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 (UK’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 (δ18Osw) 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.