1 Introduction

Alpine ice cores in the midlatitude and low‐latitude regions provide high‐resolution records of past climate and environment. High rates of ice accumulation and melting are responsible for the relatively short history of ice core records on the Tibetan Plateau as compared to the polar regions. Longer ice cores and older ice are being sought on the Tibetan Plateau for the purpose of extending the climate history in this region. The Malan and Puruogangri ice cores in the central Tibetan Plateau (Thompson et al., 2006; Wang et al., 2003) and the Dasuopu ice core in the middle of the Himalayas (Thompson et al., 2000; Yao et al., 2002) provide records of the past several thousand years. Samples from the bottom of the Dunde ice core in the northeastern Tibetan Plateau were first interpreted to be glacial‐stage ice (Thompson et al., 1989), but later proved to be a Holocene deposit (Thompson et al., 2005). The longest (308.6 m) ice core and the oldest bedrock ice so far discovered on the Tibetan Plateau is from the Guliya ice cap in the western Kunlun Mountains (Thompson et al., 1997; Yao et al., 1997). Developing a chronology for this Guliya ice core (GIC1992 hereafter), as for Tibetan ice cores in general, is challenging. Dating by layer counting is difficult for ice cores from the Tibetan Plateau because the monsoonal type precipitation pattern in this region generates weaker seasonal variation (Hou et al., 2004). For GIC1992 an age scale up to 110 ka was established down to 266‐m depth by comparing the δ18O signal with the CH 4 record from GISP2 in Greenland. Moreover, the 36Cl data suggest that the bottom ice may be older than 500 ka. Since then, the GIC1992 record has been widely used as a reference for correlating regional climate signals (e.g., Cheng et al., 2012; Chevalier et al., 2011; Cosford et al., 2008; Hayashi et al., 2017; Mahowald et al., 2011).

However, the established Guliya chronology is difficult to reconcile with several recent findings. Cheng et al. (2012) encountered inconsistencies between the δ18O record of GIC1992 and the Kesang stalagmite record. Their work suggests that the relationship between δ18O and CH 4 may be inversed, leading to a shortening of the GIC1992 age scale by a factor of 2. Meanwhile, at the Chongce ice cap (~30 km away from the GIC1992 drilling site), luminescence dating provides an upper age limit of 42 ± 4 ka for the basal sediment (Zhang et al., 2018), which is an order of magnitude lower than what the 36Cl data suggests for the bottom ice of GIC1992. Moreover, 14C dating in combination with ice flow modeling for ice cores from the Chongce ice cap indicates Holocene deposition (Hou et al., 2018), which is consistent with all other Tibetan ice cores except GIC1992. Given the proximity between the Guliya and the Chongce ice caps, these results make it difficult to argue that the large difference in age scale between GIC1992 and the other Tibetan ice cores is due to different local climate conditions in the western Kunlun Mountains (Thompson et al., 2005). All the foregoing findings raise the need for examining the GIC1992 chronology with an independent dating method.

81Kr is a cosmogenic radionuclide with a half‐life of 229 ± 11 ka. The 81Kr concentration in the atmosphere (isotopic abundance 81Kr/Kr ~1 × 10−12) is spatially homogeneous with only small changes over the past 1.5 million years (Buizert et al., 2014). These properties as well as its chemical inertness make it a desirable tracer for groundwater and ice over the age range of 40 ka to 1.3 Ma (Loosli & Oeschger, 1969; Lu et al., 2014). Meanwhile, the anthropogenic 85Kr (half‐life 10.76 ± 0.02 a), which is mainly produced by nuclear fuel reprocessing, can be used to identify any young (<60 a) components or contamination of an old sample with modern air (Winger et al., 2005). Development of the analytical method of Atom Trap Trace Analysis (ATTA) has made radiokrypton dating available to the Earth science community at large (Jiang et al., 2012). Due to the large required sample size (5–10 μL STP of krypton), so far 81Kr has been used mainly for dating groundwater, while for glacier ice only a demonstration study was conducted on large blue ice samples (~350 kg) from Taylor Glacier, Antarctica (Buizert et al., 2014). Recently, the required sample size for 81Kr‐ and 85Kr‐analysis has been reduced down to 1 μL STP of krypton, which can be extracted from about 10 kg of Antarctic ice (containing ~100 mL STP air per kg ice) or 20–40 kg of Tibetan glacier ice (25–50 mL STP air/kg; Li et al., 2011). This sample size is still too large to reassess the historic GIC1992 directly, but is sufficient for 81Kr dating of samples from the margin sites of the Guliya ice cap, as presented in this work.