Introduction

The Carnian Pluvial Episode (CPE) was a profound and widespread climatic and environmental change of the early Late Triassic1,2. It occurred between ca. 232 and 234 Ma (late Julian–early Tuvalian), and stands out as a pivotal juncture of major biological changes, with extinctions among some marine and terrestrial clades and the rise of modern-type ecosystems2. These changes were coeval with the emplacement of the Wrangellia Large Igneous Province (W-LIP), implying that the event was triggered by large-scale volcanism3,4. Coeval increased deposition of siliciclastic material and higher sedimentation rates in many marine basins, hygrophytic palynological assemblages, and paleosol profiles indicate a shift to a more humid environment and a stronger hydrological cycle in marginal marine environments5,6,7,8,9,10, especially those of the Western Tethys, with possible aridification in the Pangea interior11.

Geochemical records across the CPE in Northwestern Tethys, South China, Tibetan Plateau and Panthalassa show multiple negative carbon isotope excursions (NCIEs) associated with higher sedimentary Hg concentrations and mass independent fractionation (MIF) of Hg-isotopes towards higher values4,12,13,14. Overall, these data suggest multiple injections of13C-depleted carbon into the ocean–atmosphere system and increases in atmospheric pCO2 synchronous with higher atmospheric Hg inputs to depositional environments, likely as a result of the emplacement of the coeval W-LIP and associated emissions of volcanic gases4,5,12,13,14,15,16,17,18,19,20,21. However, studies on CPE geochemical perturbations and environmental changes have been carried out mostly in low-latitude successions (below ca. 30°N and 30°S) from marine western and eastern Tethys and Panthalassa, and terrestrial North China5,9,13,15,16,17,18,22,23,24,25, with only one record from Tasmania Basin, a southern high-latitude (ca. 70°S) region26. Higher-latitude (> 40°) areas are predicted to be more sensitive to climate change than subtropical regions due to amplified warming effects (e.g., ref27). In addition, numerical modelling also shows a latitudinal dependence in climate sensitivity to perturbations during ancient hyperthermals, especially on land28,29. Understanding the effects of past climate perturbations at different latitudes is therefore crucial for fully mapping the nature of CPE, but the occurrence and nature of the CPE from northern high latitudes has yet to be evaluated.

In this study, we present high-resolution profiles of organic matter (OM) C-isotopes, Hg concentrations, total organic carbon (TOC), total nitrogen (TN) and total sulfur (TS) from a terrestrial (fluvial-lacustrine) succession from the Junggar Basin, located in Xinjiang, northwestern China. The Late Triassic sediments in the basin were deposited at a palaeolatitude of ca. 60° N30,31,32 (Fig. 1). Here, hygrophytic sporomorph assemblages suggested more humid conditions in a portion of the studied section, possibly indicating the CPE in the area33,34. The new geochemical data and revisited biostratigraphy-based timescale shed new light on understanding the nature of CPE.

Fig. 1
figure 1

Global distribution of CPE C-isotope records and geological background of study area. (a) Paleogeography of the Carnian with locations of records of CPE’s negative C-isotope excursions5,13,15,16,17,18,20,22,24,25,26,71 (yellow dots), including the new record from the Junggar Basin (red star). Map is modified using Adobe Illustrator (version 2024, Adobe Inc., https://www.adobe.com/products/illustrator.html) after ref4. (b) Location of the studied section (red star). Map created using Adobe Illustrator (version 2024, Adobe Inc., https://www.adobe.com/products/illustrator.html). (c) Boundary between Karamay Fm. and Huangshanjie Fm. in Dalongkou section. (d) Boundary between Shaofanggou Fm. and Karamay Fm. at Dalongkou section. (e) Field photo of Dalongkou section with formation boundaries. Abbreviations: T1sh = Shaofanggou Fm., T2 − 3k = Karamay Fm., T3hs = Huangshanjie Fm.

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Geological setting

The terrestrial succession of the Junggar Basin

The Junggar Basin, which is now located in the Xinjiang Uygur Autonomous Region in Northwestern China, is a notable geological locality with a well-exposed terrestrial Permian–Triassic succession35,36,37. The studied Dalongkou section is located southwest of Santai town in the basin (Fig. 1). The section is located at the northern limb of the Dalongkou anticline. The Karamay Formation (hereafter Fm.), Huangshanjie Fm. and Haojiagou Fm. overlies the Lower Triassic Shaofanggou Fm. (Fig. 2). The Karamay Fm. lies on the Shaofanggou Fm. with an erosional contact38,39, and consists of greyish-green and grey sandstones, siltstone and mudstones. The boundary between the Karamay Fm. and the Huangshanjie Fm. is placed at the transition from a siliciclastic sequence with frequent thick sandstone layers to finer grained rocks dominated by bedded mudstones33,40 (Fig. 2). The Huangshanjie Fm. consists mainly of black to greyish mudstones and marls alternating with calcareous fine sandstones. The Haojiagou Fm. consists of interbedded yellowish-green and greyish-green mudstones, sandstones and conglomerates. The sedimentary rock succession of Dalongkou contains abundant fossils, including plants, palynomorphs, and vertebrates33,40,41,42,43,44,45,46, indicating fluvial-lacustrine environment39.We sampled a 753 m thick interval that includes the Karamay Fm., the Huangshanjie Fm. and lowermost Haojiagou Fm., in stratigraphic order from the bottom to the top (Fig. 1).

Fig. 2
figure 2

Total organic carbon (TOC), TOC:TN (TN = total nitrogen), organic carbon isotopes (δ13CTOC), mercury (Hg) concentrations, and Hg/TOC (only TOC > 0.2%) of the study section. δ13CTOC data have been separated into samples with TOC:TN ≤ 10 (black dots) and samples with TOC:TN > 10 (grey dots). Reference thresholds of TOC:TN of lacustrine algae and C3 land plants are from ref62. Data have been smoothed using LOESS regression (span = 0.1). For δ13CTOC, LOESS smoothing has been performed only on samples with TOC:TN ≤ 10. Abbreviations: NCIE = negative C-isotope excursion, P. = Period, St. = Stage, Fm. = Formation, T. = Thickness, m = metres, OL = Olenekian, SFG = Shaofanggou Fm., HJG = Haojiagou Fm., Lad-Car = Ladinian–Carnian boundary. The upper limit of background Hg values was calculated following ref61.

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Age constraints of the terrestrial succession at Dalongkou

The ages of the Huangshanjie and Haojiaggou Fms. are generally taken as Carnian–Norian and Norian (Late Triassic) respectively37,40,41,44,47. The Karamy Fm. has been previously attributed to the Middle–Late Triassic, though with great uncertainty on the placement of the Ladinian–Carnian boundary39,40,45,48. A revision of the existing biostratigraphic constraints, considering recent calibrations of diagnostic terrestrial taxa, assigns most of the Karamay Fm. to the Carnian (Supplementary Fig. S1).

Vertebrates such as Xiyukannemeyeria (Parakannemeyeria) brevirostris, Parakannemeyeria chengi, Parotosaurus sp. and Turfanosuchus dabanensis were found in the lowermost part of Karamay Fm. (10 m above its base) in the study area, including our Sects40,43,49. Turfanosuchus has been correlated to Sinokanneeyeria Fauna in the Middle Triassic Tongchuan Fm. of the Ordos Basin (North China), and to Gracilisuchus in the Carnian portion of the Chañares Fm. in Argentina50. Of note is that the transition between the uppermost Chañares Fm. and the overlying Los Rastros Fm. in Argentina records the onset of the CPE51.

In the lowermost part of the Karamay Fm. at Dalongkou (56–66 m; Supplementary Fig. S1), plant fossil Pleuromeia sp., which has a stratigraphic range spanning from the Early Triassic (Induan) to late Middle Triassic (Ladinian) in North China, has been discovered45,48,52. Annalepis cf. zeilleri is found between 70 and 100 m in the lower Karamay Fm. and its occurrence ranges from the late Early Triassic into the early Carnian45,48,53. The plant assemblage of the Karamay Fm. (above ca. 120 m) and the Huangshanjie Fm. includes Danaeopsis fecunda and Symopteris (= Bernoullia) zeilleri, along with Neocalamites carrie, Equisetites sthenoden, etc40,45. (Supplementary Fig. S1). This assemblage is similar to the Late Triassic flora of the Yanchang Group in the Ordos Basin, which belongs to the Danaeopsis-Symopteris (= Danaeopsis-Bernoullia) flora of northern China54,55, and found also within the CPE interval of the adjacent Jiyuan Basin24. This flora is considered Late Triassic in age and was associated with warm and humid climates56,57,58. An et al.55 showed that in North China Danaeopsis fecunda only occurs in the Late Triassic, starting from the Carnian55. Indeed, fossils found in the Ladinian upper Ermaying Fm. in the Ordos Basin of North China, previously attributed to Danaeopsis fecunda, have been reclassified as Danaeopsis marantacea, which occurs throughout North China and western Tethys in the Middle–Late Triassic55,59. The sporomorph assemblages in the upper Karamay Fm. and Huangshanjie Fm. (Supplementary Fig. S1) are comparable with other Late Triassic assemblages found in the Ordos Basin, Kazakhstan, and Libya40. Xenoxylon junggarensis sp. nov. is found in the middle of the Huangshanjie Fm. from 443 to 472 m, and is believed to be Norian in age46. Peng et al.33 assigned a Norian–Rhaetian age for the upper Huangshanjie Fm. on the basis of the occurrence of Lycopodiacidites-Stereisporites sporomorph assemblage.

Results

Carbon isotope compositions measured on the bulk organic carbon fraction of the rocks (δ13CTOC) have values that vary from − 30.2‰ to − 22.6‰ in the studied section (Fig. 2), with an average of − 25.78‰. The δ13CTOC values in the lowermost Karamay Fm. exhibit a long-term positive trend from 0 to 120 m. At 120 m, in the Karamay Fm., is the onset of an interval containing four NCIEs (NCIE I–IV, Fig. 2), which ends around 472 m within the Huangshanjie Fm. (Fig. 2). The NCIEs have magnitudes of 2–4‰ (Fig. 2). After the last NCIE, δ13CTOC values of the entire Huangshanjie Fm. to lower Haojiagou Fm. (top of the measured section) remain relatively stable (Fig. 2).

Only 6 samples have TS values greater than 0.1 wt.% (average value = 0.02 wt%, maximum value = 0.34 wt%; Fig. 3). TOC values vary from 0 to 9.26 wt.%, with an average value of 0.85 wt.%. 150 samples have TOC < 1 wt.%, 15 samples have TOC in the range of 1–5 wt.%, and 6 samples have TOC > 5 wt.%. There are three main intervals of higher TOC content (> 1 wt.%): at 174–180 m, 273–336 m and 443–458 m (Fig. 2), which correspond to high TOC:TN ratios (> 20).

Fig. 3
figure 3

Crossplots of geochemical data. (a) TOC versus δ13CTOC. (b) TOC:TN versus δ13CTOC, (c) TOC versus Hg (all data). (d) TOC versus Hg (only Phase I and Phase II). (e) TOC:TN versus Hg (all data). (f) TOC:TN versus Hg (only Phase I and Phase II). For the subdivision into Phase I and II see main text and Fig. 2. Reference ranges of variability of TOC:TN and δ13C of lacustrine algae and C3 land plants are from ref62.

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Hg and Hg/TOC exhibit significant variations throughout the whole Karamay Fm. and the base of the Huangshanjie Fm. (Fig. 2). Hg/TOC with TOC < 0.2 wt.% are highlighted in the figures following guidelines for Hg normalization in ref60. Hg concentrations vary from 0 to 689 ppb, with an average value of 33.9 ppb (see data in the supplementary table). Background sedimentary Hg contents are calculated to be < 45 ppb (Fig. 2) following methods in ref61. Hg is not correlated with TS (r = 0.02; Supplementary Fig. S2). TOC overall exhibits moderate correlation with Hg (r = 0.55; Fig. 3). Hg and Hg/TOC spikes are found in the interval containing the CIEs (Fig. 2). The first sharp Hg and Hg/TOC rises are found at 129 m in the Karamay Fm., slightly after the onset of the NCIE I interval (Fig. 2). The last significant Hg spikes occur at the base of the Huangshanjie Fm., i.e. from 436 to 472 m, and overlap with the last CIE (Fig. 2).

Discussion

C-isotope record of the CPE in the Junggar Basin

Based on the age constraints of the succession, our new geochemical records from the Junggar Basin from the lower Karamay Fm. to the lower Huangshanjie Fm. provide evidence of the multiple NCIEs that mark the CPE globally in marine and terrestrial basins, and are thought to have been caused by the emplacement of W-LIP (Figs. 4 and 5, and see discussion on Hg data in the next section). In sedimentary archives, the C-isotope signature of bulk OM can, however, be influenced by local variations in the relative proportion of different organic components (e.g., higher plants vs. algae) delivered to the depositional environment, and early and/or late diagenetic processes62,63. These factors could, potentially, mask global changes in the C-isotope composition of the exogenic reservoirs of the C-cycle, or produce NCIEs that are unrelated to global changes.

Fig. 4
figure 4

Correlation of data from Dalongkou section (Junggar Basin, NW China) with carbon isotope, Hg/TOC and ∆199Hg records from reference terrestrial and marine successions recording the CPE worldwide. Published C-isotopes and Hg data from W and E Tethys, and Panthalassa are from ref5,14,17,18,20,24,79. Abbreviations: J.1 = Julian 1; J.2 = Julian 2; T.1 = Tuvalian 1; T.2 = Tuvalian 2; UK = United Kingdom; HJG = Haojiagou Fm; LAD = Ladinian.

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Fig. 5
figure 5

The Carnian Pluvial Episode (CPE) at Dalongkou, Junggar Basin (Northwestern China). Major recorded lithological and floral changes are linked to the new C-isotope and Hg records. The CPE is here defined as the interval from the first recorded negative C-isotope excursion (NCIE I) to the end of the positive rebound of the last negative C-isotope excursions (NCIE IV). Main floral changes are from ref33,48. δ13CTOC and Hg trends are shown using the LOESS smoothed curves (Fig. 2). Abbreviations: P. = Period, St. = Stage, Fm. = Formation, HJG = Haojiagou Fm., Lad-Car = Ladinian–Carnian boundary.

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Diagenesis can modify the pristine δ13CTOC signature by decreasing TOC and preferentially removing 12C63. No correlation is observed between the measured δ13CTOC and TOC at Dalongkou (Fig. 3). Significant 13C-enrichments in OM due to more rapid breaking of 12C=12C bonds can be found at metamorphic and late diagenetic stages64,65. However, the preservation and colour of sporomorphs in the studied succession33 indicate low maturity, and organic geochemical analysis of the Karamay Fm. and Huangshanjie Fm. in the study area also indicates low thermal maturity66.

Changes in OM source can be proxied by the molar TOC:TN ratio because freshwater algae typically have a TOC:TN ≤ ~ 10, while C3 higher plants typically have TOC:TN > ~ 2062. TOC: TN values across Dalongkou section indicate a predominantly algal source (TOC:TN ≤ 10, Fig. 2), with short discrete intervals (i.e. 173–181 m, 288–300, 324–332 m and 444–455 m) of increased input of material from C3 higher plants (TOC:TN > 10 and TOC > 1 wt.%, Fig. 2). Freshwater algae and C3 higher plants have, in general, similar δ¹³C ranges of variability62 and δ¹³CTOC vs. TOC:TN are not correlated in our samples (r = 0.04; Fig. 3). Nevertheless, peaks of C3 higher plant contribution appear to have had an effect on bulk δ¹³CTOC resulting in noisier C-isotope intervals, especially at 288–332 m (Fig. 2). Local ecological conditions can modulate C-isotope fractionation in primary producers during photosynthesis67, and post-photosynthetic fractionation can result in different δ13C signatures among tissues68. Hence, changing fluxes of OM from different sources to the lacustrine system could potentially produce δ¹³CTOC deviations superimposed to the global atmospheric signal. In order to minimize this possible effect on the measured δ¹³CTOC values, we plotted separately δ¹³CTOC data of samples with TOC:TN ≤ 10 and TOC: TN > 10 (Fig. 2). The δ¹³CTOC curve including only data from a fairly stable lacustrine algal OM source more distinctly reveal isotopic changes that are typical of global Ladinian – Carnian δ¹³C records (Figs. 2 and 4).

The δ13CTOC data show a 2–3‰ positive trend in the lowermost part of Karamay Fm. that is consistent with the global late Ladinian – early Carnian C-isotope record (Figs. 2 and 4). A positive δ13C shift is recorded in marine carbonates (including brachiopods), and marine and terrestrial OM from the latest Ladinian through the early Carnian (Julian), and has been attributed to the reappearance of coal swamps after the coal gap following the Permian – Triassic mass extinction69. Following the δ13CTOC positive trend, four major NCIEs are recorded at Dalongkou.

The stratigraphic interval covering the four observed NCIEs falls within the age range of the CPE, despite the typically larger uncertainties associated with dating terrestrial successions (e.g., in the Ordos Basin24, Devon in UK18). NCIE I starts at 120 m in the Karamay Fm., in correspondence to the earliest occurrence of Danaeopsis fecunda, which is Late Triassic (Carnian) in age. Camarozonosporites rudis (Camarozonotriletes rudis in ref33). , is a characteristic taxon appearing during the CPE in other areas7,9,70, and is found in the Lycopodiacidites–Stereisporites informal abundance zone at Dalongkou, just after the NCIE IV interval as defined in this study (see Supplementary Fig. S1). The Norian Xenoxylon junggarensis sp. nov. is found in the middle of the Huangshanjie Fm. after the NCIE IV interval (Fig. 2 and Supplementary Fig. S1), and no distinct NCIEs are recorded above this level.

Global Carnian C-isotope records from around the world show 4–5 NCIEs in Julian 2–Tuvalian 25,18,24,26 (Fig. 4). As such, each NCIE at Dalongkou can be tentatively correlated with other terrestrial and marine CPE records (Fig. 4). Three to five major NCIEs appear in Western Tethys successions (Fig. 4) during the CPE5,16. Four distinct NCIEs were found in the terrestrial Ordos Basin of North China24 and in Tasmania Basin of Australia26. Five NCIEs are present in the terrestrial records from the UK18,71. Our record shows four main NCIEs in the Karamay Fm. – lower Huangshanjie Fm. (Fig. 2). According to the proposed chemostratigraphic correlation in Fig. 5, which is constrained with the available biostratigraphic data (see discussion above and Supplementary Fig. S1), the Julian 1–2 boundary—i.e. the biochronozones boundary around which the first NCIE of the CPE occurs[13,15,16,18,72—would be placed at Dalongkou at ca. 120 m within the Karamay Fm., and the base of the Huangshanjie Fm. would be Tuvalian in age.

Changes in hg deposition in the fluvial-lacustrine environment at Dalongkou

In the CPE interval at Dalongkou, distinct enrichments in Hg concentrations are found (Fig. 2). Hg enrichments in sedimentary records depend on a series of factors including higher input of volcanic Hg, increased delivery of terrestrial-sourced Hg (due to higher runoff, wildfires and/or oxidation of OM), increased clay minerals deposition, and Hg drawdown in anoxic environments60,73,74. Analysis of modern and ancient sediments have shown a dominant sulfidic host phase in samples with TS > 1.0% and TS/TOC > 0.35, and a dominant organic host phase in samples with TS < 1.0% and/or TS/TOC < 0.3574. Low TS content (mostly < 0.1 wt.%; Supplementary Table) and lack of correlation between Hg and TS (Supplementary Fig. S2) exclude sulfides as the main host of Hg at Dalongkou. By contrast, the relatively overall higher correlation between Hg and TOC (r = 0.55; Fig. 3) and an average TS/TOC < 0.07 suggest that OM is the main host of Hg in most of Dalongkou succession60. Nevertheless, other processes, such as changes in the relative abundance of different types of OM in sediment, variable inputs of clay, and possible wildfire activity, could have modulated the amount of Hg delivered to Dalongkou depositional environment.

We can define two main distinct modes of Hg deposition during the CPE at Dalongkou. In Phase I, which starts at the onset of the CPE (beginning of NCIE I) and ends at 272 m (Fig. 2), a low correlation between Hg and TOC (r = 0.27), and Hg and TOC:TN (r = 0.27) is found (Fig. 3). In Phase II, starting at the end of Phase I and ending at the positive rebound of the last NCIE recorded in the succession (NCIE IV) (Fig. 2), Hg and TOC have a statistically stronger correlation (r = 0.78). In Phase II, a relatively high correlation (r = 0.73) is also found between Hg and TOC:TN. Indeed, in Phase II, peaks in Hg, TOC and TOC:TN are paired (Fig. 2). Hg vs. TOC and Hg vs. TOC:TN correlations (Fig. 3) indicate that, in Phase II, the Hg enrichments depend on episodes of increased input of OM derived from C3 higher plants (Fig. 2). In this interval, most Hg concentration spikes are muted when normalized for TOC. For example, the Hg increase associated with the NCIE IV at ca. 450 m, corresponds to both high TOC (5 wt.%) and high TOC:TN (26), and the calculated Hg/TOC does not show a correspondent peak (Fig. 2). In Phase II, wildfires could also have been (partially) responsible for the observed Hg changes73, because abundant fossil charcoal is found in the upper Karamay Fm. in the Northwestern Junggar Basin75.

The absence of correlation between Hg and TOC, and Hg and TOC:TN during Phase I indicates that Hg variations are independent of host variations in this interval. Normalization to TOC does not mute the higher Hg concentrations, suggesting that Hg increases are not ascribable to increases in OM. In Phase I, the interval where the first two NCIEs are recorded, the multiple Hg increases could therefore indicate higher Hg loading in the Junggar Basin from volcanic and/or terrestrial sources. In the marine successions of the Western Tethys and Panthalassa, and the terrestrial succession of the Jiyuan Basin in North China, the NCIEs of the CPE correspond to Hg and Hg/TOC peaks that have been interpreted as volcanic pulses from the W-LIP4,24. Hg isotope records from South China and Panthalassa show higher Hg loading to these depositional environments from the atmosphere, likely sourced from W-LIP volcanism12,14. Similarly, the Hg enrichments in Phase I (the interval recording the first two NCIEs) of the C-cycle perturbations interval at Dalongkou could represent the coeval eruption of W-LIP. LIPs can lead to mercury enrichments in distal regions on a global scale due to extensive atmospheric dispersion60,76. The Hg enrichments in the Karamay Fm. and Huangshanjie Fm. (Phase I) could therefore indicate excessive Hg input during the CPE even at the high northern palaeo-latitudes where the Junggar Basin was situated.

CPE C-cycle perturbations and high-latitude vegetation response

The C-cycle perturbations, as recorded by δ13CTOC, appear to be linked to major environmental and vegetational changes in the Junggar Basin. Notably, the CIEs occur within a stratigraphic interval characterised by coarser siliciclastic (sandstone) beds, which might be related to higher runoff in the area during the CPE (Fig. 2). A lithological transition from red siltstones to grey – greyish green mudstones and sandstones occurs at the onset of NCIE I (Fig. 2). Above, another lithological transition from the greyish green sandstones of the Karamay Fm. to the thick black mudstones of the Huangshanjie Fm. could represent deepening lake levels. Lake deepening and increased frequency of turbidite sandstones are also observed during the CPE in the Ordos and Jiyuan Basins of North China24,77.

At Dalongkou, the NCIE I and the major recorded Hg/TOC peak interpreted to be of volcanic origin are coeval with the appearance of Danaeopsis fecunda and associated flora (Fig. 5). Below this point, Pleuromeia sp. is found in a thick (ca. 10 m) sandstone layer having unconformable contacts with older and younger strata, in the lowermost Karamay Fm. (Supplementary Fig. S1). Pleuromeia is thought to have been an opportunistic genus well adapted to semiarid conditions and is typically found in the Early – Middle Triassic, with some possible occurrences from the Late Triassic78. Annalepis cf. zeilleri, which occurs between ca. 70 and 100 m in the Karamay Fm. (Supplementary Fig. S1), was adapted to saline and evaporative (i.e. arid) environments45,48,53. Annalepis is mostly found in Early – Middle Triassic strata, but some relics survived into the Late Triassic – Early Jurassic45,48,53. The disappearance of Annalepis cf. zeilleri above ~ 100 m corresponds to the lithological shift from purplish-red siltstone to greyish-green mudstone. Danaeopsis fecunda was a marattialean fern that thrived in warm and humid environments during the Late Triassic55. The close temporal link between the Carnian C-cycle perturbations and this vegetation turnover strongly suggests that the CPE was responsible for significant floral changes across the entire North China, as previously hypothesised by An et al.55. Conifers, and seed ferns from the upper Karamay Fm. also suggest a warm and humid environment during the CPE45,48,57,75. A 0.3 m thick coal seam is found at 125 m in the interval of NCIE I, but below this layer there is no coal in the Karamay Fm. (Figures 2 and 5), further indicating a transition from arid to humid climate at the onset of the CPE in the Junggar Basin. Interestingly, full recovery of coal swamps and peatland from the global “coal gap” that followed the Permian – Triassic mass extinction has been previously linked to the CPE in other locations26,79,80. In addition, an Aratrisporites Abundance Zone has been detected beginning at ca. 145 m at Dalongkou33. Abundant Aratrisporites is found during the CPE also in other areas, e.g. Ordos Basin, Western Tethys and High Atlas region in Northwestern Africa7,81,82,83. The interval of hygrophytic sporomorph assemblage found in the Huangshanjie Fm. (480–500 m) correlates to the NCIE IV, which was previously interpreted as representing the entire CPE33, likely represents a discrete humid pulse within the CPE, as observed in many locations around the world5,7,18,71.

Conclusions

We analysed C-isotopes and Hg geochemistry from the Dalongkou section, formed in the ca. 60º N fluvial-lacustrine Junggar Basin (Xinjiang, Northwestern China), to identify the first northern high latitude record of the Carnian Pluvial Episode (CPE). Our data reveal a positive CIE trend followed by four NCIEs and multiple enrichments in sedimentary Hg concentrations. The positive δ13CTOC is interpreted as the global late Ladinian – early Carnian positive C-isotope excursion recorded in multiple marine and terrestrial archives and consistent in age with the available biostratigraphic constraints. The NCIEs and Hg variations recorded at Dalongkou, constrained by existing palaeobotanical biostratigraphy, can be readily correlated to the global C-isotope changes and Hg spikes found during the CPE in other geological marine and terrestrial settings. Higher Hg loading to the lacustrine environment at Dalongkou during interval recording the the first two NCIEs is interpreted to have been triggered by increased volcanic activity and/or oxidation of terrestrial OM (Phase I). Subsequently, in the interval recording the last two NCIEs, increases in Hg concentrations are linked to more elevated input of OM from C3 higher plants (Phase II).

The onset of the CPE at Dalongkou is coeval with a vegetation change marked by the first occurrence of Danaeopsis fecunda and associated macroflora. The floral change indicates an environmental change from mainly arid to more humid conditions, as also evidenced by the occurrence of a thick coal seam and a sedimentological change from purplish-red siltstone to greyish-green mudstone with frequent sandstone layers. These lines of evidence suggest that the Carnian C-cycle perturbations and consequent global warming led to intensification of the hydrological cycle and triggered a markedly more humid conditions in the high northern latitudes of the Junggar Basin, resulting in widespread re-organization of the flora, with proliferation of hygrophytic vegetations in the whole North China.

Materials and methods

We analysed 175 mudstone–siltstone samples for bulk organic carbon isotopes (δ13CTOC), Hg concentration, total organic carbon (TOC), total nitrogen (TN), and total sulfur (TS). All the samples were trimmed to remove weathered parts, and washed and crushed to ~ 200 mesh. For the δ13CTOC and TOC analyses, 1–2 g of powder was weighed for each sample, acid washed with 10% HCL in a 50 ml polypropylene centrifuge tube for 24 h to remove carbonate, then rinsed with deionized water 3–5 times until neutral, and dried at 40 °C. δ13CTOC was measured using a Thermo Fisher Delta V Advantage isotope ratio mass spectrometer at the State Key Laboratory of Biogeology and Environmental Geology of China University of Geosciences (Wuhan). Approximately 0.5–12 mg of decarbonated sample was weighed into tin caps, fed to the instrument, oxidized at 1150 °C and converted to CO2. After H2O was removed through a magnesium perchlorate absorption trap, impurities such as N2 were separated by a 70 °C chromatographic column, and the resulting CO2 was analysed by the mass spectrometer. All δ13CTOC values are reported as per mille relative to VPDB (Vienna Pee Dee Belemnite) and calibrated to international standards USGS40 (δ13C = -26.39‰), USGS24 (δ13C = -16.05‰) and UREA (δ13C = -37.32‰). The analytical precision was better than 0.2‰ (1σ) based on duplicate analysis. For total organic carbon (TOC), total nitrogen (TN) and total sulfur (TS) analyses, approximately 20 mg of decarbonated powder was packed in tin cups and measured using an Elementar Unicube in CNS mode at the School of Earth Sciences, China University of Geosciences (Wuhan). Analytical precision has been calculated on repeated analysis of internal standard OXC-12 (C = 4.47%, N = 0.15%, S = 1.20%) run every 12 samples: the relative standard deviations (RSD) are RSD = 1.36% for TOC, RSD = 2.73 for TN, RSD = 6.38 for TS. Hg concentrations were measured with a Lumex RA-915 Mercury Analyser coupled to a PYRO-915 Pyrolyser at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan). Approximately 50 mg of powder was weighed and fed to the instrument. A replicate sample and a soil standard (GBW07423; 32 ± 3 ppb Hg) were analysed every 10 samples. The results were calibrated to a soil standard GBW07381 (148 ± 11 ppb Hg). The analytical precision calculated on repeated analysis of GBW07423 standard was < 8%.