The deglacial transition on the southeastern Alaska Margin: Meltwater input, sea level rise, marine productivity, and sedimentary anoxia

Abstract

Oxygen isotope data from planktonic and benthic foraminifera, on a high‐resolution age model (44 14C dates spanning 17,400 years), document deglacial environmental change on the southeast Alaska margin (59°33.32′N, 144°9.21′W, 682 m water depth). Surface freshening (i.e., δ18O reduction of 0.8‰) began at 16,650 ± 170 cal years B.P. during an interval of ice proximal sedimentation, likely due to freshwater input from melting glaciers. A sharp transition to laminated hemipelagic sediments constrains retreat of regional outlet glaciers onto land circa 14,790 ± 380 cal years B.P. Abrupt warming and/or freshening of the surface ocean (i.e., additional δ18O reduction of 0.9‰) coincides with the Bølling Interstade of northern Europe and Greenland. Cooling and/or higher salinities returned during the Allerød interval, coincident with the Antarctic Cold Reversal, and continue until 11,740 ± 200 cal years B.P., when onset of warming coincides with the end of the Younger Dryas. An abrupt 1‰ reduction in benthic δ18O at 14,250 ± 290 cal years B.P. likely reflects a decrease in bottom water salinity driven by deep mixing of glacial meltwater, a regional megaflood event, or brine formation associated with sea ice. Two laminated opal‐rich intervals record discrete episodes of high productivity during the last deglaciation. These events, precisely dated here at 14,790 ± 380 to 12,990 ± 190 cal years B.P. and 11,160 ± 130 to 10,750 ± 220 cal years B.P., likely correlate to similar features observed elsewhere on the margins of the North Pacific and are coeval with episodes of rapid sea level rise. Remobilization of iron from newly inundated continental shelves may have helped to fuel these episodes of elevated primary productivity and sedimentary anoxia.

Climate change decouples oceanic primary and export productivity and organic carbon burial

“…Such an effect is consistent with hypothesized sources of both particulate and dissolved iron from the continental shelf during sea level rise (16) and from growing hypoxia on the shelf and upper slope during deglacial warming (34,35). Hypoxia in parts of the North Pacific during the last deglaciation may have been sustained in part by iron feedback that produced an ecological shift to a more efficient exporting ecosystem (36).…”

Megathrust splay faults at the focus of the Prince William Sound asperity, Alaska

“…Other authors speculate that deglaciation on the shelf occurred closer to 10 kya [e.g., Sheaf et al, 2003], but no direct measurements for this timing are currently available. Based on the related sediment unconformity, modern sedimentation rates recorded within the near-shore Gulf of Alaska environment are estimated at~10 mm/yr [Carlson and Molnia, 1975], and are consistent with both an ice-proximal marine sediment source and sediments derived from the Copper River delta [Carlson and Molnia, 1975;Davies et al, 2011]. As with previously published seismic surveys in the region [e.g., Carlson andMolnia, 1975, Carlson, 1989], we observe an unconformity within the upper 150 m below seafloor that defines the onset of postglacial deposition.…”

Ventilation changes in the western North Pacific since the last glacial period

“…This stratification temporarily intensified during H1 because of the expansion of the nutrient-depleted water mass down to ∼2500 m in the North Pacific . At the beginning of the Bølling, productivity in the subarctic Pacific rose rapidly (Crusius et al, 2004;Galbraith et al, 2007;Jaccard et al, 2009;Brunelle et al, 2010;Davies et al, 2011) in association with enhanced upwelling by breakdown of the glacial stratification.…”