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

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.