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.

[1] Biogenic opal, organic carbon, organic matter stable isotope, and trace metal data from a well-dated, high-resolution jumbo piston core (EW0408-85JC; 59°33.3′N, 144°9.21′W, 682 m water depth) recovered from the northern Gulf of Alaska continental slope reveal changes in productivity and nutrient utilization over the last 17,000 years. Maximum values of opal concentration ($10%) occur during the deglacial Bølling-Allerød (B-A) interval and earliest Holocene (11.2 to 10.8 cal ka BP), moderate values ($6%) occur during the Younger Dryas (13.0 to 11.2 cal ka BP) and Holocene, and minimum values ($3.5%) occur during the Late Glacial Interval (LGI). When converted to opal mass accumulation rates, the highest values ($5000 g cm À2 kyr À1 ) occur during the LGI prior to 16.7 cal ka BP, which points to a strong influence by LGI glacimarine sedimentation regimes. Similar patterns are also observed in total organic carbon and cadmium paleoproductivity proxies. Mid-Holocene peaks in the terrestrial organic matter fraction at 5.5, 4.7, 3.5, and 1.2 cal ka BP indicate periods of enhanced delivery of glaciomarine sediments by the Alaska Coastal Current. The B-A and earliest Holocene intervals are laminated, and enrichments of redox-sensitive elements suggest dysoxic-to-anoxic conditions in the water column. The laminations are also associated with mildly enriched sedimentary d 15 N ratios, indicating a link between productivity, nitrogen cycle dynamics, and sedimentary anoxia. After applying a correction for terrestrial d 15 N contributions based on end-member mixing models of terrestrial and marine organic matter, the resulting B-A marine d 15 N (6.3 AE 0.4 ‰) ratios are consistent with either mild denitrification, or increased nitrate utilization. These findings can be explained by increased micronutrient (Fe) availability during episodes of rapid rising sea level that released iron from the previously subaerial coastal plain; iron input from enhanced terrestrial runoff; and/or the intermittent presence of seasonal sea ice resulting from altered ocean/atmospheric circulation during the B-A in the Gulf of Alaska.

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[1] Outcrops of old strata at the shelf edge resulting from erosive gravity-driven flows have been globally described on continental margins. The reexposure of old strata allows for the reintroduction of aged organic carbon (OC), sequestered in marine sediments for thousands of years, into the modern carbon cycle. This pool of reworked material represents an additional source of 14 C-depleted organic carbon supplied to the ocean, in parallel with the weathering of fossil organic carbon delivered by rivers from land. To understand the dynamics and implications of this reexposure at the shelf edge, a biogeochemical study was carried out in the Gulf of Lions (Mediterranean Sea) where erosive processes, driven by shelf dense water cascading, are currently shaping the seafloor at the canyon heads. Mooring lines equipped with sediment traps and current meters were deployed during the cascading season in the southwestern canyon heads, whereas sediment cores were collected along the sediment dispersal system from the prodelta regions down to the canyon heads. Evidence from grain-size, X-radiographs and 210Pb activity indicate the presence in the upper slope of a shelly-coarse surface stratum overlying a consolidated deposit. This erosive discontinuity was interpreted as being a result of dense water cascading that is able to generate sufficient shear stress at the canyon heads to mobilize the coarse surface layer, eroding the basal strata. As a result, a pool of aged organic carbon (D 14 C = −944.5 ± 24.7‰; mean age 23,650 ± 3,321 ybp) outcrops at the modern seafloor and is reexposed to the contemporary carbon cycle. This basal deposit was found to have relatively high terrigenous organic carbon (lignin = 1.48 ± 0.14 mg/100 mg OC), suggesting that this material was deposited during the last low sea-level stand. A few sediment trap samples showed anomalously depleted radiocarbon concentrations (D 14 C = −704.4 ± 62.5‰) relative to inner shelf (D 14 C = −293.4 ± 134.0‰), mid-shelf (D 14 C = −366.6 ± 51.1‰), and outer shelf (D 14 C = −384 ± 47.8‰) surface sediments. Therefore, although the major source of particulate material during the cascading season is resuspended shelf deposits, there is evidence that this aged pool of organic carbon can be eroded and laterally advected downslope.

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