Sediment cores from Cascade Lake (68.38°N, 154.60°W) and Shainin Lake (68.34°N, 151.05°W), Arctic Alaska were selected for paleomagnetic analysis to assess 210Pb-14C age control using paleomagnetic secular variation (PSV) and relative paleointensity (RPI) features, and to quantify environmental magnetic variability during the Holocene and late Pleistocene. U-channels were studied through alternating field (AF) demagnetization of the natural remanent magnetization (NRM), and laboratory-induced magnetizations including anhysteretic remanent magnetization (ARM) acquisition, ARM demagnetization, isothermal remanent magnetization (IRM), and hysteresis experiments to determine magnetic mineralogy and grain-size variability.
Cascade Lake sediment yields a strong, well-defined characteristic remanent magnetization with average maximum angular deviation values of < 2° and average inclinations within 4° of the expected geocentric axial dipole. Correlation of inclination changes with geomagnetic field models, as well as the Burial Lake record ~ 200 km to the west, indicates a variable offset between the Cascade Lake radiometric chronology and the preferred PSV-derived age model (PSV-1), reaching a maximum offset of 1.5–2.8 kyr during the mid-Holocene. This offset likely results from either a hard-water effect or the incorporation of watershed-stored terrestrial carbon into 14C samples. The PSV-1 age model extends the Cascade Lake age model to ~ 21 ka. Cascade Lake sediment may be suitable for RPI estimation using the IRM as a normalizer, however three methods of normalization (magnetic susceptibility (kLF), ARM, and IRM) produce similar normalized remanence results.
Hysteresis experiments and S-ratios for Cascade Lake glacial till and Shainin Lake sediment supports the hypothesis that local bedrock hosts predominantly high-coercivity magnetic material. However, S-ratios from Cascade Lake (~ 21 ka to present) and Shainin Lake (~ 12.6 ka to present) do not appear consistent with Burial Lake S-ratios, and most S-ratio variability is therefore interpreted as a result of site-specific sedimentation processes and background magnetic assemblages. A Younger-Dryas-aged peak in Shainin Lake S-ratios may be revealed by the increased sensitivity of the S-ratio parameter to magnetite at high-coercivity background levels. Cascade Lake S-ratios increase from 10.3 ka to present, potentially indicating Holocene biogenic magnetite production, down-core magnetic dissolution, or eolian input from a fine-grained, low-coercivity magnetic source that is clearly distinct from eolian magnetite at Burial Lake. Anhysteretic susceptibility (kARM)/kLF may be a better indicator of this fine-grained magnetite population observed in the north-central Brooks Range, however the origin of this magnetic component remains unclear. This research highlights the potential advantages of supplementing 14C dating with additional dating methods, and will benefit from ongoing efforts to improve age control (e.g., cryptotephra exploration) and additional magnetic experiments to constrain the source of fine-grained magnetite.
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|Advisor:||Kaufman, Darrell S.|
|Commitee:||Briner, Jason P., McKay, Nicholas P., Stoner, Joseph S.|
|School:||Northern Arizona University|
|Department:||SESES - Geology|
|School Location:||United States -- Arizona|
|Source:||MAI 55/05M(E), Masters Abstracts International|
|Subjects:||Geology, Paleoclimate Science, Environmental science|
|Keywords:||Environmental magnetism, Geochronology, Late Quaternary, Paleomagnetic secular variation, Paleomagnetism, S-ratio|
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