Vegetation transitions across the Paleocene-Eocene boundary at low latitudes: Insights from a palynological study from western Indian Lignites

Abstract

Global warming at the Paleocene-Eocene boundary is considered crucial for understanding the large-scale ecological changes driven by this warming event. Early Paleogene sedimentary deposits from mid and high latitudes reveal considerable turnover attributed to early Paleogene warming, whereas low latitudinal regions are yet to be explored. Indian Plate, due to its palaeoequatorial position during the early Paleogene warming, offers detailed insights into vegetation transitions in the paleoequatorial and low latitudinal regions. The present study provides a qualitative and quantitative assessment of fossil pollen from an early Paleogene succession exposed in the Sonari Lignite Mine, Barmer Basin, Rajasthan, India. Based on the prior delineation of the Paleocene-Eocene (P-E) boundary within the succession, this study further analyses fossil pollen distributions across the boundary to identify distinct Paleocene and early Eocene (Ypresian) palynoassemblages and discuss the vegetation dynamics and compositional transitions across the P-E boundary in response to the early Paleogene warming. Both palynoassemblages are characterised by similar ecological groups (mangroves/back mangroves, fern spores, palms and inland tropical rainforests). However, their floral richness and composition vary across the boundary. The Paleocene palynoassemblage indicates the predominance of mangrove/back mangrove brackish swampy ecosystems, suggesting the presence of low-saline conditions, likely driven by high precipitation and increased runoff associated with the Indian Plate’s equatorial position and global warming events during early Paleogene. The increasing presence of palynomorphs derived from fern spores, palms and inland tropical rainforest floras towards the early Eocene (Ypresian) section indicates the expansion of a tropical rainforest ecosystem under a warm and humid climate of the early Eocene. Further, the decline of seasonal taxa (Lepidocaryum), along with the rise of moisture-loving palms and other rainforest floras in the early Eocene (Ypresian) palynoassemblage, indicates a transition of seasonal tropical forests to wet and humid tropical rainforests. In conclusion, the present study highlights how the variation in seasonality and the intensity of warm and humid climate of the Indian Plate, driven by its equatorial positioning and early Paleogene warming, shaped the vegetation patterns across the P-E boundary. With the similar ecological groups showing steady variation in their floral richness across the P-E boundary, the present study further suggests that the vegetation transitions in low latitudes were gradual, unlike the mid and high latitudes.

INTRODUCTION

The Early Paleogene (late Paleocene and early Eocene) period is characterised by global warming that led to an ‘ice-free earth’ and global eustasy (sea level changes) (Bains et al., 1999; Clementz et al., 2011; Miller et al., 1987; Sloan & Rea, 1995; Sluijs et al., 2008; Zachos et al., 1993). During this period, a series of short-term (10⁴–10⁵ years) hyperthermal events (Paleocene-Eocene Thermal Maxima, PETM ~55–52 Ma and early Eocene Thermal Maxima, ETM2 ~53.7 Ma) released massive greenhouse gases like carbon dioxide (CO₂) and methane (CH₄) to the atmosphere, raising the global temperature by 4 °C to 8 °C above the average (Dickens et al., 1995, 1997; Kennett & Stott, 1991; McInerney & Wing, 2011). The 2‰–6‰ negative carbon isotopic excursion in marine and terrestrial records, indicating the sudden release of isotopically light carbon to the ocean–atmosphere carbon system, defines the excessive globally warm events of early Paleogene (Abels et al., 2012; Lourens et al., 2005; Nicolo et al., 2007). The biotic changes in the terrestrial and marine regimes that accompany brief hyperthermal events, primarily the PETM, also underline significant changes in the global climate system (Nunes & Norris, 2006; Speijer et al., 2012). The frequency and magnitude of early Paleogene warming are analogous to current anthropogenic carbon emissions and climate change (Rush, 2022; Wing et al., 2005). Furthermore, it is anticipated that CO₂ concentration and temperature will continue to rise to the extent that they will significantly impact life on Earth. Thus, using the early Paleogene warming as a deep time analogue, several efforts have been made worldwide to understand the impact of global warming on Earth’s biotas. The early Paleogene period also witnessed large-scale tectonic movements and ecological changes that led to the extinction, evolution and diversification of terrestrial biotas (mostly floras) on a global level (Jaramillo et al., 2010; Huurdeman et al., 2021; Prasad et al., 2009, 2018; Schouten et al., 2007; Willard et al., 2019; Wing et al., 2005). The vegetation impact of early Paleogene warming across the Paleocene-Eocene (P-E) boundary varied across the latitudinal regions (Carmichael et al., 2017, 2018; Garel et al., 2014; Hollis et al., 2019; Jaramillo, 2002; Jaramillo et al., 2010; Korasidis et al., 2022; Pigg & Devore, 2010; Prasad et al., 2018; Rull, 1999; Srivastava & Prasad, 2015; Wing, 1996; Wing et al., 2005; Xie et al., 2022). Despite extensive research on the impacts of early Paleogene warming on biotas, most studies have focused on mid- and high-latitudinal regions (Bowen et al., 2004; Payros et al., 2022; Si & Aubry, 2018; Sluijis et al., 2006, 2008; Zachos et al., 2006), with limited attention given to the equatorial or lower latitudinal regions (Clementz et al., 2011; Garg et al., 2008; Jaramillo et al., 2006; Prasad et al., 2013). While the consequences of early Paleogene warming on marine biotas and vertebrates have been thoroughly researched (Speijer et al., 2012; Vieites et al., 2007; Wing et al., 2005), further investigation is required to determine the specific implications of this warming event on the terrestrial floras from the low latitudes.

The northward-moving Indian Plate was positioned near the equator during the early Paleogene (Scotese & Golanka, 1992; Sluijs et al., 2008), thereby preserving valuable evidence of how vegetation in palaeoequatiorial and low latitudinal regions responded to early Paleogene warming (Prasad et al., 2006, 2009, 2013). The Indian Plate had already entered the low latitudinal tropics in early Paleocene before situating itself at the equator around 58 Ma and shifting into the dry subtropical zone (~36 Ma) (Singh et al., 2009). The latitudinal transition of the Indian Plate, combined with favourable climatic conditions, resulted in forming organic-rich early Paleogene sedimentary deposits. Widespread, slowly rising marine transgressions during the early Paleogene led to the formation of thick lignite/coal-associated sequences on the northeastern and western continental margins of India (Patra & Singh, 2015; Paul & Dutta, 2020; Prasad et al., 2009, 2013). These sedimentary sequences have yielded several fossils of pollen, megaflora, insects, marine fish and vertebrate fauna (Bhandari et al., 2005; Clementz et al., 2011; Dutta & Sah, 1970; Garg et al., 2008; Kar & Kumar, 1986; Kumar et al., 2016; Prasad et al., 2006, 2009, 2013; Rose et al., 2006; Samant & Phadtre, 1997; Sahni et al., 2006; Shukla et al., 2014; Srivastava & Prasad, 2015; Tripathi et al., 1999). The early Paleogene warming sequences have been discussed in terms of sequence stratigraphy and fossil recovery, from the Jathang and Ranikor Barsora section of the East Khasi Hills of Meghalaya in northeast India (PETM, Prasad et al., 2018; Srivastava & Prasad, 2015), the Cambay Basin in Gujarat (ETM2, Prasad et al., 2013) and the Jaisalmer Basin in Rajasthan (PETM and ETM2, Patra et al., 2021). However, the response of vegetation to the warming has only been discussed in the East Khasi Hills of Meghalaya, northeast India (Prasad et al., 2018; Srivastava & Prasad, 2015). Further studies of early Paleogene sedimentary successions from different regions are thus required to fully understand how early Paleogene warming affected the floral communities of low latitudinal regions.

In the present study, a thick early Paleogene lignite-shale succession encompassing the P-E boundary from the Sonari Lignite Mine, Barmer Basin, western India, offers rich and diverse fossil pollen for the quantitative and qualitative assessment of vegetation change across the P-E boundary in response to the early Paleogene warming. Following this, the study examines how the vegetation pattern has transitioned across the boundary with regard to the effects of early Paleogene warming and the paleolatitudinal position of the Indian Plate. Furthermore, the paleovegetation transitions across the boundary in low latitudes versus mid and high latitudes are discussed.

GEOLOGICAL SETTING

The Barmer Basin, spanning 6800 sq. km between Lat. 24°58’ to 26°32’N and Long. 70°05’ to 72°52’E, is an approximately 200 km long, 40 km wide and 6 km deep tertiary rift basin situated in the Thar Desert of western Rajasthan (Burley et al., 2023; Compton, 2009; Roy & Pandey, 1970; Sisodia, 2006; Figure 1). The basin is bounded by NNW-SSE and NNE-SSW trending normal extensional faults. It can be traced at the northernmost end of the 600 km long northwest striking Western Indian Rift (WIR) system that developed after the separation of India from Gondwanaland and its subsequent passage over the Reunion hotspot during the Late Cretaceous-early Paleogene (Burley et al., 2023; Compton, 2009; Dolson et al., 2015; Sharma, 2007). The sedimentary deposition of Barmer Basin aligns with the phases of rifting that occurred during the early Paleogene; hence, the early Paleogene sequences in the basin are called syn-rift Basin fill succession (Compton, 2009; Kelly et al., 2014). In addition to this, the developing rift basin also experienced substantial sea level fluctuations that developed huge tertiary lignite-shale sequences within the basin (Compton, 2009; Garg et al., 2011; Parihar et al., 2016; Rana et al., 2005; Sahni et al., 2006). A ~107 m thick lignite-shale succession belonging to the Akli Formation is well-exposed in the Sonari Lignite Mine of Barmer Basin, Rajasthan. Since lignite-shale sequences are found to be enriched with rich and diverse palynofossils (Bansal et al., 2022a; Parmar et al., 2023; Prasad et al., 2013), the mine is a potential site for palaeovegetional studies.

Figure 1.

Map representing the location of the Sonari Lignite Mine, Barmer Basin, western Rajasthan, India (Modified after Compton, 2009).

Age of Sonari Lignite Mine

The Sonari Lignite Mine (Lat. 25º 57´38″ N, Long. 71º 16´ 25 E″) is an open cast mine, located nearly 30 km away in the northwest direction of the Barmer district, western Rajasthan (Figure 1). The ~107 m succession of the mine is composed of several beds of lignites (40 cm–1.5 m), carbonaceous shales (0.3 m–1 m), greenish-grey shales (0.2 m–3 m) interspersed with multiple thick bands (0.02 m–0.05 m) and beds (1 m–2 m) of yellow siltstones (Figure 2). These horizons are successively covered by mottled clay (3 m–12 m), laterites (0.1 m–5.6 m), silty sand (5 m) and sand (0.5 m) deposits at the top (not shown in Figure 2 due to their negligible palynomorph productivity). Age-diagnostic dinoflagellate cysts, including Danian species like Damassadinium californicum and Carpatella cornuta at the base and the early Eocene species Wezteliella astra towards the top part of the succession, suggest a Danian to Ypresian (63 Ma–54 Ma) age for the succession of the mine (Uddandam et al., 2023). Additionally, the succession is demarcated by the Paleocene-Eocene boundary in the succession with the presence of Apectodinium acme and Auxiodinium augustum, globally recognised marker of the PETM (Paleocene-Eocene Thermal Maxima) (Bujak & Brinkhuis, 1988; Crouch et al., 2003; Denison et al., 2021 Garg et al., 2006; Prasad et al., 2006, 2018). Thus, the rich fossil pollen recovered from the succession offers a clear and comprehensive overview of the vegetational history across the Paleocene-Eocene boundary.

Figure 2.

A. Tilia pollen diagram showing the varying palynofloral richness of defined ecological groups (mangroves/back mangroves, pteridophytic fern spores, palms and tropical rainforest floras) in the Paleocene and early Eocene sequences of the early Paleogene succession of the Sonari Lignite Mine, Barmer Basin, Rajasthan, western India. B. Pie chart with palynofloral richness percentage of defined ecological groups in the Paleocene sequence. C. Pie chart with palynofloral richness percentage of defined ecological groups in the early Eocene sequence.

MATERIALS AND METHODS

Two hundred sediment samples were collected for palynological investigations at 10 cm–20 cm intervals from the ~107 m thick succession of the Sonari Lignite Mine, Barmer Basin, Rajasthan. The fossil pollen were recovered from the base of the succession up to a thickness of 72 m (Figure 2). To retrieve the fossil pollen grains, the sediment samples were successively processed with acid solutions of HCl, HF and HNO₃ (Standard Acid Maceration method; Faegri et al., 1989). The sample processing began with crushing 10 g of each sample using a mortar and pestle. The crushed material of each sample was kept in HCl to remove carbonates. Following three rounds of washing using distilled water and decanting, the samples were kept in the HF (40%) to remove silicates. Again, after washing and decanting the samples thrice, they were treated with HNO₃ and then with a 2% KOH solution to remove the humic substances. Finally, the sample solutions were washed, and the supernatant liquid was decanted, leaving the palynologically rich residue sieved through a 10 µm sieve cloth. The final sieved material, along with a drop of polyvinyl alcohol, was smeared over the palynological glass slides and left to air dry. The slides were then covered with coverslips using Canada Balsam. These palynological glass slides were scanned under a Light Microscope (LM; Olympus DP74) to study the fossil pollen grains for palaeovegetational investigations. The recovered fossil pollen were identified by comparing their pollen morphological features with those of extant floral taxa (Table S1). The pollen analysis was conducted at the Birbal Sahni Institute of Palaeosciences, Lucknow, India. The Tilia pollen diagram (Figure 2) was created using the Tilia 1.7.16 software (Grimm, 1991–2011) to display the quantitative data of fossil pollen against the lithology of the early Paleogene succession.

RESULTS

The fossil pollen recovered from the early Paleogene succession of Sonari Lignite Mine closely resembles the extant taxa (Nearest Living Relatives or NLRs) belonging to ecological groups ranging from mangroves/back mangroves (Arecaceae- fossil genus Spinizonocolpites and Araceae- fossil genus Proxapertites), fern spores, palms (Arecaceae) and inland tropical rainforest floras (plant families like Anacardiaceae, Clusiaceae, Dipterocarpaceae, Myristicaceae etc.) (Table S1). Based on their distribution within the Paleocene-early Eocene sequence of the mine, the fossil pollen assemblage has been classified into two distinct palynoassemblages, namely, Paleocene palynoassemblage and early Eocene palynoassemblage. Quantitative data from both palynoassemblages reveal varying richness of early Paleogene taxa belonging to ecological groups across the Paleocene-Eocene boundary (Figure 2).

Paleocene palynoassemblage

The 48.5 m thick Paleocene sedimentary sequence of the early Paleogene succession displays predominance of mangroves/back mangroves (71%) over sporadically distributed tropical rainforest floras (17%), fern spores (7%) and palms (7%) (Figure 2A, B and Table S1). The mangroves/back mangrove groups in the sequence are represented by various species of two fossil taxa: Spinizonocolpites (S. echinatus, S. baculatus, S. prominatus, S. bulbospinosus and S. cf. quilonensis; NLR- Nypa fruticans) and Proxapertites (P. operculatus, P. rugulatus, P. assamicus and P. cursus; NLR- Araceae). The other fossil pollen taxa show close affinity with tropical plant families like Acanthaceae (Strobilanthes type), Amaranthaceae (Amaranthus type), Anacardiaceae (Campnosperma, Holigarna, Lannea and Semecarpus type), Asteraceae (Compositeae type), Caprifoliaceae (Lonicera type), Celestraceae (Salacia kraussii type), Clusiaceae (Garcinia and Mesua ferrea type), Cornaceae (Cornus and Nyssa type), Ctenolophonaceae (Ctenolophon type), Dipterocarpaceae (Dipterocarpus, Monotes, Shorea/Hopea and Vateriopsis type), Ebenaceae (Diospyros type), Euphorbiaceae (Croton, Endospermum, Jatropha hastate, Klaineanthus, Tetrochidium, Bridelia, Drypetes oblongifolia, D. elata, D. malabarica, Euphorbia, Flueggea, Mallotus/Macaranga and Flueggea type), Ericaceae (Erica/ Calluna/ Dabuca type), Fabaceae (Bauhinia, Brachystegia, Crudia/ Anthonotha/ Isoberlinia/ Macrolobium, Hardwickia and Trifolium type), Gunneraceae (Gunnera type), Lamiaceae (Meehania type), Malvaceae (Brownlowoideae- Brownlowia/ Pentace/ Diplodiscus/ Berrya, Durio, Pterospermum, Tilia and Firmiana type), Moraceae (Bosqueia type), Myristicaceae (Gymnacranthera canarica, Knema attenuata and Myristica malabarica type), Myrtaceae (Euryomyrtus, Melaeuca and Syzygium type), Nepenthaceae (Nepenthes type), Nyctaginaceae (Bougainvillea peruviana type), Pandanaceae (Pandacus helicopus? type), Poaceae (Cenchrus type), Polygalaceae (Polygala/Xanthophyllum type), Proteaceae (Grevillea parviflora and Lomatia type), Rutaceae (Vepris bilocularis and Zanthoxylum type), Sapindaceae (Filicium decipiens type), Sapotaceae (Madhuca and Mimusopeae/Isonandreae- Mimusops? type) and Verbenaceae (Clerodendrum type). A fossil taxa Matanomadhiasulcites show ambiguous affinity due to its overlapping resemblance with plant families; Annonaceae (Annona?) and Liliaceae (Kar, 1985; Mandal & Vijaya, 2011). The other tricolporate striate fossil pollen shows overlapping pollen morphology with extant pollen of Anacardiaceae/Burseraceae/Fabaceae or Rosaceae (Morley & Jais, 2024). Within the sequence, the prevalence of palms (Arecaceae) is displayed by fossil pollen resembling the extant taxa Arenga, Borassus/Hyphaene, Daemonorops verticillaris, Eugeissona, Korthalsia rigida, Lepidocaryum tenue, Hydriastele and Oncosperma type and a fossil taxon Kapurdipollenites. Except for a few spores like Dandotiaspora dilata and Dictyotosporites esterleae, most pteridophytic ferns in the sequence are Acrostichum (Pteridaceae).

Early Eocene (Ypresian) palynoassemblage

The fossil pollen assemblage in the early Eocene sedimentary sequence (23.5 m thick) corresponds to similar ecological groups as the Paleocene sequence, but with varying taxonomic palynofloral richness (Figure 2A, C and Table S1). As compared to Paleocene unit, the early Eocene (Ypresian) palynoassemblage, particularly in the upper part of the sequence, display notable decline in the abundance of mangroves/back mangroves (44%), whereas tropical rainforest floral richness increases by 40% (Figure 2C). In the unit, the mangrove/back mangrove groups are represented by four fossil species of Spinizonocolpites (S. echinatus, S. baculatus, S. prominatus and S. bulbospinosus; NLR- Nypa fruticans) and only one species of Proxapertites (P. operculatus, NLR-Araceae). With only 11% of fern spores (mostly Acrostichum) in the early Eocene unit, the dominance of pteridophytes has not changed substantially across the whole early Paleogene succession (Figure 2C). Moreover, the quantity of whole palm group (5%-Arenga, Borassus/Hyphaene, Calamus paspalanthus, Daemonorops verticillaris, Eugeissona, Hydriastele, Lepidocaryum tenue, Korthalsia rigida and Oncosperma) shows little fluctuation compared to what was observed in the Paleocene unit (Figure 2C). Notably, Lepidocaryum tenue exhibits a significant decline in abundance within this unit. Although the total abundance of palms and fern spores remains relatively stable in the early Eocene unit, both groups exhibit increased relative dominance when assessed per unit thickness of the entire palynofossil-rich early Paleogene sedimentary succession. This apparent rise in proportional representation is particularly notable given the reduced thickness (23.5 m) of the early Eocene interval succession compared to the underlying Paleocene strata (48.5 m). In addition, unlike the Paleocene unit, palms and fern spores exhibit continuous distribution in the early Eocene sequence, suggesting a shift in the richness of palaeoecological groups (Figure 2A). The unit also shows significant increase in the abundance of tropical rainforest floras characterised by fossil pollen resembling the plant families like Anacardiaceae (Holigarna, Lannea and Semecarpus type), Asteraceae (Compositeae type), Caesalpiniaceae (Caesalpinia type), Celastraceae (Eounymus vidali type), Clusiaceae (Garcinia and Mesua ferrea type), Cornaceae (Cornus and Mastixia arborea type), Ctenolophonaceae (Ctenolophon type), Dipterocarpaceae (Dipterocarpus, Monotes, Shorea/Hopea and Vateriopsis type), Ebenaceae (Diospyros type), Euphorbiaceae (Bridelia, Blachia umbellate, Croton, Jatropha hastate, Jatropha gossipifolia or J. podagrica, Klaineanthus, Drypetes elata, Excoecaria, Flueggea and Mallotus/ Macaranga type), Fabaceae (Copaifera, Crudia/ Anthonotha/Isoberlinia/Macrolobium, Hardwickia, Ormosia trvancorica, Trifolium and Vicia type), Malvaceae (Durio, Firmiana¸ Pterospermum and Tilia type), Myristicaceae (Gymnacranthera canarica and Knema attenuata type), Nyctaginaceae (Bougainvillea peruviana type), Olacaeae (Strombosia type), Pedaliaceae (Sesamum type), Polygalaceae (Polygala/Xanthophyllum type), Proteaceae (Lomatia type), Sapindaceae (Filicium decipiens type), Rutaceae (Vepris bilocularis and Zanthoxylum type), Sapotaceae (Mimusopeae/Isonandreae- Mimusops? type), Saxifragaceae (Saxifraga type) and Verbenaceae (Clerodendrum viscosum type).

DISCUSSION

Palaeovegetation transitions on the Indian Plate across the Paleocene-Eocene (P-E) boundary

The Paleocene and early Eocene (Ypresian) palynoassemblages of the early Paleogene deposit (a shallow marine marginal succession, Uddandam et al., 2023) of western India bring into our view a notable transition in vegetation pattern across the P-E boundary in the paleo-equatorial or low latitudinal region during the early Paleogene. Although both palynoassemblages represent the existence of a similar combination of paleoecological groups (mangroves/back mangroves, palms, fern spores and inland tropical rainforest floras), their varying dominance across the P-E boundary (Figure 2) represents the transition of vegetation pattern during the period. With 71% predominance of mangroves/back mangroves (Spinizonocolpites and Proxapertites) over other ecological groups, the Paleocene palynoassemblage represents the prevalence of a brackish swamp-dominated ecosystem during the Paleocene. The early Eocene (Ypresian) palynoassemblage, on the other hand, represents declined mangroves/back mangroves and increased palms, fern spores and inland tropical rainforest floras (particularly in the upper part of the sequence), indicating the expansion of the tropical rainforest ecosystem. The loss of three species of Proxapertites (P. rugulatus, P. assamicus and P. cursus) in the early Eocene palynoassemblage indicates the noteworthy disturbance in a swampy ecosystem during the early Eocene. Among tropical rainforests communities, a previous in-depth study on palms (an archetype of tropical rainforests biomes, Baker & Couvreur, 2013; Couvreur & Baker, 2013; Couvreur et al., 2011a; Kissling et al., 2012; Svenning et al., 2008) from the same study area reveals a huge decline in a seasonal tropical rainforest species (Lepidocaryum tenue) and increase of moisture-loving palms (Calamus paspalanthus, Daemonorops verticillaris, Eugeissona, Hydriastele, Korthalsia rigida and Oncosperma) after the P-E boundary (Parmar et al., 2023). This implies that most tropical rainforests underwent a substantial transition from a seasonal tropical rainforest during the Paleocene to warm and wet tropical rainforest in the early Eocene on the western continental margin of India. The early Eocene palynoassemblage characterised by increased abundance of fossil pollen resembling extant genera Gymnacranthera canarica, Knema attenuata, Myristica malabarica, Ormosia trvancorica etc. that are presently endemic to the perhumid rainforests of Western Ghats of Peninsular India (Tissot et al., 1994), further revealing the prevalence of wet and humid tropical rainforest during the early Eocene. In addition to this, the increase in the relative quantity of fern spores in the early Eocene palynoassemblage further attests to the expansion of moist dense rainforest providing shady habitats (Hietz, 2010) for the growth of pteridophytes during the early Eocene. The transition from seasonal tropical rainforests to wet tropical (perhumid) rainforests across the P-E boundary has also been documented from northeast India (Prasad et al., 2018). The similar ecological groups in both palynoassemblages, with varying floral composition and steady increase in abundance, indicate that the paleovegetation change in low latitudinal regions was gradual yet significant across the P-E boundary. The inference drawn in this study is in line with the assertion of progressive changes in paleovegetation across the P-E boundary in other low-latitude regions during the early Paleogene (Jaramillo et al., 2010; Korasidis et al., 2022; Prasad et al., 2018).

Early Paleogene warming and paleolatitudinal position of the Indian Plate

Early Paleogene warming has been documented as a fundamental factor driving major transitions in palaeovegetation patterns of low, mid and high latitudes across the P-E boundary (Garel et al., 2014; Jaramillo, 2002; Jaramillo et al., 2010; Korasidis et al., 2022; Pigg & Devore, 2010; Prasad et al., 2018; Rull, 1999; Srivastava & Prasad, 2015; Wing, 1996; Wing et al., 2015; Xie et al., 2022). Nonetheless, following its drifting from the seasonal to wet-humid climate zone (Prasad et al., 2018) and straddling the paleoequatorial belt during the early Paleogene (Scotese & Golanka, 1992; Sluijs et al., 2008), the Indian Plate draws attention to another factor impacting the paleovegetation pattern on the Indian subcontinent (Parmar et al., 2023; Prasad et al., 2009, 2018). The rich fossil pollen of tropical rainforest floras (primarily associated with perhumid NLRs) in the early Eocene (Ypresian) sequence indicates that the early Paleogene warming coinciding with paleoequatorial alignment of the Indian Plate (Scotese & Golanka, 1992; Sluijs et al., 2008) provided the warm and humid tropical climate for the expansion of wet tropical rainforests during the early Eocene. The warm and humid climate conditions also developed a damp-moist soil habitat under the shade of dense tropical rainforest that favoured the substantial growth of fern spores (Hietz, 2010), as indicated by their increased proportion in the early Eocene (Ypresian) assemblage. Following the high precipitation and increased runoff during the early Paleogene, driven by global warming and the equatorial position of the Indian Plate, these fern spores further eroded along with sediments deposited at river banks and transported to the depositional site. In addition, the rich mangrove/back mangrove elements, represented by Spinizonocolpites and Proxapertites, during the Paleocene and at the P-E boundary and their decline in the later part of the early Ypresian unit, indicate the prevalence of brackish swamps only until early phases of Ypresian (Figure 2). Such vegetation flourishes in coastal intertidal zones with low saline conditions controlled by the interplay between marine incursions and freshwater discharge through catchment rivers. The existence of brackish swamps in the western (present study) and northeast India (Srivastava & Prasad, 2015) during the early Paleogene is suggested to have been attributed to the high precipitation and enhanced freshwater runoff associated with warming and equatorial position of the Indian Plate. Their subsequent decline in the later Ypresian is likely linked to shifts in rainfall patterns. Thus, with the Paleocene and early Eocene (Ypresian) palynoassemblages from western India, the present study concludes that the paleovegetation transitions across the P-E boundary in low latitudinal regions, particularly the Indian subcontinent, was accompanied by the coupled effect of early Paleogene warming and the paleoequatorial position of the Indian Plate during the early Paleogene.

Palaeovegetation transitions: Low latitudes versus mid and high latitudes

Early Paleogene warming had a heterogeneous impact across the latitudinal regions (Carmichael et al., 2017, 2018; Hollis et al., 2019; Korasidis et al., 2022). The diverse floral taxonomic composition across the P-E boundary of various latitudes corresponds to different patterns of prominent transitions in palaeovegetation patterns (Korasidis et al., 2022). The mid and high latitudes have been documented to show rapid changes in vegetation pattern with the transition of conifer-dominated floral composition indicating cool temperate forest type during Paleocene to Arecaceae (palms), Bombacoideae (Malvaceae) and Picrodendraceae composing warm tropical rainforest type during Eocene (Airy Shaw, 1974; Eiserhardt et al., 2011; Forster, 1997; Korasidis et al., 2022; Linares-Palomino & Alvarez, 2005; McNeil et al., 2013; Suan et al., 2017; Svenning et al., 2008; Tryon & Tryon, 2012; Willard et al., 2019). On the contrary, in line with previous views (Jaramillo et al., 2010; Prasad et al., 2018; Korasidis et al., 2022), the present study records the gradual transitions from a high seasonal rainforest type to a low seasonality rainforest (perhumid) type in low latitudinal regions.

CONCLUSIONS

The P-E boundary in the early Paleogene succession exposed in the Sonari Lignite Mine of Barmer Basin, Rajasthan, clearly documents Paleocene and early Eocene (Ypresian) palynoassemblages, representing similar ecological groups (mangroves/back mangroves, palms, fern spores and inland tropical rainforest floras) with significantly varying floral richness and composition across the boundary.

The Paleocene palynoassemblage, which was dominated by 71% mangrove/back mangrove taxa (Spinizonocolpites and Proxapertites), represents the swamp-dominated ecosystem during the Paleocene. The early Eocene (Ypresian) palynoassemblage, on the other hand, shows an increase of ecological groups like palms, fern spores and mainly inland tropical rainforest floras that collectively imply the expansion of the tropical rainforest ecosystem during the early Eocene. A notable decline in the abundance of mangroves/back mangroves has been observed in the later phases of the Ypresian. Both palynoassemblages, thus, represent the gradual transition of vegetation pattern from a brackish swampy ecosystem to a tropical rainforest ecosystem across the P-E boundary on the western continental margin of India.

The decline of seasonal taxa Lepidocarum tenue, coupled with increased moisture-loving palms and other perhumid taxa like Gymnacranthera canarica and Ormosia travancorica, across the P-E boundary represents a transition from seasonal tropical rainforests to wet and humid tropical rainforests.

The palaeovegetation transitions across the P-E boundary on the Indian Plate were driven by both the early Paleogene warming and the palaeoequatorial alignment of the Indian Plate, following its movement from a seasonal to a wet tropical climate belt.

Unlike mid- and high latitudes, the palaeovegetation transitions in low-latitude areas during the early Paleogene warming were comparatively gradual, shifting from a highly seasonal forest type to a low-seasonality (per humid) tropical forest.