Gulf of Alaska Cryosphere and Paleoceanography in the Neogene: IODP Expedition 341 Southern Alaska

Abstract:

Neogene climate reflects the transition into a colder, more variable world dominated by the onset and intensification of major Northern Hemisphere glaciations. Integrated Ocean Drilling Program Expedition 341 recovered a cross-margin transect of pelagic facies from this subarctic, high-nutrient, low-chlorophyll modern setting to glacigenic material from the exhuming coastal mountains. The recovered sedimentary record extends from the late Miocene through the Pleistocene/Holocene. Shipboard results indicate a ~doubling in sediment accumulation rates at ~2.56 Ma at the deep-water distal Site U1417, which is interpreted as the onset of significant glacial coverage in the St. Elias Range after the Pliocene-Pleistocene transition, resulting in the start of deposition in the Surveyor Fan. At all sites, clast-rich mud and/or diamict are prevalent in the Pleistocene, indicating the persistent transport of glacigenic sediment by ice, iceberg or sea-ice rafting. An unanticipated observation is the often sharp transitions between these ice-dominated facies and more biogenic-rich, often bioturbated, muddy lithologies that might be related to increased biological productivity, enhanced nutrient supply, and/or decreased input of terrigenous sediments. All sites contained variable preservation of biosiliceous and calcareous microfossils that indicate dynamic water column productivity and/or variable sediment geochemistry from the late Miocene to the present. At deep-water sites, strong variations in environmentally sensitive planktic foraminifera, radiolarians, and diatoms record alternating warming and cooling intervals during the last 1.2 Ma. At Site U1417, planktic foraminifera and radiolarian taxa abundances indicate cooler conditions in the Pleistocene relative to the Pliocene/late Miocene. Especially high sedimentation rates during the late Pleistocene hold promise for future studies related to suborbital-scale variability in paleoproductivity and glacigenic sedimentation.