Changes of surface snow isotopes (d18O, dD, d-excess) during MOSAiC expedition

Abstract

<p>Several factors, including the movement and distribution of seawater, the amount of precipitation, and the presence of sea ice influence the Arctic Ocean's hydrology. Snow plays a pivotal role in the Arctic water cycle as it influences the amount and spatial distribution of fresh water available in the region. In the Arctic, precipitation is scarce, and snow accumulation rates are low. Consequently, snow deposited on top of sea ice remains exposed on the surface for several months. During this time, snow post-depositional processes (i.e., metamorphism, sublimation) and wind redistribution are the key drivers controlling the chemical evolution of the snowpack. Post-depositional processes affect the physical properties of snow and its isotopic composition, the latter being a key proxy in hydrology.</p> <p>The MOSAiC expedition was a year-long research project in the Arctic Ocean from 2019 to 2020. The expedition aimed to study the impacts of climate change on the Arctic region and to improve our understanding of the processes that drive the Earth's climate. This study uses the isotopic composition (δ<sup>18</sup>O, δ<sup>2</sup>H, d-excess) of top 3 cm of snow samples collected during the MOSAiC expedition. The secondary parameter d-excess helps explore the isotopic signature of post-deposition processes in surface snow due to different levels of sensitivity of δ<sup>18</sup>O and δ<sup>2</sup>H to equilibrium and kinetic fractionations. Changes in δ<sup>18</sup>O and d-excess help address the rate of snow sublimation at the surface, as well as within the snow profile. During the Arctic winter, the isotopic composition of surface snow on sea ice is mainly influenced by moisture exchange between snow and the near-surface atmosphere, taking place during sublimation and depositional processes.</p> <p>We found that the moisture exchanges between snow and the atmosphere, and snow and sea ice override the initial climate signal in the precipitated snow isotopes. The redistribution of snow due to strong Arctic winds affects the isotopic composition of snow at the surface by mixing the precipitation of different precipitation events. Our preliminary results show that average monthly continuous isotopic measurements of vapour (Brunello et al. 2023) were -15 to -25‰ more depleted compared to the average monthly surface snow throughout the winter months. This gap tends to decrease in summer. By studying changes in the surface snow’s isotopic composition in comparison to the isotopic measurements of other compartments of the Arctic water cycle (i.e., vapour, sea ice, melt ponds), we aim to find the dominant moisture sources and key post-depositional changes of snow. This will lead to an improved understanding of sea ice-snow-atmosphere interaction in the understudied Central Arctic cryo- and hydrosphere.</p>