IODP341航次之阿拉斯加湾U1417站位磁性地层学研究

MAGNETOSTRATIGRAPHY OF IODP EXPEDITION 341 SITE U1417 IN THE GULF OF ALASKA

本文运用系统的岩石磁学与古地磁学研究方法，对取自亚北极阿拉斯加湾的IODP 341航次U1417站位191.94m的综合岩芯进行了磁性地层学研究，揭示了湾内1.9Ma以来高沉积速率的年代地层框架。沉积物样品在交变退磁场下获得了稳定的剩磁方向，并显示出4个正向极性段和3个负向极性段。结合IODP 341航次初步报告中古地磁和古生物的数据，确定了U1417综合岩芯的布容-松山极性转换界线（M/B：0.78Ma）位于钻孔110.46m，Jaramillo极性亚时、Cobb Mountain极性事件和Olduvai极性亚时的记录在岩芯中的深度分别对应133.28~139.91m、158.41~162.73m和182.67~191.94m。由上述年龄控制点推算出钻孔的沉积速率在1.2Ma以来显著上升，这可能与中更新世气候转型（MPT）所导致的源区侵蚀加剧有关。综上所述，该研究不仅为阿拉斯加湾深海沉积物研究提供了有效的年代学约束，也为更新世以来地层的进一步精细划分和对比提供了重要标尺。

Gulf of Alaska （GoA）is an important area to study interaction between global climate change and subsequent tectonic processes. Because of no intervening basins lies between GoA and the source （Mt. St. Elias）, GoA provides unmixing signals to reveal history of source to sink. Integrated Ocean Drilling Program （IODP）Expedition 341 drilled 5 sites in the GoA in 2013, which were designed in a cross-margin transect from the continental shelf to distal Surveyor Fan. In this study, we present a comprehensive magnetic study of the distal site U1417 （56.57°N, 147.60°W） with water depth of 4200m. The spliced core U1417, with a whole length of 191.94m CCSF-B （an additional depth scale）and an overall recovery rate of 70.1%, was constructed from Holes U1417A to U1417D. 323 discrete samples of Site U1417 were undertaken for rock magnetism and paleomagnetism analysis. Rock magnetic studies （including temperature-dependent curves of magnetic susceptibility, hysteresis loops and isothermal remanent magnetization）indicate that the main magnetic mineral in the sediments is magnetite with lower coercivity and no appearance of hematite or greigite. Day-Plot demonstrates that all the analyzed samples are dominated by pseudo-single domain （PSD）magnetic grains. Thus reliable paleomagnetic results are available. Inclinations of characteristic remanent magnetization after alternating-field demagnetization reveals 4 normal inclination zones and 3 negative inclination zones in this site. Using anisotropy of magnetic susceptibility （AMS）, we evaluated the disturbance of all the discrete samples, and then partly modified the shipboard paleomagnetism interpretation of Cobb Mountain excursion. With the help of shipboard data and AMS, we are aware of some perturbed samples shown in Matuyama-Brunhes boundary （M/B：0.78Ma）and then redefine M/B boundary at depth of 110.46m of the core. Meanwhile, the records of the Jaramillo subchron （0.99~1.07Ma）, Cobb Mountain excursion （1.22~1.24Ma）and an incomplete Olduvai subchron （1.77~1.90Ma）are found at the depth intervals of 113.28~139.91m, 158.41~162.76m and 182.67~191.94m, respectively. The basal age of the core are extrapolated to be about 1.9Ma. Similar with ODP Site 887, an age-depth model indicates an abrupt change in the sedimentation rate at the age of 1.2Ma （ca.160m）, the sediment rate goes down from more than 10cm/ka （after the transition, 0~162.73m）to 4.08cm/ka （before the transition,162.73~191.94m）. Combining with the IODP 341 preliminary report and multichannel seismic transect results, we conclude that abrupt change should be in connection with the enhanced glacial erosion caused by the Mid-Pleistocene transition （MPT）in the provenance area. All in all, our results provide a robust framework for further stratigraphic subdivision and correlation of this area since the Pleistocene.