Reconstructing Arctic Precipitation Seasonality Using Aquatic Leaf Wax δ²H in Lakes With Contrasting Residence Times

Abstract

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.

Northward Shifts in the Polar Front Preceded Bølling and Holocene Warming in Southwestern Scandinavia

“…Terrestrial (C 28 ) and aquatic (C 22 ) n ‐alkanoic acid δ 2 H reflect the δ 2 H value of source water (Thomas et al, 2020), with a biosynthetic offset that is generally constant for a given plant community (Daniels et al, 2017; Sachse et al, 2012). Changes in the relative abundance of plant growth forms (i.e., grasses vs. shrubs) within a catchment can cause changes to δ 2 H wax due to differences in biosynthetic fractionation; however, the timing of δ 2 H wax changes at Kringlemyr is asynchronous with plant community changes as recorded by pollen in nearby Liastemmen (Paus, 1989), and we do not attempt a pollen‐based correction of our δ 2 H wax data (Text S3) (Feakins, 2013; Inglis et al, 2020; McFarlin et al, 2019).…”

Using a Paired Chironomid δ18O and Aquatic Leaf Wax δ2H Approach to Reconstruct Seasonality on Western Greenland During the Holocene

“…Aquatic plants therefore incorporate lake water hydrogen into leaf waxes mainly during the growing season (Figures 1a and 1f). Leaf wax proxy systematics at Lake N3 are discussed in detail elsewhere (Thomas et al, 2016(Thomas et al, , 2020, but in summary, sedimentary n-alkanoic acid distributions are similar to those in modern aquatic mosses, and δ 2 H of the C 24 n-alkanoic acid has very different Holocene values and trends than the C 26 and C 28 n-alkanoic acid. We therefore interpret the C 24 n-alkanoic acid to be derived mainly from aquatic mosses in the Lake N3 record (Thomas et al, 2016(Thomas et al, , 2020.…”

“…Leaf wax proxy systematics at Lake N3 are discussed in detail elsewhere (Thomas et al, 2016(Thomas et al, , 2020, but in summary, sedimentary n-alkanoic acid distributions are similar to those in modern aquatic mosses, and δ 2 H of the C 24 n-alkanoic acid has very different Holocene values and trends than the C 26 and C 28 n-alkanoic acid. We therefore interpret the C 24 n-alkanoic acid to be derived mainly from aquatic mosses in the Lake N3 record (Thomas et al, 2016(Thomas et al, , 2020. At Lake N3, aquatic plant leaf waxes incorporate lake water that contains 2 H-depleted winter snowmelt in addition to summer precipitation (Figures 1a and 1f), so we interpret C 24 δ 2 H as winter-biased mean annual precipitation δ 2 H.…”

“…and seasonality is essential to understanding future changes in the hydrological cycle (Corcoran et al, 2020;Thomas et al, 2020).…”

“…For example, at our study site on western Greenland, Lake N3, based on modeled precipitation isotopes estimated from the online isotopes in precipitation calculator (OIPC) v. 3.1, winter (October to February) precipitation is about 45‰ 2 H-and 7‰ 18 O-depleted compared to summer (June to September) precipitation (Figures 1b and 1c) (Bowen, 2020;Bowen & Revenaugh, 2003;IAEA/WMO, 2020). Where this strong seasonal precipitation isotope signal is present and preserved in proxies found in lake sediment archives, it may be used to reconstruct changes in precipitation seasonality (Nichols et al, 2009;Thomas et al, 2020). Lake water isotopic composition reflects the isotopic composition of the precipitation that falls on the surrounding catchment area and into the lake.…”