North Atlantic sea surface temperature evolution across the Oligocene–Miocene Transition from TEX86 paleothermometry

The Oligocene–Miocene Transition (OMT) includes a pronounced ~1‰ positive excursion in benthic oxygen isotope records (δ¹⁸O), reflecting Antarctic ice sheet expansion and/or deep ocean cooling, commonly referred to as the Mi-1 glaciation. At present, limited reconstructions of sea surface temperature (SST) evolution across the OMT have been published, leaving the magnitude of global cooling during Mi-1 uncertain. Here we present high-resolution (~10 kyr) SST reconstructions from IODP Site U1406 on the Newfoundland Margin (North Atlantic) using the lipid biomarker TEX₈₆ proxy, based on isoGDGT distributions. Our record shows TEX₈₆ values ranging from 0.64 to 0.76, with a ~0.04 decrease during the Mi-1 event. To assess potential non-thermal overprints on the TEX₈₆ data, we calculated GDGT-based indices, including the Branched-to-Isoprenoid Tetraether (BIT) index. BIT values are relatively high (0.4–0.8), suggesting significant input of terrestrial GDGTs that could bias TEX₈₆. However, TEX₈₆ and BIT show weak correlation (R² = 0.124), indicating limited terrestrial overprint on the TEX₈₆ signal. Furthermore, a ternary plot of brGDGT compositions shows that the Newfoundland samples differ from modern soils and peats, suggesting marine production of brGDGTs as the source of the high BIT values. These findings suggest that the Newfoundland Margin was not influenced by substantial terrestrial organic matter input across the OMT, and that TEX₈₆ provides a reliable record of SST. Translating TEX₈₆ into temperature, our record indicates warm SSTs ranging from 25 to 31 °C, with a cooling of ~2 °C during the Mi-1 event, consistent with published low-resolution alkenone-derived (U^K’₃₇) estimates (Guitián et al., 2019). Future work will focus on determining whether the observed SST cooling at Site U1406 reflects a global climate signal or is driven by latitudinal shifts in the North Atlantic SST gradient. This could be addressed using seawater oxygen isotope (δ¹⁸O_sw) reconstructions based on the combination of SST proxies and planktic foraminiferal δ¹⁸O to infer changes in surface ocean circulation, alongside comparisons with Earth System Model simulations.