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1. Using a Paired Chironomid δ 18 O and Aquatic Leaf Wax δ 2 H Approach to Reconstruct Seasonality on Western Greenland During the Holocene

   https://doi.org/10.1029/2020PA004169  

   Corcoran, Megan C. ; Thomas, Elizabeth K. ; Morrill, Carrie ( April 2021 , Paleoceanography and Paleoclimatology)

   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.

    

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2. Tracking Southern Ocean Sea Ice Extent With Winter Water: A New Method Based on the Oxygen Isotopic Signature of Foraminifera

   https://doi.org/10.1029/2020PA004095  

   Lund, David C. ; Chase, Zanna ; Kohfeld, Karen E. ; Wilson, Earle A. ( May 2021 , Paleoceanography and Paleoclimatology)

   Southern Ocean sea ice plays a central role in the oceanic meridional overturning circulation, transforming globally prevalent watermasses through surface buoyancy loss and gain. Buoyancy loss due to surface cooling and sea ice growth promotes the formation of bottom water that flows into the Atlantic, Indian, and Pacific basins, while buoyancy gain due to sea ice melt helps transform the returning deep flow into intermediate and mode waters. Because northward expansion of Southern Ocean sea ice during the Last Glacial Maximum (LGM; 19–23 kyr BP) may have enhanced deep ocean stratification and contributed to lower atmospheric CO₂levels, reconstructions of sea ice extent are critical to understanding the LGM climate state. Here, we present a new sea ice proxy based on the¹⁸O/¹⁶O ratio of foraminifera (δ¹⁸O_c). In the seasonal sea ice zone, sea ice formation during austral winter creates a cold surface mixed layer that persists in the sub‐surface during spring and summer. The cold sub‐surface layer, known as winter water, sits above relatively warm deep water, creating an inverted temperature profile. The unique surface‐to‐deep temperature contrast is reflected in estimates of equilibrium δ¹⁸O_c, implying that paired analysis of planktonic and benthic foraminifera can be used to infer sea ice extent. To demonstrate the feasibility of the δ¹⁸O_cmethod, we present a compilation ofN. pachydermaandCibicidoidesspp. results from the Atlantic sector that yields an estimate of winter sea ice extent consistent with modern observations.

    

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3. Ice Core δ 18 O Record Linked to Western Arctic Sea Ice Variability

   https://doi.org/10.1029/2019JD031023  

   Porter, Stacy E. ; Mosley‐Thompson, Ellen ; Thompson, Lonnie G. ( October 2019 , Journal of Geophysical Research: Atmospheres)

   Stable oxygen isotopes (δ¹⁸O) in the Bona‐Churchill (B‐C) ice core from southeast Alaska provide a valuable, high‐resolution history of climate variability and sea ice cover in the western Arctic over the last 800 years. Multiple ice cores have been collected from the Wrangell‐St. Elias Mountain Range; however, their δ¹⁸O records exhibit little consistency as each core offers a unique view on local, regional, and/or global climate variability. To explore the primary mechanisms influencing the isotopic signature at the B‐C site, we utilize isotope‐enabled model data, reanalysis data, and observations, which all indicate a strong connection between isotopes at the B‐C site and western Arctic climate, likely established by the location of the storm track in this region. Enriched B‐C δ¹⁸O reflects increased southerly flow and warmer waters in the Bering Sea, which modulates the heat flux through the Bering Strait and into the Arctic, thereby affecting sea ice cover in the western Arctic. The B‐C δ¹⁸O paleorecord shares some remarkable similarities (r = −0.80,p < .001) with the duration of western arctic sea ice cover reconstructed from a Chukchi Sea sediment core. Interestingly, during the Little Ice Age, enriched δ¹⁸O and reduced western Arctic sea ice are observed and may be indicative of prolonged periods of the warm Arctic/cold continents pattern and a northwestward shift of the North Pacific storm track.

    

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4. Modern Eastern Canadian Arctic Lake Water Isotopes Exhibit Latitudinal Patterns in Inflow Seasonality and Minimal Evaporative Enrichment

   https://doi.org/10.1029/2021PA004384  

   Gorbey, D. B. ; Thomas, E. K. ; Sauer, P. E. ; Raynolds, M. K. ; Miller, G. H. ; Corcoran, M. C. ; Cowling, O. C. ; Crump, S. E. ; Lovell, K. ; Raberg, J. H. ( May 2022 , Paleoceanography and Paleoclimatology)

   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.

    

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5. Synoptic and Mesoscale Mechanisms Drive Winter Precipitation δ 18 O/δ 2 H in South‐Central Alaska

   https://doi.org/10.1029/2018JD030050  

   Bailey, Hannah L. ; Klein, Eric S. ; Welker, Jeffrey M. ( April 2019 , Journal of Geophysical Research: Atmospheres)

   Measurements of oxygen and hydrogen stable isotopes in precipitation (δ¹⁸O_Pand δ²H_P) provide a valuable tool for understanding modern hydrological processes and the empirical foundation for interpreting paleoisotope archives. However, long‐term data sets of modern δ¹⁸O_Pand δ²H_Pin southern Alaska are entirely absent, thus limiting our insight and application of regionally defined climate‐isotope relationships in this proxy‐rich region. We present and utilize a 13‐year‐long record of event‐based δ¹⁸O_Pand δ²H_Pdata from Anchorage, Alaska (2005–2018,n = 332), to determine the mechanisms controlling precipitation isotopes. Local surface air temperature explains ~30% of variability in the δ¹⁸O_Pdata with a temperature‐δ¹⁸O slope of 0.31 ‰/°C, indicating that δ¹⁸O_Parchives may not be suitable paleo‐thermometers in this region. Instead, back‐trajectory modeling reveals how winter δ¹⁸O_P/δ²H_Preflects synoptic and mesoscale processes in atmospheric circulation that drive changes in the passage of air masses with different moisture sources, transport, and rainout histories. Specifically, meridional systems—with either northerly flow from the Arctic or southerly flow from the Gulf of Alaska—have relatively low δ¹⁸O_P/δ²H_Pdue to progressive cooling and removal of precipitation as it condenses with altitude over Alaska's southern mountain ranges. To the contrary, zonally derived moisture from either the North Pacific and/or Bering Sea retains relatively high δ¹⁸O_P/δ²H_Pvalues. These new data contribute a better understanding of the modern Alaska water isotope cycle and provide an empirical basis for interpreting paleoisotope archives in context of regional atmospheric circulation.

    

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