Ice Sheet Expansion in the Cretaceous Greenhouse World

doi:10.21203/rs.3.rs-1755905/v1

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Ice Sheet Expansion in the Cretaceous Greenhouse World

https://doi.org/10.21203/rs.3.rs-1755905/v1

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published 01 May, 2023

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Globally elevated temperatures during the Cretaceous extreme greenhouse climate interval were punctuated by the Valanginian Weissert event, which was characterised by a positive carbon isotope excursion, global cooling, and a glacial event approximately at 135 Ma. The inability to disentangle ocean temperature and continental ice volume trends hinders our understanding of these climate fluctuations in deep-time. We investigated the ocean temperature–ice sheet dynamics of glaciation events that occurred in the Cretaceous greenhouse world. New clumped isotope and δ¹⁸O data from sits in the Tethyan Ocean show that seawater temperatures decreased by 5°C–6°C, consistent with the development of glacial periods, and maximum ice volumes about half the size of present-day Antarctica. This cooling event provides a counter-example to other Mesozoic climate transitions driven by changes in atmospheric greenhouse gas contents. Our results suggest a robust coupling between mid-latitude seawater temperatures and polar ice volume as atmospheric CO₂ contents decline.

The Cretaceous was characterised by extreme warmth, high atmospheric pCO₂, and widespread dysoxic to anoxic conditions in the global oceans^1–3. The Valanginian episode (i.e., the Weissert event) was characterised by a positive carbon isotope excursion (CIE)^4,5, and may represent a dynamic interval of temperature variations^6,7, decreased atmospheric pCO₂^7,8, a neritic carbonate biota crisis⁹, sea level fall^10–12, and formation of polar ice sheets^13,14. Rapid sea level change and global cooling trends over short timescales imply that glacio-eustasy occurred under the greenhouse to coolhouse conditions across this episode^15,16. Previous studies have identified the former presence of ice sheets in both polar regions, including evidence from glendonite crystals¹⁷ and positive seawater δ¹⁸O values¹⁸. However, the nature, magnitude, and frequency of these climatic fluctuations are unclear, which limits our understanding of the coupling of the global carbon cycle, seawater temperatures, and ice volume in a greenhouse world.

Reconstructing seawater palaeotemperatures from high-resolution sedimentary records is the most direct way to examine secular trends in past climate. In the last decade, the tetraether index of tetraethers consisting of 86 carbon atoms (TEX₈₆)^2,6,7, magnesium/calcium ratios (Mg/Ca)¹⁹, magnesium/lithium ratios (Mg/Li)²⁰, and oxygen isotopic compositions of exquisitely preserved biogenic carbonates (δ¹⁸O_carb)⁶, have been used as palaeotemperature proxies and to quantify palaeoclimatic fluctuations in deep time. Carbonate clumped isotope (expressed with the parameter Δ₄₇) palaeothermometry is an important method for reconstructing past temperatures¹⁸. This proxy is based on the preference for ¹³C and ¹⁸O to bond with each other in the carbonate molecule, which is an effect that becomes more prominent with decreasing temperatures^21,22. An important distinction between clumped isotope and conventional δ¹⁸O thermometry is that the former does not depend on the isotopic composition of the aqueous phase from which the carbonates form, and is based on isotope exchange reactions within a single mineral phase^23,24. As such, clumped isotope thermometry is ideally suited for reconstructing palaeoclimatic conditions in deep time.

In this study, we investigated the coupling of the global carbon cycle, Southern Hemisphere seawater palaeotemperatures, and sea level changes using clumped isotope thermometry, and stable isotope and trace element data for well-preserved belemnite rostra in marine sediments from the Gyangze Basin, southern Tibet. This was an open oceanic basin formed at a palaeolatitude of ~ 48.5°S near the Indian continent during the Early Cretaceous²⁵ (Fig. 1; Supplementary Fig. 1). Diagenetically resistant, low-Mg calcite rostra of belemnites, which are extinct coleoid cephalopods, are widely distributed in Mesozoic marine sediments. Samples without evidence physical and chemical alteration were analysed and are considered to retain the primary carbonate chemistry (Methods). Our multiproxy approach suggests that reconstruction of mid-latitude Tethyan Ocean temperatures during the Vanlanginian event provide a better understanding of the climate forcing and feedbacks related to glacio-eustatic, and the interplay between environmental change, perturbation of the global carbon cycle, and ice sheet growth and decay in a greenhouse climatic system.

Belemnite selection. The well-preserved belemnite calcite is mainly non-luminescence (Supplementary Fig. 2) and has relatively low Mn (< 30–50 µg/g) and Fe (< 150 µg/g) concentrations^6,14,18 (Supplementary Figs. 3 and 4; Supplementary Tables 1–3). Previous geochemical investigations of modern and fossil mollusca have shown that Mn and Fe have low concentrations in primary biominerals, but diagenetic alteration causes enrichment of these two elements in biogenic carbonates²⁶. Significantly, belemnite calcite with high Mn and Fe concentrations also has low Sr concentration²⁷. Detailed examination of belemnite taphonomy showed that the lower Sr concentration are perhaps due to a vital and/or environment effect, and are not the result of a diagenetic effect. Electron microprobe elemental maps show that uniformly low Mn and Fe contents and compositional changes in S and Mg occur in the growth rings of the rostra (Supplementary Fig. 3). During progressive growth and calcite precipitation of the belemnite rostra, variations in S and Mg beyond the apical area were likely related to organic compounds incorporated into the carbonate biominerals. This phenomenon is attributed to a biomineralisation rather than a diagenetic process²⁸.A total of 15 samples with high Mn and Fe concentrations were excluded in this study as diagenetic alteration has made these samples unsuitable for paleoclimate interpretations.

Biostratigraphy and strontium and carbon isotope stratigraphy. The detailed radiolarian biostratigraphy used in this study is shown in Supplementary Fig. 5, which is based on the unitary association zones (UAZs) of Baumgartner et al.²⁹. The uppermost Berriasian to lower Valanginian part of the studied marine succession is well constrained by the occurrence of Thanarla pulchra, which characterises UAZ 15 in the Tethyan realm. Although radiolarians are less common, the interval of UAZ 16 is correlated with the co-occurrence of Acaeniotyle umbilicata, Godia sp., and Pantanellium squinaboli, and is assigned to the lower Valanginian. Previous studies of the evolution of Early Cretaceous radiolarians in the western Tethys have shown that positive CIE excursions coincide with the increase in the abundance of upwelling, high productivity-related Pantanellium during the upper Valanginian interval⁴. In the Manla section, the increase in abundance of Patanelliun squinaboli, along with the co-occurrence of Hemicryptocapsa capita, is interpreted to be the upper Valanginian interval. The base of the Hauterivian was placed at the base of UAZ 19 and is defined by the first occurrence of Cecrops septemporatus, which is a common specie in the Tethyan realm (Supplementary Fig. 5).

Although the radiolarian biostratigraphy is of relatively high resolution, and is consistent with the Tethyan ammonite zones and within ca. 9 Myr. Sr isotope data can yield significantly higher stratigraphic resolution and an improved geochronological framework. The ⁸⁷Sr/⁸⁶Sr isotope curve through the studied succession matches that of the uppermost Berriasian to lower Hauterivian interval, which was characterized by seawater Sr isotopic homogeneity in the Early Cretaceous. The belemnite ⁸⁷Sr/⁸⁶Sr ratios are characterised by increasing values from 0.707286 ± 0.000007 in the lowermost UAZ 15 to 0.707398 ± 0.000006 in the lower UAZ 19, which yields ages of 140.41 ± 0.33 Ma (1σ) to 133.46 ± 0.19 Ma based on the LOWESS model^14,26, respectively. Our new stratigraphic sections of the Lower Cretaceous Gyabula Formation have tight chronological constraints based on the radiolarian and ammonite biostratigraphy and high-resolution ⁸⁷Sr/⁸⁶Sr isotope data (Fig. 1 and Supplementary Fig. 6), which allows these sections to be used for stratigraphic correlations and constraining global palaeoenvironmental changes through time.

We obtained stable isotope data for belemnite rostra from three Lower Cretaceous sections (Qiabu, Longma, and Manla) in southern Tibet. The carbon isotope values of belemnite calcite (δ¹³C_bel) throughout the studied succession range from − 2.03‰ to 0.73‰. Our studied sections exhibit near uniform secular trends consistent with global δ¹³C stratigraphy, and can be divided into four successive intervals according to offsets in δ¹³C_bel values (Fig. 2; Supplementary Tables 1–3). In the pre-CIE interval, the δ¹³C_bel data are derived from the genera Belemnopsis and Hibolites, and exhibit slight variations about an average value of − 1.64‰ ± 0.30‰ during the lower to middle UAZ 16. This rather stable period is interrupted by a large positive CIE that lasted ca. 2 Myr, which is known as the Weissert CIE. The highest δ¹³C_bel values (+ 0.73‰) are reached in UAZ 18. From UAZ 18, the δ¹³C_bel data tend to delineate a distinct negative excursion up to the top of the sequence, and were mainly derived from the genus Hibolites.

Belemnite carbon isotope records and correlation with the global Weissert event. Our δ¹³C_bel data suggest that a major positive CIE is evident from the belemnite calcite data in the southern Tethyan Ocean, which is consistent with bulk organic carbon (C_org) and carbonate (C_carb) data for the northern Tethyan Ocean⁵, Boreal Realm³⁰, western North Atlantic Ocean³¹, and Pacific Ocean⁴, including shallow marine, hemipelagic, pelagic, and continental settings (Fig. 2; Supplementary Fig. 7 and Table 4). In the southern Tethyan Ocean (~ 53°S), δ¹³C values of bulk carbonate show long-term variations between − 2.5‰ and 1.5‰³⁰. This variation of 4‰ is larger than the Valanginian (2.5‰) documented by belemnite calcite data from southern Tibet (~ 48.5°S; this study). In the Northern Hemisphere, where the stratigraphic records and geochemical data for belemnite calcite are near complete, a minimum δ¹³C_bel value at the base of the lower Valanginian is followed by a pronounced increasing trend in the lower part of the upper Valanginian³⁰. The major δ¹³C_bel excursions of ~ 0.5‰ and 1.9‰ above the base of the S. verrucosum zone in Spain have been correlated to peaks at a similar stratigraphic level in France (i.e., the northern Tethyan Ocean)¹⁹. These δ¹³C_bel maxima can also be correlated to studied sections in Germany and England (i.e., the southern Boreal realm) and Russia (i.e., the Arctic Boreal realm)^5,30, where similar positive excursions are found in the middle of the P. hollwedensis and D. crassus zones, respectively (Supplementary Fig. 7). Comparison of the belemnite isotope curves for these different regions suggest that the δ¹³C_bel data exhibit similar long-term trends, but differences in the amplitudes of excursions and absolute values, which may be explained by the biogeographic separation of the sites and preservational and/or taxonomic characteristics. The Valanginian belemnite assemblages exhibit distinct regionality, and can be attributed to two biogeographic realms, which are the Boreal and Tethyan realms. The stable isotope data for the Boreal realm are mainly from the genera Arctoteuthis, Cylindroteuthis, and Lagonibelus, while the Tethyan data are mainly from Belemnopsis, Castellanbelus, Duvalia, and Hibolithes³².

The magnitudes of the CIEs in the different exogenic carbon pools vary widely. A comparison of 35 median carbon isotope values for the Weissert event shows that the smallest CIEs is 1.8‰ for marine carbonates. Intermediate CIEs of 2.2‰ and 2.4‰ are recorded by benthic foraminifers and belemnites, respectively, and the largest CIE of 5.4‰ is recorded by terrestrial plants (Fig. 3a; Supplementary Table 3). An important question is which of these archives provides the most robust data. In general, the CIE recorded by terrestrial plants⁸ is substantially larger than the CIE recorded by bulk marine carbonates and belemnites^5,30, but similar to that recorded by marine bulk organic matter at mid- to high-latitudes³³. Reconciling these discrepancies requires either that the CIE preserved by terrestrial plants and bulk marine organic matter has been amplified relative to marine carbonates, or that marine carbonates and belemnites do not accurately record the full magnitude of the CIE. It has been suggested that increases in plant productivity associated with the humid climatic conditions during the Valanginian CIE would have amplified the carbon isotopic fractionation⁵. This model is supported by the widespread coal occurrence of dense conifer forests at high latitudes¹⁶. Published total organic carbon (TOC) and hydrogen/oxygen index (HI/OI) data for Valanginian marine sediments suggest that terrestrial organic matter was preserved in distal pelagic settings, which represents a surplus of terrestrial C_org that was exported into the oceanic system³⁴. Therefore, records from bulk marine organic matter are prone to biases due mixing of marine and terrigenous C_org or changes in the algal/plant community in the bulk marine TOC pool.

Belemnite and bulk marine carbonate were affected by externally derived carbon during the Weissert event, including carbonate platform- and terrestrial-derived dissolved inorganic carbon (DIC), atmospheric CO₂, and/or methane^4,7,8. The different magnitudes of the CIE in the bulk marine and belemnite carbonate have previously been explained by the different habitat distributions of coccolithophores and belemnites, which broadly represent the epipelagic and bathypelagic DIC pools, respectively³⁵. Moreover, our clumped isotope-derived temperatures, along with other studies of belemnite rostra, suggest that the belemnites lived in deeper and colder waters than indicated by TEX₈₆ records. In the Wawal core, a CIE of 2.2‰ recorded by the benthic foraminifer Lenticulina reflects the change in deep-ocean DIC³⁶, which is similar to the median magnitudes of the CIE obtained from the belemnite rostra (2.4‰) (Fig. 2a). In contrast to coccolithophores, belemnites were predators that hunted small-sized faunas beneath the epipelagic zone at ocean temperatures of 10℃–30℃. A different vertical distribution of these two faunas is a viable hypothesis to explain the different magnitudes of the CIE recorded by bulk marine and belemnite carbonate^6,35. Given that most exogenic carbon is stored as DIC in the marine water column³⁷, our δ¹³C_bel data reflect the dynamic coupling of the oceanic carbon pool and global carbon cycling. An increase in both continental and oceanic CO₂ sequestration could have contributed to the observed late Valanginian decrease in atmosphere pCO₂, regardless of the precise mechanism that triggered ice sheet expansion.

Reconstruction of seawater temperatures and δ¹⁸O values. Global climatic perturbations have been previously documented across the Valanginian interval by oxygen and clumped isotope^13,14,18, TEX₈₆^6,7, and Mg/Ca^14,19 palaeothermometry. This decrease in seawater temperature is also recorded by our Δ₄₇ values for the belemnite rostra (Figs. 3 and 4). The belemnite Δ₄₇ values vary between 0.653‰ ± 0.010‰ (1σ) and 0.692‰ ± 0.008‰, and yield seawater temperatures of 19℃ ± 2℃ to 31℃ ± 2℃ (Supplementary Table 5). This range of Δ₄₇ values can be attributed to seawater temperature fluctuations recorded by the belemnite rostra. The temperatures are consistent for a variety of taxa, indicating there are no significant intra-specific vital effects. Our data reveal there was a long-term warming trend in the pre-CIE interval of 5.4℃ and an abrupt temperature decrease of 5.1℃ at the onset of the CIE interval. This abrupt cooling was enhanced by an additional cooling of 4.9℃ in response to the Weissert event, which corresponds to a total temperature decrease of 10℃ that lasted for < 2.4 Myr (Fig. 4). In addition, the entire Δ₄₇ record for the Weissert CIE is suggestive of cooler conditions, although a few samples have lower Δ₄₇ values suggestive of short-lived warming events within a much cooler interval. Our data also show that average seawater temperatures (24℃) for the Valanginian Weissert event at mid-latitudes (~ 48°S) are lower than the average BAYSPAR-derived SSTs of ~ 27.8℃ for the Weddell Sea (~ 54°S)⁷ and global-average TEX₈₆^H-derived SSTs of ~ 35℃². This also suggests that the Hibolithes investigated in this study records cooler waters and lived in a habitat below the thermocline^6,35.

The recorded δ¹⁸O_bel values throughout the studied succession fluctuate around 0.0‰. Starting with values of about − 0.3‰ in the lowest UZA 15, there is a slight increase of about 0.3‰ up to the upper part of UAZ 16. A progressive warming interval between the K. inostranzewi and S. verrucosum zones is characterised by average δ¹⁸O_bel values that are slightly negative at the onset of the Weissert CIE, and then increase abruptly to ~ 1‰ in the N. peregrinus zone. In the southern Tethyan Ocean (~ 48.5°S), δ¹⁸O_bel values increase steadily towards a maximum of + 0.81‰ in UAZ 18 above the Weissert CIE at approximately the S. verrucosum–N. peregrinus boundary. Thereafter, δ¹⁸O_bel values decrease by approximately 0.6‰ up to the top of UAZ 18 and remain relatively uniform at 0‰ up to the Valanginian–Hauterivian boundary. In addition, our seawater temperatures and the equilibrium relationship of Kele et al.³⁸ between temperature and α_cc/w allow the δ¹⁸O of the fluid that the inorganic calcite formed in to be determined. Changes in the oxygen isotopic composition of seawater (δ¹⁸O_sea, relative to Vienna standard mean ocean water [VSMOW]) are ~ 2‰ (− 1.3‰ to 0.4‰) (Supplementary Table 6), based on the δ¹⁸O_bel values of − 0.73‰ to 0.81‰ and temperatures of 18℃–28℃.

Implications for Early Cretaceous ice sheets Multiple proxies have revealed oceanic cooling and positive CIEs that have previously been associated with increases in carbon burial and primary productivity that likely resulted in an abrupt decrease in atmospheric pCO₂ and ice sheet formation^7,13,14. The Mesozoic to Cenozoic ice sheet history is partly constrained by δ¹⁸O records because large (i.e., tens of metres) and rapid (< 1 Myr) global sea level fluctuations should be associated with changes in δ¹⁸O values due to variations in global ice volume^39,40. The lowest sea level in the Cretaceous¹⁰ was associated with glacial polar conditions, based on present-day-like seawater δ¹⁸O values^13,41, cool temperatures^13,42,43, and occurrence of glendonites¹⁷, which together suggest that glacio-eustatic occurred at this time. Our reconstructed palaeotemperatures derived from clumped isotope palaeothermometry and calculated δ¹⁸O_sea data provide evidence for coupling of ocean temperatures in the Tethyan Ocean with the potential increase in ice volume, which suggests this drove climate change at mid- and southern latitudes.

To reconstruct the sea level fluctuations in deep time from the belemnite δ¹⁸O data, we used isotopic data from the Eastern Tethys, which contained deeper waters that were less affected by a freshwater input. During the Valanginian warm interval, seawater temperatures of 21°C coupled with a δ¹⁸O_bel value of − 0.69‰, represent an ice-free condition. This was determined from a calculated δ¹⁸O_sea value of − 1.3‰, which is broadly equivalent to the value expected for an ice-free world (i.e., sea level of 0 m) (Methods). This and the relationship between δ¹⁸O_bel and sea level change proposed by Nordt et al.⁴⁴ allow us to reconstruct sea level fluctuations across the Early Cretaceous Weissert event. Applying this approach to our stratigraphic sections yields an average sea level low stand of − 13 m that coincides with a positive δ¹⁸O_sea value of − 0.1‰ during the pre-CIE interval (Fig. 4). An abrupt rise in sea level may have occurred due to the increase in global seawater temperatures near the boundary with the main CIE, which is also linked to an increase in atmosphere pCO₂⁷.

After the main CIE, seawater temperatures decreased and sea level fell by 32 m, indicating the expansion of ice sheets and a return to another marine low stand for nearly 1 Myr. It is unlikely that the total ice volume during the Early Cretaceous was similar to the present-day, because of the warm mid-latitude seawater temperatures and the evidence that glacial events were limited in terms of the stratigraphy (thin deposits in thick formations) and geography (both polar regions). The present-day Antarctic ice sheet contains enough ice to change global mean sea level by 58 m⁴⁵. Our calculated sea level change in the Early Cretaceous ranges from 1–32 m, which is equivalent to 0–53% that of the present-day Antarctic ice sheet. We suggest that the Valanginian ice sheets were probably located on both poles. Assuming an ice thickness of 1.5 km and approximately ~ 40% of the land area being in high-latitude regions, this yields 16.5 × 10⁶ km³ of ice volume, equivalent to an isostatically adjusted glacio-eustatic component of ~ 30 m (Fig. 4; Supplementary Fig. 8 and Table 7). Ice may not have covered the entire land area within the polar circles, which is consistent with the relative warmth in northeast Siberia and coastal Australia and North America at this time, including continental thermophilic vegetation^5,46. This possibly indicates the ice sheets were thicker than assumed in the highlands.

Global palaeoclimatic reconstruction for the Early Cretaceous. Our reconstructed palaeotemperatures derived from Δ₄₇ thermometry suggest that the Valanginian CIE was characterised by climatic cooling, although a slight warming event occurred near the onset of the CIE. This global cooling trend was not interrupted by the end of the Valanginian CIE, and the coolest conditions coincided with the post-CIE interval. The proposed increase in global carbon burial, which coincided with a substantial drawdown of atmospheric pCO₂, may have contributed to ice sheet growth^7,8. The inception of an ice sheet may be sensitive to atmospheric pCO₂, based on analogous major Phanerozoic glacial events^39,44,47. Previous global circulation model (GCM) simulations for the Early Cretaceous indicate that small-scale continental glaciation can survive in Antarctica at pCO₂ = 840 ppm⁴⁰. The Δδ (δ¹³C_carb–δ¹³C_plant)-based pCO₂ reconstruction also indicates there was a ~ 40% decrease in atmosphere CO₂ levels during the Valanginian Weissert event⁸. Although there are palaeogeographic and climate ice sheet coupling model uncertainties, our results indicate that the existence of small- to medium- sized ice sheets (~ 0 − 16 × 10⁶ km³; 0 − 30 m glacio-eustatic equivalent) required atmospheric CO₂ levels to decrease to ~ 500 ppm during the Mesozoic.

Recent stratigraphic and palynological investigations have shown that a cool moist climate, probably in an open sub-tundra-like setting, similar to the modern taiga in northern North America, alternate with a glacial climate in Antarctica during the Early Cretaceous¹⁷. The present study and palaeoclimatic reconstructions^5,7 imply that the Early Cretaceous ice sheets were transient, and grew and decayed rapidly. As such, estimates of the extents of the polar circles under the greenhouse conditions are more complex and variable than for the present-day polar conditions. The late Valanginian cool interval might have been the results of enhanced nutrient influx, silicate weathering rates, organic and inorganic carbon burial, and primary productivity that led to CO₂ drawdown and subsequent global cooling^5,7,8,14. This caused ice sheet expansion and led to the long-term, Early Cretaceous, greenhouse conditions transitioning into relatively short-lived glacial periods were preceded by warm conditions that favoured formation of widespread coal deposits or black shales^7,46 and, consequently, positive CIEs (Fig. 2).

In contrast, the initial Early Cretaceous glaciation was associated with little or no cooling in both the tropical and subtropical oceans, and the prevailing high seawater temperatures were not a barrier to the initiation of polar ice sheets. Previous studies have suggested that orbital dynamics during the Late Cretaceous enhanced the activity of the hydrological cycle, which may have contributed to ice sheet formation³⁹. However, unlike the extreme warmth of the Turonian, the late Valanginian interval was characterised by an associated global cooling trend, possibly because the cold episodes at both northern and southern high latitudes allowed ice sheet expansion¹³. Alternatively, the growth of the ice sheets may have been due to increased sequestration of CO₂ in the deep ocean, similar to the reduced oceanic upwelling during Pleistocene glacial periods⁴⁷. Regardless of the exact mechanism, the temporal association of the decrease in seawater temperatures with ice sheet expansion indicates there was close coupling of the temperature of the Tethyan Ocean and polar ice volume. Our results suggest that perturbations in the carbon cycling and/or atmospheric pCO₂ had a more direct role in driving Early Cretaceous climate as compared with a coolhouse interval.

Radiolarian extraction. Five samples from the Longma section and three samples from Qiabu section were investigated for radiolarians. Hard chert samples were broken into 2–3-cm-sized pieces and washed several times to remove surface sediments. After this, the rocks were soaked in 4–5% hydrofluoric acid for 10–24 h, depending on their silica content. The acid residues were concentrated in 63 µm (bottom), 180 µm (middle), and 250 µm (top) sieves. This procedure was repeated 4–6 times until there were enough radiolarians for the biostratigraphic study. Individual radiolarian fossils were handpicked, measured, and imaged using a Hitachi TM3030 scanning electron microscope (SEM) at the School of Earth and Environmental Sciences of the University of Queensland, Brisbane, Australia. Twenty species belonging to 18 genera were identified in the two studied sections (Supplementary “Radiolarian biostratigraphy” section; Supplementary Fig. 2 and Table 1).

Belemnite preservation. More than 200 belemnite rostra samples corresponding to Hibolithes, and less commonly to Belemnopsis and Duvalia, were collected bed by bed from the Lower Cretaceous Gyabula Formation. In order to eliminate the effects of interspecies variability on the geochemical data, multiple different specimens were collected when possible from the same bed. Preservation of the belemnite rostra was assessed by a combination of petrographic observations, cathodoluminescence (CL) microscopy, SEM microscopy, and electron probe micro-analysis (EPMA) elemental mapping of polished thick-sections of the belemnites. The SEM and CL microscopy was undertaken at the State Key Laboratory of Biogeology and Environmental Geology of the China University of Geosciences, Beijing, China, which hosts a Zeiss Supera 55 scanning electron microscope and Cambridge Image Technology Limited Cl8200mk5 electron source. The EPMA was conducted with a JEOL JXA-8230 electron microprobe at State Key Laboratory of Tibetan Plateau Research Earth System, Resources and Environment (TPESRE), Beijing, China. After imaging, ~ 500 mg of powdered sample was taken from each well-preserved region of the rostra using a handheld dental drill. Depletions of Sr and enrichments of Fe and Mn are indicative of post-depositional alteration of low-Mg belemnite calcite^48,49. Calcium, Mg, Sr, Fe, and Mn concentrations were measured using solution methods at ITPCAS with a Thermo X Series inductively coupled plasma-mass spectrometry (ICP-MS). Further details are provided in the Supplementary text “Identification of the best-preserved belemnite calcite for geochemical analysis”.

Conventional stable isotope analysis. After visual (optical microscope, and CL, SEM, and EPMA investigations) and geochemical (i.e., trace element) screening, 20 calcite rostra from Manla section, 50 calcite rostra from Qiabu section, and 31 calcite rostra from Longma section were analysed for stable carbon and oxygen isotopes. Stable isotope analyses were performed on ~ 300 µg powdered calcites at the Laboratory for Stable Isotope Geochemistry, Institute of Geology and Geophysics, China Academy of Sciences, Beijing, China and were measured by a mass spectrometry (Thermo Fisher Scientific MAT 253) coupled to a Gas bench II preparation system. The analytical precision was better than ± 0.15‰ for δ¹³C and ± 0.20‰ for δ¹⁸O, based on replicate analyses of internal laboratory calcite standard TB-1 and TTB-1. All isotope ratios are presented in per mil (‰) related to the Vienna Pee Dee Belemnite (VPDB) standard.

Strontium isotope stratigraphy. ⁸⁷Sr/⁸⁶Sr ratios were measured after Sr separation from other elements in the belemnite rostrum by cation exchange chemistry using AG50W-X4 resin. The ⁸⁷Sr/⁸⁶Sr ratios were measured with a Nu Plasma II multi-collector ICP-MS at TPESRE. For biogenic calcite, 1–3 mg of powdered sample is needed. The NIST SRM 987 international Sr standard was used to assess the instrumental stability. Total procediral blanks were 300 pg for Sr. Data were normalized to the mean value of 0.710248 ± 13 (2sigma; n = 41) for SRM 987. Measured ⁸⁷Sr/⁸⁶Sr ratios were normalized to ⁸⁶Sr/⁸⁸Sr = 0.1194 to correct for mass fractionation. The long-term reproducibility (2sigma) was < ± 0.000010 for routine analyses. The ⁸⁷Sr/⁸⁶Sr ratios were projected onto the global marine ⁸⁷Sr/⁸⁶Sr calibration curve (Look-up Table )2³ to obtain numerical ages (Supplementary Fig. 6 and Tables 1–3).

Clumped isotope thermometry. We undertook carbonate clumped isotope analyses in the newly established experimental systems at TPESRE. Twenty-nine representative belemnite rostra of Belemnopsis (n = 2) and Hibolithes (n = 27) from the Early Cretaceous Gyabula Formation deposits in the southern Tethys that appeared to retain primary textures were analysed to estimate the formation temperature of the biogenic calcite. For each analysed sample, the preservation state was assessed with the same visual and geochemical methods used for the C-O isotope analyses to facilitate subsequent seawater paleotemperature calculations.

The detailed analytical procedures for the clumped isotope measurements are described in the Supplementary section “Clumped Isotope Analyses” and are only summarised here. Firstly, 5.5–6.0 mg of each powdered sample was digested with 1–2 mL of phosphoric acid (ρ = ~1.93 g/cm³; ~104%) in an acid bath at 90°C for 15 min. After acid digestion, the CO₂ was quickly captured in a stainless-steel U-trap with liquid nitrogen, and gas impurities, such as water vapor, were removed. Purification was then carried out in stainless steel vacuum tubes at 23°C to remove potential contaminants, which involved passing the gas through a cryogenic water trap (stainless steel tubing immersed in a liquid nitrogen-ethanol slurry at − 90°C ± 5°C), an adsorption U-trap filled with Porapak Q (stainless steel tubing immersed in a liquid nitrogen-ethanol slurry at − 15°C ± 0.5°C), and another cryogenic water trap (− 90°C ± 5°C) to further remove traces of water vapor and organic contaminants. Finally, the purified CO₂ gas samples were measured with a Finnigan MAT 253 plus gas source isotope ratio mass spectrometer configured to simultaneously collect m/z = 44–49 ion beams, with a pressure adjustment that produced a mass 44 signal of 16 V to obtain raw Δ₄₇ data. Each analysis included 8 sets of measurement sequences that consists of 4.5 off-peak cycles, 8 on-peak cycles, and then 4 off-peak cycles. Equilibrium CO₂ standard gases prepared at 1000°C and 25°C were also analysed with the samples for normalising the sample Δ₄₇ values. The raw Δ₄₇ values were corrected for non-linearity and baseline background effects of the mass spectrometer by empirical calibration⁵⁰, and then standardised from a working gas reference frame to an absolute reference frame (ARF), which allows for interlaboratory comparison^51,52. We further constructed a standard transfer function (STF)^53,54 to standardise our samples and the effects from different acid digestion temperatures⁵⁵.

Six internal carbonate standards (BACS, BZ, IAEA-C1, ISTB, ETH3, and ETH4) were analysed multiple times for calibration proposes. During the course of the analyses, the mean Δ₄₇ values and standard deviations (σ) of these six standards were 0.719‰ ± 0.011‰ (n = 11), 0.497‰ ± 0.018‰ (n = 27), 0.362‰ ± 0.015‰ (n = 15), 0.728‰ ± 0.018‰ (n = 10), 0.709‰ ± 0.008‰ (n = 4) and 0.513‰ ± 0.003‰ (n = 2), respectively. We also undertook frequent analyses of these standard materials to check the internal consistency of repeated measurements within and between analytical sessions. The Haipu (Beijing, China) isotopic standard CO₂ tank gas (δ¹⁸O = − 20.117‰ [VPDB]; δ¹³C = − 21.495‰ [VPDB]) was used as the working reference gas during the clumped isotope analyses, and isotopic ratios were calculated using the Brand parameter set⁵⁶. In this study, we used the thermometry equation of Kele et al.³⁸ (Eq. 1) to reconstruct the precipitation temperatures of biogenic calcite:

$${\text{Δ}}_{47}\text{ }\text{=}\text{ }\text{0.0449}\left(\text{±}\text{ }\text{0.001}\right)\cdot \frac{{\text{10}}^{\text{6}}}{{\text{T}}^{\text{2}}}\text{ }\text{+}\text{ }\text{0.167(}\text{±}\text{ }\text{0.01}\text{)}$$

where T is the temperature in Kelvin. In an open marine setting, this equation represents equilibrium fractionation between calcite and water and yield δ¹⁸O_sea values within the expected range²⁸.

Estimating sea levels and ice volumes from the δ ¹⁸ O data. To convert changes in δ¹⁸O_bel values to those in sea level, we assumed that the δ¹⁸O_sea value with a global sea level of 0 m would be − 1‰, because of the absence of ice, and reconstructed similar relative ice volume trajectories for any hypothetical ice-free baseline value during the greenhouse world. We used the calcite palaeothermometry equation of Kele et al.³⁸:

10³ln (¹⁸α_cc/w) = 16.8×10³/T–26 (2)

where T is the temperature in Kelvin. A δ¹⁸O_bel value of − 0.69‰ and temperature of 23°C from Δ₄₇ thermometry best represent this ice-free climate condition during the Early Cretaceous. According to plate tectonic and palaeogeographic reconstructions, the Gyangze Basin was a marginal marine basin adjacent to the Indian continent in the studied interval¹¹. Although evaporation and weathering inputs may affect the δ¹⁸O values of belemnite calcite and seawater, the study area is thought to have been an open marine environment at mid-latitude, based on preserved marine fauna, and thus these are expected to have had minimal effects on our calculations.

To calculated sea-level change, we used the equation of Nordt et al.⁴⁴:

SLC = (δ₀ – δ) × 10/Δ_10msl × F_iv × 0.68 (3)

where SLC is the change of sea level in metres, δ₀ = − 0.69‰ is the δ¹⁸O_bel value when sea level is 0 m, and δ is the δ¹⁸O_bel value at the time of interest. The Δ_10msl is assumed to be the change in δ¹⁸O_sea expected from a 10 m change in sea level due to glacio-eustasy in the Mesozoic greenhouse world⁴⁴. F_iv represents the fraction of the δ¹⁸O_bel values attributed to changes in δ¹⁸O_sea, which is 0.33 and explained by changes in ice volume⁵⁷. It is then assumes that the isostatic adjustment from apparent to eustatic sea level change is 0.68⁵⁸.

All study data are included in the article and/or Supplementary Materials.

Acknowledgements

This work was supported by Second Tibetan Plateau Scientific Expedition and Research Program (Grant No. 2019QZKK0708), the National Natural Science Foundation of China BSCTPES project (Grant No. 41988101), the National Natural Science Foundation of China (41988101-0106, 42102021), the Chinese Academy of Sciences, Strategic Priority Research Program (Grant No. XDA20070301), and the Special Research Assistant Funding Project of Chinese Academy of Sciences (2021000015).

Author contributions

T.W., S.H. and Q.Z. conceived the idea of this study, T.W. and S.H. performed research and all the experiment analysis. Q.Z., G.L., F.C., C.W., Z.L., J.X., J.S. and X.M. contributed to data analysis, discussion and participated in the fieldwork; T.W. and X.M. identified the fossils. S.H., T.W. and J.X. contributed to clumped and stable isotopic analyses; T.W., S.H., Q.Z. and Y.Y. worked on trace geochemical element testing (⁸⁷Sr/⁸⁶Sr ratios); T.W. J.X. and S.H. performed Petrographic analysis (CL, SEM, EPMA); and T.W., S.H., L.D, Q.Z. wrote the manuscript with contributions from all authors.

Ethics declarations

Competing interests

The authors declare that they have no conflict of interest.

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Ice Sheet Expansion in the Cretaceous Greenhouse World

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