Stacked (averaged) record of ice age cycles over the last 3 million years

transcript

Lisiecki and Raymo, 2004 (as plotted by Clark et al. 2006)

Summary Plio-Pleistocene Ice Cycles• One school maintains that ice sheet behavior evolved as a result of

gradual drift toward cooler climate (greenhouse gas induced?). According to this logic, the early Pleistocene, ice sheet waxing/waning was restricted to high latitudes, where integrated summer insolation is dominated by 41 k variability (Huybers, Tziperman, etc.)

• Another school argues that ice sheets always responded on 23 kyr timescales as well, but that the signal was simply “cancelled” by opposite phased behavior in the north/south poles (Raymo)

• The 100 kyr variability of marine 18O can viewed EITHER as a signal of deep ocean temperature variability (Shackleton) OR as the result of non-linearities in large ice sheet dynamics (Clark et al.).

Note that carbon cycle variability plays a role in all these ideas. Makes it imperative to obtain a quantitative CO2 record through the Plio-Pleistocene. These ideas are all ice sheet centric.

Summary Plio-Pleistocene Ice Cycles• What do we know about either CO2 OR other aspects of the climate system

through the major transitions of the Plio-Pleistocene?

----Greenhouse gases may be coming, if old enough ice on Antarctica can be found.

-----There is some evidence from the tropical ocean/atmosphere.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

deMenocal, 1995, Science

K. Lawrence et al., Science 2006

The role of oceanic transport of heat

Here’s theheat balance inthe modernocean.

Additional aspects of Milankovitch theory

Pattern of phase of paleotemperature records

--The 23 kyr precessional effect on insolation is antiphased between the hemispheres. No such pattern is actually observed in the temperature response (ice cores are the best example). The North must somehow transmit the phase to the South!

Layer counting chronologycan be transferred to Antarctic ice coresBy aligning the gas records (which must be synchronous)

Vostok

Site 1089

MD97-2120

O2/N2 indicative of bubble close-off processes

From Kawamura et al. 2007

Vostok

Site 1089

MD97-2120

-360 0

1400 50 100 150 200 250 300 350

EPICA ICE COREProxies for temperature (red) and sea ice extent (blue)

(Wolff et al.)

Age (kyr B.P.)

(‰) (%)

0 50 100 150 200 250 300 350

MD97-2120

The ice core proxy for sea ice cover mirrors the SST proxy in the S.W. Pacific sediment core

(Wolff et al.)

(Pahnke et al.)

Age (kyr B.P.)

Pattern of phase of paleotemperature records

--The 23 kyr precessional effect on insolation is antiphased between the hemispheres. No such pattern is actually observed in the temperature response (ice cores are the best example). The North must somehow transmit the phase to the South, if you believe Milankovitch!

--What about the tropics? What’s the phase of the response, and what season is most critical for forcing? Does the dating of the penultimate deglaciation suggest that the link between insolation and climate lies in the tropical ocean?

from Imbrie et al.Paleoceanography,1993

The tropical oceans also clearly follow the precessional forcing

Bard, G-cubed

S.E. Asian Monsoon Variability in a Chinese speleothem recordappears to be a direct response to local insolation (precession)

Yuan et al. Science, 2004

(25°N)

Wang et al.

Chinese speleothem record of S.E. Asian Monsoon variability

The east west asymmetryin response to uniform heating leads to a change in the mean state of sea surface that is forced directly by the the magnitude of the heating (i.e. precessionallyrelated insolation changes).

Low Oct. insolation

High Oct.insolation

Forcing the dynamicsof the equatorialPacific

from Clementet al. 1999Paleoceanography

This change in the mean state arises from a change inthe statistics of El Niño.

Surface air temperature anomaly associated with thelast El Niño warm event (1997-1998)

Direct dating of18O record using U/Thin marine sediments

Timing is inconsistent withforcing from summer insolationat 65°N

From Henderson and Slowey,2000, Nature

Most recent compilationof sea level estimatesfrom coral reef terraces

All the samples whoseU/Th dates satisfy stringentcrosschecks on accuracy

Pattern of phase of paleotemperature records--The 23 kyr precessional effect on insolation is antiphased between the

hemispheres. No such pattern is actually observed in the temperature response (ice cores are the best example). The North must somehow transmit the phase to the South, if you believe Milankovitch!

--What about the tropics? What’s the phase of the response, and what season is most critical for forcing? Does the dating of the penultimate deglaciation suggest that the link between insolation and climate lies in the tropical ocean?

Statistics of carbon dioxide variability --Large amplitudes of the 100 kyr and 413 kyr cycle--Proper phase to explain some of the apparent insolation/climate non-

linearities?

Suggestion that 100 kyr cycle involves deep ocean temperature, not ice volume

Greenhouse gases clearly participate in the ice age cyclesof 100 kyr

Example ofvariabilityin deep oceancarbonthat operateson 413 kyr timescales

Pattern of ocean heat transport??

--What was the variability in meridional heat transport? Could ocean dynamical effects give rise to feedbacks that amplify the ice age cycles?

Can Ice Age Cycles Be Treated as a Forced Response of a Linear System? (Test of Milankovitch Theory)

--If one uses a seemingly plausible time constant of >10 kyr then the coherency of the 18O signal with insolation and the phase of the 23 kyr and 41 kyr response (determined by accepting an age of 125 kyr for the last interglacial period) conform to that expected of a linear system forced by Northern Hemisphere summer insolation. Milankovitch is in good shape!

However….one could also model the system by assuming that the “prime mover” involves the effect of orbital geometry on tropical ocean dynamics or the effect of orbital geometry on the carbon cycling (CO2 content of the atmosphere),especially if slightly earlier dates are accepted for the ages of the last interglacial period (128-130 kyr). Milankovitch may need some tweaking!

Summary

Connection between the strength of the North Atlantic“conveyor” and the ice ages (from Raymo et al., 1994)

The most famous example of un-forced instability isthe climate record from the Greenland ice core

Evolution of severaltypical events

Entire record

figure from Ganolpolski et al., 2001, Nature

“Instability” of the North Atlantic surface ocean is clearly associated with cold episodes of the last 500,00 years

from McManus et al., 1999, Science

Mechanisms of (unforced) climate instability on millennial timescales

• Ice sheet behavior--Large ice sheets may be prone to surging. For example, if the ice insulates the base as the ice cap grows, and the base is heated by a constant geothermal heat flux

• Ocean’s overturning circulation--perturbations to the salt balance of the high latitude ocean can disrupt the production of deep water and, therefore, alter the poleward delivery of heat (which then could lead to additional feedbacks)

• Tropical ocean/atmosphere interaction--the statistics of ENSO could, in principle change abruptly, which might give rise to climate variability on a global scale.

The “surge” behavior of large ice sheets might produce a flood of icebergs intothe high latitude surface waters

Most North Atlanticdeep sea sedimentcores show evidenceof detrital layersrich in sediment thatwas scraped fromHudson’s bayregion.

Here’s an examplefrom Gerard Bond’swork.

South Atlantic Record of Antarctic Iceberg Rafting

Kanfoush et al., 2000, Science

If one looks closely, is there a pervasive 1500 year cycle of icebergrafting in the North

Atlantic Ocean?

from Bond et al., 2001, Science

It’s an open question

The iceberg flux history of theNorth Atlantic raises the possibility of some kind of “stochastic resonance”,

where an omnipresent 1500 year cycle is simply amplified through association with some other (perhaps less regular)

instability.

MacAyeal, Paleoceanography, 1993

Possible “Binge/Purge”cycle of the LaurentideIce Sheet

Climate over the Last Glacial CycleDansgaard-Oeschger Oscillations and Heinrich Events

Taken from: Alley, 1998: Nature, 392, 335 - 337

Mechanisms of (unforced) climate instability on millennial timescales

• Ice sheet behavior--Large ice sheets may be prone to surging if the ice insulates the base as the ice cap grows, and the base is heated by a constant geothermal heat flux

• Ocean’s overturning circulation--perturbations to the salt balance of the high latitude ocean can disrupt the production of deep water and, therefore, alter the poleward delivery of heat (which then could lead to additional feedbacks)

• Tropical ocean/atmosphere interaction--the statistics of ENSO could, in principle change abruptly, which might give rise to climate variability on a global scale.

The heat budget of the modern ocean(according to L. Talley)

To what extent is this pattern of temperature associated with the ocean?

Examples of ocean involvement can be found throughout the global oceans: first, in sediment cores

from the North Atlantic…

from Sachs and Lehman, 1999, Nature

…In cores from the deep Southern Ocean(4700 meters)

0 20 40 60 80 100 120 140

Age (kyr)

Summit (Greenland)Air temperature

RC11-83/ODP Site 1089Antarctic Deepwater Chemistry

13C/12C of benthicforaminifera

(in ‰)

18O/16O of ice(in ‰)

from Charles et al., 1996, EPSL;Ninnemann et al., 1999, AGU Monograph 112

…in cores from the Santa Barbara Basin

laminated massive

from Behl and Kennett, 1996, Nature

Observations of the ice age ocean are, as yet, insufficient to discriminate between these alternative mechanisms of instability

Stable carbon isotopes in seawaterdelineate the tongue of nutrient-depleted NADW extending southto the A.A. Circumpolar Current

Stable carbon isotopes in deepsea sediment cores suggestsomething changed…

The pattern suggests invasion ofSouthern Ocean Water into thedeep N. Atlantic at the close ofthe last ice age.

But, such maps say nothing aboutrates of overturning. 14C wouldbe a much more diagnostictracer in this respect.

Figure from Alley and Clark, 1999, Ann Rev. Earth+Planet. Sci

The distribution of sites, especially in Southern Hemisphere, is too sparse to provide much constraint

Activity ratio of231Pa increases whenthe rate of flushingdecreases

Other radionuclides(aside from 14C)can be transported bythe ocean’s overturningcirculation.

from McManus et al. 2004

The most famous example of un-forced instability isthe climate record from the Greenland ice core

Evolution of severaltypical events

Entire record

figure from Ganolpolski et al., 2001, Nature

Hysteresis of the ocean’s conveyor

from Rahmstorf, Nature, 1995

www.ccpo.odu.edu/.../ ocean_405_lecture_supp.html

The balance of evaporation-precipitation over the Atlantic Ocean basin is negative,resulting in a net export of fresh water to the Pacific Ocean.

1 Sverdrup (Sv) = 1,000,000 m3 per second

Broecker’s original“Salt Oscillator” Model

Inter-basinal exchangegradually modulatessalinity of Atlantic

Paleoceanography, 1990

The “purge” phase might produce a flood of icebergs intothe high latitude surface waters

Possible sensitivity to N. Atlantic freshwater forcing

12 14 16 18 20 22 kyr B.P.

Freshwater forcingdeduced from theobserved rise in sea level

Record of iceberg rafting in N. Atlantic sediment cores

“Backlogging” of 14Cin the atmospheresuggests slowdown of global ocean overturning

Greenland ice core record

Clark et al, 2002, Science

Surface temperature change in a coupled Ocean/Atmospheremodel resulting from a shutdown of the thermohaline circulation

from Schiller et al., 1997Climate Dynamics

Other models don’t show the same effects in theSouthern Ocean upon shutdown of the THC….

from Manabe and Stouffer, 1999

However, with the condition of bottom waters at their freezing point, Antarctic freshwater balance may also have been important

One possible view of the evolution of thermohaline circulation:

from Keeling and Stephens, 2001, Paleoceanography

Layer counting chronologycan be transferred to Antarctic ice coresBy aligning the gas records (which must be synchronous)

Stacked (averaged) record of ice age cycles over the last 3 million years

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