Age scale of the air in the summit ice: Implication for glacial‐interglacial temperature change

Abstract

Abstract. The air occluded in ice sheets and glaciers has, in general, a younger age (defined as the time after its isolation from the atmosphere) than the surrounding ice matrix because snow is first transformed into open porous firn, in which the air can exchange with the atmosphere. Only at a certain depth (tim-ice transition) the pores are pinched off and the air is definitely isolated from the atmosphere. The tim-ice transition depth is at around 70 rn under present climatic conditions at Summit, central Greenland. The air at this depth is roughly 10 years old due to diffusive mixing, whereas the ice is about 220 years old. This results in an age difference between the air and the ice of 210 years. This difference depends on temperature and accumulation rate and did thus not remain constant during the past. We used a dynamic tim densification model to calculate the tim-ice transition depth and the age of the ice at this depth and an air diffusion model to determine the age of the air at the transition. Past temperatures and accumulation rates have been deduced from the •5180 record using time independent functions. We present the results of model calculations of two paleotemperature scenarios yielding a record of the age difference between the air and the ice for the Greenland Ice Core Project (GRIP) and the Greenland Ice Sheet Project Two (GISP2) ice cores for the last 100,000 years. During the Holocene, the age difference stayed rather stable

What was the surface temperature in central Antarctica during the last glacial maximum?

“…Based on total gas content measurements Martinerie et al [20] describe the close o¡ density as a function of temperature and pure ice density. A present day ¢rn air study at central Greenland shows that the air is isolated at a density 14 kg m 33 lower than estimated from total gas content measurements [6]. This di¡erence is due to the presence of a non-di¡usive zone above the COD.…”

“…While vage at Byrd station is only a few hundred years under present day conditions, reaching up to V1 kyr for the LG, this di¡erence reaches several thousand years for Vostok. As for the Greenland debate [6] we will compare calculated vage estimates for di¡erent temperature scenarios to observed vages. To obtain an experimental vage for Vostok the procedure is as follows (see Section 4 for details and discussion) : (1) The Byrd station and Vostok isotope records are put on the same time scale by synchronizing the records.…”

“…For the Greenland sites an empirical estimate of vage was deduced from climatic events recorded simultaneously in the gas and in the ice record, namely the atmospheric CH 4 concentration and N 18 O [6]. The reconstruction was thus based on data from one core.…”

“…Generally the two terms are separated by one to two orders of magnitude, the age of the ice at the ¢rn^ice transition being in the range of centuries to millennia and the age of the air being a few decades at most. The densi¢cation was calculated according to Schwander et al [6] making use of a dynamical version of the empirical densi¢cation model by Herron and Langway [6,18] from the surface density (taken as 350 kg m 33 ) to a density of 550 kg m 33 and the semi-empirical model by Pimienta and Barnola [19] for denser strata. The model includes the heat transfer in the ¢rn.…”

“…The model allows to investigate which temperature results in a vage compatible with observed vage. With this method it was possible to show that the Greenland surface temperature derived from bore hole temperature is the more probable one for the fast temperature changes in the LG [6]. We will apply the vage method to the Vostok ice core to evaluate the surface temperature reconstructions derived from water isotope (T WI ) and bore hole temperature (T BH ) over the last 50 kyr.…”