Changing hydroclimate seasonality in SW Iberia over the Last Deglaciation and Holocene revealed using leaf-wax isotopes

The western Mediterranean is projected to experience more frequent and extreme drought and flood events in response to anthropogenic climate change, increasing its vulnerability to water scarcity and associated socioeconomic and ecological impacts. Reconstructions of past hydroclimatic conditions help constrain the natural hydrological response to different forcing mechanisms and reveal links between hydroclimate and other climate system components, improving our understanding of future change. Here, we examine hydroclimate isotope changes in southwest (SW) Iberia over the past 28 thousand years (kyr) using the stable hydrogen composition of leaf-waxes (δ²H_wax) in combination with published pollen and sea-surface temperature data from an Iberian Margin marine sediment core (SHAK06-5K). Additionally, we investigate whether the carbon isotope composition of leaf-waxes (δ¹³C_wax) may provide complementary hydroclimatic information using δ¹³C_wax values of contemporary vegetation. Our δ²H_wax record closely correlates with the abundance of heathland in SW Iberia, as well as changes in precession-driven boreal spring/early summer insolation, indicating that δ²H_wax in SW Iberia records spring/early summer hydroclimate conditions. When combined with reconstructions of winter hydroclimate based on pollen records, this record provides a basis for assessing changes in the seasonality of precipitation over orbital timescales. During the Late Pleniglacial and Late Glacial (28 – 11.7 ka), low δ²H_wax values, combined with the expansion of heathland and steppe taxa, indicate low rates of spring/early summer evapotranspiration, cooler temperatures and reduced hydroclimate seasonality, due to precession-driven low boreal spring/early summer insolation. In the Early Holocene, high δ²H_wax values, combined with reduced heathland and expansion of temperate arboreal taxa, indicate increased spring/early summer temperatures and evapotranspiration and enhanced hydroclimate seasonality driven by a boreal spring/early summer insolation maximum. During the mid-to-late Holocene, lower δ²H_wax values, heathland expansion and decline in arboreal taxa indicates reduced hydroclimate seasonality, with higher rates of spring/early summer precipitation combined with reduced evapotranspiration and lower winter precipitation. The close correlation between δ¹³C_wax values in our record and pollen data from the same core, as well as δ¹³C_wax values of contemporary vegetation from SW Iberia, indicates that this signal was heavily influenced by the physiology of the dominant vegetation types over this period. These findings highlight the sensitivity of regional hydroclimate to seasonal insolation and temperature, and the close link between hydroclimate and vegetation physiology.