Porewater salinity and δ18O reveal a kilometer-deep subsurface pathway for glacial meltwater into the Gulf of Alaska

Porewaters recovered from IODP Sites U1419 (687 m water depth), U1420 (248 m), and U1421 (719 m) reveal that continental meteoric waters permeate the SE Alaska margin near Bering Glacier. Salinities <20 are found more than 800 m below the sea floor (>1000 m below modern sealevel). Several discrete zones of low salinity (and chlorinity) appear to track seismic-stratigraphic boundaries, implying relatively rapid lateral transport along zones of high porosity and permeability to maintain vertical salinity gradients of ~0.1/m in discrete zones. Sediments at these sites are underconsolidated, and alternate between ice-proximal coarse-grained diamict, and fine-grained biogenic hemipelagic mud. Sediment accumulation rates are >1 m/ka. Porewater profiles are consistent with a component of upward diffusive venting of meteoric waters on the shelf (Site U1420) in Bering Trough, and on the continental slope just below Bering Trough (Site U1421), but not on the continental slope away from the trough (Site U1419). Oxygen isotope measurements help to 'fingerprint' different freshwater end members and trace the water to its continental sources. This subsurface pathway for transport of freshwater from coastal Alaska likely varied dynamically in response to climate and the well-known ice loading that influenced hydrostatic heads and the availability of freshwater in the recharge zone in both Holocene 'Neoglacial' and during the Last Glacial Maximum. Stable isotope data from benthic foraminifera from the Alaska Margin suggest abrupt changes in rates of deep venting of low-salinity, low- δ18O waters, and put temporal constraints on the hydrodynamics. For example, a deglacial benthic foraminiferal low-δ18O spike of ~1o/oo (~14,300 yr BP) follows about a century after apparent freshening of near-surface waters (~14,400 yr BP), which in turn follows a few centuries behind retreat of the Bering Glacier onto land and the abrupt cessation of glacial-marine sedimentation (~14,800 yr BP). Documentation of this subsurface pathway for freshwater transport on glaciated margins has broad implications for mechanisms of deglaciation, ice-ocean interaction, and interpretations of paleoceanographic records in many ice-proximal settings.