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The Editor,

Journal of Glaciology

Sir,

Dating of tropical and Equatorial ice cores drilled in high mountain glaciers is difficult because seasonal variations can be traced only in the upper part. Modelling of the ice flow is difficult at depth due to the rapid thinning of the ice layers. Fortunately, atmospheric trace gases whose lifetime exceeds the inter-hemispheric mixing time (~1year) are tracers on a global scale. By combining several gases, it is possible to attribute the age of an unknown ice layer by comparison with other well-dated ice-core records (Chappellaz and others, 1997 a; Landais and others, 2003). Methane and the isotopic composition of atmospheric O218Oatm) are preferentially used for such purposes because of their suitable atmospheric turnover time (~10 years for CH4 and ~ 1500 years for atmospheric O2).

We recovered one ice core to bedrock (117.06 m long) in 2001, and two more, also to bedrock, (108.83 and 95.80 m) in 2002 on the col of East Rongbuk Glacier (28°01′N, 86°58′ E; 6518ma.s.l.) on the north slope of Qomolangma (Mount Everest) (Fig. 1). Hereafter we refer to the 117.06 m core as “core 2001 ”, and the 108.83 m core as “core 2002”. East Rongbuk Glacier covers an area of 48 km2 with a length of 14 km. Borehole temperatures at the 108.83 m core site range from −8.9°C at 10 m to a minimum of −9.6°C at 20 m, then increase slightly to −8.9°C at the bottom. Measurements of δ18Oatm and methane were performed at the Laboratoire des Sciences du Climat et de l’Environnement (LSCE) and the Laboratoire de Glaciologie et Geophysique de l’Environnement (LGGE) respectively. Details of the measurements are available elsewhere (Chappellaz and others, 1997b; Landais and others, 2003).

Fig. 1. Location map of ice-core drilling site.

The narrow range of our δ18Oatm results (−0.09% to 0.26%) with respect to the Greenland Ice Sheet Project 2 (GISP2) ice core (Fig. 2; note the different vertical scales) suggests no glacial origin of the bottom ice. But we cannot decipher whether the bottom ice originated from the early or late Holocene, because the narrow δ18Oatm band of our cores runs across the GISP2 δ18Oatm profile during these two periods. If early-Holocene, the age of core 2001 at 102.5 m depth or of core 2002 at 87.5 m depth should be 100 E > 8000 years if adopting the GISP2 δ18Oatm time-scale. However, an 80.4 m annually dated ice core recovered from the same East Rongbuk Glacier has a maximum age of 154 years (Kang and others, 2002; Qin and others, 2002). This excludes any possibility of early-Holocene origin for the bottom ice of our cores.

Fig. 2. Ice-core methane and δ18Oatm profiles of the GRIP, GISP2, Himalayan core 2001 and core 2002. All the methane results are on the LGGE−University of Bern internal scale (Chappellaz and others, 1997b), which excludes the experimental bias for comparison. The 700 ppbv pre-industrial CH4 level depicted by polar ice cores is added for clarity.The −0.09% and 0.26% δ18Oatm horizons of the summit core panel are the minimum and maximum of the Himalayan ice cores, respectively.

Both methane profiles show a rapid increase during the industrial period and low methane concentrations (a little below 700 ppbv pre-industrial methane level) in the very bottom sections, but these values are still much higher than the methane concentrations during the middle Holocene (~570 ppbv) as depicted by the Greenland Ice Core Project (GRIP) ice core (Fig. 2), even after taking into account the interpolar methane gradient (35 ppbv for the period 250−1000 years bp: Chappellaz and others, 1997b; Houweling and others, 2000). This further indicates the age of the bottom ice to be late Holocene.

To estimate precisely the age of the bottom ice, we prepared a CH418Oatm phase plane by using the GRIP CH4 and GISP2 δ18Oatm records, then superimposed our CH4 and δ18Oatm pairs (Fig. 3). All the CH4 and δ18Oatm pairs from the bottom 2 m of our cores are situated within BOX 1 (in Fig. 3). The ages of the CH4 and δ18Oatm pairs of the GRIP and GISP2 ice cores within BOX 1 are in the range 1498-2055 years BP, confirming the late-Holocene origin of the bottom ice. Furthermore, we exclude any possibility of connecting the CH418Oatm pairs within BOX 1 to the early Holocene. Otherwise, some of the CH418Oatm pairs within BOX 2 (in Fig. 3; from shallower sections, thus younger than those in BOX 1) would be dated at middle Holocene (Fig. 3), a period that is beyond any possible connection to BOX 2, even after considering the cumulative effects of the experimental uncertainty (Chappellaz and others, 1997a; Landais and others, 2003) and the methane interpolar gradient (Chappellaz and others, 1997b; Houweling and others, 2000). The ice-core base is dated at ~ 1650 years BP when applying a simple flow model. Thus our ice cores will provide extremely high-resolution paleo- climate records for the Himalayan region.

Fig. 3. The CH418Oatm pairs of the Himalayan ice cores superposed on the GRIP−GISP2 CH418Oatm phase plane. The ages are GISP2 time-scale (Meese and others, 1994) after correction of gas−ice time difference, and the GISP2−GRIP chronologies are in good agreement back to 3300 years BP (Southon, 2002), avoiding bias for our dating due to its younger age.

Acknowledgements

Thanks are due to S. Kang and many other scientists, technicians, graduates and porters for hard work expertly carried out in the field. Special thanks are extended to B. Bellier and S. Cherrier for chemical measurement, and to M. Bender for the GISP2 δ18Oatm data. This work was supported by the Chinese Academy of Sciences (100 Talents Project and KZCX1-10-02), the Natural Science Foundation of China (90411003), the Centre National de la Recherche Scientifique, France, and the U.S. National Science Foundation (ATM 0139491).

28 October 2003

References

Chappellaz, J., Brook, E.,Blunier, T. and Malaize, B.. 1997a. CH4 and •18O of O2 records from Antarctic and Greenland ice: a clue for stratigraphic disturbance in the bottom of the Greenland Ice Core Project and the Greenland Ice Sheet Project 2 ice cores. J. Geophys. Res., 102(C12), 26,547-26,557.
Chappellaz, J.. and 7 others. 1997b. Changes in the atmospheric CH4 gradient between Greenland and Antarctica during the Holocene. J. Geophys. Res., 102 (D13), 15,987-15,997.
Houweling, S.., Dentener, F. and Lelieveld, j.. 2000. Simulation of preindustrial atmospheric methane to constrain source strength of natural wetlands. J. Geophys. Res, 105 (D13), 17,243-17,255.
Kang, S. and 6 others. 2002. Twentieth century increase of atmospheric ammonia recorded in Mt. Everest ice core. J Geophys. Res., 107(D21). (10.1029/2001JD001413.)
Landais, A. and 12 others. 2003. A tentative reconstruction of the last interglacial and glacial inception in Greenland, based on new gas measurements in the GRIP ice core. J Geophys. Res., 108(D18). (10.1029/ 2002JD003147.)
Meese, D. A. and 8 others. 1994. The accumulation record from the GISP2 core as an indicator of climate change throughout the Holocene. Science, 266(5191), 1680-1682.
Qin, D. and 6 others. 2002. Preliminary results from the chemical records of an 80.4 m ice core recovered from East Rongbuk Glacier, Qomolangma (Mount Everest), Himalaya. Ann. Glaciol., 35, 278-284.
Southon, j. 2002. A first step to reconciling the GRIP and GISP2 ice-core chronologies, 0-14 500 yr bp. Quat. Res., 57(1), 32-37.