DOI: 10.25377/sussex.21610635
title: Data for paper: Permafrost in the Cretaceous supergreenhouse.

description: Data for paper published in Nature Communications, 26 Dec. 2022. Gamma-ray (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 time50, thus establishing a 4.34 myr long floating astronomical time scale for the Well Lingtai. Using the same method to the Luohe Fm of the Well Wuqi created a 4.19 Myr long floating astronomical time scale (sedimentation rate: 8.24 cm/kyr, dominated cycle: 33.09 m) (Supplementary 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 405-kyr, 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). We are grateful to the PETROCHINA CHANGQING OILFIELD COMPANY for granting permission to use the subsurface data of the wells. <br> Abstract: 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.