Paleoenvironmental reconstruction of the Campanian-Maastrichtian boundary using nannofossil assemblages in the Mannar Basin, Sri Lanka

Abstract

The study examined calcareous nannofossil assemblages across the Campanian-Maastrichtian boundary in the Mannar Basin, Sri Lanka, to reconstruct paleoenvironmental and paleoclimatic conditions. Thirteen drill-cutting samples at 10 m spacing from a 130 m interval (2920-3050 m; CLPL-Dorado 91H/1z) were studied by light microscopy (Axiocam ERc 5s; Zen 3.1) and scanning electron microscopy (SEM), identifying 37 species, of which 21 were confidently classified. Quantitative analyses using PAST5, including diversity indices and principal component analysis (PCA) of ecological ratios, revealed distinct depth-linked shifts in community structure. Persistent genera (Cylindralithus, Cyclagelosphaera margerelii, Praeprinsius tenuiculus, Watznaueria) indicate broad tolerance across the interval. Late Campanian assemblages contain taxa consistent with cooler, nutrient-rich surface waters, whereas early Maastrichtian assemblages show warmer, oligotrophic and more unstable conditions, including the appearance of Lucianorhabdus, a recognised warm-oligotrophic indicator in Upper Cretaceous successions. Indicator signals are interpreted at the species level. Micula decussata is associated with cooler, nutrient-rich states, whereas M. murus is linked to warm, oligotrophic conditions. Indicator species analysis identified Uniplanarius sissinghii as typical of early Maastrichtian environments, while Microrhabdulus undosus and Microrhabdulus decoratus were indicative of the late Campanian. PCA results show that the first two components explain over 90% (92.8%) of total variance, which represent a specialist–generalist axis (PC1) and a temperature–nutrient axis (PC2), consisting of recognised ecological gradients in nannofossil assemblages. Taphonomic processes, particularly overgrowth and partial dissolution observed in SEM, affected preservation and may have reduced taxonomic resolution and led to misidentification, emphasising the importance of preservation assessment in paleoenvironmental interpretations. These findings refine the understanding of southern Indian Ocean paleobiogeography across the Campanian– Maastrichtian transition and demonstrate the value of multivariate, species-level nannofossil analysis that resolves short-lived cooling events within an overall warming trend.