A micropalaeontological perspective on changes in bottom-water conditions at the Iberian Margin during the last glacial cycle

Changes in deep-ocean carbon storage during the last glacial cycle were closely linked to variations in deep-water oxygenation, which reflects the combined influences of ocean ventilation and export production. Yet the relative importance of these processes remains difficult to disentangle. Whilst the Last Glacial Maximum (LGM) and subsequent deglacial changes in Atlantic Ocean circulation have been extensively studied, significantly less is known about the Marine Isotope Stage (MIS) 5a/4 transition, a period of global cooling and rapid glacial inception, and the subsequent full glacial conditions of MIS 4, for which palaeoclimatic records remain scarce. Here we use benthic foraminiferal assemblage records from sediment cores retrieved at bathyal (∼2.65 km) and abyssal (∼4.7 km) depths on the Iberian Margin to investigate changes in export productivity and bottom-water oxygenation between ∼50 and 85 ka. These records are compared with benthic foraminiferal assemblage records from the Iberian Margin spanning the last ∼23ka. We find that bathyal sites are characterised by overall productive, high export flux environments throughout all glacial stages, with assemblages dominated by infaunal foraminifera. In contrast, deeper abyssal sites are characterised by oligotrophic conditions with assemblages primarily composed of epifaunal foraminifera. Peak glacial conditions of the LGM and MIS 4 are represented by a distinct faunal assemblage at both depths, reflecting changes in the quality and seasonality of organic carbon supply rather than enhanced productivity. In contrast, assemblages during the most extreme climatic conditions associated with Heinrich Stadials (HSs) 1 and 6, are distinct from all others, indicating reduced bottom-water oxygenation and major hydrographic perturbations. Assemblages during the MIS 4/HS 6 interval resemble those during the LGM/HS 1 interval, indicating broadly similar benthic conditions during these two important periods of the last glacial cycle. Overall, the reconstructed changes in benthic foraminifera appear to be driven primarily by organic carbon flux, highlighting the dominant influence of changes in food supply, while significant oxygenation changes were largely restricted to Heinrich Stadials. This implies that, despite potential variability in deep-water oxygenation, benthic conditions on the western Iberian Margin were not strongly oxygen-limited during the last glacial cycle, and demonstrates the value of benthic foraminiferal assemblages for disentangling biological and physical controls on past deep-ocean conditions.