Aging of ice

West Antarctica, but not at the very dry sites in East Antarctica. In addition to such continual counting methods, one can determine the absolute age of ice by identifying the fallout from volcanic eruptions of known age. 

We conclude from the correlations of these four sets of data that the calculation of Dansgaard et al., [41] of the age of ice versus depth in the Greenland ice cap seems to be correct with an error of not more than a couple of years, at least over the last 800 years. We conclude that the climate variations in Greenland, southern Japan, and southern California have had the same periodicities for the last 800 years or more. 

Hemming S. R., Broecker W. S., Sharp W. D., Bond G. C., Gwiazda R. H., McManus J. F., Klas M., and Hajdas I. (1998) Provenance of Heinrich layers in core V28-82, northeastern Atlantic Ar-40/Ar-39 ages of ice-rafted hornblende, Pb isotopes in feldspar grains, and Nd—Sr—Pb isotopes in the fine sediment fraction. Earth Planet. Sci. Lett. 164(1-2), 317-333. 

Fig. 17.29 The thickness of the annual layers of ice in the Vostok core (Fig. 17.28) decreases with increasing depth in the upper part of the core and approaches a constant value below a depth of about 1,600 m. This property causes the age of ice in a long core to vary non-linearly with depth (Derived from Figure 1 of Lorius et al. 1985)...

Another family of feedbacks arises because the radical differences in the albedo (reflectivity) of ice, snow, and clouds compared to the rest of the planetary surface, which causes a loss of the absorption of solar radiation and thereby cools the planet. Indeed, the high albedo of snow and ice cover may be a factor that hastens the transition into ice ages once they have been initiated. Of course, the opposite holds due to decreasing albedo at the end of an ice age. As simple as this concept may appear to be, the cloud-albedo feedback is not easy to quantify because clouds reflect solar radiation (albedo effect) but absorb... 

It is clear from the records of ice ages (see, e.g., Fig. 1-2) that Earth can have and has had climates that are different from our current state. Other, more extreme possibilities have been suggested, each of which could be stable for considerable periods of fime. The frozen "snowball Earth" already mentioned is analogous to present-day Mars, where its CO2 green-... 

Obtaining an accurate and detailed depth-age relationship for an ice core is, of course, a necessary task for learning paleoclimate histories. Approximate time scales can be calculated using numerical models of ice and heat flow for the core site (Reeh, 1989), constrained by estimates of the modem accumulation rate and by measurements of ice thickness from radio-echo-sounding surveys. 

Moreover, in the large scale, in the great depths of the ice cap containing ice laid down 10,000 years ago and more in the last ice age, the ice is more depleted in the heavy isotopes than can be found in any modern day precipitation. 

Glaciochemical horizons are intervals of core with substantially higher or lower than average concentrations of certain chemical constituents. If a historical event of known age can be correlated with the event horizon in the core, the assigned age of that interval can be used to confirm the depth-age relationship which has been determined from seasonal variations or other dating methods. In addition, in deep ice where annual layers are too thin to count seasons reliably and dating is only possible by model calculations [15,30], these horizons provide check points for calculated ages. 

The 6180 in Byrd core melted ice as a function of depth has been measured by mass spectrometry [4,5]. Since the 6180 scale depends on the temperature of the ocean water that developed into snow flakes [6], accurate dating of the core itself is necessary to reveal the temperature history of the ocean surface water. Oeschger et al., [7] measured the 14C contents of C02 extracted from 3 tons of ice melted i n situ, at depths of 100, 175, 270, and 380 m near the Byrd site, their 14C ages for 270- and 380-m depths are 1300 700 and 3000 500 years, respectively. 

Iron concentrations as a function of depth in the Antarctic Vostok ice core, together with mean COq concentrations in air trapped in ice, versus mean age of air. Source From Martin, J. (1990). Paleoceanography 5, 1-13. 

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