Eocene−Oligocene glaciation on a high central Tibetan Plateau

The nature of glaciation (bipolar vs. unipolar) during the Eocene–Oligocene transition (EOT) remains unresolved. Here, we report the occurrence of frost marks, ice-rafted debris (IRD), and glendonites from the Upper Eocene to Lower Oligocene Niubao Formation (Fm.) deposited in a proglacial lake above glaciolacustrine conglomerates and diamictite facies in the Lunpola Basin, central Tibetan Plateau (CTP). Magnetostratigraphy dates these cryo-spheric deposits to ca. 36.2–31.8 Ma, synchronous with a stratigraphic interval containing IRD offshore of SE Greenland and in the Barents, Chukchi, and Laptev Seas, suggesting a strong continental-oceanic coupling. Our results provide robust continental evidence for intermittent cryospheric processes in the midlatitude Northern Hemisphere during the late Eocene and EOT. The global cold snap EOT-1 influenced already glacierized high-altitude mountains, lowering equilibrium line altitudes (ELAs) of glaciers and leading to local development of ice fields, ice caps, and valley glaciers with proglacial lake systems, such as the one recorded in the Niubao Fm. The record of IRD, glendonites, and frost marks before the onset of EOT-1 points to an active cryosphere on a plateau already elevated by ca. 36.2 Ma.


INTRODUCTION
During the Cenozoic, widespread mountain glaciations on Earth are thought to have occurred no earlier than the Miocene and Pliocene (Hutchinson et al., 2021), with the Eocene-Oligocene transition (EOT) characterized by unipolar glaciation restricted to Antarctica (Zachos et al., 2001). However, increasing evidence from the EOT indicates that polar ice also existed in the Northern Hemisphere (e.g., Tripati and Darby, 2018), consistent with bipolar glaciation. The recognition of ice-related structures and cold-water authigenic minerals in lacustrine deposits from plateau settings provides terrestrial evidence for past cryospheric processes (Ao et al., 2020), and therefore for evaluating the occurrence of glaciations during the EOT.
Here, we report the occurrence of frost marks, glendonites, and ice-rafted debris (IRD) from the Niubao Formation (Fm.) in the Lunpola Basin, central Tibetan Plateau (CTP). Their age is constrained between 36.2 and 31.8 Ma by magnetostratigraphic and radiochronologic dating. These ice-related structures in the midlatitudes of the Northern Hemisphere record terrestrial cryospheric processes during the EOT. This new finding reveals an active continental cryosphere with plateau glaciation in central Asia.

CHRONOSTRATIGRAPHY
The age of the studied section is well constrained by magnetostratigraphy and radiochronology. Han et al. (2019) obtained an absolute U-Pb age of 35.31 ± 0.93 Ma from zircon in a tuff 108 m above the base of section 382 (Fig. 1C), producing a general chronologic framework. For the present study, detailed paleomagnetic sampling was performed at mostly 0.2-0.7 m intervals through section 382 ( Fig. 1; Figs. S1A and S1B) to further constrain *wuchi-hua@foxmail .com; junlingpei@163 .com the chronology. Stepwise thermal demagnetization was applied to two sets of specimens (286 in total) from the samples. Well-defined characteristic remanence magnetization (ChRM) directions were recognized in 192 samples (Fig. S2). Based on virtual geomagnetic pole (VGP) latitudes, five pairs of normal (N1-N5) and reversed (R1-R5) magnetic polarity intervals were observed (Fig. 1). Constrained by the radiochronologic dating by Han et al. (2019), the observed magnetic polarities were correlated with chrons C16r to C12n of the Geologic Time Scale 2012 (GTS2012; Gradstein and Ogg, 2012), yielding an age range of ca. 36.7-30.6 Ma for the studied section, constraining the occurrence of ice-related structures to ca. 36.2-31.8 Ma (Fig. 1C).

CRYOGENIC STRUCTURES
Three groups of cryogenic structures occur in the middle unit of the Niubao Fm.: (1) frost marks, (2) ice-rafted debris (IRD), and (3) glendonite pseudomorphs after ikaite. Frost marks on bedding planes are the most common, char-acterized by radiating, dendritic, and featherlike shapes typical of ice crystals (Figs. 2A, 2B, 2E, and 2F). The marks have straight and sharp margins, straight to slightly curved lineations, and radiate outward from nuclei. They are geometrically similar to modern frost structures (Figs. 2C, 2D, and 2G). The Eocene frost marks are typically 2-5 cm long (≤50 cm) and 2-5 mm wide, often linked. They branch into each other with a recurrent pattern and a uniform angle (<30°) between branches. Host dolomite facies are fine grained (<10 μm grain size; Figs. S3A and S3B) and appear to lack evidence for significant recrystallization. The intermittent occurrence of frost marks on bedding planes indicates that the Lunpola lake basin experienced strong seasonal temperature variations and winters characterized by water temperatures below 0 °C (Girard et al., 2015).
IRD features appear as angular and subrounded dropstones (outsized clasts) encased in laminated sediments, at both outcrop (Figs. 3A-3C) and petrographic scale (Figs. 3D-3F). The clasts deform the underlying sediments, showing bending, rupture, and penetration structures at their base and drape and onlapping structures at their top (Figs. 3D-3F). The clasts are widely scattered in the laminated sediments and associated with glendonites. Disruption of the lower contact and onlapping geometries of lamination suggest that the clasts sank to the lake bottom, deforming underlying sediments , and were subsequently draped by laminated sediments. The outsized clasts are thus interpreted as dropstones that sank from drifting ice floes and icebergs into the lake bottom . The dropstones show vertical to steeply dipping orientation of their long axes with respect to bedding planes (Figs. 3A-3C), like other IRD worldwide (Rodríguez-López et al., 2016Le Heron et al., 2021).
Glendonites occur mostly as euhedral blocky carbonate crystals or aggregates with pyramidal faces, with a displacive growth form in the laminated sediments (Figs. 2H-2N). Crystal size is 0.1-1.5 mm, and, in cross section, the shape is blocky or rhombic. Penetrative, blocky twins are common, with obtuse angles (95°-125°) (Fig. 2I). Sedimentary laminae curve around the structures as they do around authigenic phosphatic nodules, suggesting that glendonite precursors formed during early diagenesis, prior to sediment compaction. Cathodoluminescence images show that the glendonites have been diagenetically altered (Figs. 2K-2N; Figs. S3C and S3D), preserving secondary porosity (Fig. S3C) due to dehydration and volumetric reduction during the conversion of ikaite to more stable polymorphs . Natural ikaite forms between -2 °C and +7 °C (Vickers et al., 2022). The occurrence of glendonites after ikaite in sediments hosting frost marks and IRD indicates that freezing temperatures occurred intermittently during deposition of the Niubao Fm.

ICE-CONTACT FAN CONGLOMERATES AND DIAMICTITES
The lower unit of section 382 consists of decameter-thick conglomerates containing striated clasts, as well as interbedded diamictite facies with dropstones (Figs. 3G-3I), indicating deposition by proglacial or subglacial processes in an ice-contact, coarse-grained fan (e.g.

DISCUSSION AND CONCLUSIONS Eocene-Oligocene Glaciation
The EOT involved a major environmental and climatic change that lasted for ∼790 k.y., leading to the first major glaciation in Antarctica and to a global cooling event from a largely ice-free greenhouse to an icehouse world (Zachos et al., 2001;Hren et al., 2013).
Although it is generally believed that the Paleogene cryosphere was related to a unipolar Antarctic glaciation, our data from continental facies of central Tibet indicate plateau glaciers during the Eocene. Our results are based on 19 stratigraphic occurrences of ice-related structures (frost marks and IRD) and coldwater-related authigenic minerals (glendonites after ikaite) recovered from the 156-m-thick interval. Freezing conditions prevailed at least seasonally, leading to an exceptional record of dropstones produced by melting ice floes and icebergs on the lake and by the growth of ikaite in the lacustrine sediments of a proglacial environment. Paleomagnetic dating of the host strata indicates that these cryospheric deposits formed at 36.2-31.8 Ma, synchronous with an interval of sea ice and IRD accumulation offshore of SE Greenland and in the Barents, Chukchi, and Laptev Seas (Fig. 4) (Tripati and Darby, 2018).

Paleogene Uplifted Central Tibetan Plateau
The timing and magnitude of the uplift of the CTP are controversial (Fig. S4). The first estimate of the paleoelevation of the Lunpola Basin, ∼4.5 km above sea level (asl) at >35 Ma, came from pedogenic carbonate δ 18 O data (Rowley and Currie, 2006). A high elevation (∼3 km asl) during the Eocene-Oligocene (45 and 30 Ma) for the proto-Tibetan Plateau in the CTP region has also been proposed (Wang et al., 2008;Spicer, 2017). Our previous studies indicated an active Cretaceous plateau cryosphere (Wu and Rodrí-  (Spicer, 2017). In contrast, revised magnetostratigraphic (Fang et al., 2020) and paleontologic data (Su et al., 2019) from the basin suggest that elevations of central Tibet were generally low (<2.3 km asl) at ca. 39.5-37.0 Ma and high (3.5-4.5 km asl) at ca. 26 Ma. Xiong et al. (2022) calculated that it rose from <2.0 km asl at 50-38 Ma to >4.0 km asl by 29 Ma. Furthermore, Li et al. (2022) proposed that the CTP rose from 1.5 km asl during the middle-late Eocene (40-36 Ma) to >3 km between 36 and 29 Ma. Our sedimentologic evidence for cryospheric processes in the CTP indicates that the basin repeatedly experienced freezing during 36.2-31.8 Ma. The 19 horizons with frost marks suggest winter freezing, and the occurrence of dropstones during the global cold snap EOT-1 suggests glacier calving. We also found evidence of ice rafting and cool waters in the lake for at least five stages before the EOT-1, and two glacial stages after the global events EOT-1 and Oi-1.
Globally, EOT temperature was likely higher than present, reaching ∼32 °C in the western Pacific Ocean (Straume et al., 2022), and making the minimum altitude needed for freezing conditions in the Tibetan EOT higher than present. The onset of EOT cooling into the Oligocene icehouse probably triggered a drop in the paleo-equilibrium line altitudes (ELAs) of Tibetan glaciers. For comparison, during the Last Glacial Maximum ca. 20 ka, the ELAs fell by 0.65-0.8 km in the central hinterland of the Tibetan Plateau (Jiang et al., 2019). If we apply this value to the Cenozoic Tibetan glaciers experiencing EOT global cooling, the paleo-ELAs would have dropped below 4 km asl, making the snouts of EOT Tibetan glaciers reach even lower altitudes than their controlling ELAs (<4 km; e.g., 1.4 km snout-head altitude gradient of modern Pasterze Glacier, European Alps; Le Heron et al., 2022). The proglacial lake of the Eocene-Oligocene Niubao Fm. may well represent the sedimentary record of a retreating plateau glacier front (cf. Le Heron et al., 2022).
The long duration of these Eocene-Oligocene cryospheric processes (36.2-31.8 Ma) and their occurrence in the midlatitude CTP (Xie et al., 2021) suggest that they were not caused solely by the forcing mechanism triggering EOT cooling-which lasted only ∼790 k.y. (Hutchinson et al., 2021)-but also by the altitude that the plateau had already reached. Thus, we argue that the CTP had reached a high paleo-altitude (3-4 km asl) by 36.2 Ma, controlling the altitudinal distribution of ELAs and Cenozoic glaciers. The interval recorded in the Lunpola Basin by ice-related structures likely involved a topographic transition from an intramontane basin surrounded by prominent mountain ranges to a plateau terrain.

ACKNOWLEDGMENTS
We are grateful to Ron Strachan for editorial work on the manuscript, and to D.P. Le Heron and two anonymous reviewers for providing us with helpful comments and suggestions that improved the manuscript. We thank K.L. Zhao and X.G. Lang for helpful discussions, and L.F. Hou, H.S. Huang, Zh. Shi, and Y. Chen for assistance in the laboratory. This research was funded by the National Natural Science Foundation of China (grants 41972115, 41930218, and 91855213) and by the "Convocatoria de Ayudas para la recualificación del sistema universitario Español 2021-2023, Financiado por la Unión Europea-Next Generation EU," Vicerrectorado de Investigación, Universidad del País Vasco (UPV/EHU), to J.P. Rodríguez-López. This work is a contribution to the Research Group of the Basque Government IT-1602-22.