Integrated imaging fingerprints of anoxic concretions track 7000 years of abrupt and extreme Arctic winter climate change

Paleo data play a critical role in constraining the future evolution of climate in the Arctic, our planet`s most rapidly warming region. However, most proxies capture past change during the brief summers, when biological indicators are synthesized, and the availability of liquid water as well as the absence of snow allows sediment transport. As a result, far less is known about climate variability in other seasons. This bias is of consequence, because simulations hint at more extensive winter variability. Also, these changes affect regional and global climate by impacting surface albedo, glacier melt, and biodiversity. This work helps close this knowledge gap, by providing a 7000-year long record of abrupt and extreme Arctic winter climate change. For this purpose, we analyzed lake sediments – sensitive recorders of surface change, taken from a high-altitude basin – amplifying the imprint of winter season change, on Svalbard – a High Arctic climate change hotspot. By characterizing variations in geochemistry with X-Ray Fluorescence (XRF), density with Computed Tomography (CT), and anoxia with Hyperspectral Imaging (HI), we provide multiple lines of evidence for the presence of concretions that formed after extreme winters triggered widespread anoxia owing to residual lake ice feedbacks. Comparison with ice core records and climate model simulations suggests that these abrupt shifts were triggered by volcanic eruptions. Our findings highlight the potential of imaging techniques to fingerprint the geochemical imprint of winter climate change, and further challenge the notion that Holocene climate was stable.