HR: 1340h
AN: GP53B-1224    [Abstracts]
TI: Environmental magnetic and physical grain size data of surface sediments across a variety of depositional environments in the Gulf of Alaska region
AU: * Rosen, G P
EM: rosengp@ufl.edu
AF: University of Florida, Department of Geological Sciences, 241 Williamson Hall, PO BOX 112120, Gainesville, FL 32611-2120, United States
AU: Jaeger, J M
EM: jaeger@geology.ufl.edu
AF: University of Florida, Department of Geological Sciences, 241 Williamson Hall, PO BOX 112120, Gainesville, FL 32611-2120, United States
AU: Stoner, J S
EM: jstoner@coas.oregonstate.edu
AF: College of Oceanic & Atmospheric Sciences, Oregon State University, 104 COAS Administration Building, Corvallis, OR 97331-5503, United States
AU: Channell, J E
EM: jetc@geology.ufl.edu
AF: University of Florida, Department of Geological Sciences, 241 Williamson Hall, PO BOX 112120, Gainesville, FL 32611-2120, United States
AB: Modern depositional environments vary significantly across the Gulf of Alaska (GoA) region depending on sediment type, proximity to glacial sediment sources, and relative balance of terrestrial input and biological productivity. Multi-core samples were collected from fjord, continental shelf and deep sea fan sites along the southern GoA margin in 2004 (cruise EW0408). Depositional environments range from ice-proximal glacimarine to biologically productive ice-free bays. As expected, magnetization intensities (NRM, ARM, IRM) and magnetic susceptibility (MS) are a strong function of biogenic dilution as measured by organic carbon (TOC) and biogenic silica content. In organic rich sediments, located in the south of the study area, the magnetic grain-size parameter kARM/k (anhysteretic susceptibility divided by susceptibility) indicates fine-grained magnetic material (kARM/k = ~14) throughout the radioisotopically defined (²¹⁰Pb, ²³⁴Th) biological surface mixed layer (typically 5-20 cm thick). Just below the base of this mixed layer, magnetic grain size coarsening (to kARM/k values of ~2) occurs over several tens of cm. This coarsening can be attributed to dissolution of fine-grained (possibly biogenic) magnetite, accompanied by the formation of authigenic iron sulfides. Sedigraph particle-size analyses show no down-core coarsening corresponding with the kARM/k profile, implying that the kARM/k parameter is sensitive to magnetic grain sizes largely below the ~0.5 μm lower limit of the Sedigraph. Sediments with low biogenic content, from the northern part of the study area, are characterized by low TOC concentrations and stronger magnetization intensities and susceptibilities. Here, in shelf environments, kARM/k values are not reflected in physical grain size data, implying that the two parameters are again sensitive to different grain size (and mineralogical) populations. For near-shore environments influenced by river plumes, down-core kARM/k changes (dominated by changes in k) tend to parallel physical grain size changes in the sortable silt population. The apparent magnetic coarsening driven by the higher susceptibility is most likely associated with an observed strengthening in river discharge velocity, which would increase the total amount of sortable silt and magnetite transported within the plume in the coastal ocean.
DE: 1512 Environmental magnetism
DE: 1861 Sedimentation (4863)
DE: 3022 Marine sediments: processes and transport
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting