LITHOFACIES-BASED INTERPRETATIONS OF ENVIRONMENTAL MAGNETIC DATA OF LATE QUATERNARY GULF OF ALASKA SEDIMENTS

Marine strata are widely used to interpret environmental change with respect to paleoclimatic variability and fluctuating depositional conditions. Deep sea and anoxic basin sediments are commonly used, whereas expanded sedimentary sequences preserved on continental margins have been less commonly studied. On the Gulf of Alaska (GoA) margin a high-resolution record of past glacial activity is preserved. Jumbo piston core EW0408-85JC, spanning >19ky, was collected from the upper slope off the Bering Glacier and is comprised of predominantly glacimarine and paraglacial lithogenic sediments. Age control is achieved through radiocarbon dating, comparison of paleomagnetic directional data with new Holocene global field model predictions for the site, and ²¹⁰Pb profiles. Environmental magnetic data illuminates regional glacial and interglacial conditions representing the Last Glacial Maximum (LGM), deglaciation, onset of neoglaciation, and the Little Ice Age (LIA) which are corroborated by lithologic facies transitions observed in x-radiographs. kARM/k (anhysteretic remanent magnetization/magnetic susceptibility), a proxy for magnetic grain size, ranges from 0.5-2.5 (mean 1.25, standard deviation 0.58) in this core. GoA multicores from a variety of depositional environments and lithologies span kARM/k values of ~0.1 (very coarse silt/sand) to 14 (very fine muds/clays). LGM strata are pebbly poorly sorted diamicts, yet kARM/k exhibits no distinct coarsening but does contain high-frequency, low-amplitude changes compared to overlying Holocene facies. Deglacial sediments are homogenous bioturbated muds characterized by changes in magnetic mineralogy (20% shift in s-ratios indicate changes in proportion of low and high coercivity minerals) and magnetic grain size (kARM/k shows an abrupt fining [by ~0.75] and subsequent gradual coarsening into the Neoglacial), which we speculate corresponds to reworking and increased weathering of a newly deglaciated shelf. Neoglacial strata contain finely laminated sequences and coarsest kARM/k values (0.5) which fine over time (reaching 2.5) except for a coarsening during the LIA (~1.5). The use of x-radiographs to view sedimentary structures and bedding type provides a lithologic foundation for interpreting variability in magnetic data.