The Arctic hydrological cycle is predicted to intensify as the Arctic warms, due to increased poleward moisture transport during summer and increased evaporation from seas once ice‐covered during winter. Records of past Arctic precipitation seasonality are important because they provide a context for these ongoing changes. In some Arctic lakes, stable isotopes of oxygen and hydrogen (δ18O and δ2H, respectively) vary seasonally, due to seasonal changes in precipitation δ18O and δ2H. We reconstruct precipitation seasonality from Lake N3, a well‐dated lake sediment archive in Disko Bugt, western Greenland, by generating Holocene records of two proxies that are produced at different times of the year, and therefore record different lake water seasonal isotopic compositions. Aquatic plants synthesize waxes throughout the summer, and their δ2H reflects winter‐biased precipitation δ2H at Lake N3, whereas chironomids synthesize their head capsules between late summer and winter, and their δ18O reflects summer‐biased precipitation δ18O at Lake N3. During the middle Holocene at Lake N3, aquatic plant leaf wax was strongly2H‐depleted, while chironomid chitin was18O‐enriched. We guide interpretations of these records using sensitivity tests of a lake water and energy balance model, where we change precipitation amount and isotope seasonality inputs. The sensitivity tests suggest that the contrasting trends between proxies were likely caused by an increase in precipitation amount during all seasons and an increase in precipitation isotope seasonality, in addition to proxy‐specific mechanisms, highlighting the importance of understanding lake‐ and proxy‐specific systematics when interpreting records from sediment archives.
Arctic precipitation is predicted to increase this century. Records of past precipitation seasonality 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 (δ2H) 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 that growing season lake water δ2H in one lake reflects summer precipitation δ2H, while the other reflects amount‐weighted annual precipitation δ2H and evaporative enrichment. Aquatic plant wax δ2H in the “summer lake” is relatively constant throughout the Holocene, perhaps reflecting competing effects of local summer warmth and increased distal moisture transport due to a strengthened latitudinal temperature gradient. In contrast, aquatic plant wax δ2H in the “mean annual lake” is 100‰2H depleted from 6 to 4 ka relative to the beginning and end of the record. Because there are relatively minor changes in summer precipitation δ2H, we interpret the 100‰2H depletion in mean annual precipitation to reflect an increase in winter precipitation amount, likely accompanied by changes in winter precipitation δ2H 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 reconstruct past changes in precipitation seasonality at sites with strong precipitation isotope seasonality and minimal lake water evaporative enrichment.
more » « less- NSF-PAR ID:
- 10456702
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 35
- Issue:
- 7
- ISSN:
- 2572-4517
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
more » « less
Abstract Lacustrine δ2H and δ18O isotope proxies are powerful tools for reconstructing past climate and precipitation changes in the Arctic. However, robust paleoclimate record interpretations depend on site‐specific lake water isotope systematics, which are poorly described in the eastern Canadian Arctic due to insufficient modern lake water isotope data. We use modern lake water isotopes (δ18O and δ2H) collected between 1994–1997 and 2017–2021 from a transect of sites spanning a Québec‐to‐Ellesmere Island gradient to evaluate the effects of inflow seasonality and evaporative enrichment on the δ2H and δ18O composition of lake water. Four lakes near Iqaluit, Nunavut sampled biweekly through three ice‐free seasons reflect mean annual precipitation isotopes with slight evaporative enrichment. In a 23° latitudinal transect of 181 lakes, through‐flowing lake water δ2H and δ18O fall along local meteoric water lines. Despite variability within each region, we observe a latitudinal pattern: southern lakes reflect mean annual precipitation isotopes, whereas northern lakes reflect summer‐biased precipitation isotopes. This pattern suggests that northern lakes are more fully flushed with summer precipitation, and we hypothesize that this occurs because the ratio of runoff to precipitation increases with latitude as vegetation cover decreases. Therefore, proxy records from through‐flowing lakes in this region should reflect precipitation isotopes with minimal influence of evaporation, but vegetation changes in lake catchments across a latitudinal transect and through geologic time may influence the seasonality of lake water isotopic compositions. Thus, we recommend that future lake water isotope proxy records are considered in context with temperature and ecological proxy records.
-
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.more » « less
-
more » « less
Abstract In reconstructing past changes in precipitation or evaporation from the geologic record, paleoclimatologists sometimes employ the dual‐biomarker isotope method. This method requires that two co‐occurring sedimentary lipids are derived from different biological sources, and that their compound‐specific2H/1H ratios record different aspects of the hydrologic cycle. Several studies have used this approach, typically by comparing the2H/1H ratios of
n ‐alkyl lipids (δ2Hwax) thought to be sourced from aquatic versus terrestrial plants that gain their hydrogen atoms from lake water and soil water, respectively. Yet confidently fingerprintingn ‐alkyl lipid sources continues to be a challenge because the wax inputs of different plant types vary across biomes, lake types, and time. New research in theJournal of Geophysical Research‐Biogeosciences by Hollister et al. (2022,https://doi.org/10.1029/2022jg006903 ) utilizes a combination of three independent metrics to demonstrate that for Arctic lakes, mid‐chainn ‐alkanoic acids can be sourced principally from aquatic plants whereas long‐chainn ‐alkanoic acids derive from a mixture of aquatic and terrestrial plants. By cataloging wax compound distributions and compound‐specific H and C isotopes of many new plant species, their efforts will strengthen future biomarker paleoclimatology and reinforce previous applications of the dual‐biomarker approach in high‐latitude lakes. The identification of a lake system with a strong aquatic plant wax signal in the sediments should motivate future targeting of similar lakes for reconstructing past moisture with the dual‐biomarker method. -
more » « less
Abstract The water balance of the Arctic tundra is shifting as permafrost stability, seasonality, and the ratio of precipitation to evaporation respond to amplified Arctic warming. While in some northern tundra locations there has been a notable increase in the number of water bodies, generally, the tundra landscape has experienced a decline in the number and area of lakes. We analyzed changes in small lake count (<10,000 m2), large lake count (>10,000 m2), and lake surface area across the periglacial tundra of western Greenland, using historical satellite and aerial imagery and weather data from the late 1960s to present. Overall, we found a decrease in lake count (21%) and surface area (2%) across our study region. Specifically, smaller ponds were particularly prone to change, with decreases of 28% in count and 15% in surface area. Shrinking lakes often became revegetated by both emergent aquatic and terrestrial vegetation, which captures potential successional trajectories following Arctic lake drying. Additionally, while annual precipitation may be increasing, it occurred primarily during the winter months in the form of snow, which may or may not contribute to the overall growing season water budget. Conversely, the peak growing season months of June, July, and August all have experienced significant increases in potential evaporation rates, thus likely creating a water deficit for much of the growing season. These results suggest that a large section of deglaciated Greenland appears likely to become drier in the summer months, which may result in widespread ecological consequences.
