Brief communication: New evidence further constraining Tibetan ice core chronologies to the Holocene

There is considerable controversy regarding the age ranges of Tibetan ice cores. The Guliya ice core was reported to reach as far back as ∼760 ka (kiloannum, i.e. 1000 years), whereas chronologies of all other Tibetan cores cover at most the Holocene. Here we present ages for two new ice cores reaching bedrock, from the Zangser Kangri (ZK) glacier in the northwestern Tibetan Plateau and the Shulenanshan (SLNS) glacier in the western Qilian Mountains. We estimated bottom ages of 8.90±0.570.56 ka and 7.46±1.461.79 ka for the ZK and SLNS ice core respectively, further constraining the time range accessible by Tibetan ice cores to the Holocene.

Brief Communication: New evidence further constraining Tibetan ice core chronologies to the Holocene

There is considerable controversy regarding the age ranges of Tibetan ice cores. The Guliya ice core was reported to reach as far back as ~ 760 ka (thousand years), whereas chronologies of all other Tibetan cores cover at most the Holocene. Here we present ages for two new ice cores reaching bedrock, from the Zangser Kangri (ZK) glacier in the northwestern Tibetan Plateau and the Shulenanshan (SLNS) glacier in the western Qilian Mountains. We estimated bottom ages of 8.90 ± 0.57/0.56 ka BP and 7.46 ± 1.46/1.79 ka BP for the ZK and SLNS ice core respectively, constraining the time range accessible by Tibetan ice cores to the Holocene.

Apparent discrepancy of Tibetan ice core <i>δ</i>¹⁸O records may be attributed to misinterpretation of chronology

Ice cores from the Tibetan Plateau (TP) are widely used for reconstructing past climatic and environmental conditions that extend beyond the instrumental period. However, challenges in dating and interpreting ice core records often lead to inconsistent results. The Guliya ice core drilled from the northwestern TP suggested a cooling trend during the mid-Holocene based on its decreasing δ18O values, which is not observed in other Tibetan ice cores. Here we present a new high-resolution δ18O record of the Chongce ice cores drilled to bedrock ∼30 km away from the Guliya ice cap. Our record shows a warming trend during the mid-Holocene. Based on our results as well as previously published ice core data, we suggest that the apparent discrepancy between the Holocene δ18O records of the Guliya and the Chongce ice cores may be attributed to a possible misinterpretation of the Guliya ice core chronology.

Apparent discrepancy of Tibetan ice core δ¹⁸O records may be attributed to misinterpretation of chronology

Ice cores from the Tibetan Plateau (TP) are widely used for reconstructing past climatic and environmental conditions that extend beyond the instrumental period. However, challenges in dating and interpreting ice core records often lead to inconsistent results. The Guliya ice core drilled from the northwestern TP suggested a cooling trend during the mid-Holocene based on its decreasing δ18O values, which is not observed in other Tibetan ice cores. Here we present a new high-resolution δ18O record of the Chongce ice cores drilled to bedrock ~ 30 km away from the Guliya ice cap. Our record shows a warming trend during the mid-Holocene. Based on our results as well as previously published ice core data, we suggest that the apparent discrepancy between the Holocene δ18O records of the Guliya and the Chongce ice cores may be attributed to a possible misinterpretation of the Guliya ice core chronology.

Age ranges of the Tibetan ice cores with emphasis on the Chongce ice cores, western Kunlun Mountains

An accurate chronology is the essential first step for a sound understanding of ice core records. However, dating ice cores drilled from the high-elevation glaciers is challenging and often problematic, leading to great uncertainties. The Guliya ice core, drilled to the bedrock (308.6 m in length) along the western Kunlun Mountains on the north-western Tibetan Plateau (TP) and widely used as a benchmark for palaeoclimate research, is believed to reach >500 ka (thousand years) at its bottom. Meanwhile other Tibetan ice cores (i.e. Dasuopu and East Rongbuk in the Himalayas, Puruogangri in the central TP and Dunde in the north-eastern TP) are mostly of Holocene origin. In this study, we drilled four ice cores into bedrock (216.6, 208.6, 135.8 and 133.8 m in length, respectively) from the Chongce ice cap ∼30 km to the Guliya ice core drilling site. We took measurements of 14C, 210Pb, tritium and β activity for the ice cores, and used these values in a two-parameter flow model to establish the ice core depth–age relationship. We suggested that the Chongce ice cores might be of Holocene origin, consistent with the other Tibetan ice cores except Guliya. The remarkable discrepancy between the Guliya and all the other Tibetan ice core chronology implies that more effort is necessary to explore multiple dating techniques to confirm the age ranges of the TP glaciers, including those from Chongce and Guliya.

Age of the Tibetan ice cores

An accurate chronology is the essential first step for a sound understanding of ice core records, however, dating of ice cores drilled from the high elevation glaciers is challenging and often problematic, leading to great uncertainties. The Guliya ice core, drilled to bedrock (308.6 m in length) from the northwestern Tibetan Plateau (TP) and widely used as a benchmark for paleoclimate research, is believed to reach > 500 ka (thousand years) at its bottom. Meanwhile other Tibetan ice cores (i.e., Dasuopu and East Rongbuk in the Himalayas, Puruogangri in the central TP, and Dunde in the northeastern TP) are mostly of the Holocene origin. In this study, we drilled ice cores to bedrock from the Chongce ice cap ~ 30 km from the Guliya ice core drilling site. We performed measurements of 14C, 210Pb, tritium and β-activity for the ice cores, and used these values in a two-parameter flow model to establish the ice core depth-age relationship. The modeled ages of two Chongce ice cores at the ice-bedrock contact are 8.3 ± 6.2/3.6 ka B.P. and 9.0 ± 7.9/3.6 ka B.P. respectively. The significant discrepancy between the Guliya and all other Tibetan ice core chronologies calls for a revisit of this legend ice core record.

Correlation between precipitation and temperature variations in the past 300 years recorded in Guliya ice core, China

The Guliya ice cap, on the crest of the Kunlun Shan, central Asia, is an ideal site for acquiring ice cores for climate-change studies. Detailed analyses of the precipitation index (glacier accumulation) and the temperature proxy (δ18O) recorded in the Guliya ice core since 300 years BP show that precipitation correlates with temperature in this region. Climate conditions in the Guliya region since 300 years BP can be separated into three periods: warm and wet from AD 1690 to the end of the 18th century; cold and dry from the 19th century to the 1930s; and warm and wet again since the 1940s. During this period, the climate exhibits just two phases: warm/wet and cold/dry. Comparison of the temperatures and the precipitation recorded in the Guliya ice core shows that variations of temperature and precipitation in the region correlate quite well. However, changes in the precipitation regime appear to lag behind those of the temperature by 20–40 years. We believe this results from the larger heat capacity of the ocean relative to that of the land. Hence, ocean temperatures and corresponding evaporation rates change more slowly than do continental conditions. Additionally, however, positive feedback processes, such as increasing temperatures and precipitation improving vegetation, moisture retention and, hence, local convective precipitation probably play an important role. In this paper, we explain how the timescale of evolving vegetation and the feedback mechanism between precipitation and the temperature could help explain why the changes in precipitation lag those of temperature by 20–40 years over long periods. Taking this time lag into account, we should be able to predict future precipitation trends, based on observed temperature trends.