Permafrost in the Cretaceous supergreenhouse

Earth’s climate during the last 4.6 billion years has changed repeatedly between cold (icehouse) and warm (greenhouse) conditions. The hottest conditions (supergreenhouse) are widely assumed to have lacked an active cryosphere. Here we show that during the archetypal supergreenhouse Cretaceous Earth, an active cryosphere with permafrost existed in Chinese plateau deserts (astrochonological age ca. 132.49–132.17 Ma), and that a modern analogue for these plateau cryospheric conditions is the aeolian–permafrost system we report from the Qiongkuai Lebashi Lake area, Xinjiang Uygur Autonomous Region, China. Significantly, Cretaceous plateau permafrost was coeval with largely marine cryospheric indicators in the Arctic and Australia, indicating a strong coupling of the ocean–atmosphere system. The Cretaceous permafrost contained a rich microbiome at subtropical palaeolatitude and 3–4 km palaeoaltitude, analogous to recent permafrost in the western Himalayas. A mindset of persistent ice-free greenhouse conditions during the Cretaceous has stifled consideration of permafrost thaw as a contributor of C and nutrients to the palaeo-oceans and palaeo-atmosphere.

The Lower Cretaceous Luohe sandstone is mainly composed of 430-110 m thick cross-beds generated by migration and climbing of huge sand dunes (up to a maximum height of 352 m) 6,7 . The palaeowind orientations in the Ordos Basin during the Cretaceous were easterly or north-easterly, suggesting that the prevailing winds were westerlies 8 (Supplementary Fig. 2a). The Valanginian-Hauterivian annual mean surface air temperature (at 1.5 m height above the ground surface) for the desert basin has been modelled as ≤0ºC ( Supplementary Fig. 2b) 9 .
Supplementary Note 2⎼Stratigraphic architecture and sedimentology of the Luohe Fm.
The Luohe Fm attains thicknesses of >30 m in the studied outcrops and shows superimposed sets of large-scale (gigantic) cross-bedded sandstones with a maximum thickness of 10 m. The sandstones are fine-grained, very well sorted and show a reddish brown colour, minor faults and fractures and a pristine preservation of small-scale to largescale sedimentary structures ( Supplementary Fig. 3).
Superimposed cross-bedded sets show an average thickness of 3 m (range 1-10 m) ( Supplementary Fig. 3a). Lamina sets in the foresets show a constant direction of dip and coherent orientation with alternating dm-to cm-thick tabular laminae with cm-to mm-thick laminae; a single lamina can be traced laterally for more than 20 m and shows inverse grading. Geometrical relationships between laminae of underlying and overlying laminasets are variable; dip angles vary from 1º to 30º among superimposed cross-bedded sets. The bottomsets of cross-bedded sets show pinching out of wedge-shaped downward termination of foreset laminae (Supplementary Fig. 3b and c) interbedded with inversely graded superimposed lamina sets forming subcritically climbing translatent strata ( Supplementary Fig. 3d).
The outcrops of the Luohe Fm display large-scale cross-bedded sets showing a hierarchy of stratigraphic surfaces. Internally, foresets show inclined surfaces dipping in the same direction as the foreset lamination covered by downlapping lamination of the overlying cross-bedded set (reactivation surfaces 'R' in Supplementary Fig. 3a). Foreset deposits also contain surfaces (superimposition surfaces 'S' in Supplementary Fig. 3a) that cross-cut reactivation surfaces and above which overlying cross-bedded sets show downlapping geometries. Both types of surfaces (R and S) are contained and crosscut by main bounding stratigraphic surfaces (marked 'IS'), which dip in the opposite direction to the dip direction of the large foresets ( Supplementary Fig. 3a). The highest order hierarchical surfaces are aeolian supersurfaces ('SS') that crosscut the IS surfaces as well as the sets of cosets that IS bound and contain surfaces R and S ( Supplementary Fig. 5).
Sandstone wedges appear always to be associated with SS surfaces in the studied outcrops (Supplementary Figs. 4 and 5). The wedges penetrate the underlying cross-bedded sandstones and are truncated by SS surfaces and overlain by downlapping large-scale cross-bedded facies ( Fig. 1b and Supplementary Figs. 4b-d, and 5a⎯c).
A sedimentary succession 30 m thick formed by fine-grained very well-sorted sandstones showing large-scale (gigantic) cross-bedding is interpreted to have resulted from the accumulation of migrating aeolian dunes 10⎯13 . The inclined sandy wedges in the foresets indicate grain flow deposits ( Supplementary Fig. 3b, and c) merging to windripple laminations, indicative of a common facies pattern observed in aeolian dune bottomsets 14⎯17 due to alternation of grain flow caused by gravitational instability and wind reworking on aeolian dune foresets 18 . The variable dips among cross-bedded sets correspond to different orientation of three-dimensional sections of the sets, showing low angles when sections are perpendicular to prevailing wind direction and high angles in sections parallel to palaeowind transport direction 19 . Subcritically climbing translatent strata showing inverse grading ( Supplementary Fig. 3d) resulted from the migration of wind ripples 20⎯24 .
The hierarchy of bounding surfaces indicates that dunes had crescentic crests and superimposed smaller dunes migrating on a major parent bedform, together conforming to a complex dune (draa) 9,13,14,25,26 . Surfaces "R" are reactivation surfaces formed by reactivations of foreset sedimentation and/or the effect of secondary winds on the dune foreset 14,26,27 . Surfaces "S" are superimposition surfaces due to the migration of superimposed dunes on a parent bedform 12,26 . "IS" surfaces are interdune surfaces, formed by the migration of an interdune depression over the stoss slope of the underlying aeolian dune 25,28,29 . Overall, these were complex dunes, and the interdune surfaces constituted interdraa surfaces 30 . The main bounding surfaces (surface SS) are aeolian supersurfaces that separate genetically related aeolian desert sequences (erg sequences) and are considered the response of allogenic forcing on the desert system 31 water-level changes, including climate change, synsedimentary tectonics and erg migration 31⎯37 .
In summary, the studied outcrops of the Luohe Fm in the Ordos Basin represent the sedimentary record of migrating draas (complex dunes that can attain heights of >350 m in modern deserts worldwide) whose development was interrupted by changing allogenic conditions that triggered the formation of aeolian supersurfaces (SS in Supplementary Fig. 5). The studied sandstone wedges appear always in discrete stratigraphic horizons associated with erg sequence boundaries (aeolian supersurfaces, SS) (Supplementary Figs 4b⎯d, and 5a⎯c).

Supplementary Note 3⎼Sandstone wedge sedimentology.
Sandstone wedges have been identified in three different outcrops of the Luohe Fm. Outcrop one (Fig. 1b, and Supplementary Figs. 4, and 6e) shows two distinct levels of wedges separated by trough-cross bedded aeolian sandstones with tangential downlapping of aeolian toeset sediments on the wedge tops. Outcrop two (Supplementary Figs. 3 and 6a⎯d) shows two wedges penetrating aeolian dune cross-bedded sets. Outcrop three (Figs. 1c⎯d, 2 a⎯e and Supplementary Fig. 5) shows ten wedges concentrated in two discrete horizons bounding three draa successions (Fig.  1c⎯d). The wedges are up to 2 m high and are estimated to be up to c. 1 m wide (orthogonal to their axial planes). The heights are thought to be minimum values because the tops of all of the wedges are truncated by erosion surfaces, and so an unknown amount of wedge material has been eroded away.
The sandstone wedges shows similar morphological and sedimentological characteristics in the three outcrops ( Fig. 2 6); (viii) the overall shapes of many wedges is irregular, with opposite sides often being nonsymmetrical and abruptly changing orientation and/or width (Fig. 2a, b and Supplementary Fig. 6e).
We interpret the sandstone wedges as permafrost-related wedges after considering two lines of evidence: (i) the wedges do not show the typical features of giant desiccation cracks, or tectonic/tensional structures; (ii) the wedges show numerous morphological affinities to relict sand wedges and composite-wedge pseudomorphs of Proterozoic to Holocene age, and their palaeogeographic and palaeoclimate setting correlates well with recent permafrost-aeolian plateau desert systems from the western Himalayas. We use the standard terminology for periglacial wedges set out by (ref. 38 ).
The morphology and infills of the wedges from the Cretaceous aeolian dune sandstones from the Luohe Fm are rare in the sedimentary record of aeolian dunes worldwide 10 . Desiccation cracking often occurs in desert basins and can generate giant cracks, often polygonal, both in ancient 39 and recent desert basins 40 . However, such cracking develops in basin floors formed by silty and muddy (cohesive) playa lake systems rather than sandy aeolian dune deposits. Ancient faulting and fracturing in aeolian sandstones are normally postdepositional due to both tectonic deformation 41 and weathering 42 . Tension (tensile) cracks can develop due to gravitational slope instability but not in flat, nearly palaeohorizontal interdune surfaces over a non-cohesive substrate (aeolian sands) and lacking synsedimentary infill. The geometries, dimensions, and penetration depths of all these cracking mechanisms are incompatible with the features and morphometric properties of the wedges observed in the Luohe Fm.

Supplementary Note 4⎼Quaternary analogue of the Cretaceous permafrost wedges.
A Quaternary analogue for the Cretaceous permafrost wedges is provided by sand wedges that penetrate crossbedded aeolian dune facies from the Late Pleistocene Kittigazuit Fm, Hadwen Island, Tuktoyaktuk Coastlands, NT, Canada ( Supplementary Fig. 7) 43 . The aeolian sand-dune deposits of the Kittigazuit Fm are widespread in the Tuktoyaktuk Coastlands 44⎯46 . The dunes developed in a permafrost environment, and their deposits are frozen, bonded with pore ice, and contain visible ice veins and lenses (1-5 mm thick); thaw depths that can reach 1.5 m or more 44 . The Kittigazuit Fm contains permafrost wedges that crosscut the host cross-bedded aeolian dune facies 47 ( Supplementary Fig. 7). The upper part of the wedges is truncated, and the tops of the wedges resumed upward growth within aeolian sand-sheet deposits. Deposition of aeolian sand within the Kittigazuit Fm took place under conditions of continuous permafrost and shows morphological affinities with the permafrost wedges observed in the Cretaceous Luohe Fm of China, including (i) permafrost wedges overlain by laminated aeolian sands (1 in Supplementary Fig.  8a, b); (ii) local upturned strata in aeolian host sands that show a sharp contact with wedge margins (2 in Supplementary Fig. 8a, b); (iii) wedge showing a margin with a stepped shape (3 in Supplementary Fig. 8a, b); (iv) sharp truncation of the wedge's top covered by downlapping laminated aeolian sandstones (4 in Supplementary Fig.  8a, b); (v) rejuvenated top central part of the wedge (5 in Supplementary Fig. 8a, b); (vi) downward narrowing of the wedge with clear internal vertical lamination (6 in Supplementary Fig. 8a, b); (vii) wedges overlain by non-deformed and laminated aeolian sands (7 in Supplementary Fig. 8a, b); and (viii) local downturned strata of host aeolian sandstone (8 in Supplementary Fig. 8a, b).
Additional similarities between permafrost wedges in the Pleistocene Kittigazuit Fm and the Cretaceous Luohe Fm are shown in Supplementary Fig. 9. They include: (i) sand veins in the wedge toe (1 in Supplementary Fig. 9a, b); (ii) downward bending of wedge toe into laminated aeolian sandstones (2 in Supplementary Fig. 9a, b); (iii) sharp and nearly orthogonal contact between wedge margins and stratification in the host aeolian sandstone (3 in Supplementary  Fig. 9a, b); (iv) stepped margin of the wedge entering into the wedge infill (4 in Supplementary Fig. 9a, b); (v) internal vertical lamination parallel to the wedge margins (5 in Supplementary Fig. 9a, b); (vi) downward bending of internal lamination towards the wedge toe (6 in Supplementary Fig. 9 a, b); and (vii) upward widening of the wedge and a stepped margin (7 in Supplementary Fig. 9a, b).

Supplementary Note 5⎼Recent environmental analogue from the Western Himalayas.
The cold aeolian dunefield suffered two transgressions in 2007 and 2013 expanding the lake shore as recorded in satellite imagery (Fig. 4, and Supplementary Fig. 10). These transgressions partially submerged the aeolian dunefield margin and flooded some interdune depressions. It is possible to observe an ice floe in the lake surface (Fig.  4a-d and ice floes in interdune areas of the adjacent dunefield ( Fig. 4e-g).
Previous systematic palaeomagnetic work on the Early Cretaceous was performed in the Shaozhai area of Ordos Basin 5 . Such work relied on the geomagnetic polarity time scale GTS2004 (ref. 49 ), whereas we used MHTC2012 to obtain a more accurate dating by magnetostratigraphy. Based on the virtual geomagnetic polarity (VGP) latitudes ( Supplementary Fig. 11), 24 pairs of normal (N1 to N24) and reverse (R1 to R24) magnetic polarities are observed in the Shaozhai Section 5 . The observed magnetic polarities are correlated with chrons CM5n-CM12r.1n of the geomagnetic polarity time scale MHTC12, yielding the age range of 134.0 to 126.1 Ma for the measured section ( Supplementary Fig. 11), while the top boundary of the Luohe Fm is 129. 4 Ma.
GR logs of Well Lingtai and Well Wuqi were selected for cyclostratigraphic analysis of the early Cretaceous in Ordos Basin (Supplementary Fig. 1d). One of them, Well Lingtai, is adjacent to the Shaozhai Section where previous work on magneticstratigraphy was carried out, while the Well Wuqi is close to the section where the permafrost wedges crop out (Supplementary Fig. 1d).
Analysis of the cyclostratigraphy of the Luohe-Yijun Fm enables the construction of a floating astronomical time scale for the Lower Cretaceous strata in the Ordos Basin of China. We selected logs from two wells (Well Lingtai and Well Wuqi) to investigate their cyclostratigraphy. The obvious 7.54 cm/kyr sedimentation rate (H0 significance levels lower than 0.1%) and apparent 30.49 m sedimentation cycle (confidence levels greater than 95%) in the Well Lingtai ( Supplementary Fig. 12a) provide a significant 405-kyr astronomical signal. The filtering of 405-kyr cycle was used to convert depth to time 50 Fig. 12b). The cyclostratigraphic age defined by the top boundary of the Luohe Fm (ca. 129. 4 Ma, in MHTC12), combined with the identification of an excellent stratigraphic datum for lithostratigraphic correlation, defined by the stratigraphic contact between purplish red medium bedded, middle-grained sandstones, and the underlying red thick-bedded fine-grained sandstones with large high-angle tabular cross-bedding or medium-bedded conglomerates, allowed us to obtain an astronomically corrected duration of 133.74-129. 4 Ma for the Luohe-Yijun Fm in Well Lingtai, and 133.59-129. 4 Ma for the Luohe Fm in Well Wuqi, respectively ( Fig. 7 and Supplementary Fig. 11). Differences between the age framework established by the two wells and our recalibrated magnetic stratigraphic age is an error of just under 405kyr, indicating that the results are plausible. In addition, it is noteworthy that a significant obliquity signal can be traced after 133 Ma in the evolutionary FFT spectral analysis of Well Wuqi (Fig. 7).
In geologic history, the climatic significance of the obliquity is that it drove the occurrence of glacial periods, representing a cooling or cold climate 51,52 . The Eocene-Oligocene transition marks the passage from early greenhouse conditions to modern icehouse ones in the Cenozoic, while the significant signal of obliquity in some sedimentary records is an indication of strong astronomical constraints during this climatic transition 53,54 . Similarly, in both the Oligocene-Miocene as well as the Middle Pleistocene, the emergence of the glacial period shows a clear obliquity signal in the astronomical cycle 51,52 . Further, the Late Ordovician Hirnantian glaciation has been confirmed to have been forced by obliquity 55 . Therefore, our study may provide a record of glaciation during the warm Early Cretaceous, and which may be correlated with the Hauterivian cold snap, before the early Barremian pulse 56 .
By Gaussian filtering, the 405-kyr cycle exhibits a change from high to low amplitude at the bottom of the Luohe Fm, while the ~40-kyr filter maintains a relatively low amplitude over time. A similar phenomenon has been documented in the case of the Mi-1 glaciation event 51 . Our record then confirms a glacial event, which correlates with global palaeoclimate proxies worldwide (Fig. 7), including IRD and glendonites in Svalbard, Australia and Alaska 57⎯59 , and postdates the Weissert Event 60,61 .
A minimum eccentricity will place the Earth in an unusually limited position for seasonal variation. The lower the obliquity, the less solar irradiation the polar regions receive, which favours the creation of ice sheets 51,62 . We presume that from 132. 49 Ma onwards, the polar summer continued to cool, and the ice sheets expanded in the Northern Hemisphere with the appearance of minimum eccentricity and a decline in obliquity. After this, the eccentricity increased rapidly and the obliquity changed from low to high amplitude, the orbital-climate effect then gradually disappeared, and the ice volume decreased 51,62 . The evolutionary FFT spectrum and the variation of eccentricity and obliquity cycles can be well correlated with the horizon of the permafrost sandstone wedges, which also confirms the accuracy of the revised palaeomagnetic age framework. showing the stratigraphic and sedimentological relationships between the wedges and the host aeolian sandstones. Upward-pointing yellow arrows indicate wedges. Aeolian grain flow deposits sharply overlie the wedge. See detail in Supplementary Fig. 6e.  Supplementary Fig. 5 | Cretaceous permafrost wedges and aeolian dune architecture. a, Field photograph of permafrost sandstone wedges horizons hosted in aeolian dune sandstones of the Luohe Fm. b, Ten permafrost sandstone wedges are identified (labelled "w1" to "w10"). Detailed sedimentological observations of the wedges can be seen in Fig. 2 and Supplementary Fig. 6. c, Aeolian architecture is based on the recognition of aeolian bounding surfaces hierarchy 10,31 . "SS" aeolian supersurface; "IS" interdune surface; "S" superimposition surface; "R" reactivation surface. Permafrost sandstone wedges are marked in blue colour.