Late Quaternary paleomagnetism and environmental magnetism at Cascade and Shainin lakes, north-central Brooks Range, Alaska

Abstract

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.