral=left coiling) (cold water planktonic foraminifera) in two sediment cores from North Atlantic.
Ice core chronology transferred to sediment core by matching common features in both records.
The vertical bars indicate the dates use to align the sediment records with the Greenland records (tie points).
Note the high values of the linear correlation coefficient between the records (0.82 and 0.84) after alignment, indicating that from 67% to 71% of the variance in one record can be “explained” (in the statistical sense) by the variance in the other record, assuming zero lag.
Bond et al. (1999)

11. Correlation between Greenland & North Atlantic Sediment RecordsMD MD
Example of transfert of ice core chronology to marine sediment records.
Top: N. pachyderma left coiling in North Atlantic cores MD (gray) and MD (black).
Bottom: NGRIP d18O on GICC05 timescale. Interstadials numbered.
Sediment record chronologies obtained from alignment of pachy records to Greenland d18O (specifically, midpoints of transition into interstadials are used).
Vertical dashed lines = tephra horizons within both the marine & ice core records.
NGRIP is the most recent long ice core drilled in Greenland.
Austin & Hibbert (2012)

12. Correlation between Greenland & Arabian Sea Records
As already stated, there is evidence for DO-type of variability far away from Greenland, such as in the Arabian Sea.
The study of Schulz et al. was among the first to provide evidence for such variability outside the circum North Atlantic.
The study relied on sediment cores raised from the northeastern Arabian Sea and bathed today by anoxic waters (OMZ=oxygen minimum zone).
Schulz et al. (1998)

13. Correlation between Greenland & Arabian Sea Records
The total organic carbon (TOC) sediment records show DO-type variability.
Note that the chronology of the 2 sediment cores was tuned to Greenland d18O (top) and then checked by radiocarbon dates on planktonic foraminifera picked from the two cores.
Schulz et al. (1998)

14. Analogues of DO events in Antarctica
Do DO events have analogues in Antarctic Ice cores?

15. d18O in Cloud Vapor & Condensate
Of course, d18O can also be measured on Antarctic ice cores. In fact, dD records (proportional to H-2/H1- ratio), not d18O, records are most generally reported for Antarctic ice cores. dD is subject to the same Rayleigh distillation process as d18O. So dD could also be used to estimate paleo-temperatures.
Dansgaard (1964)
Fig: Broecker (2002)

16. The Global Meteoric Water Line
Like O-18 and O-16, light hydrogen H-1 and heavy hydrogen H-2 (deuterium) are both fractionated during evaporation at the ocean surface.
As a result, there is a positive relationship between dH-2 (also noted dD) and d18O of precipitation.
This relationship is called the (global) meteoric water line.
Both dD and d18O of precipitation tend to be small in cold regions and large in warm regions.
Clark & Fritz (1997)

17. dD and d18O have been measured on several ice cores in Antarctica, some of which are shown here.

18. dD & CO2 Records from Antarctic Ice Cores
based on dD
The upper panel shows EDC dD expressed in terms of temperature (“EDC” stands for “EPICA Dome C”, where EPICA is the name of the project that collected and generated the above records)
The lower panel shows the concentration of CO2 measured on air trapped in different Antarctic ice cores:
Taylor Dome (brown), Vostok (green), and EDC (other colors).
Horizontal dashed lines are mean values for different time intervals.
Arabic numerals = Marine Isotopic Stage (MIS); Roman numerals = glacial terminations or deglaciations (e.g., T_I = most recent termination).
Note the strong apparent correlation between both records, … which in fact motivates more or less explicitly much of ongoing climate research.
Lüthi et al. (2008)

19. dD & CH4 Records from Antarctic Ice Cores
CO2 is not the only gas that can be measured on air trapped in Antarctic ice cores. CH4 (another greenhouse gas, as we have seen) can also be measured on such air samples.
Bottom: EDC dD
Middle: EDC CH4
Insert: Bottom section of EDC CH4 and dD + benthic d18O stack of Lisiecki and Raymo (2005) (see previous class about this record).
Clearly, oceanic and atmospheric properties changed in tandem.
Top: Vostok CH4
MIS are shown at the bottom
Loulergue et al. (2008)

20. Map showing ice core location discussed in next panel
Brook et al. (2005)

21. Synchronization of Greenland & Antarctic Records
CH4 can be measured both on Greenland and Antarctic ice cores.
Since the residence time of CH4 in the atmosphere is of the order of 10 years, variations of this gas should be synchronous on centennial and longer time scales between the hemispheres to a good approximation.
Thus, CH4 can be used to synchronize ice core records from Greenland and Antarctica.
This approach has been pioneered by Thomas Blunier and colleagues and later applied by other glaciologists.
Here is an example of such application, showing that Antarctic isotopic records seem to have correlatives to Greenland isotopic records (DO events).
The upper panel shows measurements of another property measured in air trapped in ice cores: the oxygen-18 to oxygen-18 ratio of molecular O2. Molecular oxygen has an estimated residence time of ca. 2 millennia in the atmosphere, so paired measurements of d18O of trapped O2 in Greenland and Antarctic ice cores can also be used to synchronize gas records from both polar regions but generally with less precision than CH4 (which shows larger changes and resides over a much shorter amount of time in the atmosphere).
Brook et al. (2005)

22. A “Bipolar Seesaw” ? EPICA (2006)
The ice core records synchronization based on CH4 suggests that isotopic (i.e., temperature) changes in Greenland and Antarctica were not similar.
A striking example is provided by the cold phase preceding DOs 12 and 8 (Heinrich events 4 and 5, more on these events later):
when Greenland was apparently cold (stadial), Antarctica experienced apparent warming (and at the onset of DO12 and DO8, Antarctica started to cool).
This out-of-phase relationship between isotopic records from Greenland and Antarctica led to the concept of the “bipolar seesaw”.
EDML: EPICA Dronning Maud Land
EDC: EPICA Dome C
EPICA (2006)

23. A “Bipolar Seesaw” ? EPICA (2006)
There appears to be a positive linear correlation between the amplitude of the Antarctic warming and the duration of the contemporaneous stadial interval in Greenland during MIS3.
Thus, long stadials in Greenland are generally synchronous with large temperature changes over Antarctica.
EPICA (2006)

24. Heinrich Events
Heinrich events (Hes) provide another prominent example of suborbital climate variability in the North Atlantic and also in the far field.

25. Sediment cores from the North Atlantic contain layers that are rich in ice rafted debris (IRD) and lithic grains, particularly in the so-called “IRD belt”.
Six of such “Heinrich layers” have been identified for the last glacial period (Helmut Heinrich seems to have been the first one to recognize the paleoceanographic significance of these layers).
The coarse material (IRD and lithic grains) present in open-ocean sediments is thought to have been part of icebergs and (or) sea ice, which would have reached the open ocean and melted there recurrently during the last glacial period.
Hemming (2004)

26. This map is a compilation of sediment cores in which Heinrich layers have been identified (in 2004).
Hemming (2004)

27. Spatial Extent of HE-type Variability
As for DO events, there is evidence that Heinrich events were not confined to the circum-North Atlantic but were in fact at least hemispheric or even perhaps global in extent.
The next slide shows records located in this map and showing extrema during Heinrich events: a SST record from the Alboran Sea (western Mediterranean Sea), a TOC record from the Arabian Sea (see above in this ppt), and a d18O record from a speleothem (stalagmite) sampled from the Hulu Cave in eastern China (speleothems are calcium carbonate precipitates; they have become an important archive in paleoclimatology in particular because properties such as d18O can be measured with high temporal resolution and because speleothem samples can be “absolutely dated” using the thorium-230/uranium-234 technique).
Hemming (2004)

28. Hemming (2004) From top to bottom: GISP2 d18O
% carbonate in DSDP 609 sediment core (from the eastern North Atlantic)
3) % N. pachyderma left coiling and alkenone-based SST records from Alboran Sea core
4) TOC record from Alboran Sea
5) d18O record from Hulu cave.
Note evidence for extrema in each record, some of which may synchronous with Heinrich events.
Hemming (2004)

29. Core-Top Calibration of UK’37 to Temperature
Indian
Some phytoplanktonic species (of the class Prysmeniophycae) synthesize long-chain alkenones with 37, 38, or 39 carbon atoms.
These chains are either di- or tri-unsaturated.
For example C37:2 stands for a molecule with 37 carbon atoms and which is di-unsaturated.
It has been shown in laboratory experiments that the above index (UK’37) is a positive linear function of temperature. A similar relationship was observed between UK’37 measured on alkenones extrated from sediment core tops and annual mean sea surface temperature (SST).
This plot is an example of core-top calibration.
Muller et al. (1998)

30. Distribution of the thickness of the Heinrich layers.
Hemming (2004)

31. Planktonic d18O values in Heinrich layers from North Atlantic sediments are generally characterized by low values.
The low d18O values should be due to low d18O of surface waters, since these waters are generally observed to be cold, not warm, during HEs (remember than cooling tends to increase d18O and warming tends to decrease d18O).
The light d18O values of surface waters might have resulted from the melting of icebergs and (or) sea ice that carried the IRD.
Hemming (2004)

32. Relationship between Dansgaard-Oeschger & Heinrich Events

33. HEs documented in North Atlantic sediment records seem to have occurred during stadials in Greenland d18O records.
Clement & Peterson (2008)

34. One of the first attempts to relate DO and HE events (work of Gerard Bond).
Top panel: N. pachydema left coiling
G. Bond
Fig: Broecker (2002)

35. Subtropical North Atlantic Temperatures 60 to 30 kyrs ago
SST reconstruction during MIS3 from Bermuda Rise sediment (alkenone method).
Calibration equation to T is based on laboratory experiments and is very close to the core-top calibration of Muller et al. (1998; see above in this ppt).
Sediment core chronology was obtained by matching common features in the Bermuda Rise SST record and in the GISP2 d18O record.
Using 146 tie points, a linear correlation of 0.83 between the two records was found.
Sachs & Lehman (1999)

36. Mechanisms of DOEs and HEs
This is a very active field of research.

37. Cartoon of Climate Variability
The shaded area describes schematically the spectrum of a typical climate record: overall “red” (i.e., with largest variance in the smallest frequencies) with (sometimes) a local maximum in the millennial band.
The thin vertical lines are the frequencies of various gravitational agents forcing the earth climate (orbital forcing in the low frequencies and tidal forcing in the high frequencies).
The fundamental problem re. millennial scale climate variability is that there is no known external agent that could force the climate system at these frequencies: the millennial band is in a gap in the forcing spectrum.
Thus, DO and HE events would be manifestation of the internal dynamics of the glacial climate.
Munk et al. (2002)

38. Mechanisms of Dansgaard-Oeschger Events:
Changes in the ocean circulation
Sea-ice dynamics
Tropical ocean-atmosphere processes …
Mechanisms of Heinrich Events:
Binge-purge of the Laurentide Ice Sheet
Repetitive failure of ice dams of a Hudson Bay Lake
Ice shelf build-up & collapse in the Hudson Strait …
Different mechanisms have been proposed to explain the DO events and the HEs.
Please note that several of the mechanisms listed above may apply to both DO and HE events (e.g., changes in ocean circulation and sea-ice dynamics).
Note also that the list is not exhaustive and that the proposed mechanisms may not be exclusive (e.g., a change in ocean circulation may be associated with a change in sea cover).
Among these mechanisms, a change in ocean circulation, particularly in the North Atlantic, is perhaps the most popular explanation for millennial-scale variability.
Today, the North Atlantic Ocean is the site of a vigorous meridional overturning circulation: warm surface waters flow northward (the Gulf Stream is a key component of this flow) and cold deep waters flow southward (the deep western boundary current, a relatively strong southward-flowing current along the western boundary of the North Atlantic, is a key component of this flow). For both flows, the volume transport is of the order of 10 Sv (1 Sv = 10^6 m^3/s). Since surface waters are warmer than deep waters, this MOC leads to a net heat transport northward. Should the MOC and this heat transport be reduced, the circum North Atlantic is expected to become colder, thereby potentially explaining the stadials and the HEs. Numerical experiments with climate models support elements of this scenario.
Sea ice is a good candidate to explain rapid climate change, e.g., sea ice cover can vary quickly (cf. inter-annual variability of sea ice cover in the modern Arctic Ocean), and it is characterized by a strong albedo (so it could lead to the so-called ice-albedo feedback).
Some authors have also argued that processes in the tropical regions may play a key role in millennial-scale variability.
The mechanisms to explain HEs are more specific.
The binge-purge hypothesis argues that a growing Laurentide Ice Sheet (ice cap above the North American continent during the last glacial period) becomes eventually instable, leading to “purges”, e.g., iceberg discharges.
Another idea invokes the presence of an ice dam at the mouth of the Hudson Strait. It would have retained water from a lake. Recurrent failure of the dam would have release large amount of freshwater and ice in the North Atlantic.
Still another idea relies on the build-up and collapse of an ice shelf in the Hudson Strait (today, large ice shelves are found along the Antarctic perimeter but not in the northern hemisphere).

39. A ROLE OF OCEAN CIRCULATION CHANGE?
Schematic of surface circulation in North Atlantic based on surface drifters deployed in the 90s (see first class).
Note the Gulf Stream and North Atlantic Current (NAC). The NAC is generally warm and salty compared to the surrounding waters. When flowing northward, NAC releases heat to the overlying cold atmosphere. This heat release is thought to play a major role in the zonal asymmetry of winter climates between NE America and NW Europe (not unchallenged idea!).
Should the NAC turn more zonal, temperature in European landmasses might drop.
Fratantoni (2001)

40. MERIDIONAL CIRCULATION IN THE ATLANTIC OCEAN
AAIW
MOW
NADW
AABW
This figure, borrowed from the first class, reminds us the different deep water masses flowing in different directions are present in the modern Atlantic. The North Atlantic Deep Water (NADW), which is relatively warm and salty, is sandwiched between the Antarctic Bottom Water (AABW) and the Antarctic Intermediate Water (AAIW), which are relatively cold and fresh.
Data: M. McCartney, L. Talley, J. Swift

41. Meridional Overturning Circulation (MOC) near 25oN in the Atlantic Ocean
south
north
Sea surface
16 – 18 Sv
1000 m
DT = 15oC
16 – 18 Sv
Sea floor
The figure is a schematic description of the meridional fluxes of volume and heat near 25 degrees north in the North Atlantic. The meridional circulation at this latitude is currently monitored as part of the RAPID program.
Northward Heat Transport = r Cp (16 x 106) (15) = 1 x 1015 W
volume fluxes from Longworth & Bryden (2007)

42. Multiple Equilibria of Thermohaline Convection
HEAT
WATER
HEAT
WATER q
Dr < 0
Dr > 0
Dr > 0
Dr < 0
Henry Stommel showed that the flow (q) between two different reservoirs characterized by different temperatures (T) and salinities (S) and exchanging heat and freshwater with another reservoir (an extreme simplification of the meridional overturning circulation in the ocean) can exhibit multiple equilibria.
You show it! (problem set) q
“low latitudes”
“high latitudes”
from Stommel (1961)

43. Meridional Heat Transports in Atmosphere & Ocean
NOT SHOWN IN CLASS.
This figure shows observational estimates (with error bars) of the meridional heat fluxes in the ocean and atmosphere.
The total (ocean+atmosphere) flux tends to compensate the net radiative heat gain at low latitudes (ca. 40S and 40N) and the net radiative heat loss at high latitudes (ca. poleward of 40S and 40N).
Note the amplitude of the heat fluxes as well as the dominance of atmospheric heat transport over oceanic heat transport at high northern latitudes.
Wunsch (2007)

44. Surface Air Temperature Change Caused by Collapse of North Atlantic MOC
in a Climate Model
NOT SHOWN IN CLASS:
Numerical experiments with climate models show that, when freshwater is artificially added at the sea surface in the North Atlantic, the North Atlantic MOC and the attendant meridional heat flux tend to decrease, leading to cooling of surface air at high northern latitudes.
Such experiments are often taken as evidence for a role of ocean circulation change in past climate change, in particular in the Heinrich events.
Albeit instructive, these experiments are not without limitations (climate models may suffer from various sources of systematic errors, and the very design of such “freshwater hosing experiments” can be questioned).
Vellinga & Wood (2002)

45. Hysteresis Behavior of the Meridional Overturning Circulation
North Atlantic SST
NOT SHOW IN CLASS:
Left panel: if state (1) is far on the left in the upper branch of the hysteresis, a given change in freshwater balance (blue arrow) would be reversible: the system would transit to state (2) but it would come back along the same trajectory to state (1) if a change in freshwater balance of similar amplitude but different sign (red arrow) is applied.
Middle panel: if state (1) is not that far on the left in the upper branch, the same change in freshwater balance (blue arrow) would exceed a threshold value (vertical dashed line), leading to a fundamentally (much colder) state (2). The application of a change in freshwater balance of similar amplitude but different sign (red arrow) would bring back the system to state (1) but along a different trajectory.
Right panel: if state (1) is even closer to the threshold, it will again transit to a fundamentally different state (2) upon the application of a change in freshwater balance, but the application of a change in freshwater balance of similar amplitude but different sign (red arrow) would not bring the system back to state (1) but to a different state (3).
Results from idealized models suggest that the meridional overturning circulation in the North Atlantic might exhibit this hysteresis behavior.
Freshwater balance
Freshwater balance
Freshwater balance
Stocker & Marchal (2000)