Late Cretaceous (Santonian to Campanian) Palynological Records and Paleoclimatic Signiﬁcance from Borehole ZKY2-1, Songliao Basin

: The global temperature gradually decreased from the Cretaceous Santonian to Campa-nian, while angiosperms evolved rapidly and gradually became dominant. The Songliao Basin, NE China, contains abundant fossil palynomorphs from the Santonian to Campanian age. A thorough investigation of fossil palynomorphs in borehole ZKY2-1 of the SW Songliao Basin was performed, reconstructing the vegetation and paleoclimate transition from the Santonian–earliest Campanian (lower Nenjiang Formation) to the late Campanian (Sifangtai Formation). Eighty form-genera from borehole ZKY2-1 have been identiﬁed. Three palynomorph assemblages were identiﬁed: the Schiza-eoisporites – Cyathidites – Inaperturopollenites assemblage, Schizaeoisporites–Classopollis–Retitricolporites assemblage, and Schizaeoisporites–Aquilapollenites–Tricolporopollenits assemblage, from bottom to top. Based on palynological analysis from ZKY2-1 and other boreholes in the Songliao Basin, angiosperm pollen proportion in the Sifangtai Formation is signiﬁcantly higher than in the lower Nenjiang Formation, indicating rapid angiosperm spread from late Santonian to Campanian. Palynological records indicate relatively humid climate during this period; the content of cool palynological types increased from the lower Nenjiang Formation to the Sifangtai Formation, suggesting a transition from warm to cool climate during the late Santonian–earliest Campanian to the late Campanian. The new palynological evidence from the Songliao Basin reveals a global cooling on land and sea during the late Santonian–Campanian period. This climate change may further promote angiosperm spread during the Late Cretaceous period


Introduction
The Cretaceous was a typical greenhouse climate period with a series of major geological events, such as large igneous provinces, the Cretaceous Normal Superchron, the Oceanic Anoxic Event, the explosion of life, and mass extinctions [1][2][3][4][5][6].Among them, climate change from a hot greenhouse to a cool greenhouse is remarkable [1,7].The Middle Cretaceous is a typical hot greenhouse; however, the global temperature decreased gradually during the Santonian to Campanian periods [1,7,8].The Late Cretaceous temperature data are mainly from marine sediments [1,9,10], with only a few records coming from nonmarine Cretaceous [11,12].Therefore, studying the terrestrial response to climatic change during the Cretaceous period is very important.
In this study, palynomorphs from borehole ZKY2-1 are investigated to establish t palynological biostratigraphy of the area, and provide the palynological data in the sou west, which makes the research data from the Songliao Basin more perfect.The study veals the late Santonian to Campanian paleovegetation and paleoclimate significance borehole ZKY2-1 in the southern part of the basin.Research concerning the Songliao Basin is mainly concentrated in the center and north, and less concentrated in the south, especially in the southwest [27].Ostracods from the lower Nenjiang Formation (K 2 n) (late Santonian to earliest Campanian) and Sifangtai Formation (K 2 s) (middle to late Campanian) were recently recovered from borehole ZKY2-1 located in the SW Songliao Basin [27].
In this study, palynomorphs from borehole ZKY2-1 are investigated to establish the palynological biostratigraphy of the area, and provide the palynological data in the southwest, which makes the research data from the Songliao Basin more perfect.The study reveals the late Santonian to Campanian paleovegetation and paleoclimate significance of borehole ZKY2-1 in the southern part of the basin.

Geological Setting
The Songliao Basin extends from northeast to southwest, spanning the central part of Heilongjiang Province, the northeast part of Liaoning Province, the eastern part of Inner Mongolia Autonomous Region, and the western part of Jilin Province.It is about 750 km long from north to south, 370 km wide from east to west, and covers an area of about 26 km × 104 km [33] (Figure 1).The Songliao Basin has three evolution stages: a fault depression period (Huoshiling-Yingcheng Formations), a depression period (Denglouku-Nenjiang Formations), and a tectonic inversion period (Sifangtai-Yi'an Formations).The former belongs to the volcanic rift basin and the intracontinental depression basin [40].The Songliao Basin can be divided into five first-order tectonic units: the western slope area, northern dip area, central depression area, northeast uplift area, and southwest uplift area.Borehole ZKY2-1 is located in the southwest uplift area (Figure 1).The Cretaceous sediments formed in the basin are predominantly clastic rocks.From bottom to top, the rock stratigraphic framework consists of the Huoshiling Formation (K 1 h), Shahezi Formation (K 1 s), Yingcheng Formation (K 1 y), Denglouku Formation (K 1 d), Quantou Formation (K 2 q), Qingshankou Formation (K 2 qn), Yaojia Formation (K 2 y), Nenjiang Formation (K 2 n), Sifangtai Formation (K 2 s), and Mingshui Formation (K 2 m) [23,41].The Songliao Basin is rich in fossils, especially the Late Cretaceous strata [22].The chronostratigraphic framework of the Songliao Basin has been established using magnetic, astronomical, isotopic, biostratigraphic, and quantified stratigraphic data [15,17,22,23,25,26,32,42,43] (Figure 1).K 2 n is divided into five distinctive members, including K 2 n 1 , K 2 n 2 , K 2 n 3 , K 2 n 4 , and K 2 n 5 , from bottom to top [17].K 2 n is considered late Santonian to middle Campanian, with the Santonian/Campanian boundary at the lower K 2 n 2 [22,35,36,40,44].K 2 n 1 and the lower K 2 n 2 sequences mainly consist of dark gray or black mudstones and grayish to green silty mudstones intercalated with thin carbonates and deposited in deep to subdeep lake environments [30].The upper K 2 n 2 and K 2 n 3 , K 2 n 4 , and K 2 n 5 consist of grayish mudstones, gray argillaceous siltstones, brownish red mudstones, and sandstones deposited in a shallow lake to delta environment [41].K 2 s comprises sedimentary facies representing riverine and lacustrine systems and other associated environments [45].The K 2 s is considered late Campanian in age [22,35,37].

Materials and Methods
Borehole ZKY2-1 is located in the southwestern Songliao Basin (43°56'59" N, 122°33'45" E), east of the Central Asia orogenic belt.The borehole is buried shallowly, and the fossils are well preserved, forming a continuous stratum from the Cretaceous Yaojia Formation to the Paleogene Taikang Formation in the southwest part of the basin.This study mainly focuses on the lower K2n to the K2s (240-495 m).The samples were collected at intervals of approximately 1-3 m.Twenty-five samples were selected for analysis.The lower K2n (346-495 m) is mainly black or dark gray silty mudstone, while K2s (240-345 m) is mainly gray-green argillaceous siltstone.All research materials are stored in the Microbiology Paleontology Laboratory of China University of Geosciences (Beijing).
Palynological analyses were performed using samples weighing 30-50 g.The samples were first treated with 10% HCl and 40% HF to remove carbonates and silicates, followed by KOH treatment to discolor organic matter.Then, KI and ZnCl2 were combined into a heavy liquid (2.0 g/cm3) that was used to separate the palynomorphs by flotation.Palynomorph identification and counting were performed at 650× magnification and, if necessary, at 800× magnification by using a Carl Zeiss microscope (Axiolab 5, Carl Zeiss AG, Oberkochen, Germany).Photos were taken using an AxioCam MRc5 digital camera

Materials and Methods
Borehole ZKY2-1 is located in the southwestern Songliao Basin (43 • 56 59 N, 122 • 33 45 E), east of the Central Asia orogenic belt.The borehole is buried shallowly, and the fossils are well preserved, forming a continuous stratum from the Cretaceous Yaojia Formation to the Paleogene Taikang Formation in the southwest part of the basin.This study mainly focuses on the lower K 2 n to the K 2 s (240-495 m).The samples were collected at intervals of approximately 1-3 m.Twenty-five samples were selected for analysis.The lower K 2 n (346-495 m) is mainly black or dark gray silty mudstone, while K 2 s (240-345 m) is mainly gray-green argillaceous siltstone.All research materials are stored in the Microbiology Paleontology Laboratory of China University of Geosciences (Beijing).
Palynological analyses were performed using samples weighing 30-50 g.The samples were first treated with 10% HCl and 40% HF to remove carbonates and silicates, followed by KOH treatment to discolor organic matter.Then, KI and ZnCl 2 were combined into a heavy liquid (2.0 g/cm 3 ) that was used to separate the palynomorphs by flotation.Palynomorph identification and counting were performed at 650× magnification and, if necessary, at 800× magnification by using a Carl Zeiss microscope (Axiolab 5, Carl Zeiss AG, Oberkochen, Germany).Photos were taken using an AxioCam MRc5 digital camera (Carl Zeiss AG, Oberkochen, Germany).Considering the region's nature, palynomorph identification in this study was mainly based on relevant books [15,19,20].
CorelDRAW (2019, Corel, Ottawa, Canada) was used for drawings.The data shown were analyzed using Excel (Word Processing System 2019, Kingsoft, Beijing, China) to make line charts, and then the whole picture was drawn in CorelDRAW.
Three assemblages are divided by identification and analysis of palynomorphs of the index taxa.

Stratigraphic Correlation between Boreholes ZKY2-1 and SK1
The Late Cretaceous integrated stratigraphic framework has been established based on borehole SK1 [22,23,36,37,39].Therefore, comparing other boreholes and borehole SK1 is very important (Figure 6).A large-scale lake invasion occurred during sedimentation of the lower K 2 n 1 and lower K 2 n 2 , depositing two sets of black mudstone, shale, and oil shale [48,49].The bottom of both K 2 n 1 and K 2 n 2 in boreholes SK1 and ZKY2-1 are mainly characterized by black shale and mudstone.Thus, the boundary of K 2 y/K 2 n 1 and K 2 n 1 /K 2 n 1 can be easily recognized [50].Owing to tectonic uplift, there is a widespread stratigraphic hiatus between K 2 n and K 2 s [51].From K 2 n to K 2 s of borehole SK1, there may be 3.8 myr of stratigraphic hiatus [38].In borehole ZKY2-1, the stratigraphic hiatus is much longer than that of borehole SK1 because there are no K 2 n 3 to K 2 n 5 strata [27].K 2 s is composed of dark gray, gray mudstone, and grayish green, light gray siltstone [52].Borehole SK1 mainly revealed greenish gray mudstone and silty mudstone, light gray fine sandstone and siltstone, and grayish brown silty mudstone [53].ZKY2-1 revealed mainly gray and dark gray mudstone, siltstone, fine sandstone, and medium sandstone, similar to borehole SK1.
ated by Gao and others in 1976 includes Aquilapollenites, which are widely distributed globally and mainly concentrated in Asia, North America, and in Europe north of 35°N.Aquilapollenites have also been found in central Africa, Malaysia, and Australia.In the Late Cretaceous, the Aquliapollen type was reported in the Rocky Mountains of the United States, Alaska, and the lower reaches of the Mississippi River; and Canada, Siberia, Japan, France and the United Kingdom; It is mainly found in the Songliao Basin, Sanjiang Basin, and Jiayin Region of Heilongjiang Province in northeast China [17].The palynomorphs in borehole ZKY2-1 have typical characteristics of the Late Cretaceous.
By comparing borehole ZKY2-1 with borehole SK1 and analyzing the age of the palynomorph assemblages, K2n 1 in borehole ZKY2-1 is dated as late Santonian, K2n 2 is determined as the latest Santonian-earliest Campanian, the period of K2s is assigned as late Campanian.The age of the lower Nenjiang Formation to Sifangtai Formation of borehole ZKY2-1 lasted from late Santonian to late Campanian, but the middle Campanian strata are missing.The established chronostratigraphic framework provides the basis for the following vegetation analysis and climate change.Palynomorphs and ostracods play an important role in biostratigraphic correlation in the Songliao Basin.In borehole ZKY2-1, K 2 n 1 is represented by the Schizaeoisporites-Cyathidites-Inaperturopollenites assemblage, with gymnosperm pollen predominating.High concentrations of Cyathidites and Schizaeoisporites are found in the fern spores, while Osmundacidites were found to a lesser level.Pinuspollenites dominate in the gymnosperm pollen, while Pinaceae and Classopollis have varying content levels.Tricolpopollenites from angiosperm pollen were found.In borehole SK1 [24], Schizaeoisporites were predominant in the fern spores, while Cyathidites and Osmundacidites were found in varying proportions.The gymnosperms pollen was abundant in Pinaceae, Classopollis, and Pinuspollenites.The abundance of Tricolpopollenites distinguishes angiosperm pollen.Generally, the palynomorph assemblage of K 2 n 1 in borehole ZKY2-1 is similar to that in borehole SK1.A simple comparison was made between borehole ZKY2-1 and borehole SK1.In borehole SK1, K 2 n 1 is represented by the Cypridea gracila-Cypridea gunsulinensis assemblage [30].In borehole ZKY2-1, K 2 n 1 is represented by the Mongolocypris magna-Cypridea ardua-Cypridea acclinia assemblage [27].The two are very similar and comparable.
In borehole ZKY2-1, the lower K 2 n 2 is represented by the Schizaeoisporites-Classopollis-Retitricolporites assemblage.The fern spore Schizaeoisporites predominates.Gymnosperm pollen contains a significant content of Classopollis.Angiosperm pollen, such as Asteropollis, Retitricolpites, and Retitricolporites, are present in certain amounts.The abundance and diversity of palynomorphs increased gradually from bottom to top.In borehole SK1, the palynomorph assemblage of the lower K 2 n 2 is represented by the Lythraites-Callistipollehites-Schizaeoisporites assemblage [24], with fern spores of Schizaeoisporites predominating in K 2 n 2 .The content of Classopollis is high in gymnosperm pollen, and three colpi characterize angiosperm pollen.The number and species of palynomorphs gradually increase from K 2 n 1 to K 2 n 2 .Generally, the palynomorph assemblage of K 2 n 2 in borehole ZKY2-1 is similar to that of borehole SK1.A simple comparison was made between borehole ZKY2-1 and borehole SK1.In borehole SK1, K 2 n 2 is represented by the Mongolocypris magna-Mongolocypris heiluntszianensis assemblage [30].In borehole ZKY2-1, the lower K 2 n 2 is represented by Ilyocyprimorpha netchaevae-Scabriculocypris trapezoids [27].It is suggested that the ostracod assemblage of the two boreholes is similar.Compared with borehole SK1, the ostracod and palynomorphs from the upper K 2 n 2 to K 2 n 5 are not identified.
In borehole ZKY2-1, the palynomorph assemblage of K 2 s belongs to the Schizaeoisporites-Aquilapollenites-Tricolporopollenits assemblage.Cyathidites is dominant in the fern spores, Schizaeoisporites and Gabonisporites are common.Pinuspollenites and Inaperturopollenites are high in gymnosperms pollen; Classopollis and Araucariacites are common.Tricolpopollenites and Retitricolporites are high in angiosperm pollen; Callistopollenites and Asteropollis are common.In borehole SK1 [43], the fossil palynomorphs in the K 2 s assemblage are dominated by gymnosperm pollen, followed by fern spores and angiosperm pollen.Generally, the palynomorph assemblage of K 2 s in borehole ZKY2-1 is similar to that of borehole SK1.A simple comparison is made between borehole ZKY2-1 and borehole SK1.In borehole SK1, K 2 s is represented by the Talicypridea amoena-Metacypriskaitunensis-Ziziphocyprissimakovi assemblage [30].In borehole ZKY2-1, K 2 s is represented by the Lycopterocypris profunda-Talicypridea reticulata-Renicypris?Renalata [27].The ostracod is very similar and comparable.
The widespread distribution of Schizaeoisporites is an important characteristic of the Late Cretaceous palynomorph assemblage [26].Nichols and Sweet [54] regarded the triporate type as an indication of the age of Santonian.Callistopollenites are common in Late Cretaceous palynomorph assemblages in the Northern Hemisphere (e.g., in the Edmonton Formation, Alberta, Canada) [55]; Li et al. [15] regarded Callistopollenites as late Santonian to Campanian.Rugubivesiculites are often found in the Campanian [18].In the Lower Cretaceous sediments of Spain [56] and Sweden [55], Appendicisporites may have survived up to the Santonian period [57].According to Nichols and Jacobson [57], Aquilapollenites first appeared during the Campanian.In contrast, according to Nichols and Sweet [54], Aquilapollenites originated during the Santonian-Campanian.The Aquliapollen type created by Gao and others in 1976 includes Aquilapollenites, which are widely distributed globally and mainly concentrated in Asia, North America, and in Europe north of 35 • N. Aquilapollenites have also been found in central Africa, Malaysia, and Australia.In the Late Cretaceous, the Aquliapollen type was reported in the Rocky Mountains of the United States, Alaska, and the lower reaches of the Mississippi River; and Canada, Siberia, Japan, France and the United Kingdom; It is mainly found in the Songliao Basin, Sanjiang Basin, and Jiayin Region of Heilongjiang Province in northeast China [17].The palynomorphs in borehole ZKY2-1 have typical characteristics of the Late Cretaceous.
By comparing borehole ZKY2-1 with borehole SK1 and analyzing the age of the palynomorph assemblages, K 2 n 1 in borehole ZKY2-1 is dated as late Santonian, K 2 n 2 is determined as the latest Santonian-earliest Campanian, the period of K 2 s is assigned as late Campanian.The age of the lower Nenjiang Formation to Sifangtai Formation of borehole ZKY2-1 lasted from late Santonian to late Campanian, but the middle Campanian strata are missing.The established chronostratigraphic framework provides the basis for the following vegetation analysis and climate change.

Vegetation Change during the Santonian-Campanian Period
The Cretaceous period was critical for the origin and spread of angiosperms [58,59].Although angiosperms may have appeared for a long time [60], angiosperm pollen is diffi-cult to identify until after deposition of the Barremian strata [19].The late Early Cretaceous epoch is considered significant for the early distribution of angiosperm pollen; nevertheless, the number and diversity are still relatively limited [17,58,61].Angiosperms proliferated and flourished rapidly, replacing gymnosperms as the dominant group gradually during the Late Cretaceous.In the Late Cretaceous, the Santonian-Lower Campanian period was crucial for rapid angiosperm proliferation [14].Based on the data from borehole ZKY2-1 and other boreholes/outcrops in the Songliao Basin, the rapid radiation process of angiosperms is described in detail.
Coniferous forest, evergreen broadleaf forest, deciduous broadleaf forest, shrub, and herb are the classification of palynomorphs vegetation types [17].
According to the palynomorph assemblage reaction of the late Santonian-Campanian period in borehole ZKY2-1, the K 2 n 1+2 record comprises high amounts of coniferous forest and some content of herb.Coniferous forest and herb are the dominant vegetation types.As herb content increases, coniferous and evergreen broadleaf forests have some content throughout the sedimentation of the K 2 s.The vegetation types were herb and broadleaf mixed forest during this period.A comparison analysis between K 2 n 1+2 and K 2 s reveals that from late Santonian-earliest Campanian to late Campanian, the vegetation type of NE Asia changed from coniferous forest and herb to herb and herb-broadleaf mixed forest (Figure 7, Table 2).
The Cretaceous period was critical for the origin and spread of angiosperms [58,59] .Although angiosperms may have appeared for a long time [60], angiosperm pollen is difficult to identify until after deposition of the Barremian strata [19].The late Early Cretaceous epoch is considered significant for the early distribution of angiosperm pollen; nevertheless, the number and diversity are still relatively limited [17,58,61].Angiosperms proliferated and flourished rapidly, replacing gymnosperms as the dominant group gradually during the Late Cretaceous.In the Late Cretaceous, the Santonian-Lower Campanian period was crucial for rapid angiosperm proliferation [14].Based on the data from borehole ZKY2-1 and other boreholes/outcrops in the Songliao Basin, the rapid radiation process of angiosperms is described in detail.
Coniferous forest, evergreen broadleaf forest, deciduous broadleaf forest, shrub, and herb are the classification of palynomorphs vegetation types [17].
According to the palynomorph assemblage reaction of the late Santonian-Campanian period in borehole ZKY2-1, the K2n 1+2 record comprises high amounts of coniferous forest and some content of herb.Coniferous forest and herb are the dominant vegetation types.As herb content increases, coniferous and evergreen broadleaf forests have some content throughout the sedimentation of the K2s.The vegetation types were herb and broadleaf mixed forest during this period.A comparison analysis between K2n 1+2 and K2s reveals that from late Santonian-earliest Campanian to late Campanian, the vegetation type of NE Asia changed from coniferous forest and herb to herb and herb-broadleaf mixed forest (Figure 7, Table 2).Late Santonian to early Campanian angiosperm pollen Callistopollenites, Retitricolpites, Tricolpopollenites, Asteropollis, and Triphyllopollis are found in K 2 n 1 and K 2 n 2 of borehole ZKY2-1.In K 2 n 1+2 of borehole ZKY2-1, the content of angiosperm pollen is very low (0%-4.4%).Considering that the number of K 2 n 1+2 in borehole ZKY2-1 is small and there may be errors, we compare and comprehensively analyze the palynomorph data for K 2 n 1 and lower K 2 n 2 in the Houjingou section with Yan Jingjing, and show the average proportion of angiosperm pollen (0%-12.3%)[42].In contrast, during the middle and late Campanian periods, angiosperm pollen Aquilapollenites are found in K 2 s of borehole ZKY2-1.In K 2 s of borehole ZKY2-1, the content of angiosperm pollen increases significantly (0.5%-46.4%).The number and abundance of angiosperm pollen increases significantly from K 2 n 1+2 to K 2 s (Figure 8), among which Tricolpopollenites and Retitricolporites flourished gradually, and Aquilapollenites appeared in K 2 s.The angiosperm pollen also increased rapidly in the Songliao Basin and Jiayin Basin (NE China) from Santonian to late Campanian, [15,26,43,62,63].In summary, in East Asia, angiosperm pollen experienced rapid radiation from late Santonian to late Campanian.

Climate Change during the Late Santonian-Campanian Period
Palynomorphs are abundant in the Songliao Basin, which can reflect the nonmarine Cretaceous paleoclimate, especially the paleoclimatic of East Asia [17,26,43].The palynomorphs from borehole ZKY2-1 reveal minimal humidity change from K2n 1+2 to K2s (or the early Santonian to late Campanian).However, there is an obvious drop in temperature from K2n 1+2 to K2s.
Classopollis thrives in an arid and hot environment [64][65][66].The content of Classopollis decreased from K2n 1+2 to K2s.Pinaceae was the most common in semiarid-to subhumidtype climate [67][68][69][70] in borehole ZKY2-1.Abietineaepollenites and Pinuspollenites were dominant in the semiarid-type climate [68,69].From K2n 1+2 to K2s, the contents of semiaridsemihumid, semiarid, and semihumid components are relatively stable.The Ephedripites, representing a dry climate, are very low in both K2n 1+2 and K2s, suggesting the climate was not very arid.Briefly, the dry and humid conditions during the late Santonian to Campanian periods are mainly semiarid-semihumid, with fluctuations from arid to humid.However, further study of humidity is needed in the future.

Climate Change during the Late Santonian-Campanian Period
Palynomorphs are abundant in the Songliao Basin, which can reflect the nonmarine Cretaceous paleoclimate, especially the paleoclimatic of East Asia [17,26,43].The palynomorphs from borehole ZKY2-1 reveal minimal humidity change from K 2 n 1+2 to K 2 s (or the early Santonian to late Campanian).However, there is an obvious drop in temperature from K 2 n 1+2 to K 2 s.
Classopollis thrives in an arid and hot environment [64][65][66].The content of Classopollis decreased from K 2 n 1+2 to K 2 s.Pinaceae was the most common in semiarid-to subhumidtype climate [67][68][69][70] in borehole ZKY2-1.Abietineaepollenites and Pinuspollenites were dominant in the semiarid-type climate [68,69].From K 2 n 1+2 to K 2 s, the contents of semiaridsemihumid, semiarid, and semihumid components are relatively stable.The Ephedripites, representing a dry climate, are very low in both K 2 n 1+2 and K 2 s, suggesting the climate was not very arid.Briefly, the dry and humid conditions during the late Santonian to Campanian periods are mainly semiarid-semihumid, with fluctuations from arid to humid.However, further study of humidity is needed in the future.
The palynomorphs from borehole ZKY2-1 can indicate the temperature of climate, and among them are hot-type Cheirolepidiaceae (Classopollis); warm-type Bryophyta (Foraminisporis), Osmundaceae (Osmunacidites), Araucariaceae (Callialasporites), Taxodiaceae (Inaperturopollenites); and cool-type Taxodiaceae (Concentrisporites, Exesipollenites), Coniferophyta (Pinaceae, Abietineaepollenites, Pinuspollenites, Cedripites, Parvisaccites, Piceaepollenites, Piceites) [64][65][66][67][68]71].In K 2 n 1+2 , the content of hot palynomorphs is 30.8%, the content of warm Minerals 2023, 13, 338 15 of 19 palynomorphs is 28.2%, and the cool palynomorphs is 41%.In K 2 s, the content of hot palynomorphs is 2.1%, the content of warm palynomorphs is 27.7%, and the cool palynomorphs is 70.2%.Summarizing, in the late Santonian-Campanian period, the vegetation generally showed that hot and warm types decreased from K 2 n 1+2 to K 2 s, and the cool-type increased from K 2 n 1+2 to K 2 s, indicating that the temperature decreased from late Santonian to early Campanian K 2 n 1+2 to late Campanian K 2 s (Table 3).The change in Santonian to Campanian temperature in the Songliao Basin is supported by the palynomorphs and plants in Jiayin Basin, which is located northeast of the Songliao Basin [62].The Santonian flora of the Jiayin Basin indicate relatively hot to warm climate, while the late Campanian to Maastrichtian flora of the Jiayin Basin imply a relatively cool climate [62,75].The paleoclimate of the Jiayin Basin is consistent with the overall climatechange response revealed in borehole ZKY2-1 in the late Santonian to Campanian period, suggesting that palynomorphs of borehole ZKY2-1 indicate adaptation to the climate during the late Santonian-Campanian period.According to oxygen isotope and TEX86 recorded in marine sediments, the global temperature decreased from late Santonian to late Campanian [1,7,[76][77][78][79], which is also consistent with the palynological analysis of ZKY2-1.In combination with the dryness and humidity, that is, from the late Santonian to late Campanian period, the overall climate changed from a semiarid-semihumid warm climate to a semiarid-semihumid cool climate (Figure 9).
A series of evidence confirmed a decline in global temperature [1,7] and rapid radiation of angiosperm pollen [38,80] during the Late Cretaceous.However, there are few studies on the relationship between global temperature decline and angiosperm pollen evolution during the Late Cretaceous.This study on borehole ZKY2-1 clearly shows that the decline of global temperature during the Santonian-Campanian may have promoted the rapid radiation of angiosperm pollen.Because angiosperm pollen could adapt to more complex environments, including changeable or cold temperatures, the cooling during the late Campanian period may have been conducive to the adaptation and radiation of angiosperm pollen, and the second is not conducive to the prosperity of ferns spores and gymnosperm pollen, which objectively further promotes the further radiation of angiosperm pollen.Nevertheless, this study only puts forward a preliminary hypothesis and the relationship between the two needs further in-depth study.

Conclusions
Palynomorphs are abundant in the Late Cretaceous stratigraphy in the Songliao Basin, indicative of climate change and vegetation evolution.Borehole ZKY2-1 provides valuable materials for studying palynomorphs in the Songliao Basin.Borehole ZKY2-1 can be well correlated with borehole SK1, with K2n 1 dated as late Santonian, K2n 2 determined as the latest Santonian-earliest Campanian, and K2s assigned as late Campanian.
From late Santonian-earliest Campanian to late Campanian, the vegetation type of NE Asia changed from coniferous forest and herb to herb and broadleaf mixed forest.In addition, angiosperm pollen was radiated rapidly from late Santonian to late Campanian.
Although humidity was relatively stable, with semiarid to semihumid climate during the late Santonian to Campanian period, the temperature changed from a warm climate during the late Santonian-earliest Campanian to a cool climate during the late Campanian.The decline of global temperature may have promoted the evolution and expansion of angiosperm pollen.

Figure 1 .
Figure 1.(A) Schematic diagram of Songliao Basin (SLB) and (B) the location of borehole ZKY2 I-North Plunge Zone; II-Northeast Uplift Zone; III-Central Deposition Zone; IV-West Slo Zone; V-Southeast Uplift Zone; VI-Southwest Uplift Zone.

Figure 9 .
Figure 9. Percentage distribution map of typical thermally adaptable species in borehole ZKY2-1 and a pie chart of typical temperature changes for the Santonian-Campanian stage.