Ukpabi, Ndubuisi; Seidenkrantz, Marit-Solveig (2026): High resolution Holocene coccolithophore assemblages, absolute abundances, and hydrographic index (CHI) calculations from sediment core DA12 11/2 ST1 GC01, Iceland Basin [dataset bundled publication]. PANGAEA, https://doi.org/10.1594/PANGAEA.996263

This dataset provides high resolution coccolithophore assemblage counts, absolute abundances, ecological contrast indices, and derived hydrographic metrics from marine sediment core DA12 11/2 ST1 GC01, retrieved from the Iceland Basin at 2120 m water depth. The record spans the entire Holocene and consists predominantly of dark grey, fine grained calcareous ooze with occasional ash layers. Coccolithophore assemblages were analysed at 5 cm intervals, corresponding to an average temporal resolution of ~70–80 years in the upper Holocene. Each sample was freeze dried, homogenized, deflocculated, and mounted for light microscopy and scanning electron microscopy. A minimum of 300 coccoliths were identified to species level per sample, following established taxonomic criteria. Absolute abundances were calculated from the total number of coccoliths in 0.01 g of sediment using field of view geometry and subsample mass. As stated in the dataset, "C = N g−1 FOV = Field of Vision (fields counted)". Species included Calcidiscus leptoporus, Coccolithus pelagicus, Gephyrocapsa spp., Emiliania huxleyi, Helicosphaera carteri, Syracosphaera pulchra, Umbilicosphaera spp., Pontosphaera discopora, Pontosphaera japonica, and Braarudosphaera bigelowii. Ecological preferences of taxa were used to construct four Ecological Contrast Indices (thermal, salinity, nutrient, and stratification/mixing). These contrasts were combined to produce the Coccolithophore Hydrographic Index (CHI), a dimensionless proxy summarizing warm–cold, saline–fresh, oligotrophic–nutrient rich, and stratified–mixed ecological states. CHI values were log transformed, z scored, and tanh scaled to generate standardized hydrographic anomalies and percentile based state classifications. The dataset includes raw CHI, log₁₀(CHI), CHI_zscore, CHI_sx, percentile thresholds (Q10, Q90), and trend classifications derived from rolling slope analysis. This dataset provides a continuous, quantitative record of coccolithophore community structure and hydrographic variability in the Iceland Basin throughout the Holocene. It is intended for paleoceanographic reconstructions, proxy comparison studies, and multiproxy analyses of North Atlantic surface water dynamics.