<p> Hydrogen isotope ratios (δD) in various types of leaf waxes, including n-alkanoic acids, extracted from lacustrine sediments are becoming increasingly popular for understanding past climate changes. Leaf-wax δD values track precipitation δD, and provided that controls on precipitation δD are known, changes in leaf wax δD can be used to reconstruct these changes in the past. Seventy-six sediment samples from Andrew Lake, Adak Island, Alaska, extending to 4800 years ago (4.8 ka) were analyzed for δD of <i> n</i>-alkanoic acids. δD values of isolated C<sub>28</sub> <i> n</i>-alkanoic acids show a strong inverse correlation with October-May storminess (days with >19 mm of precipitation) over the meteorological record (r<sup>2</sup> = 0.58, p < 0.02), and a similarly strong correlation with total precipitation amount. This implies that isotopes in precipitation on Adak Island are strongly influenced by the amount effect. Shifts in precipitation amount and storminess are associated with shifts in the North Pacific hydroclimate, which is driven by the Aleutian Low during fall and winter. Low δD values indicate high precipitation amount/storminess on Adak Island, which correlates well to a weak or westward Aleutian Low as inferred from other sites, while high δD values indicate lower precipitation amount and generally correlate with a stronger or more eastward Aleutian Low. Results from Adak Island were combined with evidence from previously published paleoclimate studies from southern Alaska and the Yukon to reveal a more complete spatial picture of hydroclimatological changes for the last 5000 years. Variability in δD since 3.5 ka is related to changes in North Pacific atmospheric circulation patterns including the Aleutian Low. High precipitation amount/storminess during the Medieval Climate Anomaly (∼850 to 1050 AD) on Adak Island is consistent with evidence from other studies that suggest a weakened Aleutian Low. Evidence of decrease precipitation during the early Little Ice Age (∼1200 to 1500 AD) gave way to wetter conditions during the latter half (∼1500 to 1900 AD). A wet late LIA is consistent with the results from coastal studies in southern Alaska, suggesting another period of a weakened Aleutian Low. This study is part of a multi-proxy investigation involving two lakes on Adak Island including analyses of pollen, biogenic silica, chironomids, isotopes in diatoms, and other proxies to help increase our knowledge about past hydroclimate in the North Pacific.</p>
| Identifer | oai:union.ndltd.org:PROQUEST/oai:pqdtoai.proquest.com:1537814 |
| Date | 19 June 2013 |
| Creators | Vaillencourt, David A. |
| Publisher | Northern Arizona University |
| Source Sets | ProQuest.com |
| Language | English |
| Detected Language | English |
| Type | thesis |
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