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doi:10.2204/iodp.proc.341.101.2014

Background

Geological setting

Continental margin strata in southern Alaska are the product of sediment derived from the Yakutat Terrane and several antecedent Mesozoic–modern accreted terranes that compose much of the northern North American Cordillera (Fig. F4) (Plafker, 1987; Plafker et al., 1994). The Yakutat Terrane was likely excised from western Canada and translated to the northwest along the dextral Queen Charlotte-Fairweather Fault system (Plafker, 1987; Plafker et al., 1994; Landis, 2007; Perry et al., 2009). The age of initial Yakutat-North America subduction, when the leading edge of the microplate encountered the Aleutian Trench and initiated flat-slab subduction, is poorly constrained but may have occurred as early as ~25 Ma (Benowitz et al., 2011; Finzel et al., 2011). Increased uplift has occurred in the past 10 m.y. associated with the convergence of increasingly thick crust and the formation of a syntaxial bend in the Pacific/North American plate boundary (Plafker et al., 1994; Rea and Snoeckx, 1995; White et al., 1997; Enkelmann et al., 2010). Ongoing collision and flat-slab subduction of thick (up to 35 km) oceanic plateau crust and cover strata (Christeson et al., 2010; Worthington et al., 2012) is constructing the present high topography of the Chugach and St. Elias Ranges (Pavlis et al., 2004; Eberhart-Phillips et al., 2006; Gulick et al., 2007). It also causes active tectonic deformation spread throughout southern Alaska and northwestern Canada (Fig. F3) (Plafker et al., 1994; Mazzotti and Hyndman, 2002; Pavlis et al., 2004; Spotila et al., 2004; Eberhart-Phillips et al., 2006; Meigs et al., 2008; Spotila and Berger, 2010; Enkelmenn et al., 2010; Worthington et al., 2010; Finzel et al., 2011). Focused deformation occurs in two indentor corners (Fig. F3), one in the west that is experiencing thin-skinned young deformation (Bruhn et al., 2004) and the other in the east (Seward corner) (Spotilla and Berger, 2010; Enkelmann et al., 2010) where intense strain and high exhumation rates (“tectonic aneurysm”) are associated with the change from strike-slip to collision (Enkelmann et al., 2008, 2010; Elliot et al., 2010; Koons et al., 2010, 2013). The active deformation front for this convergence cuts diagonally from the eastern syntaxis near Mt. St. Elias along the Malaspina Fault, reaching Icy Bay, and then across the shelf at the Pamplona Zone and down the slope to the Aleutian Trench (Fig. F3), thereby linking Yakutat-North America deformation structures with the Pacific-North America faults (Bruns, 1983; Worthington et al., 2010; Gulick et al., 2013). The northward boundary, or backstop, of deformation within the St. Elias orogen is debatable because of extensive ice cover and may include the Chugach-St. Elias Fault or the Contact Fault (Fig. F5) (Spotila and Berger, 2010; Enkelmann et al., 2010; Bruhn et al., 2012). However, there are suggested far-field effects >1000 km away (Mazzotti and Hyndman, 2002; Redfield et al., 2007).

The Yakutat Terrane consists of Eocene to modern sedimentary rocks of the Kulthieth, Poul Creek, and Yakataga Formations, which are primarily siliciclastic marine and glacimarine strata interbedded with volcanics and coal beds (Plafker et al., 1994). The Kulthieth Formation sediments are the oldest, deposited at ~60–35 Ma, and are composed of nonmarine to shallow-marine deltaic feldspathic and micaceous sandstone and siltstone formed during a relative sea level regression (Risley et al., 1992; Plafker et al., 1994; Perry et al., 2009). The Poul Creek Formation conformably overlies the Kulthieth, ranging in age from Oligocene to Miocene (Risley et al., 1992; Plafker et al., 1994). The Poul Creek Formation is characterized by a high abundance of argillaceous sediment that is in part glauconitic and organic rich, representing deposition during a general marine transgression in outer shelf/slope environments. It also contains waterlain basaltic tuff, andesitic breccia, pillow lava, and related deposits of the intertonguing Cenotaph volcanics (Plafker, 1987; Risley et al., 1992; Plafker et al., 1994). The Yakataga Formation represents glacimarine siliciclastic shelf/slope deposits dating from the onset of St. Elias glaciation in the late Miocene (~5.5 Ma) to the present (Plafker et al., 1994; Lagoe et al., 1993; Risley et al., 1992; Plafker, 1987; Eyles et al., 1991). It consists of a lower diamict interval, possibly deposited by debris flows, which transitions upward into sandstone, diamict, and mudstone with occasional coquinas and boulder pavements. The uppermost Yakataga Formation mostly comprises facies that indicate significant development of the shelf ice margin (paraglacial mudstones to boulder pavements).

Volumetrically, the major potential contributors of sediment deposited on the Yakutat Terrane during the Cenozoic are the Wrangellia, Alexander, Chugach, and Yukon-Tanana Terranes (Fig. F4). The Kulthieth and Poul Creek Formations are likely derived from a parent source in British Columbia (Cowan, 1982; Plafker, 1987; Plafker et al., 1994; Landis, 2007; Perry et al., 2009). At the time of deposition of the Kulthieth and Poul Creek Formations, sediment transport pathways likely originated from the east, deriving material from the Intermontaine Belt (Cache Creek, Nisling, and the Stikine Terranes), Coast Belt (Coast Plutonic Complex and Central Gneiss Complex), and Insular Belt (Wrangellia, Alexander, Chugach, and Yakutat Terranes) in British Columbia (Gehrels and Berg, 1994; Plafker et al., 1994). The Wrangellia Terrane consists of a lower section of Permian to Middle Triassic limestone, chert, and pelitic strata. These units are overlain by 4–5 km of Middle Triassic mafic lava flows and associated mafic and ultramafic rocks, providing a diagnostic isotopic and compositional marker of sediment derived from the Wrangellia Terrane (Jones et al., 1977; Nokleberg et al., 2000; Trop et al., 2002). The Alexander Terrane consists of Precambrian(?) through Middle(?) Jurassic sedimentary, metamorphic, and plutonic rocks (Gehrels and Berg, 1994) with distinctive magnetic mineralogies (Cowan et al., 2006). The Chugach Terrane consists mainly of highly deformed, weakly metamorphic Upper Cretaceous graywacke and slate. These accretionary prism strata are interpreted to have been mainly derived from plutonic and volcanic rocks of the Coast Plutonic Complex in British Columbia (Dumoulin, 1987). The final major terrane that may have contributed sediment to the Yakutat Terrane is the Yukon-Tanana Terrane. Polydeformed metamorphic rocks of this terrane consist mainly of quartz-mica schist, quartzite, metarhyolite, and gneissic plutonic rocks (Foster et al., 1994). Protolith ages are only partly known but include middle Paleozoic radiometric dates for some of the igneous rocks and Devonian paleontologic ages for some of the carbonate strata (Gehrels and Saleeby, 1987; Plafker, 1987; Haeussler et al., 2006).

The provenance of the Yakataga Formation is likely a combination of detritus supplied mostly from the onshore Neogene thrust belt (Fig. F5). The Yakutat Group, Kulthieth, Poul Creek, and Yakataga Formations are exposed in the thrust belt. Potential sources of sediment in the “backstop region” include of the Chugach and Prince William Terranes, with possible minor contributions from more inboard terranes (Fig. F5). The Chugach and Prince William Terranes are a subduction complex welded onto the continent when the Kula-Farallon Ridge subducted beneath the margin, resulting in high-temperature, low-pressure metamorphism (Plafker et al., 1994; Pavlis et al., 2004). The Chugach and Prince William Terranes are lithologically similar and have been considered one composite terrane (Kusky et al., 1997). Age distributions of detrital zircons suggest metasedimentary flyschoidal rocks of the Chugach Terrane are derived from inboard accreted terranes of British Columbia (Haeussler et al., 2006; Perry et al., 2009). The lithologies of the Chugach Terrane are dominated by graywacke flysch and mélange units with substantial basaltic constituents. These mafic units have undergone very low grade metamorphism in the westernmost terrane, increasing in metamorphic grade to amphibolite-facies and phyllitic units of the Orca and Valdez Groups in the Prince William Terrane in the St. Elias region (Fig. F5) (Plafker et al., 1994; Haeussler et al., 2006). The Valdez Group of the Prince William Terrane north of the St. Elias Mountains is characterized by volcaniclastic graywacke and argillite with subordinate metavolcanic strata and weakly foliated green glassy tuff (Plafker et al., 1994). Volcanic rocks of the southern margin of the Valdez Group also include tholeiitic pillow basalts with island-arc to mid-ocean-ridge basalt (MORB) compositions (Plafker et al., 1994). The Orca Group dominates the Prince William Terrane within the St. Elias Range. As with the Valdez Group, this group is a deep-sea flysch complex with abundant oceanic basaltic rocks, sheeted dikes, and gabbroic intrusions of an ophiolite complex and underlies the Bagley Icefield along the Contact Fault system, from which the Bering Glacier sources (Plafker et al., 1994; Richter et al., 2006). Yakataga Formation strata accumulated in the later stages of the accretionary history of the Yakutat Terrane. Sedimentary petrography of Yakataga lithic fragments reveals sedimentary, metasedimentary, and volcanic rock fragments. Zircon fission track, U-Th/He, and U/Pb analyses of the Yakataga Formation indicate intermixing of sediment derived from two or more sources, likely the Chugach-Prince William composite terrane and recycling of Poul Creek and Kulthieth rocks (Plafker et al., 1994; Enkelmann et al., 2008; Perry et al., 2009; Witmer et al., 2009).

Physical and oceanographic setting

The morphology of the Gulf of Alaska shelf has been strongly influenced by active tectonics and glacial deposition overprinted by glacial erosion (Carlson et al., 1982; Elmore et al., 2013). Locally, the shelf bathymetry has been shaped by glacial and tectonic forces that produced five distinct shelf-crossing troughs and valleys in the study area: the Alsek Sea Valley, the Yakutat Sea Valley, the Bering Trough, the Kayak Trough, and the Hinchinbrook Sea Valley (Figs. F1, F6). These have U-shaped profiles, contain poorly sorted, glacially derived diamict along their flanks, and presumably formed during the advance of major ice streams and glaciers (Carlson et al., 1982). The extent of grounded ice covering the shelf during the last sea level lowstand is poorly constrained, but it may have ranged from only the across-shelf valleys to the entire shelf (Fig. F7) (Molnia, 1986; Manley and Kaufmann, 2002). There is evidence of grounded ice to the shelf edge within the shelf-crossing troughs based on erosional surfaces mapped in the Bering Trough (Berger et al., 2008a) and Yakutat and Alsek Sea Valleys (Elmore et al., 2013). Little sampling has occurred on the continental slope or on Surveyor Fan. Grab samples recovered diamict from the continental slope, which likely dates to the LGM (Molnia and Sangery, 1979). The jumbo piston Core EW0408-85JC on the continental slope at Site U1419 reveals slow sedimentation (<1 mm/y) of Holocene-age silty clay in the vicinity of the Alaska Coastal Current (Davies et al., 2011). Jumbo piston Core EW0408-87JC on the proximal Surveyor Fan (Site U1418) also records slow sedimentation (<1 mm/y) of likely Holocene-age silty clay that may be derived from intermediate nepheloid layers supplied with sediment from the Alaska Coastal Current (Jaeger et al., 2008).

The modern oceanographic environment in the Gulf of Alaska is characterized by high wintertime wave and wind energy, pronounced coastal currents, and deepwater gyres (Stabeno et al., 2004). The regional meteorology of the Gulf of Alaska is chiefly affected by energetic storms associated with the Aleutian Low Pressure System (ALPS). The Gulf of Alaska experiences mean wind speeds and frequency of gale-force winds similar to those of the western and central North Pacific (Stabeno et al., 2004). Cyclonic motion of the subarctic gyre drives circulation in the outer Gulf of Alaska, with the inner Gulf being dominated by the Alaska Coastal Current, a wind- and buoyancy-forced coastal jet (Fig. F6) (Stabeno et al., 2004). The southern boundary of the subarctic gyre, the North Pacific drift, diverges as it impinges on the North American continental margin, with the northward branch becoming the Alaska Current (Fig. F6). The Alaska Current dominates flow along the southwestern and southern Alaska continental slope, eventually transitioning into the Alaskan Stream farther to the west. Several large, semipermanent to seasonal eddies form within the Alaska Current, leading to mixing of water masses across the continental margin (Stabeno et al., 2004).

Water and sediment are dispersed on the continental shelf by the Alaska Coastal Current (Feely et al., 1979; Stabeno et al., 2004; Weingartner et al., 2005). The current is generally confined to within 40 km of the coast and has flow velocities occasionally exceeding 50 cm/s and mean annual transport of ~106 m³/s (Royer, 1982; Stabeno et al., 1995). Although primarily wind driven, the Alaska Coastal Current is enhanced by a baroclinic response to coastal freshwater discharge to the Gulf of Alaska (Royer, 1981, 1982). This freshwater flux is delivered via a series of small mountainous drainages that experience high precipitation rates (2–6 m/y) due to adiabatic cooling of warm, moist air associated with the cyclonic storm systems of the ALPS (Weingartner et al., 2005). Freshwater and sediment discharge are lowest in winter, when much of the precipitation is stored as snow, and peak in late summer/fall, when meltwater and precipitation fluxes are greatest (Stabeno et al., 2004; Neal et al., 2010). Runoff creates a sharp shallow (<50 m) halocline of salinity contrast >3 psu over the shelf in the fall, but during the winter it is mixed (>100 m) and much more subdued (contrast of ~1 psu) as a result of strong storms (Stabeno et al., 2004). Strong cyclonic winds dominate from fall through spring, peaking in December and January at 9 m/s, and fast (>14 m/s) easterly coastal jets forced by mountain topography can occur (Stabeno et al., 2004). These winds lead to downwelling conditions and near-bed (3 m above bed) nontidal maximum currents of 0.15 m/s in the summer and >0.3 m/s in the winter (Hayes and Schumacher, 1976; Hayes, 1979). Wave energy is highest in the winter months, decreasing in the summer. Monthly mean significant wave heights at National Oceanic and Atmospheric Administration (NOAA) Buoy 46001 (Fig. F6, inset) averaged over 25 y are 3.5 ± 2 m (1σ), and maximum significant wave heights are 14 m in November and December (Gilhousen et al., 1983). Summer significant wave heights average 1.5 m at Buoy 46001, but maxima of 6–10 m can occur.

Regional productivity in the Gulf of Alaska is strongly influenced by the interaction of high freshwater runoff and regional meteorology dictated by the ALPS. During fall through spring, strong cyclonic winds associated with the ALPS support onshore surface Ekman transport and downwelling on the shelf, along with storm-induced vertical mixing (Stabeno et al., 2004; Childers et al., 2005). During summer, the onshore winds and subsequent downwelling conditions relax, allowing occasional brief periods of coastal upwelling in this dominantly downwelling system (Stabeno et al., 2004). Primary productivity in the Gulf of Alaska in the winter is inhibited by low insolation and enhanced vertical mixing that limits the near-surface residence time of algae. Algal blooms on the shelf are related to early spring increased solar irradiance, wintertime replenished nutrient supply, and the onset of water column stratification leading to enhanced algal residence time in surface waters. Productivity near the coast remains relatively high through early summer but is followed by a reduction in summer because of nutrient limitation created by the strong halocline (Childers et al., 2005; Stabeno et al., 2004). Key nutrients (nitrate, silicic acid, and phosphate) are derived mostly from the subsurface ocean via open-ocean upwelling, onshore Ekman transport, tidal pumping, and storm or eddy mixing (Childers et al., 2005). Nutrients delivered by the fluvial system include iron and silicic acid (Stabeno et al., 2004). Relatively few data are available on the cycling of iron in this system, although shelf processes and surface water discharge may play a role in regulating surface-ocean iron concentrations (Stabeno et al., 2004; Schroth et al., 2009; Wu et al., 2009; Davies et al., 2011; Addison et al., 2012). The central Gulf of Alaska is a HNLC region (Stabeno et al., 2004); here primary productivity is likely limited by micronutrients such as iron (Boyd et al., 2004; Tsuda et al., 2005). Sources of iron to the central basin include curl-driven upwelling, eolian dust (Mahowald et al., 2005), advection of dissolved iron from the continental shelf and slope (Chase et al., 2007; Lam and Bishop, 2008), and terrestrial runoff (Stabeno et al., 2004; Royer, 2005). Expedition 341 drill sites span from the productive nitrate-limited shelf system to the iron-limited open-ocean system.

Northwestern Cordilleran ice sheet dynamics

The glacial history of the Gulf of Alaska margin has been constructed through a combination of surface outcrop sampling (e.g., Lagoe et al., 1993; Lagoe and Zellers, 1996; White et al., 1997), scientific drilling (Rea, Basov, Janecek, Palmer-Julson, et al., 1993; Rea and Snoeckx, 1995; Prueher and Rea, 1998), and industry well cuttings (Lagoe et al., 1993; Zellers, 1995; Lagoe and Zellers, 1996). The chronology of glaciation of the Alaskan margin has been mostly established through paleomagnetic and diatom/radiolarian biostratigraphic control at Ocean Drilling Program (ODP) Site 887 (Barron et al., 1995). Supporting information comes from tephrochronology, paleomagnetic, and biostratigraphic control at Deep Sea Drilling Project (DSDP) Site 178 (von Huene et al., 1973; Lagoe et al., 1993), and foraminiferal biostratigraphy in industry wells on the shelf (Zellers, 1995) and in the onshore exposures of the Yakataga Formation (Lagoe et al., 1993). A summary of glacial history and the associated sediment record is shown in Figure F2. Alpine glaciation along the margin is speculated to have initiated as early as ~7 Ma (Lagoe et al., 1993) and increased in coverage by 5.5 Ma (Lagoe et al., 1993; Rea and Snoeckx, 1995; White et al., 1997), when elevation of the Chugach-St. Elias mountain belt was sufficient to trap precipitation from storms generated in the Gulf of Alaska. Initial onset of tidewater glaciation, Lagoe et al.’s (1993) “Glacial Interval A,” is linked to the appearance of diamict in the Yakataga Formation exposed at Yakataga Reef, dated to ~5.5 Ma from a K-Ar glauconite age and at Site 887 from ~5 Ma (Krissek, 1995) to 4.3 Ma (Rea and Snoeckx, 1995) (Fig. F2). A reduction in glacimarine sedimentation correlating with the ~4.5–2.8 Ma mid-Pliocene warm period (MPW; Shackleton et al., 1995) is observed in marine and nonmarine records, though timing varies between different locales in the Gulf of Alaska region. In outcrop and continental shelf samples, the MPW lasts from 4.2 Ma to 3.5–3.0 Ma (Lagoe and Zellers, 1996). At Site 887, the MPW lasts from 3.6 to 2.8 Ma (Rea et al., 1993; Rea and Snoeckx, 1995). Renewed onset of intense glaciation after ~3 Ma, “Glacial Interval B” (Lagoe et al., 1993), is characterized by an increase in ice-rafted debris (IRD) accumulation at 2.6 Ma within deep-sea records (Lagoe et al., 1993; Krissek, 1995; Prueher and Rea, 1998) and by thick successions of diamict in onshore exposures (Lagoe et al., 1993).

At ~1 Ma, the rate of terrigenous sedimentation at Site 887 doubled, likely caused by widespread glacial advance associated with the mid-Pleistocene transition (MPT) that may be responsible for erosion of a series of U-shaped shelf-crossing troughs (Carlson, 1989; Lagoe et al., 1993; Rea and Snoeckx, 1995; Elmore et al., 2013). This glacial intensification is referred to as “Glacial Interval C” (Berger et al., 2008a). Since the onset of the MPT a series of 100 k.y. glacial–interglacial cycles characterize the Late Pleistocene climate record (Clark et al., 2006; McClymont et al., 2013). Recent high-resolution seismic reflection profiles in the Bering Trough image glacial erosion surfaces that extend to the shelf edge, likely correlating with widespread Pleistocene glacial advances potentially associated with Glacial Interval C (Berger et al., 2008a). The northwestern lobe of the Cordilleran ice sheet was present on the shelf during the LGM (Fig. F7) (Davies et al., 2011), although the full extent of ice cover is poorly known (Molnia, 1986; Manley and Kaufman, 2002). Terrestrial records suggest that the regional LGM expression lasted from 23,000 to 14,700 calendar years before present (cal y BP), with evidence for millennial-scale cooling and transient glacial readvances during deglaciation (Engstrom et al., 1990; Mann and Peteet, 1994; Briner et al., 2002; Hu et al., 2006; Davies et al., 2011). Retreat of the Bering Glacier off the continental shelf following the LGM likely occurred after 16,000 cal y BP, as indicated by peat accumulation in parts of the Bering foreland, and had apparently retreated well onshore by 14,700 cal y BP (Peteet, 2007; Davies et al., 2011).

Continental margin and Surveyor Fan stratigraphy

Varying degrees of glacial erosion, tectonic deformation, and rock exhumation in southern Alaska and northwestern British Columbia since the Miocene (Lagoe et al., 1993; Rea and Snoeckx, 1995; Enkelmann et al., 2010; Spotila and Berger, 2010) supplied sediment into the Gulf of Alaska, leading to periodic depositional pulses on the continental margin and Surveyor Fan (Stevenson and Embley, 1987; Lagoe et al., 1993; Rea and Snoeckx, 1995; Zellers, 1995; Worthington et al., 2010; Reece et al., 2011).

The seismic stratigraphy of the Bering Trough between the Kayak Island Zone and Pamplona Zone (Fig. F3) has been imaged at several scales (Fig. F8). Regional seismic surfaces, deformation structures, and seismic sequences are observed (Table T1; Figs. F9, F10), but age control based on cuttings from industry wells is limited (Zellers, 1995). Taken together, the geometries of the upper and lower sedimentary packages within the Bering Trough record glacial dynamics of the shelf-crossing Bering Glacier (Fig. F11, F12).

Depositional basins inboard of the Khitrov Ridge along the continental slope west of the Bering Trough contain a sedimentary record of glacial–interglacial sedimentation overprinted by active tectonic deformation (Fig. F13). The seismic facies at this site are interpreted to represent contrasting hemipelagic (interglacial) and glacimarine (glacial maxima) cyclicity in sediment lithofacies.

The Neogene record of sedimentation on the Surveyor Fan, a terrigenous depocenter that comprises the majority of the Alaskan Abyssal Plain (Fig. F6), comes from DSDP Leg 18 and ODP Leg 145 drilling. Leg 18 drilled and cored five sites across the southwestern corner of the Surveyor Fan, across the Aleutian Trench, and up the slope of the accretionary prism (Fig. F1) (Kulm, von Huene, et al., 1973). In 1992, Leg 145 occupied an additional site (887) in the far southwestern Gulf of Alaska on the Patton-Murray Seamounts (Figs. F1, F6) (Rea, Basov, Janecek, Palmer-Julson, et al., 1993). At DSDP Site 178, terrigenous turbidites, gravelly to diatomaceous mud, and claystone of the Surveyor Fan overlie marine chalk, barren clay, and basaltic basement of Pacific plate crust (Fig. F14) (Kulm, von Huene, et al., 1973). The Chirikof and Surveyor Channel systems control present-day Surveyor Fan morphology and sediment distribution, but unlike other large deep-sea channels they are not associated with major fluvial systems or submarine canyons (Fig. F6) (Ness and Kulm, 1973; Stevenson and Embley, 1987; Carlson et al., 1996; Reece et al., 2011). The Surveyor Channel is >700 km long with three major tributaries (Ness and Kulm, 1973; Stevenson and Embley, 1987; Carlson et al., 1996; Reece et al., 2011). Early studies divided the Surveyor Fan into two major sequences (Ness and Kulm, 1973; von Huene and Kulm, 1973; Stevenson and Embley, 1987), termed “upper” and “lower,” that were based on sedimentation rates and differences in acoustic facies imaged in 2-D seismic reflection profiles. The boundary between the two sequences hypothetically represented a shift from a lower, coarser grained facies to an upper, finer grained facies possibly associated with Surveyor Channel inception and its influence on fan sediment distribution during deposition of the upper sequence (Ness and Kulm, 1973; Stevenson and Embley, 1987). Reece et al. (2011) used reprocessed U.S. Geological Survey (USGS) and 2004 high-resolution and 2008 crustal-scale seismic reflection data to correlate stratigraphic changes and fan morphology through time. In contrast to the previous interpretation of two seismic sequences within the fan, they recognized three sequences (I to III) that are regionally extensive deposits likely related to increases in exhumation on land and regional response to global changes in climate (Figs. F14, F15). Two-way traveltime (TWT) thickness (isopach maps) for the three sequences shows a changing depositional history on the Surveyor Fan (Reece et al., 2011). Sequence I depocenters are prevalent in topographic lows between basement highs, showing no significant spatial variation, which reflects infilling of preexisting Pacific plate topography. Deposits of Sequences II and III exhibit a distinct change in the locus of accumulation to shelf proximal depocenters that thicken into the Yakutat slope, with Sequence III being thicker and covering a much larger area (Fig. F16).

The correlation between seismic reflection profiles projected into the stratigraphy at Site 178 places tentative ages on sequence boundaries (Fig. F14). The Sequence I/II boundary occurs ~330 m within the drilled interval at Site 178 and is placed at ~5 Ma, near the beginning of Glacial Interval A, based on ⁴⁰Ar/³⁹Ar dating of ash layers (Hogan et al., 1978). At 130 mbsf, the Sequence II/III boundary is tentatively dated ~1 Ma based on correlation with a magnetic polarity reversal identified at Site 178 (von Huene et al., 1973), making it coincident with the onset of Glacial Interval C. Both sequence boundaries are synchronous with doubling of terrigenous sediment fluxes observed at Site 887 at ~5 and ~1 Ma, but based on this chronology no regional sequence boundary projected into Site 178 correlates with the onset of Glacial Interval B (Reece et al., 2011).

Spatial variability in seismic facies and stratigraphy reveals the temporal evolution of fan stratigraphy. The thickening of Sequences II and III into the Yakutat Terrane continental slope is evidence of the long-term connection of the Surveyor Fan to the Yakutat shelf (Stevenson and Embley, 1987; Reece et al., 2011). Because of the dextral transform motion of the Pacific plate and Yakutat Terrane along the Queen Charlotte-Fairweather Fault, Surveyor Fan provenance likely varied from southern Coast Mountains sources in older fan sediment to St. Elias Range sources in younger fan sediment, similar to the sedimentary strata on the Yakutat microplate (Fig. F17) (Perry et al., 2009). The onset of Glacial Interval A may have led to reorganization of fan sedimentation by spurring Surveyor Channel genesis (Reece et al., 2011). The youngest phases of the Surveyor Channel created shelf-proximal depocenters at the base of the Yakutat Terrane slope. Glacial Interval C, with its corresponding ice advances to the shelf edge, markedly increased sediment flux to the Surveyor Fan via an extended Surveyor Channel. The Surveyor Channel system is a unique deepwater sediment delivery pathway because of its glacial source and its trench terminus, both of which may contribute to the Surveyor’s ability to maintain a major channel and evade avulsion over long periods of time (Stevenson and Embley, 1987; Reece et al., 2011).

Tectonic-climate interactions

Climatic influence on the width, structural style, and distribution of deformation in mountain belts is well established through analog, numerical, and analytical modeling studies based on critical wedge theory (Willett, 1999; Roe et al., 2006; Stolar et al., 2006; Whipple, 2009). Generally, an increase in erosional intensity through glacial or fluvial processes is predicted to accelerate rock exhumation and decrease orogen width and relief (Whipple and Meade, 2004; Roe et al., 2006). In the Chugach-St. Elias Mountains, spatial patterns along the windward side of the orogen including increased exhumation rates and exhumation of rocks from greater depth, a relative deficiency of activity along the leeward side, and relatively shallow particle exhumation pathways are all indicative of a coupled tectonic-climate “wet prowedge” system (Fig. F5) (Berger et al., 2008a, 2008b; Meigs et al., 2008). Based on apatite (U-Th)/He thermochronometry, in conjunction with offshore seismic data and modeling results, Berger et al. (2008a) proposed that a structural reorganization of the St. Elias orogen occurred in association with the onset of Glacial Interval C and the MPT. The proposed structural reorganization includes initiation of a large-scale backthrust onshore and deactivation of faults in the offshore frontal portion of the wedge (Fig. F5). However, faulting has remained active in the St. Elias Range, primarily associated with the Pamplona Zone deformation front (Bruns and Schwab, 1983; Chapman et al., 2008; Plafker et al., 1994; Meigs et al., 2008). Recent modeling (Malavieille, 2010; Simpson, 2010) suggests that the extent of active faulting and folding in a frontal wedge is highly dependent on the details of mass redistribution by climate drivers and the magnitude of incoming sediment load.

However, the localization of exhumation solely along the windward equilibrium line altitude, as indicated by apatite (U-Th)/He thermochronometry (Berger et al. 2008b), has been questioned based on reinterpretation of bedrock samples and observations of detrital thermochronometry from glaciers draining from the St. Elias Range (Enkelmann et al., 2008, 2010). A purely tectonic explanation has been proposed for the observed patterns of exhumation. These patterns are interpreted not as reflecting a temporal increase in exhumation rates over the Pleistocene, but rather as being simply driven by a southward progression of the Yakutat fold-and-thrust belt, perhaps influenced by the westward arrival of the leading edge of the thicker sedimentary cover and crust of the Yakutat Terrane (Meigs et al., 2008; Enklemann et al., 2009, 2010; Christenson et al., 2010; Worthington et al., 2012). However, onshore thermochronometry data alone cannot uniquely distinguish between orographically versus tectonically controlled temporal changes in erosion (Meigs et al., 2008).

Gulf of Alaska paleoceanography

The Gulf of Alaska, located in the subarctic northeast Pacific Ocean, is an important component of the Northern Hemisphere climate system. Modern observations indicate linkages between the ALPS atmospheric conditions, North Pacific circulation, and marine ecosystem productivity, yet paleoceanographic data describing past changes in this system are sparse. Previous paleoceanographic studies of the Gulf of Alaska have been limited to lower temporal resolution records retrieved from lower sediment accumulation rate locales that were chosen to avoid dilution by turbidites and to remain above the carbonate compensation depth (Zahn et al., 1991; McDonald et al., 1999; Galbraith et al., 2007; Gebhardt et al., 2008). The Late Pleistocene millennial-scale climate variability typical of the North Atlantic has been inferred from earlier work in the Gulf of Alaska but has only recently been confirmed for the Expedition 341 region (Barron et al., 2009; Davies et al., 2011; Addison et al., 2012).

Core EW0408-85JC, collected on Khitrov Ridge at Site U1419, provides a detailed record of the last deglacial period to the present based on lithofacies analyses, siliceous microfossils, organic matter composition and biogenic silica concentrations, redox-sensitive metals, and oxygen isotope data from planktonic and benthic foraminifers, all tied to a high-resolution age model (44 ¹⁴C dates spanning 17,400 y) (Fig. F18). Surface water freshening likely due to glacial meltwater input began at 16,650 ± 170 cal y BP during an interval of relatively ice proximal sedimentation probably sourced from the Bering Glacier (Davies et al., 2011). A sharp lithofacies transition from diamict to laminated hemipelagic sediments indicates retreat of regional outlet glaciers onto land or into fjords at 14,790 ± 380 cal y BP. A sudden warming and/or freshening of the Gulf of Alaska surface waters corresponds to this lithofacies transition and coincides with the Bølling interstadial of Northern Europe and Greenland. Cooling and/or higher surface water salinities returned during the Allerød interval, coincident with the Antarctic Cold Reversal, and continued until 11,740 ± 200 cal y BP, when onset of warming coincided with the end of the Younger Dryas (Davies et al., 2011). Two laminated opal-rich intervals (deglacial Bølling-Allerød and early Holocene) reveal discrete periods of enhanced water column productivity that likely correlate to similar features observed elsewhere on the margins of the North Pacific and are coeval with episodes of rapid sea level rise (Barron et al., 2009; Davies et al., 2011; Addison et al., 2012).

Proxy-based Holocene productivity was consistently higher than during the colder periods of expanded regional glacial activity. The finding of low productivity during the glacial and stadial intervals is consistent with previous findings from the open Gulf of Alaska but is inconsistent with the hypothesis that such changes are the result of higher upper-ocean stratification during cold intervals (Sigman et al., 2004; Jaccard et al., 2005). The Bølling-Allerød interval is laminated and enriched in redox-sensitive metals. Sedimentary δ¹⁵N is higher than normal seawater values in these laminated intervals. Based on these observations, enhanced nitrate utilization and/or relatively weak denitrification may have occurred in the North Pacific at these times (Addison et al., 2012). Remobilization of iron from continental shelves may have helped to fuel elevated primary productivity and hypoxia during these episodes (Davies et al., 2011; Addison et al., 2012).

A temporal correspondence exists between water column and sedimentation events observed in Core EW0408-85JC and last deglacial–early Holocene climate. In addition to the Bølling interstadial diamict–laminated facies transition, an early termination of the Bølling-Allerød warm interval observed in Core EW0408-85JC relative to the North Atlantic appears in a number of high-latitude North Pacific records (Davies et al., 2011). When compared to the δ¹⁸O ice core records from Antarctica (Ruth et al., 2007) and Greenland (Andersen et al., 2006; Rasmussen et al., 2006; Svensson et al., 2006), the planktonic oxygen isotope pattern of Core EW0408-85JC bears some similarity to both of these records, lending support to the idea that North Pacific climate records reflect both North Atlantic and Southern Ocean forcing (Mix et al., 1999; Davies et al., 2011).

Site survey data acquisition

In 2004, 1800 km of high-resolution multichannel seismic (MCS) reflection profiles were collected in the Gulf of Alaska aboard the R/V Maurice Ewing (Fig. F8). For the shelf and fjord profiles, the source was a dual 45/45 inch³ generator-injector (GI) air gun array producing frequencies that provide 3–5 m vertical resolution. For the deeper water lines over the Surveyor Fan, dual 105/105 inch³ GI guns were used. Processing included trace regularization, normal move-out correction, bandpass filtering, muting, frequency-wave number (f-k) filtering, stacking, water-bottom muting, and finite-difference migration (Gulick et al., 2007; Berger et al., 2008a).

In 2008, ~1250 km of MCS reflection data and ~500 km of wide-angle seismic refraction data were acquired in the Gulf of Alaska as part of the St. Elias Erosion and Tectonics Project (STEEP; Fig. F8). The primary tectonic survey targets included the offshore Yakutat-North America deformation front, the offshore St. Elias orogenic wedge, known as the Pamplona Zone fold-and-thrust belt, and the Yakutat Terrane crustal structure. The survey also targeted the Transition Fault (results presented in Christeson et al., 2010; Gulick et al., 2013) and the offshore zone of seismicity in the Pacific plate known as the Gulf of Alaska Shear Zone (Gulick et al., 2007; Reece et al., 2013). Primary stratigraphic targets included mapping the shelf and Surveyor Fan sediments to basement.

In 2011, ~3022 km of MCS reflection and ~600 km of wide-angle seismic refraction data were acquired with supporting sonobuoy refraction data (Fig. F8). The purpose of the Gulf of Alaska seismic mapping program was to image the distal parts of the Surveyor and Baranof Fans and the northern Pacific plate crustal structure that these sediment bodies interact with. The program was funded through the USGS as part of the United States assessment of Article 76 of the Law of the Sea Convention. The survey design targeted two areas for assessment and a series of science targets to better understand the tectonic-sedimentary system within the Gulf of Alaska. The two regions where the Alaskan deep-sea fans cross the U.S. Exclusive Economic Zone are where the Surveyor Fan system interacts with Aja Fracture Zone, potentially forming a thickened deposit, and with the Baranof Fan (Walton et al., in press). Science targets included the Surveyor Channel and its interaction with the Aleutian subduction zone, the Aja Fracture Zone, the Chirikof Channel, and the channels of the Baranof Fan. Site U1417 is in the imaged distal part of the Surveyor Fan to the north and west of Surveyor Channel.

Acquisition parameters for both the 2008 and 2011 seismic reflection data included a seismic source of 36 Bolt air guns with a total volume of 6600 inch³ fired every 50 m. Receivers were hydrophones in an 8 km long solid streamer with a 12.5 m channel spacing. Common midpoint spacing was 6.25 m. Seismic data processing included trace regularization, normal move-out correction, bandpass filtering, muting, stacking, and f-k migration using Paradigm Geophysical FOCUS software (detailed seismic processing work flow in Worthington, 2010). Vertical resolution at the seafloor for this data set is 20–30 m.

Locations of a 1975 USGS survey and a 1979 survey by Western Geophysical are shown in Figure F8 (black lines crossing shelf and slope). These seismic profiles provide ~30 m vertical resolution and reliably image up to 4 s TWT of the subsurface, ~3.5 km at 1750 m/s seismic velocity. These profiles image major faults, fault-related folds, and unconformities, roughly illustrating stratigraphic and structural relationships along the margin (Bruns, 1983, 1985; Bruns and Schwab, 1983; Lagoe et al., 1993; Zellers, 1995). Also shown in Figure F8 (thin black lines crossing slope and fan) are single-channel seismic profiles acquired as part of the 1980s USGS GLORIA program. Within the Surveyor Fan, these lines generally penetrated to basement (ocean crust) and provided a ~30 m vertical resolution at the seafloor. Processing of the 1970s USGS data used in Reece et al. (2011) and Worthington et al. (2010) included bandpass filtering, muting, normal move-out correction, and stacking. The 1980s USGS data processing included trace editing and balancing, muting, and bandpass filtering.

In addition to MCS data, high-resolution subbottom profiles and multibeam bathymetry exist for the Expedition 341 region. High-resolution subbottom profiles were collected during the 2004 Maurice Ewing cruise using a Bathy 2000-P compressed high-intensity radar pulse (CHIRP) subbottom profiler. High-resolution (5–20 m²) multibeam sonar data were collected at the proposed drill sites. At water depths <800 m, a SIMRAD EM1002 midwater high-resolution multibeam sonar was used, and in deeper water (i.e., Site U1418), a STN ATLAS Hydrosweep DS-2 multibeam sonar was used. Additionally, in 2005, >162,000 km² of high-resolution (~100 m²) multibeam sonar data were collected along the base of the Yakutat slope in the Gulf of Alaska in support of the United Nations Law of the Sea extended continental shelf investigation (Fig. F1). Data were collected aboard the R/V Kilo Moana and postprocessed at the University of New Hampshire Center for Coastal Studies (USA; Mayer et al., 2005; Gardner et al., 2006). Vertical accuracy is ~0.3%–0.5% of the water depth. Additional high-resolution SeaBeam multibeam data were collected in 1988 by NOAA on the continental slope and proximal Surveyor Fan southwest of Kayak Island.

The supporting site survey data for Expedition 341 are archived at the IODP Site Survey Data Bank and available at the Academic Seismic Portal hosted by the University of Texas Institute for Geophysics (www.ig.utexas.edu/sdc/).

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