Magnetostratigraphy Study Since 1.9 Ma Of IODP Expedition 341 Site U1417 In The Gulf Of Alaska

Gulf of Alaska is located in the subarctic northeast Pacific,bounded by the mountains to the east,north and west,and open to the south.It is an crucial area to figure out the climate change by exploring a highly erosive glacial system.Hence IODP Expedition 341 drilled here in 2013 to reveal the tectonic and environmental responses form the margin sequences.Using multiple magnetic and rock magnetic measurements,this study aims to provide a robust magnetostratigraphic framework for Gulf of Alaska.After magnetic hysteresis loop?loop?,isothermal remanent magnetization?IRM?,temperature-dependent curves with magnetic susceptibility?k-T?,S-ratio,anhysteretic remanent magnetization?ARM?and other rock magnetic measurements indicated that the dominant magnetic minerals of Site U1417 were magnetites with lower coercivity.The magnetite is pseudo single domain?PSD?and grain size of magnetic minerals shifted slightly.Most samples obtained stable characteristic remanent magnetization?ChRM?after alternating field demagnetion at 20 mT.Anisotropy of magnetic susceptibility?AMS?was used to evaluate the reliability of samples and then the obvious perturbed samples are removed.With the help of shipboard data,Matuyama-Brunhes boundary?M/B:780 ka?of the site was determined at depth of110.46 m.Meanwhile.Jaramillo subchron?1.07⁰.99 Ma?and an Olduvai subchron?1.95¹.77 Ma?were found at 139.91¹33.28 m,191.94¹82.67 m respspectively.The shipboard results of paleomagnetism in the Preliminary Report of the Exp.341 was modified about the depth of Cobb.Mountain excursion and it was determined at162.73¹58.41m.The basal age of Site U1417 is¹.9 Ma.After establishment of the magnetostratigraphy in Site U1417,an age-depth model was depicted by age control points.Comparing with the shipboard paleomagnetic data of ODP Site 887,it was found that the two sites both have an abrupt change in sedimentation rate at 1200 ka.Combining the seismic reflector and Monte Carlo simulations results,it is suggested that the abrupt change in sedimention rate is a reflection of the Middle-Pleistocene Transition.Using?¹⁸O interpretation in Site U1417 form present to 800 ka,it indicates that low magnetic susceptibility zones correspond with MIS 16,MIS 14,MIS 12,MIS 10 and MIS 6.Combining high Ba/Al and BSi in glacials in 1500⁵00 ka?Müller et al,2018,although samples are very sparse?,the low magnetic susceptibility zones was assumed to be caused by the expansion of the Northern Cordillera Ice Sheet?NCIS?during the glacial period.Fe inputs from glacial debris results in phytoplankton productivity events.Phytoplankton bloom thus dilute the content of magnetite in the sediment and high concentration of organic matter thining magnetic minerals.A further confinement of age model based on the above magnetostratigraphy is surely needed for more concrete discussion.