SOURCES AND DISPERSAL OF ICE RAFTED SIKU EVENTS IN THE NE PACIFIC

Four discrete ice-rafted debris (IRD) peaks (IRD MAR (mass accumulation rate) greater than 12 g cm^-2 kyr^-1) from the Gulf of Alaska continental slope have been radiocarbon dated within the past 42 ka at Integrated Ocean Drilling Program (IODP) Site U1419. These peaks, termed Siku Events (S) by Walczak et al. (2020), occur in phase with Pacific Ocean ventilation suggesting a close coupling of ice-ocean dynamics. We examine the origin of S1-S4 over the past 42 ka based on sedimentological and micropaleontological characteristics and their paleoenvironmental implications. The major source of IRD is the largest Cordilleran Ice Sheet outlet glacier, the Bering Ice Stream, which retreated through its shelf-crossing trough. At U1419, epoxy grain mounts of rock fragments from the IRD peaks include sandstone, siltstone, argillite, and basalt, which have a provenance consistent with the Bering Glacier drainage basin. Smaller IRD peaks in between the Siku events at U1419 contain marble and granitoid rock fragments with a southern Alaskan provenance (Alexander Terrane). The presence of far-travelled grains occurs in intervals when sea-ice associated diatoms are absent, which suggests iceberg drift through open water along the Alaskan coast.

IODP Site U1421, lies on the slope seaward of the Bering Trough, 39 km to the east of Site U1419. At U1421, four IRD events are correlative with S1-S4, and likely represent the destabilization/retreat phase of the marine terminus in the tidewater glacier cycle. Species of the foraminifer, Elphidium dominate IRD peaks at U1421 but are less prominent at U1419, suggesting reduced meltwater influence at U1419. Sea-ice diatoms are present in diatom-rich samples at U1421 and intermittently at U1419, suggesting the possibility of both sea ice rafting and icebergs as sources of IRD at these sites near the coast.

Few Northeast Pacific radiocarbon-dated IRD records exist for comparison, however two central Alaska Gyre sites have peaks corresponding with S1-S4. The magnitude of each peak does not always match the correlative peak at U1419 but this can be attributed to varying iceberg bypass or survivability, which likely depends on surface currents, meltwater production or sea surface temperatures. Siku Events resulted from both temperate tidewater glacier dynamics and oceanographic conditions related to climate change.