Rising atmospheric CO₂ concentrations: the overlooked factor promoting SW Iberian Forest development across the LGM and the last deglaciation?

Abstract. Across the last deglaciation, the global atmospheric concentration of carbon dioxide (pCO₂) increased from ~180 to ~280 ppm. However, the impact of pCO₂ changes on vegetation across the last deglaciation remains poorly understood. Under full glacial low pCO₂, plants experienced constraints on photosynthesis. Therefore, a significant reduction in pCO₂ limitation should have impacted local and regional vegetation dynamics across deglaciation. We hypothesise that deglacial pCO₂ rise could have (1) led to a gradual reduction of the physiological constraint promoting forest response when moisture availability was sufficient, despite low temperatures; and (2) amplified the response of forest development to warmer and wetter conditions. The high-resolution analysis of terrestrial (pollen, C₂₉:C₃₁ organic biomarker) and marine (alkenone-derived Sea Surface Temperature, C_{37: 4}%, and long-chain n-alkanes ratios) indicators, using a direct land-sea direct comparison, in the Iberian margin site Integrated Ocean Drilling Program (IODP) U1385 ("Shackleton site") throughout the Last Glacial Maximum (LGM) and last deglaciation allowed us to track and compare the changes with shifts in global pCO₂. The LGM is characterised by a grassland-heathland mosaic type ecosystem, triggered by cool and moderately humid conditions but low pCO₂ levels may have exacerbated physiological drought and suppressed forest development. During Heinrich Stadial 1 (HS1)the coldest and most arid conditions combined with sustained low pCO₂ values precluded forest development and resulted in the dominance of Mediterranean steppe or semi-desert vegetation. The Bølling-Allerød (BA) is characterised by a temperature optimum (warmest SSTs and forest development) and variable moisture condition, while increasing pCO₂ which contributed to the highest levels of forest development. Within the BA, significant SW Iberian forest development occurred at ~15 cal kyr B.P. associated with an increase in pCO₂ above 225 ppm. During the Younger Dryas (YD), cool temperatures combined with sufficient moisture availability allowed the maintenance of a grassland-forest mosaic, the increasing values of pCO₂ in this period should have offset the low temperature as well as the moisture levels and allow the forest to persist. The overlooked role of pCO₂ could explain an intriguing feature observed in Site U1385 and other Iberian margin records, namely the near absence of forest during the LGM and HS1 but relatively high forest cover during the YD. Our study aims to shed light on the influence of climatic factors (temperature and moisture availability) together with pCO₂ as thresholds on forest response to deglacial climate changes across the Iberian Peninsula.