Arctic precipitation is predicted to increase this century. Records of past precipitation seasonality 17 provide baselines for a mechanistic understanding of the dynamics controlling Arctic precipitation. We present an approach to reconstruct Arctic precipitation seasonality using stable hydrogen isotopes (δ 2 H) of aquatic plant waxes in neighboring lakes with contrasting water residence times, and present a case study of this approach in two lakes on western Greenland. Residence time calculations suggest 21 that growing season lake water δ 2 H in one lake reflects summer precipitation δ 2 H, while the other reflects amount-weighted annual precipitation δ 2 H and evaporative enrichment. Aquatic plant wax δ 2 H in the "summer lake" is relatively constant throughout the Holocene, perhaps reflecting competing 24 effects of local summer warmth and increased distal moisture transport due to a strengthened latitudinal temperature gradient. In contrast, aquatic plant wax δ 2 H in the "mean annual lake" is 100‰ 2 H-depleted from 6 to 4 ka relative to the beginning and end of the record. Because there are relatively 27 minor changes in summer precipitation δ 2 H, we interpret the 100‰ 2 H-depletion in mean annual precipitation to reflect an increase in winter precipitation amount, likely accompanied by changes in winter precipitation δ 2 H and decreased evaporative enrichment. Thus, unlike the "summer lake," the "mean-annual lake" records changes in winter precipitation. This dual-lake approach may be applied to 31 reconstruct past changes in precipitation seasonality at sites with strong precipitation isotope seasonality and minimal lake water evaporative enrichment.

The Arctic hydrological cycle is predicted to intensify as climate warms due to increased poleward moisture transport and greater evaporation in areas previously covered by sea ice (Bintanja & Selten, 2014;Kopec et al., 2016). These mechanisms have different seasonal expressions: reduction of sea ice during fall and winter months causes an increase in local evaporation and thus more fall and winter precipitation, whereas strengthening of the meridional moisture gradient due to increasing temperature causes an increase mainly in summer precipitation (Bintanja & Selten, 2014). Changes in Arctic precipitation seasonality may influence feedbacks between hydroclimate, atmospheric and ocean circulation, and plant communities, and are important for projections of sea level rise and ice sheet dynamics (

We report estimated stable isotope compositions of depositional waters and paleoprecipitation from the Cretaceous Arctic to further elucidate the role of the global hydrologic cycle in sustaining polar warmth during that period. Estimates are based on new hydrogen isotopic analyses of n-alkane biomarkers extracted from Late Cretaceous and mid-Cretaceous terrestrial deposits in northern Alaska and the Canadian High Arctic. We integrate these new results with earlier published work on oxygen isotopic analyses of pedogenic siderites, dinosaurian tooth enamel phosphates, and pedogenic clay minerals from the same field areas. Average Late Cretaceous δD values of −143‰ VSMOW corresponded with average δ18O values of −24.1‰ VSMOW, and average mid-Cretaceous δD values of −106‰ VSMOW corresponded with average δ18O values of −22.1‰ VSMOW. The distributions of water isotope δD and δ18O values from Cretaceous Arctic deposits do not intersect with the Global Meteoric Water Line, suggesting an apparent deuterium excess ranging from about 40 to 60 per mil. We considered several possible explanations for these Cretaceous results including (1) mass-balance changes in zonal patterns of evaporation and precipitation at lower latitudes, (2) concentration of 2H in leaf tissue waters from continuous transpiration by coniferous paleofloras during the Arctic growing season, and (3) concentration of 2H in the groundwaters of methane-emitting Arctic soils.

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