No 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.

Erosion, sediment production, and routing on a tectonically active continental margin reflect both tectonic and climatic processes; partitioning the relative importance of these processes remains controversial. Gulf of Alaska contains a preserved sedimentary record of the Yakutat Terrane collision with North America. Because tectonic convergence in the coastal St. Elias orogen has been roughly constant for 6 My, variations in its eroded sediments preserved in the offshore Surveyor Fan constrain a budget of tectonic material influx, erosion, and sediment output. Seismically imaged sediment volumes calibrated with chronologies derived from Integrated Ocean Drilling Program boreholes show that erosion accelerated in response to Northern Hemisphere glacial intensification (∼2.7 Ma) and that the 900-km-long Surveyor Channel inception appears to correlate with this event. However, tectonic influx exceeded integrated sediment efflux over the interval 2.8-1.2 Ma. Volumetric erosion accelerated following the onset of quasi-periodic (∼100-ky) glacial cycles in the mid-Pleistocene climate transition (1.2-0.7 Ma). Since then, erosion and transport of material out of the orogen has outpaced tectonic influx by 50-80%. Such a rapid net mass loss explains apparent increases in exhumation rates inferred onshore from exposure dates and mapped out-of-sequence fault patterns. The 1.2-My mass budget imbalance must relax back toward equilibrium in balance with tectonic influx over the timescale of orogenic wedge response (millions of years). The St. Elias Range provides a key example of how active orogenic systems respond to transient mass fluxes, and of the possible influence of climate-driven erosive processes that diverge from equilibrium on the million-year scale. O rogenesis reflects the balance of crustal material entering a mountain belt to undergo shortening and uplift versus material leaving the orogen through exhumation, erosion, and sediment transport (1-5). Perturbations in the influx/efflux from the orogen are expected to result in predictable changes in deformation within the orogen as it attempts to reestablish equilibrium (3). The long-term sink for sediment transported out of mountain belts is often in the deep sea, particularly in large submarine fans where sediments accumulate at anomalously high rates (>10 cm/ky) compared with deep-sea pelagic sedimentation (6-8). Even higher sedimentation rates (>100 cm/ky) proximal to glacially eroded regions (9-14) imply that wet-based glaciers are extremely efficient agents of erosion. Observations and modeling have argued that erosion rates can influence tectonic processes (15)(16)(17)(18)(19), but the timescales of adjustment, and the role of landscape disequilibrium, remain unclear. For example, exceptionally high local sedimentation rates (100-1000 cm/ky) recorded on the century timescale (13) SignificanceIn coastal Alaska and the St. Elias orogen, over the past 1.2 million years, mass flux leaving the mountains due to glacial erosion exceeds the plate tectonic input. This...

Tectonically active coastal regions of the world recently have been suggested to supply the bulk of sediment from land to the oceans. Seabed sampling on the continental shelf and in coastal embayments of the north-east Gulf of Alaska (Alsek River to Prince William Sound) was performed to examine the temporal and spatial variability of sediment accumulation in a mountainous coastal setting. Cores of varying lengths (30-300 cm) were collected at 84 stations to provide information on sedimentary processes using radiochemical ( 210 Pb and 137 Cs) techniques. Four types of 210 Pb activity profiles were observed, dominantly reflecting steadystate sediment accumulation. However, nonsteady-state profiles also were measured, resulting in part from episodic deposition near glacier-fed rivers and on the Copper River Delta. Sediment accumulation rates in the eastern half of the study area are highest at midshelf depths (#100 m) (≥10 mm yr −1 ) and near rivers draining the Bering Glacier (#20 mm yr −1 ). On the Copper River Delta, sediment accumulation rates are highest for the delta front (> 20 mm yr −1 ) and decrease westward along the sediment dispersal route. Total annual sediment accumulation is 90-140×10 6 tons yr −1 on the shelf in the study area. Annual sediment accumulation for the total marine environment in the study area (including Icy and Yakutat Bays) exceeds 250×10 6 tons yr −1 , potentially making this region the largest sink for sediment in North America. Spatial patterns in sediment accumulation on the shelf are similar between centennial and Holocene time-scales, reflecting the dominance of the Copper River and Bering and Malaspina glaciers as sediment sources. Temporal variability in accumulation rates between centennial and Holocene time-scales exists for portions of the study area near fiords and demonstrates the considerable changes that occur in sediment supply during glacial advances and retreats.

Isostatic uplift of tectonically stable, passive margin lithosphere can preserve a record of paleo-shoreline position by elevating coastal geomorphic features above the infl uence of nearshore wave activity. Conversely, depositional ages and modern elevations of these features can provide valuable information about the uplift history of a region. We present a numerical model that combines sea-level oscillation, subaerial exposure, a precipitation-karstifi cation function, and isostatic uplift to explore the dynamic geomorphic behavior of coastal carbonate landscapes over multiple sea-level cycles. The model is used to estimate ages of coastal highstand depositional features along the Atlantic coast of north Florida. Numerical simulations using current best estimates for Pleistocene sea-level and precipitation histories suggest ages for Trail Ridge (1.44 Ma), the Penholoway Terrace (408 ka), and the Talbot terrace (120 ka) that are in agreement with fossil evidence. In addition, model results indicate that the rate of karstifi cation (void space creation or equivalent surface lowering rate) within the north Florida platform is ~3.5 times that of previous estimates (1 m/11.2 k.y. vs. 1 m/38 k.y.), and uplift rate is ~2 times as high as previously thought (0.047 mm/yr vs. 0.024 mm/yr). This process has implications for landscape evolution in other carbonate settings and may play an underappreciated role within the global carbon cycle.

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Time-series measurements of chloride (Cl 2 ) concentrations in lagoon and pore waters of an estuary on the east coast of Florida (Indian River Lagoon) demonstrate exchange of lagoon surface water to depths of ,40 cm in the sediment in less than 46 h. The exchange rate may be as fast as 150 cm d 21 based on models of the decay in the amplitude of diurnal temperature variations and the time lag of maxima and minima of the temperature variations at depths of 15 and 30 cm below the sediment-water interface. These flow rates indicate a minimum residence time of 0.33 d for the pore water. Considering the small tides and waves, rate of the exchange, and large number of bioturbating organisms in the Indian River Lagoon, the exchange of water is driven largely by bioirrigation. The exchange provides a greater flux of excess radon-222 from the sediment to the lagoon than would occur from diffusion alone. The exchange also pumps oxygenated water into the sediments, thereby enhancing organic carbon remineralization and the flux of nitrogen from sediments to the lagoon water. High rates of exchange across the sediment-water interface indicate that marine sources are volumetrically more important than terrestrial sources to submarine groundwater discharge in the permeable sediments of this estuary.

Glacimarine sedimentary deposits within the basins of Muir Inlet, a 48-km-long silled fjord, are interpreted from complimentary sets of high-resolution, seismic-refl ection profi les using known glacial-advance and retreat history. Two prominent glacial erosion surfaces are identifi ed: the lowest attributed to the Last Glacial Maximum (LGM) advance and the upper coincident with the Little Ice Age (LIA) advance. The LGM ice sheet, which advanced onto the continental shelf, was 1700 m thick in Muir Inlet and eroded bedrock, whereas the thinner LIA ice did not. LGM deposits >300 m thick occur beneath the LIA erosion surface in the deepest basins. Evidence for earlier Neo glacial advances is present in subaerial deposits; however, Neoglacial sediments preserved within the marine record are restricted to one depositional package on the entrance sill. Volumes of LIA retreat sediments were calculated within basins. An average annual sediment fl ux was calculated by modeling the duration of sediment contributed from Muir Glacier and from tributary glaciers and side-entry sources. The annual sediment fl ux ranged from 1.3 × 10 6 m 3 /yr to 4.6 × 10 7 m 3 /yr and increases logarithmically with increasing drainage basin area, similar to fl uvial systems. This sediment fl ux does not only represent bedrock erosion. Additional sediment is contributed from persistent tributary glaciers and from LGM sediment stored within deeper basins. Basin-wide refl ections characterize the most common seismic facies and indicate that strata are horizontal and continuous across each basin, confi rming the importance of sediment gravity fl ows originating from sills and sloping fjord walls.