Early to Middle Miocene in the North Sea Basin: proxy-based insights into environment, depositional settings and sea surface temperature evolution

Abstract. Two Miocene successions from onshore Denmark (Sønder Vium, or Sdr. Vium, well) and offshore Norway (2/11-12S well) have been correlated using dinoflagellate cyst (dinocyst) biostratigraphy. This correlation provides a valuable opportunity to compare Early to Middle Miocene dinocyst assemblages and palynofacies across environmentally contrasting, yet time-equivalent, depositional settings in the North Sea Basin. Both successions were deposited during the Miocene Climatic Optimum (MCO; ∼ 16.9–14.7 Ma), a prominent global warming phase within the Miocene. Our analyses integrate palynofacies and dinocyst biostratigraphy to reconstruct depositional settings, changes in the environment, and climatic conditions through the Burdigalian–Langhian interval. The palynofacies data, including terrestrial/marine palynomorph ratios (P/D index), indicate a distal marine setting in the offshore 2/11-12S well (low P/D index, dominance of marine palynomorphs) and a more proximal marine setting at Sønder Vium (high P/D index, higher terrestrial input). Dinocyst assemblages also mirror this environmental gradient. Oceanic to outer neritic taxa, such as Impagidinium spp. and Nematosphaeropsis spp., are abundant offshore but rare or absent onshore. Conversely, Homotryblium tenuispinosum, indicative of inner neritic settings, is present onshore but appears only once in the offshore well. The presence of thermophilic taxa (e.g., Melitasphaeridium choanophorum and Polysphaeridium zoharyi) in both wells during the late Burdigalian to early Langhian suggests sustained warm sea surface temperatures (SSTs) across the basin. Warm temperatures are supported by lipid-biomarker-derived (TEX86) SST estimates from the 2/11-12S well. Moreover, a distinct occurrence pattern of Polysphaeridium zoharyi is concurrent with the warmest SSTs during the late Burdigalian to early Langhian, corresponding to the MCO. The later appearance of cooler-water taxa, such as Habibacysta tectata, in the latest Langhian also coincides with a decrease in SST, indicating a climatic shift towards cooler conditions.