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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...

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Sediment traps and echo sounder profiles collected between 1984 and 1987 were used to determine the rate and style of sediment infilling of a fjord basin with a temperate tidewater glacier at its head. McBride Glacier retreated rapidly (0.25 km/yr, 1984-1986; 0.05 km/yr, 1986-1987) enlarging a 90-to 100-m-deep basin adjacent to the terminus. Highly turbid meltwater is discharged subglacially at the base of the water column and rises to the surface forming turbid overflow plumes. Sedimentation rates from suspension settling decrease exponentially with distance from the subglacial stream; from 24 g/cm 2 /day at the terminus to less than 1 g/cm 2 /day, 1.3 km away.The total annual volume of sediment deposition within the ice-proximal basin of McBride Inlet was 3.6 x 10 6 m 3 between 1984 and 1985,3.0 x 10 6 m 3 between 1985 and 1986, and 1.7 x io 6 m 3 between 1986 and 1987. This sediment is contributed by suspension deposition from meltwater plumes, bedload transported by the subglacial stream, direct meltout of glacial debris, ice rafting, and side-entry sources. Rapid glacier flow and large amounts of meltwater and precipitation, a result of the temperate climate, enhance the production and delivery of sediment to the fjord.In McBride Inlet, meltwater discharge is the main ice-proximal sediment source. During 1984-1986, 67 percent of the total volume was deposited by suspension settling alone. Sediment infills the irregular bathymetry of the predepositional basin, producing a smooth, flat basin floor. Within this basin, sediment accumulation rates are as high as 13 m/yr, 300 m from the glacier.Major sediment sources are episodic, including sediment gravity flows, and daily and seasonal variations occur in meltwater discharge. Within temperate Ijords a thick sequence of glacial marine sediment rapidly accumulates, filling proximal basins with interlaminated sand/silt and mud.

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New radiocarbon and sedimentological results from the Gulf of Alaska document recurrent millennial-scale episodes of reorganized Pacific Ocean ventilation synchronous with rapid Cordilleran Ice Sheet discharge, indicating close coupling of ice-ocean dynamics spanning the past 42,000 years. Ventilation of the intermediate-depth North Pacific tracks strength of the Asian Monsoon, supporting a role for moisture and heat transport from low-latitudes in North Pacific paleoclimate. Changes in 14C age of intermediate waters are in phase with peaks in Cordilleran ice-rafted debris delivery, and both consistently precede ice discharge events from the Laurentide Ice Sheet, known as Heinrich Events. This timing precludes an Atlantic trigger for Cordilleran Ice Sheet retreat, and instead implicates the Pacific as an early part of a cascade of dynamic climate events with global impact.

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.