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 is the fastest-warming region on Earth, with mean annual temperatures expected to increase 10°C by the end of the century under the most extreme scenarios (IPCC, 2021). Arctic precipitation is also expected to increase as local evaporation from newly ice-free seas and poleward moisture transport increase (Bintanja & Andry, 2017) (Singh et al., 2017). This rapid climate change is exacerbated by positive feedbacks primarily linked to decreasing albedo (Holland & Bitz, 2003). Records of past Arctic climate change, including water cycle change, can provide crucial long-term perspectives to understand how this region will respond to future climate change.

Paleo water isotope records can elucidate how the Arctic water cycle responded to past climate changes. We analyze the hydrogen isotope composition (δ2H) of plant‐derived n‐alkanoic acids (waxes) from Lake Qaupat, Baffin Island, Nunavut, Canada, to assess moisture sources and seasonality during the past 5.8 ka. We compare this record to a sedimentary ancient DNA (sedaDNA)‐inferred vascular plant record from the same lake, aiming to overcome the uncertainty of plant community impacts on leaf waxes. As the sedaDNA record reveals a stable plant community after the colonization of Betula sp. at 6.1 ka, we interpret plant wax δ2H values to reflect climate, specifically mean annual precipitation δ2H. However, the distributions of n‐alkanoic acid homologs suggest that aquatic mosses, which are not represented in the sedaDNA record, may become more abundant towards the present. Therefore, we cannot exclude the possibility that changes in the plant community cause changes in the plant wax δ2H record, particularly long‐chain waxes, which become less abundant through this record. We find that Lake Qaupat mid‐chain plant wax δ2H is enriched coincident with high Labrador Sea summer surface temperature, which suggests that local moisture sources in summer and early autumn have the greatest impact on precipitation isotopes in this region.

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