Isotope measurements of the Arctic water cycle and exchange processes between seawater, sea ice, and snow during MOSAiC

For the past two decades, the Arctic water cycle changed rapidly due to surface air temperatures (SATs) increasing at twice the global rate. Terrestrial ice (i.e. Greenland Ice Sheet) and marine sea-ice loss, alterations of ocean circulation patterns, and shifting atmospheric moisture sources and transport are some of the most pronounced changes caused by the Arctic amplification, fostering increased humidity levels. Stable water isotopes (δ18O, δ2H) and the secondary parameter d-excess are valuable tracers for hydrological changes, including how these shifts may affect the global climate system. However, it is only recently that we are using precipitation and water vapor networks to resolve water isotope patterns and processes in the Arctic. However, a fully coordinated study of the entire water cycle attributes year-long including sea ice, ocean water, vapor, and precipitation has until recently has been absent. The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition provided a unique opportunity to collect, analyze, and synthesize discrete samples of the different hydrological compartments in the central Arctic, covering a complete one-year seasonal cycle using a combination of ship-based, the pan-Arctic Water Isotope Network (PAPIN). These observations can lead to new insights into coupled ocean-atmosphere climate processes operating in the Arctic, especially during extreme events, sea ice formation, sea ice retreat, and during a dichotomy of synoptic weather patterns over the MOSAiC-year. We present the isotopic traits of more than 2,200 discrete samples (i.e., seawater, sea ice, snow, brines, frost flowers, lead ice, ridge ice, and precipitation) collected during MOSAiC. Snow has the most depleted δ18O values (-16.3 ± 9.1‰; the number of samples N=306), whereas seawater is the most enriched δ18O compartment (-1.5 ± 0.9‰; N=302) of the Arctic water cycle. Precipitation throughout the Arctic Basin varied from -10‰ to -35‰. Snow profiles are gradually enriched in δ18O from top to bottom by ~20‰ partially due to sublimation of deposited snow, as well as snow metamorphism and its effects on the water isotopes. Second-year ice (SYI) is isotopically relatively depleted in δ18O (-4.2 ± 2.6‰; N=200) compared to first-year ice (FYI) (-0.7 ± 2.1‰; N=635) and insulated FYI (i.e. FYI grown at the bottom of SYI) (-1.7 ± 2.4‰; N=214). The latter is likely caused by post-depositional exchange processes with snow. Open water leads (-1.6 ± 2.4‰; N=137) and melt ponds (-2.1 ± 2.7‰; N=109) on the surface of sea ice contribute to the moistening of the atmosphere in the Arctic on a regional scale. Our dataset provides an unprecedented snapshot of the present-day isotopic composition of the Arctic water cycle during an entire year. The coupling of these discrete samples data with the continuous measurements of atmospheric water vapor may shed light on the relative contribution of snow, sea ice, seawater, open water leads, and melt ponds both spatially and temporally to regional and local moisture levels in the Arctic. Stable water isotopes will ultimately contribute to resolving the linkages between sea ice, ocean, and atmosphere during the critical transition from frozen ocean to open water conditions

Hydroclimatic Controls on the Isotopic (δ18 O, δ2 H, d-excess) Traits of Pan-Arctic Summer Rainfall Events

Arctic sea-ice loss is emblematic of an amplified Arctic water cycle and has critical feedback implications for global climate. Stable isotopes (delta O-18, delta H-2, d-excess) are valuable tracers for constraining water cycle and climate processes through space and time. Yet, the paucity of well-resolved Arctic isotope data preclude an empirically derived understanding of the hydrologic changes occurring today, in the deep (geologic) past, and in the future. To address this knowledge gap, the Pan-Arctic Precipitation Isotope Network (PAPIN) was established in 2018 to coordinate precipitation sampling at 19 stations across key tundra, subarctic, maritime, and continental climate zones. Here, we present a first assessment of rainfall samples collected in summer 2018 (n = 281) and combine new isotope and meteorological data with sea ice observations, reanalysis data, and model simulations. Data collectively establish a summer Arctic Meteoric Water Line where delta H-2 = 7.6.delta O-18-1.8 (r(2) = 0.96, p 0.75 parts per thousand/degrees C) were observed at continental sites, while statistically significant temperature relations were generally absent at coastal stations. Model outputs indicate that 68% of the summer precipitating air masses were transported into the Arctic from mid-latitudes and were characterized by relatively high delta O-18 values. Yet 32% of precipitation events, characterized by lower delta O-18 and high d-excess values, derived from northerly air masses transported from the Arctic Ocean and/or its marginal seas, highlighting key emergent oceanic moisture sources as sea ice cover declines. Resolving these processes across broader spatial-temporal scales is an ongoing research priority, and will be key to quantifying the past, present, and future feedbacks of an amplified Arctic water cycle on the global climate system

Arctic Snow Isotope Hydrology: A Comparative Snow-Water Vapor Study

The Arctic’s winter water cycle is rapidly changing, with implications for snow moisture sources and transport processes. Stable isotope values (δ18O, δ2H, d-excess) of the Arctic snowpack have potential to provide proxy records of these processes, yet it is unclear how well the isotope values of individual snowfall events are preserved within snow profiles. Here, we present water isotope data from multiple taiga and tundra snow profiles sampled in Arctic Alaska and Finland, respectively, during winter 2018–2019. We compare the snowpack isotope stratigraphy with meteoric water isotopes (vapor and precipitation) during snowfall days, and combine our measurements with satellite observations and reanalysis data. Our analyses indicate that synoptic-scale atmospheric circulation and regional sea ice coverage are key drivers of the source, amount, and isotopic composition of Arctic snowpacks. We find that the western Arctic tundra snowpack profiles in Alaska preserved the isotope values for the most recent storm; however, post depositional processes modified the remaining isotope profiles. The overall seasonal evolution in the vapor isotope values were better preserved in taiga snow isotope profiles in the eastern Arctic, where there is significantly less wind-driven redistribution than in the open Alaskan tundra. We demonstrate the potential of the seasonal snowpack to provide a useful proxy for Arctic winter-time moisture sources and propose future analyses

Isotopic signatures of snow, sea ice, and surface seawater in the central Arctic Ocean during the MOSAiC expedition

The Arctic Ocean is an exceptional environment where hydrosphere, cryosphere, and atmosphere are closely interconnected. Changes in sea-ice extent and thickness affect ocean currents, as well as moisture and heat exchange with the atmosphere. Energy and water fluxes impact the formation and melting of sea ice and snow cover. Here, we present a comprehensive statistical analysis of the stable water isotopes of various hydrological components in the central Arctic obtained during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in 2019–2020, including the understudied Arctic winter. Our dataset comprises >2200 water, snow, and ice samples. Snow had the most depleted and variable isotopic composition, with d18O (–16.3%) increasing consistently from surface (–22.5%) to bottom (–9.7%) of the snowpack, suggesting that snow metamorphism and wind-induced transport may overprint the original precipitation isotope values. In the Arctic Ocean, isotopes also help to distinguish between different sea-ice types, and whether there is a meteoric contribution. The isotopic composition and salinity of surface seawater indicated relative contributions from different freshwater sources: lower d18O (approximately –3.0%) and salinities were observed near the eastern Siberian shelves and towards the center of the Transpolar Drift due to river discharge. Higher d18O (approximately –1.5%) and salinities were associated with an Atlantic source when the RV Polarstern crossed the Gakkel Ridge into the Nansen Basin. These changes were driven mainly by the shifts within the Transpolar Drift that carried the Polarstern across the Arctic Ocean. Our isotopic analysis highlights the importance of investigating isotope fractionation effects, for example, during sea-ice formation and melting. A systematic full-year sampling for water isotopes from different components strengthens our understanding of the Arctic water cycle and provides crucial insights into the interaction between atmosphere, sea ice, and ocean and their spatio-temporal variations during MOSAiC

Arctic snow isotope hydrology:a comparative snow-water vapor study

Abstract The Arctic’s winter water cycle is rapidly changing, with implications for snow moisture sources and transport processes. Stable isotope values (δ¹⁸O, δ²H, d-excess) of the Arctic snowpack have potential to provide proxy records of these processes, yet it is unclear how well the isotope values of individual snowfall events are preserved within snow profiles. Here, we present water isotope data from multiple taiga and tundra snow profiles sampled in Arctic Alaska and Finland, respectively, during winter 2018–2019. We compare the snowpack isotope stratigraphy with meteoric water isotopes (vapor and precipitation) during snowfall days, and combine our measurements with satellite observations and reanalysis data. Our analyses indicate that synoptic-scale atmospheric circulation and regional sea ice coverage are key drivers of the source, amount, and isotopic composition of Arctic snowpacks. We find that the western Arctic tundra snowpack profiles in Alaska preserved the isotope values for the most recent storm; however, post depositional processes modified the remaining isotope profiles. The overall seasonal evolution in the vapor isotope values were better preserved in taiga snow isotope profiles in the eastern Arctic, where there is significantly less wind-driven redistribution than in the open Alaskan tundra. We demonstrate the potential of the seasonal snowpack to provide a useful proxy for Arctic winter-time moisture sources and propose future analyses

Hydroclimatic controls on the isotopic (δ18 O, δ2 H, d-excess) traits of Pan-Arctic summer rainfall events

Arctic sea-ice loss is emblematic of an amplified Arctic water cycle and has critical feedback implications for global climate. Stable isotopes (δ18O, δ2H, d-excess) are valuable tracers for constraining water cycle and climate processes through space and time. Yet, the paucity of well-resolved Arctic isotope data preclude an empirically derived understanding of the hydrologic changes occurring today, in the deep (geologic) past, and in the future. To address this knowledge gap, the Pan-Arctic Precipitation Isotope Network (PAPIN) was established in 2018 to coordinate precipitation sampling at 19 stations across key tundra, subarctic, maritime, and continental climate zones. Here, we present a first assessment of rainfall samples collected in summer 2018 (n = 281) and combine new isotope and meteorological data with sea ice observations, reanalysis data, and model simulations. Data collectively establish a summer Arctic Meteoric Water Line where δ2H = 7.6⋅δ18O–1.8 (r2 = 0.96, p 0.75‰/°C) were observed at continental sites, while statistically significant temperature relations were generally absent at coastal stations. Model outputs indicate that 68% of the summer precipitating air masses were transported into the Arctic from mid-latitudes and were characterized by relatively high δ18O values. Yet 32% of precipitation events, characterized by lower δ18O and high d-excess values, derived from northerly air masses transported from the Arctic Ocean and/or its marginal seas, highlighting key emergent oceanic moisture sources as sea ice cover declines. Resolving these processes across broader spatial-temporal scales is an ongoing research priority, and will be key to quantifying the past, present, and future feedbacks of an amplified Arctic water cycle on the global climate system

Snowpit stable isotope profiles during the MOSAiC expedition

In the designated snowpits on the MOSAic expedition a 100cm3 sample of snow was measured for density in the field, approximately every 3 vertical centimetres. The same snow volume was melted for salinity measurements and sealed into glass jars onboard Polarstern. As a result, δ18O and δ2H isotope composition was measured at the same intervals as density and salinity profiles, included in the snowpit dataset bundle. These samples were then transported to the WSL laboratory in Switzerland to analyse the stable water isotopes (δ18O and δ2H). This dataset provides details on the date and coordinates of the snowpit event, the height of the sample in the snow profile, and the corresponding isotope composition. This dataset includes the isotopic composition of the samples obtained in the winter months (October 2019 to May 2020). After analysis of the samples, and comparing the values to a parallel dataset (doi:10.1594/PANGAEA.948511), it was identified that our samples needed to be corrected due to evaporative fractionation during sample storage. The corrected values are included in this published dataset and we recommend that future studies using this dataset use these corrected values only. The correction was done by calculating the mean of this dataset to the mean of the parallel dataset and correcting for the difference. As a result, the δ18O was corrected by -6.4‰, the δ2H was corrected by -36.4‰. The new values are located in the columns 'δ18O H2O [‰ SMOW] (Corrected)' and 'δD H2O [‰ SMOW] (Corrected)'. We would like to emphasise that calculations of d-excess values for this dataset need interpreting carefully

Hydroclimatic controls on the isotopic (δ¹⁸ O, δ² H, d-excess) traits of pan-Arctic summer rainfall events

Abstract Arctic sea-ice loss is emblematic of an amplified Arctic water cycle and has critical feedback implications for global climate. Stable isotopes (δ¹⁸O, δ²H, d-excess) are valuable tracers for constraining water cycle and climate processes through space and time. Yet, the paucity of well-resolved Arctic isotope data preclude an empirically derived understanding of the hydrologic changes occurring today, in the deep (geologic) past, and in the future. To address this knowledge gap, the Pan-Arctic Precipitation Isotope Network (PAPIN) was established in 2018 to coordinate precipitation sampling at 19 stations across key tundra, subarctic, maritime, and continental climate zones. Here, we present a first assessment of rainfall samples collected in summer 2018 (n = 281) and combine new isotope and meteorological data with sea ice observations, reanalysis data, and model simulations. Data collectively establish a summer Arctic Meteoric Water Line where δ²H = 7.6⋅δ¹⁸O–1.8 (r² = 0.96, p < 0.01). Mean amount-weighted δ¹⁸O, δ²H, and d-excess values were −12.3, −93.5, and 4.9‰, respectively, with the lowest summer mean δ¹⁸O value observed in northwest Greenland (−19.9‰) and the highest in Iceland (−7.3‰). Southern Alaska recorded the lowest mean d-excess (−8.2%) and northern Russia the highest (9.9‰). We identify a range of δ¹⁸O-temperature coefficients from 0.31‰/°C (Alaska) to 0.93‰/°C (Russia). The steepest regression slopes (>0.75‰/°C) were observed at continental sites, while statistically significant temperature relations were generally absent at coastal stations. Model outputs indicate that 68% of the summer precipitating air masses were transported into the Arctic from mid-latitudes and were characterized by relatively high δ¹⁸O values. Yet 32% of precipitation events, characterized by lower δ¹⁸O and high d-excess values, derived from northerly air masses transported from the Arctic Ocean and/or its marginal seas, highlighting key emergent oceanic moisture sources as sea ice cover declines. Resolving these processes across broader spatial-temporal scales is an ongoing research priority, and will be key to quantifying the past, present, and future feedbacks of an amplified Arctic water cycle on the global climate system