Paleoclimate and Paleovegetation Significance of Paleogene Sporopollen–Algae Assemblage in the Eastern Portion of the South China Sea

Abstract

Paleoenvironmental knowledge holds significant scientific value for elucidating the evolutionary history of Earth’s crust and for guiding the exploration of oil and gas resources. This study presents a comprehensive analysis of sporopollen–algae fossils from Well DW-2, located in the Zhu II Depression of the eastern South China Sea. The sampled strata of the well were from 0 to 3230 m in depth, with a substantial number of sporopollen–algae fossils identified within the Enping and Zhuhai Formations spanning from 2501 to 3227 m. Two distinct sporopollen–algae assemblages were delineated from bottom to top. The sporopollen characteristics reveal a transition from tropical and subtropical to subtropical and temperate plant communities, reflecting the warm and humid climate during the formation of the Enping Formation and the relatively cool and dry climate during the formation of the Zhuhai Formation. A significant number of marine dinoflagellate fossils were identified, and the observed changes in algal characteristics suggest that the Enping Formation was accumulated in a shallow marine environment, while the Zhuhai Formation was predominantly formed in a transitional setting between marine and terrestrial conditions at the shelf margin. These findings not only enhance our understanding of the paleoenvironmental conditions in the South China Sea region but also have broader implications for reconstructing global paleoclimates and identifying potential hydrocarbon reservoirs in similar sedimentary basins.

1. Introduction

The study of the Paleogene environment is of guiding significance for revealing the history of crustal evolution and oil and gas exploration. With the exploration and utilization of marine oil resources becoming an increasingly important part of energy strategy, micropaleontological studies have played an important role in recording sedimentary environments and constructing biostratigraphic frameworks in various basins [1]. The Pearl River Mouth Basin in the South China Sea has a typical “continental-to-marine” sedimentary evolution sequence, with significant changes in sedimentary environments. Among them, the Zhu II Depression is the largest, deepest, and thickest Cenozoic depression in the basin and is also a key area for deep-water oil and gas exploration [2]. Therefore, paleoenvironmental reconstruction and paleobiological studies are of great significance for the stratigraphic division and oil and gas exploration in the deep-water areas of the eastern South China Sea.

Due to limitations in deep-water drilling technology, oil and gas exploration has focused on the shallow-water areas of the northern continental shelf of the South China Sea, resulting in insufficient understanding of the stratigraphy, biology, and oil and gas potential in the deep-water areas of the Pearl River Mouth Basin. The Zhu II Depression, located in the southern Pearl River Mouth Basin, is a large hydrocarbon-bearing depression [3] surrounded by uplifted areas with modern water depths ranging from 200 to 2000 m [4]. Previous studies have conducted extensive research on the sedimentary environments of this area, but due to the limited number of wells, the conclusions drawn from paleobiological data are not unified, and there is a lack of rigorous direct evidence, which restricts further oil and gas exploration in the region.

This paper focuses on the paleobiological study of Well DW-2 in the deep-water area of the Zhu II Depression (Figure 1), with a modern water depth of more than 700 m and a total drilling depth of 3230 m. A large number of Paleogene sporopollen–algae fossils were discovered in the strata below 2500 m. The study aims to reconstruct the paleoclimate and paleovegetation characteristics during the development of the Enping and Zhuhai Formations, providing scientific guidance for revealing the stratigraphic development laws and oil and gas exploration in the eastern South China Sea.

2. Materials and Methods

Sporopollen and algae fossils were analyzed for a total of 40 samples from the section from 2501 to 3227 m in depth of the Well DW-2. The samples were generally collected with a spacing range of 6–21 m from the well’s lithic fragments, with the exception of two core samples taken from the depths of 3054–3154 m of the well.

The laboratory processing followed the conventional methods for spore and pollen analysis. Initially, HCl (10%) was added to remove calcium carbonate, and the samples were washed until neutral. Subsequently, HF (18%) was used to remove siliceous components. After acid treatment, the samples were enriched using a washing and sieving technique. The sieve mesh size was 8 μm, and the washing process was conducted in an ultrasonic cleaner for no more than 2 min. After washing and sieving, materials larger than 8 μm, including spores, pollen, and planktonic algae fossils, were concentrated in test tubes.

The identification and counting of fossils were performed using a LEICA biological microscope made by the Leica Mikrosysteme Vertrieb GmbH, Wetzlar, Germany. For this study, the basic requirement for obtaining a statistically meaningful spore and pollen assemblage was to identify more than 100 spore and pollen grains per sample. The results of the identification were expressed as the percentage content of fossils.

3. Results

The lithological characteristics of the Enping Formation and the Zhuhai Formation reflect the changes in the sedimentary environment of the Pearl River Estuary Basin in different geological periods. The Enping Formation is mainly composed of marine mudstone deposits mixed with a small amount of argillaceous limestone, while the Zhuhai Formation is characterized by shallow sea/land interaction deposits, and its lithology is characterized by interbedded limestone, silty limestone, and sandstone. A total of 125 species of sporopollen–algae fossils were identified for the 40 samples from the Enping and Zhujiang Formations, including 18 species of fern spores, 9 species of gymnosperm pollen, 56 species of angiosperm pollen, and 42 species of planktonic algae. The spores and pollen fossils include Pinuspollenites, Quercoidites minutus, Quercoidites microhenrici, Polypodiisporites, and Polypodiaceaesporites. The common taxa include Taxodiaceaepollenites hiatus, Dicolpopollis kockelii, Cupuliferoipollenites, Caryapollenites, Juglanspollenites, Gothanipollis, Salixpollenites, Alnipollenites, Engelhardtioidites, Liquidambarpollenites, and Ulmipollenites (Figure 2 and Figure 3).

Among the planktonic algae, the majority are marine dinoflagellates. Common genera include Cordosphaeridium (heart-shaped sphere algae), Homotryblium (semi-sac algae), Lingulodinium machaerophorum (small tongue-shaped algae), Operculodinium (lid algae), Polysphaeridium subtile (slender-tubed multi-sphere algae), Spiniferites (spiny armor algae), and Systematophora (tufted algae). Freshwater algae, such as Pediastrum (discoid algae) and Leiosphaeridia (smooth-surfaced spherical algae), occur in smaller quantities (Figure 4).

3.1. Sporopollen Assemblages

Assemblage 1 (2920–3227 m): Gothanipollis bassensis–Quercoidites–Polypodiaceaesporites Assemblage

In this assemblage, angiosperm pollen is dominant (38.0–65.0%), with gymnosperm pollen (12.5–38.0%) and fern spores being common (10.1–32.1%) and having comparable abundances. Gymnosperms are primarily represented by pine pollen (9.3–27.0%), with Pinuspollenites being the dominant genus. Other taxa such as Abietineaepollenites, Podocarpidites andiniformis, and Podocarpidites nageiaformis are occasionally seen. Fern spores are dominated by the Polypodiaceae family (9.8–27.4%), with common species including Polypodiaceaesporites, Polypodiisporites perverrucatus, and Polypodiisporites. Other taxa such as Polypodiaceoisporites, Pterisisporites undulatus, Cyathidites, Granulatisporites, Lygodiumsporites, and Osmundacidites are sporadic.

Assemblage 2 (2500–2920 m): Pinuspollenites–Alnipollenites–Polypodiaceaesporites Assemblage

In this assemblage, angiosperm pollen dominates (38.0–65.0%), followed by gymnosperms (20.8–50.0%), with fern spores being common (6–23.8%). Gymnosperms are still primarily represented by pine pollen (19.8–48.0%), with a significant increase in abundance compared to the previous assemblage. Common taxa include Abietineaepollenites, Piceapollis, Pinuspollenites, Podocarpidites andiniformis, Podocarpidites nageiaformis, Podocarpidites, and Tsugaepollenites igniculus. Angiosperms are dominated by the genus Quercoidites (0–28.0%, averaging 19.7%), though its abundance has decreased significantly compared to the previous assemblage. Fern spores are still dominated by the Polypodiaceae family (6–20%), though their abundance has decreased compared to the previous assemblage. Common species include Polypodiaceaesporites, Polypodiisporites, Polypodiisporites perverrucatus, and Polypodiaceoisporites.

3.2. Algal Assemblages

Assemblage 1: Homotryblium tenuispinosum–Hystrichosphaeridium tubiferum Assemblage

This assemblage is characterized by a high abundance and diversity of dinoflagellate cysts, with contractile cysts being dominant. The genus Cleistosphaeridium is particularly abundant, with Cleistosphaeridium diversispinosum being the most common species. The genera Homotryblium and Hystrichosphaeridium are also abundant, with Homotryblium tenuispinosum and Hystrichosphaeridium tubiferum being particularly common. Green algae such as Pediastrum, and acritarchs such as Granodiscus and Leiosphaeridia are sporadic.

Assemblage 2: Polysphaeridium subtile–Lingulodinium machaerophorum Assemblage

Compared to the previous assemblage, the abundance and diversity of dinoflagellate cysts in this assemblage are reduced but still significant, with contractile cysts remaining dominant. The genus Cleistosphaeridium shows no significant change in type or abundance. The notable changes are the increased abundance of Polysphaeridium subtile, Lingulodinium machaerophorum, and Polysphaeridium zoharyi, making them the dominant species in this assemblage. The transparent cyst Lejeunecysta hyalina and the genus Lejeunecysta are occasionally seen. Green algae and acritarchs have similar types as appeared in the previous assemblage and are only occasionally seen.

Assemblage 3: Systematophora–Spiniferites Assemblage

The abundance and diversity of dinoflagellate cysts in this assemblage are significantly reduced compared to the previous ones, with no dominant species. Taxa that were dominant in the previous assemblages, such as Cordosphaeridium, Cleistosphaeridium, Homotryblium, Hystrichosphaeridium, Polysphaeridium, and Lingulodinium machaerophorum, are only sporadically seen. Operculodinium centrocapum and Polysphaeridium subtile are nearly extinct in this assemblage. In contrast, Systematophora and Spiniferites are more abundant and become characteristic taxa. Green algae such as Pediastrum and acritarchs such as Dictyotidium, Granodiscus, and Leiosphaeridia are only sporadically seen.

4. Discussion

4.1. Comparison of the Findings from Well DW-2 with Other Regional Geological Records

(1)

Enping Formation

During the Eocene/Oligocene transition, significant global climatic changes occurred, with a substantial drop in temperature and important shifts in vegetation types. In the Eocene, tricolpate and tricolporate pollen types were well-developed, characterized by the dominance of evergreen broadleaf oak pollen (Quercoidites), as well as common occurrences of tropical and subtropical taxa such as Rutaceoipollis, Rhoipites, Euphorbiacites, Nyssapollenites, Retitricolporites, Rhamnacidites, and Sapindaceidites. By the Oligocene, these pollen types decreased significantly, while pollen of Pinaceae (pine family), deciduous broadleaf trees like Ulmipollenites (elm), Juglanspollenites (walnut), and Betulaceae (birch family) increased. These changes are widely reflected in the late Eocene to early Oligocene spore/pollen assemblages across China, such as in the Shacheng Formation and Dongying Formation in the Bohai Bay Basin [6], the Qianjiang Formation in the Jianghan Basin, and the Yacheng and Lingshui Formations in the Ying-Qiong Basin [7].

Lei Zuoqi (1996) established eight spore/pollen assemblages in the Cenozoic of the Pearl River Mouth Basin, including the “Dicolpopollis kockelii–Gothanipollis bassensis” assemblage in the Enping Formation (

Table 1. Comparison of palypollen assemblage between the Zhu II Depression and surrounding areas.

Strata	Lei Zuoqi, 1996 [8]	Zhao Fei, 2017 [5]	Qin Jungan, 2016 [7] Zhang Yifan, 2017 [10]	This Study
Zhuhai Fm.	Pinuspollenites– Alnipollenites	Pinuspollenites– Alnipollenites	Polypodiisporites– Quercia evergreen– Quercia decidouous	Pinuspollenites– Alnipollenites– Polypodiaceaesporites
Enping Fm.	Gothanipollisbassensis Stover– Dicolpopollis kockelii		Quercoidites- Dicolpopollis kockelii– Polypodiisporites	Gothanipollis Bassensis Stover– Quercoidites– Polypodiaceaesporites

) [8]. This assemblage is characterized by the dominance of small evergreen broadleaf oak pollen (Quercoidites minutus) and small deciduous broadleaf chestnut pollen (Cupuliferoipollenites minitrimatus), with high abundances of tropical and subtropical tricolpate and tricolporate pollen types. Dicolpopollis kockelii is well-developed, and Gothanipollis bassensis is common. This is similar to the Enping Formation assemblage in the LS33-1-1 well in the Qiongdongnan Basin, which is characterized by the “Quercoidites–Dicolpopollis kockelii–Polypodiisporites” assemblage, dominated by Quercoidites (including Quercoidites microhenrici and Quercoidites minutus) with abundant Dicolpopollis kockelii and the absence of Gothanipollis bassensis [9].

These comparisons indicate that Assemblage 1 “Gothanipollis bassensis–Quercoidites-Polypodiaceaesporites” is consistent with the Enping Formation, characterized by the dominance of Quercoidites and the continuous presence of Dicolpopollis kockelii and Gothanipollis bassensis.

The Enping Formation in the Pearl River Mouth Basin has always been dominated by continental strata, and the occurrence of dinoflagellate cysts is not widespread within this formation. Rich dinoflagellate cysts have only been reported in the Enping Formation of Well BY7-1-1 in the Zhu II Depression [11]. The algal fossils discovered in the Enping Formation of Well DW-2 show essentially the same overall characteristics as those in Well BY7-1-1.

In Well DW-2, the interval from 2501 to 2920 m is dominated by Cleistosphaeridium. The interval from 2920 to 3227 m contains two assemblages: the lower part is characterized by the Homotryblium tenuispinosum–Hystrichosphaeridium tubiferum–Cordosphaeridium gracile assemblage, while the upper part is characterized by the Polysphaeridium subtile–Lingulodinium machaerophorum assemblage. Mao Shaozhi et al. (1996) established the Homotryblium tenuispinosum–Hystrichosphaeridium tubiferum assemblage in the Enping Formation of Well BY7-1-1, which is essentially consistent with the lower assemblage in Well DW-2 [11]. Additionally, types such as Homotryblium tenuispinosum and Cordosphaeridium gracile, which appear at the top of the Enping Formation, have also been found in Well PY33-1-1 in the Pearl River Mouth Basin, extending into the Zhuhai Formation [12]. Therefore, the dinoflagellate cyst data also indicate that the interval from 2920 to 3227 m in Well DW-2 should correspond to the upper part of the Enping Formation.

(2)

Zhuhai Formation

The interval from 2500 to 2900 m in Well DW-2 has established the “Pinuspollenites–Alnipollenites–Polypodiaceaesporites” assemblage. This assemblage is comparable to the “Alnipollenites–Pinuspollenites” assemblage established by Lei Zuoqi in the Zhuhai Formation of the Pearl River Mouth Basin [8]. The abundance of Pinaceae and Betulaceae pollen reflects the cooling trend during the Oligocene, a feature that is well represented in the Zhuhai Formation strata across the South China Sea region. For example, Well WHC14-1-1 in the western Pearl River Mouth is characterized by high abundances of Pinaceae pollen and significant amounts of Betulaceae pollen and Ulmipollenites [8].

This assemblage is also comparable to the “Alnipollenites–Pinuspollenites” assemblage in the Zhuhai Formation of Well LW5 in the northern deep-water area of the South China Sea, as well as the lower part of the Lingshui Formation in the Qiongdongnan Basin [6], which is characterized by the pollen assemblage of Pinuspollenites–evergreen Quercus–deciduous Quercus. All these assemblages are marked by high abundances of Pinaceae pollen. Therefore, the palynological analysis results indicate that the interval from 2500 to 2900 m in Well DW-2 corresponds to the Zhuhai Formation strata.

The development characteristics of dinoflagellate cysts in Well DW-2 are distinct from those in most other regions of the South China Sea. In the Pearl River Mouth Basin, dinoflagellate cysts are generally abundant in the Zhuhai Formation, while they are relatively scarce in the Enping Formation, with only a few reports from Wells BY7-1-1 and PY33-1-1 (

Table 2. Comparison of trichoflagellate assemblage between Zhu II Depression and surrounding areas.

Strata	Mao Shaozhi, 1996 [11]	Ma Xinxiang, 1996 [12]	This Study
Zhujiang Fm.	Polysphoeridium subtile– Lingulodinium machaerophorum	Homotryblium tenuispinosum– Systematophora ancyrea
Zhuhai Fm.	Homotryblium plectilum– Cordosphaeridium gracile		Systematophora–Spiniferites
		Homotryblium Abbreviatum– Cordosphaeridium gracile
Enping Fm.	Homotryblium tenuispinosum– Hystrichosphaeridium tubiferum		Polysphaeridium subtile– Lingulodinium machaerophorum
			Homotryblium tenuispinosum– Hystrichosphaeridium tubiferum

) [11,12]. However, in Well DW-2, the Enping Formation contains rich dinoflagellate cysts, whereas the Zhuhai Formation shows a sharp decrease in both abundance and diversity of these fossils. This discrepancy may be closely related to the unique geographical location of Well DW-2 during that period.

In the interval from 2500 to 2900 m in Well DW-2, dinoflagellate cysts are relatively scarce, with marine types being dominant. The key taxa include Systematophora and Spiniferites. This assemblage is comparable to the following: (1) the upper Enping Formation to lower Zhuhai Formation in Well PY33-1-1, characterized by the Homotryblium abbrevilatum–Cordosphaeridium gracile assemblage [12]; (2) the upper Zhuhai Formation to lower Zhujiang Formation in Well PY33-1-1, marked by the Systematophora ancyrea–Homotryblium tenuispinosum assemblage [12]; (3) the lower Zhuhai Formation in Well BY7-1-1, defined by the Cordosphaeridium gracile–Homotryblium plectilum assemblage [10]. Additionally, the Homotryblium plectilum–Cordosphaeridium gracile assemblage zone in the late Oligocene Y-2-X well from the Qiongdongnan Basin also shares similar features with Assemblage 3 in this study [13], characterized by the development of Spiniferites and Homotryblium. These comparisons suggest that the interval from 2500 to 2900 m in Well DW-2 likely corresponds to the upper part of the Zhuhai Formation.

4.2. Paleoclimate and Paleovegetation Significance of the Paleogene Sporopollen–Algae Assemblage from the Well DW-2

The global climate transition from a relatively warm and humid greenhouse state to a colder icehouse state during the Eocene/Oligocene interval is well reflected in the two spore/pollen assemblages established in the Enping and Zhuhai Formations. In the Enping Formation of Well DW-2, high abundances of tropical and subtropical evergreen broadleaf forest taxa such as Quercoidites (oak pollen) and Polypodiaceaesporites (water fern spores) are observed, along with a significant presence of tropical swamp plants. Characteristic fossils, including Dicolpopollis kockelii (Koch’s double-groove pollen), Rutaceoipollis (rue pollen), Rhoipites (lacquer tree pollen), and Ilexpollenites (holly pollen), indicate the widespread occurrence of tropical and subtropical tree species, while the peak abundance of Gothanipollis bassensis (representing tropical shrub species) is also noted. The high occurrence of tropical and subtropical spore/pollen assemblages reflects the warm and humid climate during the deposition of the Enping Formation.

In contrast, the Zhuhai Formation is characterized by the presence of Pinuspollenites (pine pollen), which thrives in cooler environments, and the characteristic fossil Alnipollenites (alder pollen), representing warm temperate deciduous broadleaf forest species. This indicates a trend toward cooler and drier conditions compared to the Enping Formation. The overall spore content in the Zhuhai Formation is higher than that of dinoflagellate cysts, with pine pollen being dominant due to its strong dispersal capability. The shift in spore/pollen characteristics from the Enping to the Zhuhai Formation reveals a transition from tropical and subtropical to subtropical and temperate spore/pollen assemblages, reflecting the relatively cooler and drier climate during the formation of the Zhuhai Formation. This change corresponds to the global cooling event at the end of the Eocene and the beginning of the Oligocene. In the Pearl River Mouth Basin, the transition from the Gothanipollis bassensis–Dicolpopollis kockelii spore/pollen assemblage in the Enping Formation to the Pinuspollenites–Alnipollenites assemblage in the Zhuhai Formation is a reflection of this global climatic change. The analysis of Well DW-2 shows a sequence from the evergreen oak pollen–marine dinoflagellate assemblage to the Pinuspollenites–evergreen oak pollen–Polypodiaceaesporites assemblage, indicating a transition from tropical and subtropical to subtropical and temperate spore/pollen assemblages and reflecting the shift from warm and humid to cooler and drier climates. The interval from 2870 to 2990 m in the well is the most important sedimentary record of this transition.

The dinoflagellate cysts discovered in this well are primarily characterized by contractile cysts with forked protrusions, while appressed and chambered cysts are rare. Contractile cysts with forked protrusions are typical of marine dinoflagellates [14]. Genera such as Spiniferites, Hystrichosphaeridium, and Cordosphaeridium indicate a typical open-sea environment and are most abundant during the marine transgression phase (marine dinoflagellate fossils in China). Species with complex protrusions, such as Polysphaeridium, Homotryblium, and Systematophora, represent normal shallow-sea environments, while cysts with smooth or finely ornamented outer walls, such as Dictyotidium, Granodiscus, and Leiosphaeridia, indicate nearshore shallow-sea environments [15] and are also common in terrestrial lake basins of the Cenozoic [8]. Freshwater taxa such as Pediastrum are only sporadically observed. In the Enping Formation, the spore/pollen and algal assemblages have high planktonic algal content, reaching up to 53.1%, with most being marine dinoflagellates and only low or sporadic occurrences of freshwater algae. From 3227 to 3020 m, marine dinoflagellates are abundant, while freshwater dinoflagellates such as Pediastrum and Leiosphaeridia continuously appear in small amounts from 3227 to 2930 m, indicating a shallow-sea environment with continuous freshwater input. The sudden decrease in marine dinoflagellates starting at 3020 m, along with basin uplift and shallowing of the water body, marks a shift from shallow-sea to coastal environments, eventually leading to complete uplift. The distinct changes in marine dinoflagellate abundance in the Enping Formation—high, low, and then high again—well illustrate the sensitivity of fossil types in strata to the strength of marine influence. The decrease in marine dinoflagellates and increase in freshwater planktonic algae indicate a regression or freshwater lake input at that time. Thus, it is indicated that the Enping Formation in Well DW-2 was formed in a shallow shelf sea, and this finding can be further supported by the predominantly lithological mudstone layers in the studied well.

In the Zhuhai Formation, the content of planktonic algae decreases, with continuous occurrences of Pediastrum, indicating that the location is not far from the continent. The presence of marine dinoflagellates such as Polysphaeridium subtile [10] suggests marine transgression. From 2900 m, the well section subsides again until 2500 m, with occasional occurrences of marine dinoflagellates, though not as abundant as during the Enping Formation period, indicating that the water body remains shallow and in a coastal environment. This finding can be further supported by the lithological evidence that a certain number of coarser and high-energy sand sediments appeared, indicating the intermittent terrestrial environment. Therefore, the Zhuhai Formation in this well was mainly formed in a transitional marine/terrestrial environment at the shelf margin. Based on the content of planktonic algae, the lower part of the analyzed well section (2830–3227 m) can be identified as an algal-rich interval, with a thickness of nearly 400 m, where the content of planktonic algae in the spore/pollen and algal assemblages reaches 22.3–53.1%. However, the most striking feature of this algal-rich interval is not only its thickness and high planktonic algae content but also the fact that the vast majority of planktonic algae in this interval are marine dinoflagellates, which is unique compared to previous analyses of the Paleogene in the Pearl River Mouth Basin.

5. Conclusions

• The research focuses on stratigraphic significance and environmental implications. Two distinct spore/pollen assemblages and three algal assemblages were identified, aligning closely with the regionally established fossil types of the Enping and Zhuhai Formations.
• The transitions observed in spore/pollen characteristics reflect a shift from tropical and subtropical assemblages to subtropical and temperate ones, indicative of the warm and humid climate during the formation of the Enping Formation and the relatively cooler and drier conditions during the formation of the Zhuhai Formation.
• The algal data suggest that the Enping Formation was deposited in a shallow marine setting, whereas the Zhuhai Formation was primarily formed in a transitional marine/terrestrial environment along the shelf margin. These findings can be further supported by the regional lithologic evidence,
• These findings offer insights into the stratigraphy, paleoenvironment, and biological diversity of the region, and a pathway for assessing the oil and gas resource potential in the South China Sea.