A varve chronology for glacial retreat and hydroclimate changes during Heinrich Stadial 1 on the Northwest European margins

Heinrich Stadial 1 (HS 1; 18-15 ka) was a climatically dynamic period during the Last Deglaciation, which is thought to have been driven by alterations in Atlantic Meridional Overturning Circulation (AMOC), and which impacted terrestrial systems around the Northern Hemisphere. The event is usually divided into substages suggesting that conditions were not stable throughout the period. The middle- and high-latitude land masses adjacent to the North Atlantic were likely to have been most strongly impacted by such AMOC changes. However, very few records exist from these areas due to extensive ice cover which persisted over much of Europe and North America at this time, and because the impact of changing AMOC conditions on the melting ice masses and wider landscape is difficult to assess. This paper uses annually laminated lake-sediment cores from a glaciolacustrine system in South Wales to generate the first chronology of southern British Irish Ice Sheet (BIIS) deglaciation and associated environmental changes through HS 1. Microfacies analysis of two laminated core sections from Llangorse Lake yields a merged master varve chronology which spans 3506 ± 143 years. This chronology is tethered to an absolute age scale at the youngest point of the varve section using radiocarbon dates and extrapolating the Bayesian age model to date the onset and end of varve formation. The varve thickness data shows that an ice-dammed glacial lake formed circa 18.84 cal ka BP (20,034–17,648) and transitioned into a cold-climate lake system ∼18.75 cal ka BP (19,939–17,553). This later, nival lake persisted for ∼3400 years until ∼15.34 cal ka BP (16,207–14,463), spanning the period of HS 1, after which the lake transitioned to non-varved sediments. The LLaMVC21 varve thickness record is related to i) millennial scale transition from drier to wetter conditions during Heinrich Stadial 1, and ii) inter-annual to decadal variability in hydroclimate, with three multi-decadal periods of decreased varve thickness detected. The findings here supports a wetter late HS 1 with the timing of the change between drier and wetter conditions broadly consistent with more distant records in more northerly latitudes.