Late Cenozoic Ice-Rafting Records from Leg 145 Sites in the North Pacific: Late Miocene Onset, Late Pliocene Intensification, and Pliocene—Pleistocene Events

Abstract

Ice-rafted debris (IRD) in marine sediments provides a valuable direct indicator of the presence of glacial ice at sea level on surrounding continents, and is therefore a useful tool in paleoclimatic studies. The location of sites drilled during Ocean Drilling Program Leg 145, combined with the length, stratigraphic integrity, and good age control of the sediments recovered, provides an opportunity to examine the Late Cenozoic record of North Pacific ice rafting at locations within a few hundred kilometers of potential IRD sources. Ice-rafting records for Sites 881, 883, and 887 have been determined, with temporal resolution ranging from one sample per 10 k.y. to one sample per 50 k.y. The mass accumulation rate (MAR) of IRD in the coarse sand-sized fraction of each sample has been calculated from the abundance of coarse sand (wt%), the IRD abundance within the coarse sand fraction (vol%), shipboard measurements of sediment dry-bulk density, and average sedimentation rates constrained by shipboard paleomagnetic and biostratigraphic data. The oldest occurrences of IRD in Leg 145 cores are at least late Miocene in age, and variations in IRD MARs prior to 2.6 Ma are consistent with interpretations of an older episode of tidewater glaciation (~6 to ~4.2 Ma) and a mid-Pliocene warm interval (-4.2 to -3.0-3.5 Ma) recorded in the Yakataga Formation of coastal Alaska. The age of initial IRD input is younger at Deep Sea Drilling Project (DSDP) sites farther south. IRD MARs at all three sites increase markedly at 2.6 Ma, recording the onset of continental-scale glaciation in the Northern Hemisphere. An accompanying increase in the abundance of volcanic ash may indicate a linkage between volcanism and climate that merits additional study. Pliocene-Pleistocene IRD MARs decrease from the Gulf of Alaska towards the west and from the northwesternmost Pacific towards the east, which identifies the Alaskan coast and the Kurile/Kamchatka margin as major IRD sources. At least eight episodes of increased IRD MARs can be correlated among Leg 145 sites, and most of these can be correlated to IRD records from DSDP Sites 579 and 580. These increases also correlate to North Pacific-wide episodes described previously, indicating that the Leg 145 IRD MAR records contain the major features of North Pacific paleoclimatic history. Pliocene-Pleistocene IRD MAR fluctuations at Sites 881 and 887 exhibit quasi-periodic cyclicity with average durations near orbital values. These results suggest that additional high-resolution studies of sediments from Sites 881 and 887 may yield the first North Pacific IRD records comparable in detail to IRD records from the North Atlantic.