Bulk d 18 O thus far

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Bulk 18O thus far

Depth in core (cms)

18O(‰)

Expected effect of temperature on the oxygen isotopic signal in calcite

Note the similarity to actual temperature (This means that the temperature effect on 18O usually dominates the salinity effect)

Variables that must be considered• Solar irradience• albedo (snow and ice cover, including sea

ice)• greenhouse gas content of atmosphere• surface temperature distribution• ocean heat transport (wind-driven and

overturning)• winds

From J. Hansen, 2008GISS atmospheric GCM results

Descent of snowlines gives a sense of the temperature depression at higher elevations, at various latitudes

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Most accessible example is Mauna Kea, Hawaii

Glacial moraines can beMapped about 800 metersDown from summit.

Surface of the ice age oceanreconstructed on the basisof changes in biogeographyin sediment cores (CLIMAP project)

Modern distribution ofsea surface temperature

How was this map created?

Biogeography ofplanktonic foraminifera in modernsediments.Examples shown here are“polar” (top panel) and “tropical” species.One can scale the relativeabundance of these taxato the temperatureof the overlying waters. Thisis the basis for the so-calledCLIMAP reconstruction of theice age earth.

Theassemblagesin core topsediments areresolved into“factors”(principle components) andregressed ontoobservedsea surfacetemperature in themodern ocean

CLIMAP Project Results

Descent of snowlines gives a sense of the temperature depression at higher elevations, at various latitudes

are needed to see this picture.

The climate sensitivity inferred from the last ice agedepends critically on the temperature patterns assumed for the tropics

CLIMAP results (from microfossil assemblages)

New and improvedCLIMAP

Hostetler and Mix, Nature, 1999

from Barrows et al. 2005

East-west gradients

reduced

Distribution of temperature

observations from the

Last Glacial Maximum

Fortunately, there are other means of deducing temperature

in the ocean

From Lea, 2005

Examples ofMg/Ca derived temperature

Gas chromatographic spectrum, showing the sensitivity ofsedimentary alkenone unsaturation to temperature (T. Herbert, 1998)

“Unsaturationindex” (ratio ofdi- and tri-unsaturatedcompounds to total)

From Bard et al. 2001

Noble gases in aquifers give a measure of ground temperature variability

Water ages as it flows from recharge zone through the aquifer

Distribution of aquifers with glacial age water(and the inferences of temperature change from noble gas measurements)

Layer counting chronology can be extended to other archivesby “wiggle matching” distinctive events

Example of changes in pollen assemblages in a famousEuropean lake sediment core (Grande Pile)

The dynamics ofcontinental vegetationmight also createa feedback, throughalbedo effects.

From “the Biome project”(animations are downloadable)

What about sea ice?(difficult to reconstruct directly, because not much sediment rains

down from beneath sea ice)

Surface of the ice age oceanreconstructed on the basisof changes in biogeographyin sediment cores (CLIMAP project)

Modern distribution ofsea surface temperature

Distribution ofsea ice inferredfrom diatom taxa(Gersonde et al. 2004)

What about greenhouse gases?(measured directly from ice cores)

EPICA ICE CORE

Ice core measurements of N2O over the last 80,000 years

GreenlandTemp. history

Antarctic

methane

What about strength of the sun?We don’t have any direct

information, but we do know the distribution (seasonality) of solar

radiation)

The direct effect of eccentricity is small. However, many climate recordsshow considerable variability at frequencies matching theeccentricity cycle (413 ky and 96 kyr).

The changes in axial tilt

Precessional effects are complicated, because the two hemispheres are out of phase…

However, the amplitude of the precessional cycle varies through timein a way that makes it a convenient “tuning fork”

Departure of incoming solar radiation asa function of latitude and orbital geometry(zero point is definedhere as an arbitrary reference orbit)

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Read it and weep, baby!!!

Depth in core (cms)

18O(‰)

Once established,large ice sheets can createtheir own climate, to someextent.

A strategy that is now relatively common is to use coupled ocean atmosphere models of intermediate complexity to investigate the components of ice age cooling

Another example of a different model of intermediate complexity (from Weaver et al. 1998, Nature)

The global heat contentof the ocean doesn’tchange much in thismodel,even though thepattern of temperaturechange isdisrupted. Theimplication from thismodel is that oceancirculation is nota real amplifier.

The various model results, though they deal with the surface of the ocean and atmosphere, raise the issue of what was happening in the deep ocean during the

last ice age

Hydrography of the glacial ocean

from Adkins et al., 2002, Science

from Adkins and Schrag, EPSL, 2003

Porewater profiles offer the opportunity to reconstruct LGM salinity

How to separate the “ice volume effect” (i.e. sea level) from temperature in foram oxygen isotope records of the last ice age cycle

Schrag et al., Science, 1996

Salinity (or 18O) measured inthe sediment porewaters usuallyshows a remnant maximum some30-50 meters below the sea floor.This maximum reflects the (nowdiffused) relict of when the oceanswere last saltier--i.e. the last ice age.If diffusion rates of porewaters are known, and startingpoints are modelled, then one can reconstructthe salinity (or 18O composition) of the oceanduring the last ice age.

In principle, upper ocean density gradients can be reconstructed with some fidelity using easily measured tracers

For example, in core top sediments (i.e. most recently deposited)

Overlying bottom waterdensity

Measured oxygen isotopes in benthic foraminifera

from Lynch-Steiglitz et al., 2001; G3

As the amount of polar ice increases, e.g. during ice ages, the ocean is progressively enriched in 18O (and also saltier). The sediments record these changes continuously. If only 1‰ is related to this ice volume effect, then the remainder must be the result of temperature effects.

The magnitude of 18O change varies across the tropics

from Broecker, 1985

So there’s pretty good evidence that much of the world’s deep ocean was near its freezing point

during the last ice age. What about other aspects of the deep ocean?

It’s a nutrient tracer that is measurable in sediments becauseCd substitutes for Ca in the CaCO3 lattice of foraminifera, roughly in proportion to its dissolved concentration in seawater

The distribution of water masses can be assessed by changes in thegeometry of nutrient tracers. One example is Cadmium.

Distribution of nutrient-like tracers in the ice ageAtlantic Ocean tend to show a change from themodern state to a “two layer” state, where nutrientrich water invades the deep Atlantic well to the north.

from Marchitto and Broecker, 2006

Pacific

-0.8-0.40.00.40.8≥1.2

GEOSECS C13

Carbon isotope (nutrient tracer) distribution in the oceanDistribution of carbon isotopes in the present ocean.The large separation between ocean basins suggest thatwe can use this tracer as another measure of deep oceancirculation.

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

Example: Changes in carbon isotopes over the last deglaciation

Summary: Even though we now have a fairly legitimate idea of magnitude of temperature change

over the last ice age cycle, it’s presently STILL unclear what the principle feedbacks might have

been that led to the large amplitude (~5°C) temperature change.

We’ll take this issue up again in a few slides….

First let’s look at the ultimate forcing

Astronomical theory of the ice ages

An empirical (statistical) connection between orbital parameters and climate can be easily demonstrated--the very same rhythms characterizing the orbital parameters also characterize nearly every climate record ever produced. Fortunately, the conclusion that climate is linked to orbital change does not require a very accurate timescale.

BUT, what is (are) the physical mechanisms linking the seasonal changes in insolation to climate on longer timescales??

The direct effect of eccentricity is small. However, many climate recordsshow considerable variability at frequencies matching theeccentricity cycle (413 ky and 96 kyr).

The changes in axial tilt

Precessional effects are complicated, because the two hemispheres are out of phase…

However, the amplitude of the precessional cycle varies through timein a way that makes it a convenient “tuning fork”

Departure of incoming solar radiation asa function of latitude and orbital geometry(zero point is defined here as an arbitrary reference orbit)

The orbital changes lead toa seasonal redistribution ofincident solar energy onthe order of about 10%.

Astronomical theory of the ice ages

(VERY) BRIEF HISTORY OF THE PROBLEM

The idea of an astronomical link to the ice ages was first proposed in the 19th century. Croll concentrated on the eccentricity of the Earth’s orbit. He was obliged to explain how exceedingly subtle changes in insolation resulting from eccentricity could be amplified to produce ice age cycles. He was the first to point out the importance of ice-albedo feedback--once established, a large ice tends to keep things cool.

Milankovitch (1930’s and 1940’s) then formalized the astronomical theory of climate to include the precessional cycles and the tilt cycle. His suggestion was that summer insolation at high northern latitudes was the key: snow accumulates all the time at high latitudes, but if summer melting were reduced, then ice sheets could grow. He emphasized 65°N as the most important latitude: ice is heterogeneously distributed at this latitude now, so it could wax and wane most readily there.

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

The first TEST is to use the 18O curve as a measure of ice volume, and, therefore, as a basic gauge of the state of glaciation.

--If one uses a seemingly plausible time constant of >10 kyr (huge ice sheets don’t grow and decay immediately), 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!

Bulk d 18 O thus far

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