Drivers of Stable Water Isotope Anomalies in a Temperate Alaskan Ice Core on the Juneau Icefield

Siegel, Paloma

Undergraduate Honors Thesis

Drivers of Stable Water Isotope Anomalies in a Temperate Alaskan Ice Core on the Juneau Icefield Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/undergraduate_honors_theses/s1784n13c

Abstract

Ratios of stable oxygen and hydrogen isotopes in water are a well-established proxy for regional temperature and can be measured in snow and ice to reconstruct current and past climate trends. This study focuses the on Matthes-Llewelyn Ice Core, a 294m deep ice core drilled in 2019 on the Juneau Icefield. Specifically, we examine the potential drivers of measured water isotope anomalies in the upper 24.5m of the ice that coincide with the presence of liquid water in a ~5cm thick firn aquifer. We pursue two hypotheses to examine the driver(s) of these anomalies. First, we consider the influence of shifts in evaporation source temperature through storm trajectory pathway modeling and sea surface temperature (SST) analysis. Next, we consider the impact of post-depositional processes on the glacial water isotope composition because they may alter measured signals and influence how ice core water isotope records are interpreted in melt-impacted glaciers. Analysis uses the Simple Water Isotope Model (SWIM), the Hybrid Single-Particle Lagrangian Integrated Trajectory (HySPLIT) model, and National Center for Environmental Prediction (NCEP) reanalysis SST data. SWIM temperature reconstructions for the ML Ice Core water isotope record indicate that the evaporation source temperature in the upper 24.5m of the ice core decreased by approximately 5˚C. We investigate the SWIM outputs with HySPLIT modeled daily air parcel back-trajectories for one year prior to and directly after the estimated age of the ice at the firn-ice transition. We then compared the HySPLIT outputs to SST reanalysis plots to determine if storm paths shifted to a cooler location, or if specific regions of the North Pacific cooled surficially. Last, we analyzed snow and ice samples from the 2021 JIRP field season to examine potential post-depositional processes pertaining to meltwater and melt/re-freeze cycles. Combined HySPLIT modeling and SST analysis do not support the hypothesis that evaporative source temperature shifts caused the water isotope anomalies at and above the firn-ice transition, thus suggesting that post-depositional processes occurring englacially possibly led to the observed data spikes. Preliminary 2021 snow and ice lens analysis indicates that mean deuterium excess values are significantly higher in refrozen melt layers, perhaps alluding to post-depositional alteration. We cannot confirm that post-depositional processes are occurring, but we are confident that atmospheric/climatological shifts are not driving the observed anomalies.

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