Ice core records provide an important proxy for climate change.

Long ice core records have been drilled on both the Greenland and Antarctic ice sheets. Ice core records extend back between 800 and 800,000 years.

Simplified model of global atmospheric circulation.

 - fractionation factor

How do we date Ice Cores? Dating of the upper portion of the ice core record can determined using layer counting. Layer counting within cores will typically be applicable to age ranges up to 10 – 20 ky.

Ice formed near the divide will be plastically deformed (thinned) with depth and layer counting cannot be used. Ages can be determined by ice flow modeling. The ice sheet is considered to be frozen to a horizontal base, for modeling purposes.

Dating of ice cores can also use “wiggle-matching” techniques. Timing of July insolation minima at 65° N can be used to assign ages to the timing of ice volume build up as indicated by the marine isotope record.

Gas bubbles trapped in the glacial ice can be used to determine the atmospheric gas concentration at the time of closure. The local temperature reconstructions from the  18 O data can then be directly compared to the atmospheric gas concentrations. The relationship between atmospheric CO 2 concentration and temperature is irrefutable.

The Greenland data show very rapid and very large fluctuations in the the temperature compared to the Antarctic data, although the large-scale warming and cooling trends are similar. This suggests that the driver of rapid climate change is primarily located in the northern hemisphere, where the polar region contains much more open water than in Antarctica. Rapidly disintegrating ice sheets over open ocean could result in changes in atmospheric or oceanic circulation patterns and provide strong positive feedback for temperature change when the ocean goes from a white reflector to dark absorber of solar radiation – or vice versa.

Loess Marine sediment and loess records. Oxygen isotope measurements of benthic foraminifera provide a proxy of global ice volume change. Loess deposition in north- central China is greatest during cold and dry glacial periods. Soils form during warm interglaciations when the summer monsoon is enhanced. Paleomagnetic record of past 2 million years is shown on the right.

Northern hemisphere ice sheets during the LGM (~20 ka).

Neogloboquadrina pachyderma (polar water foraminifera)

The North Atlantic Ocean distribution of the planktonic foraminifera Neogloboquadrina pachyderma at the Last Glacial Maximum (LGM). Compared to the modern distribution (dashed line near Greenland ) this "foram" found a much wider geographic range at the LGM. This data set shows that the polar front moved very far to the south and traveled more zonally across the LGM Atlantic.

Ocean circulation will redistribute heat on the earth’s surface. Deep water formation in the North Atlantic will be strongly controlled by temperature and salinity content of the water.

Position of the polar front based on presence of G. pachyderma in marine sediment cores. What happened between 11,000 – 10, C yr (13,000 – 12,000 cal. yr) ago?

Neogloboquadrina pachyderma (polar water foraminifera)

HO, H3 & H6: smaller number of lithic fragments and different provenance (European source?).

BINGE PHASE- The gradual accumulation of ice on the Laurentide ice sheet led to a gradual increase in its mass, over 6000 years. PURGE PHASE- Once the sheet reached a critical mass, the soft, unconsolidated sub-glacial sediment formed a "slippery lubricant" over which the ice sheet slid, lasting around 750 years. Geothermal heat caused the sub-glacial sediment to thaw once the ice volume was large enough to prevent the escape of heat into the atmosphere. The mathematics of the system are consistent with a 7,000-year periodicity, similar to that observed if H3 and H6 are indeed Heinrich events. BINGE-PURGE MODEL (MacAyeal, 1993) Kitchen-based oscillator model. Container on axle slowly fills with water until container becomes unstable (water level rises above axle) and reaches tipping point. Spills contents and then flips upright again to refill with water.

EXTERNAL FORCING: AMOC (Atlantic Meridonial Overturning Circulation) Slowdown: 1. A change in the mode of oceanic circulation favors a reduction in North Atlantic Deep Water formation and an AMOC slowdown. With reduced convection, the subsurface oceanic layer progressively warms, increasing the rate of basal melting under the Labrador Sea ice shelf. 2. Once the depth of crevasses in the ice shelf represents a high proportion of the thinning ice shelf, this latter abruptly collapses. A pronounced peak in calving is produced. 3. The missing buttressing effect previously exerted by the ice shelf favors a strong ice-stream acceleration, thus transporting inland-ice and detritus into the Atlantic that translate (i) into a sea-level rise of up to 2 m and (ii) into the Heinrich layer observed in marine sediments.

(A) Location of core sites with records discussed in text (red dots). Marcott S A et al. PNAS 2011;108: ©2011 by National Academy of Sciences

(A) δ18O record from Antarctic EPICA (European Project for Ice Coring in Antarctica) Dronning Maud Land ice core (37) on revised age model (38). Marcott S A et al. PNAS 2011;108: ©2011 by National Academy of Sciences Lower Mg/Ca ratios in benthic forams relate to colder temperatures. Empirical studies have shown that more Mg +2 can be incorporated into the calcite lattice structure of CaCO 3 with warmer water temperatures. Antarctic EPICA (European Project for Ice Coring in Antarctica NGRIP (North Greenland Ice Core Project)

Schematics of the Heinrich event 1. Alvarez-Solas J, Ramstein G PNAS 2011;108:E1359-E1360 ©2011 by National Academy of Sciences

Heinrich Events appear to coincide and may represent severe D-O (Dansgaard-Oeschger) events. Ultimate mechanism triggering the slowdown in the AMOC may be small gradual changes in solar output (i.e., similar to Little Ice Age event).

European pollen records are consistent with the Greenland ice core (Grip) record. During the YD interval arctic herbs dominate the pollen record as glacial conditions returned to Europe. The YD interval ended abruptly and warm conditions returned to Europe.

Did the drainage of glacial Lake Agassiz into the North Atlantic trigger the YD?

Shutting down NADW production will impact surface ocean currents, such as the Gulf Stream. Think about the impact on northern Europe.