Holocene hydroclimate in southern and arctic Alaska inferred from diatom oxygen isotopes and data – model comparisons

• Main Author: Broadman, Ellie
• Format: Thesis
• Language: English
• Published: 2021
• Subjects: QC Physics; Arctic; Arctic Ocean; Bering Sea; Gulf of Alaska; Pacific; aleutian low; glaciers; Sea ice; Alaska
• Online Access: https://openknowledge.nau.edu/id/eprint/5733/; https://openknowledge.nau.edu/id/eprint/5733/1/Broadman_2021_holocene_hydroclimate_southern_arctic_alaska_inferred_fr.pdf; https://openknowledge.nau.edu/id/eprint/5733/2/Broadman_2021_holocene_hydroclimate_Appendix_D.xlsx

Rapid anthropogenic warming in Alaska is causing drastic changes to the region’s hydroclimate, as evidenced in part by melting glaciers, diminishing sea ice, and changing precipitation patterns. These rapid changes are superimposed upon the naturally occurring, long-term variability characteristic of ocean and atmospheric systems in the North Pacific and Arctic Ocean basins. One particularly important source of natural climate variability in this region is the Aleutian Low, an atmospheric pressure cell that modulates the paths of winter storms in the Gulf of Alaska and Bering Sea. The influence of the Aleutian Low on terrestrial hydroclimate is well documented on intra- and interannual timescales over the instrumental period in southern Alaska, and to some degree in Arctic Alaska. However, Aleutian Low variability on centennial to millennial timescales remains uncertain over the course of the current interglacial period (the Holocene); the findings of existing reconstructions contradict one another, and few studies address the pressure cell’s influence on past hydroclimate in northern Alaska. Furthermore, there is evidence that suggests the presence of modern teleconnections between variability in the extent and duration of Arctic sea ice and lower latitude circulation, including the Aleutian Low. However, no studies have interrogated such a teleconnection through the Holocene. In addition to these dynamical uncertainties, efforts to reconstruct past hydroclimate are complicated by the challenges of deciphering a climate signal from proxy systems, particularly those pertaining to oxygen isotopes, which are influenced by numerous climate and environmental variables.To improve our understanding of Holocene hydroclimate variability in sub-Arctic and Arctic Alaska, I have used a variety of approaches focused on lacustrine diatom oxygen isotope datasets and model experiments. These approaches include: 1) multi-proxy paleoclimate analyses conducted on lake sediments collected at individual sites; 2) comparing proxy datasets at neighboring sites; 3) compiling and synthesizing paleoclimate datasets across Alaska and neighboring lands and oceans; 4) comparing modern observations and paleo data with output from a coupled ocean-atmosphere general circulation model; and 5) interpreting paleo oxygen isotope data using a simple hydrologic and isotope mass balance model. Results from this dissertation support a growing body of evidence that the Aleutian Low strengthened between the middle and late Holocene (around 5 – 4 ka cal BP), and suggest this change may have occurred concomitant with an increase in the annual duration and extent of sea ice in the western Arctic Ocean. This finding indicates that the observed modern relationship between North Pacific atmospheric circulation and high latitude hydroclimate is a more persistent feature of Holocene hydroclimate than was previously demonstrated. Finally, my results reveal that these synoptic-scale influences on hydroclimate are not always evident at individual sites, because site-specific factors can dampen a regional climate signal. In particular, I demonstrate the importance of groundwater derived from receding glaciers in the hydrologic and isotope mass balance of lakes adjacent to mountain glaciers, which is an often over-looked component of paleoenvironmental reconstructions from lacustrine oxygen isotope datasets. In light of these findings, valuable future research directions for Alaska’s Holocene hydroclimate include: 1) developing methods to identify the influence of synoptic-scale climate influences on lake water isotopes; 2) proxy system modeling, and corresponding monitoring studies; and 3) the development and application of simplistic modeling frameworks.