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doi:10.2204/iodp.pr.341.2014

Scientific objectives

The aim of drilling the southern Alaska margin is to obtain a Neogene–recent sedimentary record of NCIS glaciation and its influence on tectonic processes and relationship with regional and global paleoclimatic changes. Focus is placed on establishing the timing and locus of NCIS expansion during the Pliocene and Pleistocene and its impact on surface processes and freshwater and sediment fluxes to the subarctic Pacific Ocean. Erosion and sediment redistribution during glacial–interglacial cycles may have a direct effect on mountain building and deformation; thus, one goal is to determine the timing of changes in deformation patterns and sedimentary fluxes to the continental shelf and the adjacent deep-sea sediment fan. Priority is placed on documenting the NCIS response to global climate forcing. A unique component of Expedition 341 is the availability of extensive adjacent onland studies of glacial and tectonic processes and existing seismic coverage on the margin, which, when coupled with the age and stratigraphic controls provided by drilling, will allow for a more complete source-to-sink study of the depositional history, glacial record, and sequence stratigraphic significance of these strata. Sampling the rapidly accumulating Neogene glacimarine sediments will document the spatial and temporal behavior of the geomagnetic field at extremely high temporal resolution. Such data are missing from this part of the planet and are required to assess the geodynamo processes that control secular variation and geomagnetic polarity reversals.

To address these objectives, we cored, logged, and analyzed sedimentary records from a five-site depth transect from the distal Surveyor Fan to the zone of active deformation on the outer shelf to recover strata that contain a record of the most significant climate and tectonic events of the southern Alaska continental margin with varying temporal resolution and stratigraphic completeness. Of particular note are stratigraphic intervals that have the potential to preserve records of the key phases of the evolution of the NCIS, such as the late Miocene inception of tidewater glaciation, warm early Pliocene events, large-scale early Pleistocene expansion of glacial coverage, and the mid-Pleistocene glacial intensification leading to the onset of highly erosive ice streams (Fig. F2). The expected chronostratigraphy and integrated multidisciplinary sediment provenance and climatic proxy record–based reconstructions of glacial dynamics are fundamental to understanding tectonic-climate interplay and the processes responsible for developing high-latitude continental margin stratigraphy.

The specific scientific objectives for Expedition 341 are as follows:

1. Document the tectonic response of an active orogenic system to Pliocene and mid-Pleistocene climate change.

Our fundamental hypothesis is that the St. Elias orogen has undergone perturbation that has markedly changed the patterns and rates of deformation and exhumation in the orogenic wedge (Figs. F2, F5). Enhanced glacial erosion associated with the MPT and the establishment of highly erosive ice streams led to substantial mass redistribution in the wedge, shutting down existing regions of active deformation and refocusing the deformation and exhumation patterns of the orogen (Berger et al., 2008a; Worthington et al., 2008, 2010; Chapman et al., 2008). Testing the hypothesis that the MPT led to rapid intensification of erosion along the windward side of the mountain range first requires that we establish the baseline erosion conditions in the orogen prior to this climate perturbation, which involves integration of results of the Surveyor Fan seismic reflection data sets (Fig. F16) and sediment mass fluxes and provenance records from the most distal Site U1417. Documenting climatic influence on enhanced exhumation requires establishing a connection between a change in sediment provenance to more windward source rocks (e.g., Yakutat terrane) and establishing glacial conditions, which will require sediment accumulation rates and provenance records from Sites U1417 and U1418, as they likely contain a distal (and complete) and proximal record of the Surveyor Fan sequences, respectively, but are still within a reasonable drilling depth. Lastly, addressing the hypothesis that the onset of ice streams completely altered the deformation and exhumation patterns in the orogenic wedge (Berger et al., 2008a) will require age control, sediment accumulation rates, and provenance records from the more proximal Sites U1418–U1421 near the Bering Glacier, where tectonic deformation patterns have been shown to evolve with sedimentation (Worthington et al., 2010). Drilling will also allow testing the alternative hypothesis that rates of exhumation and erosion have not changed in the past 5 m.y. and that the locus of exhumation has steadily progressed southeastward with the encroachment of thicker crust (Enkelmann et al., 2008, 2009, 2010).

2. Establish the timing of Neogene advance and retreat phases of the northwestern Cordilleran ice sheet to test its relation to global ice sheet dynamics.

Previous scientific drilling on glaciated margins similar to southern Alaska has provided a rich and detailed examination of global Neogene ice dynamics including DSDP Legs 18 (Kulm, von Huene, et al., 1973) and 28 (Hayes, Frakes, et al., 1975); ODP Legs 105 (Srivastava, Aurthur, Clement, et al., 1987), 113 (Barker, Kennett, et al., 1990), 119 (Barron, Larsen, et al., 1989), 145 (Rea, Basov, Janecek, Palmer-Julson, et al., 1993), 152 (Larsen, Saunders, Clift, et al., 1994), 178 (Barker, Camerlenghi, Acton, et al., 1999), and 188 (O’Brien, Cooper, Richter, et al., 2001); and IODP Expedition 318 (Escutia, Brinkhuis, Klaus, and the Expedition 318 Scientists, 2011). We expect that drilling on this glaciated margin will complement these expeditions by filling a substantial gap in knowledge of North American ice dynamics by establishing the timing of advance and retreat phases of the NCIS throughout the Quaternary. Establishing the timing of NCIS advance–retreat cycles will address a major challenge in Quaternary paleoclimatology, which is to know the extent to which glacial-age climate change was a synchronous worldwide event and what the driving mechanisms were for potentially propagating millennial-scale warming–cooling cycles around the globe (oceanic, atmospheric, or both) (Clapperton, 2000; Mix et al., 2001; Hill et al., 2006). Although many records provide strong evidence for millennial-scale climate change in the northeast Pacific in the Quaternary (Clark and Bartlein, 1995; Behl and Kennett, 1996; Hendy and Kennett, 1999; Grigg et al., 2001; Hendy and Cosma, 2008; Davies et al., 2011), the timing and character of these variations in relation to North Atlantic or Southern Ocean records are still unknown, largely because of uncertainties resulting from the scarcity of high-resolution records from the region. The Expedition 341 drilling transect addresses these issues because the regional climate and oceanography are highly sensitive to atmospheric-oceanic dynamics in the North Pacific (Bartlein et al., 1998). The Expedition 341 drilling program provides a higher resolution chronology and a more complete record of glacial activity in the St. Elias orogen, which will allow us to assess the timing of the changes relative to the established records of global forcing (i.e., global δ¹⁸O stack).

3. Conduct an expanded source-to-sink study of the complex interactions between glacial, tectonic, and oceanographic processes responsible for creation of Neogene high-latitude continental margin sequences.

The Gulf of Alaska margin offers the opportunity for expanded study of the complex interactions between glacial, tectonic, and oceanographic processes responsible for creation of one of the thickest, most complete Neogene high-latitude continental margin sequences (Stevenson and Embley, 1987; Lagoe et al., 1993; Worthington et al., 2010; Reece et al., 2011). In southern Alaska, high sediment accumulation rates driven by the interaction of glacial processes with a dynamic tectonic setting have resulted in the substantial growth of this continental margin (Figs. F11, F12, F16) (Lagoe et al., 1993; Plafker et al., 1994; Willems, 2009; Worthington et al., 2010), and this drilling transect coupled with the onshore work accomplished as part of STEEP to examine sediment production and transfer will allow us to document the depositional history, glaciological record, and sequence stratigraphic significance of these strata in a source-to-sink context. To test hypothetical models of glacial-sequence formation for temperate glacimarine settings (Dunbar et al., 2008) and those specific to Alaska (Fig. F11) (Powell and Cooper, 2002; Willems, 2009), we make use of the extensive seismic coverage on the shelf and the age and stratigraphic controls provided by our drilling program. Emphasis will be placed on documenting how the sedimentary “signals” of tectonic- and climate-induced changes in sediment production vary through the morphodynamic elements of glacimarine sediment dispersal systems.

4. Understand the dynamics of productivity, nutrients, freshwater input to the ocean, and surface and subsurface circulation in the northeast Pacific and their role in the global carbon cycle.

Drilling on the Gulf of Alaska continental margin will create a high-resolution (millennial) view of variability in productivity and water column circulation under a range of different forcings, including global-scale factors such as insolation and CO₂ and regional factors such as sea ice and runoff. The North Pacific is currently a low-salinity region, which inhibits large-scale intermediate and deepwater formation (Emile-Geay et al., 2003). However, evidence exists for enhanced Pacific meridional overturning circulation that has antiphase activity with Atlantic meridional overturning circulation (Okazaki et al., 2010; Menviel et al., 2012). The dynamics of Northern Hemisphere freshwater and precipitation fluxes to the respective Atlantic and Pacific Oceans may be the primary control on large-scale intermediate and deep water-column circulation in the Pacific. Drilling also will allow us to compare and contrast the magnitude and scales of variability in water-column productivity between glacial, interstadial, and interglacial conditions, which differ in detail such as insolation forcing, sea level, and so on. Productivity maxima events are widespread around the rim of the North Pacific (e.g., Mix et al., 1999). At Site U1419, a high-resolution chronology based on nearby shallow piston coring links these events locally to episodes of global sea level rise, leading Davies et al. (2011) to conclude that remobilization of iron and other limiting nutrients from continental shelves and inundated estuaries during sea level rise (e.g., Lam and Bishop, 2008; Severmann et al., 2010) contributes to events of productivity and hypoxia around the margins of the North Pacific. Assessing this hypothesis will require finding similar events associated with earlier sea level rises in the region.

A consequence of this episodic enhancement of productivity coupled with glacial-induced changes in terrigenous carbon supply is variability on a range of timescales and forcing conditions in the fluxes and burial of C, N, and Si and plankton assemblages on the Gulf of Alaska margin (Davies et al., 2011; Addison et al., 2012). In general, the paleoproductivity questions addressed by this expedition are fairly unique and significant because this understudied region is very productive and hosts important fisheries and ecosystems (Stabeno et al., 2004). Drilling offers a unique opportunity to study how the past Gulf of Alaska marine ecosystem behaved during earlier periods of warmth, several of which are likely models for future warming trends.

5. Document the spatial and temporal behavior during the Neogene of the geomagnetic field at extremely high temporal resolution in an undersampled region of the globe.

Over the last decade, our understanding of the paleomagnetic record during the Pliocene–Pleistocene has improved substantially, providing new techniques and significantly improving stratigraphic resolution and reliability. Resulting largely from ODP/IODP drilling of ocean sediments, we now know that the strength of Earth’s magnetic field (paleointensity) varies globally on suborbital timescales for at least the last 1.5 m.y. (e.g., Channell et al., 2009), that short-duration (millennial or less) geomagnetic polarity events (magnetic excursions) are not only real but also common components of field behavior (e.g., Lund et al., 2001, 2005; Channell et al., 2002), and that polarity transitions display complex though reproducible behaviors (e.g., Channell et al., 1998; Clement, 2004; Mazaud et al., 2009), all hinting at the dynamics that drive geomagnetic change. New records have also resulted in improved chronologies (Channell et al., 2008), allowing increasingly reliable temporal calibrations and improved resolution of magnetic stratigraphic techniques. Global relative paleointensity (RPI) stacks providing orbital resolution tuning targets extend back over 2 m.y. (Valet et al., 2005), suborbital resolution stacks extend back over 1.5 m.y. (Channell et al., 2009), and multimillennial records are available for the last 75 k.y. (Laj et al., 2004). Though constrained by fewer records, a western Pacific regional stack since 3 Ma (Yamazaki and Oda, 2005) provides additional tuning targets that may prove to be globally applicable. The RPI record is being refined through ongoing IODP research, and its integration with other stratigraphic and absolute dating techniques was a primary objective of North Atlantic IODP Expeditions 303 and 306 (e.g., Channell, Kanamatsu, Sato, Stein, Alvarez Zarikian, Malone, and the Expedition 303/306 Scientists, 2006). RPI, together with the recognition of short-duration polarity events (e.g., ODP Leg 172 Science Party et al., 1998, 2001; Guyodo and Valet, 1999; Channell et al., 2002; Singer et al., 2002), provides additional stratigraphic opportunities through what has been termed the geomagnetic instability timescale (Singer et al., 2002).

However, much of this understanding is derived from data obtained from a limited part of the world. Historical data, dynamo models, and some paleomagnetic records attest to the importance of regions of concentrated flux that result in longitudinal asymmetry of the geomagnetic field. These asymmetries include subdued secular variation in the Pacific relative to the Atlantic hemisphere and regions of concentrated geomagnetic flux (flux lobes or bundles) over Canada and Siberia at ~60°N latitude (Fig. F19). If truly long-lived, these asymmetries imply that the structure of the geodynamo reflects lower mantle control, possibly through regulation of the long-term heat flux from the core (Bloxham and Gubbins, 1987; Bloxham, 2000). As such, these mantle-controlled nonaxisymmetric flux concentrations could provide organizing structures that may control much of the dynamics of the geomagnetic field.

The ability to develop magnetic stratigraphies that allow regional to global correlation over a range of timescales will be important to the success of IODP drilling in southern Alaska. Preliminary paleomagnetic results from the Gulf of Alaska (Davies et al., unpubl. data) suggest that the proposed sites record geomagnetic field variability consistent on submillennial scales with independently dated Holocene paleosecular variation records from Alaskan lakes (Geiss and Banerjee, 2003) and across North America (Lund, 1996). Long paleomagnetic time series constrained by independent chronologies from radiocarbon dating, tephrachronology, and stable isotope stratigraphies would allow Pacific paleomagnetic secular variation and relative paleointensity to be compared with the many records from the Atlantic (e.g., Channell, 1999, 2006; Stoner et al., 2000; Lund et al., 2005). Outside of reversals and excursions, few of these studies focused on the directional record, concentrating instead on relative paleointensity. By linking paleomagnetic directions and intensity between these regions, we will be able to assess geomagnetic persistence, a signature of the mantle’s influence on the geodynamo and the paleomagnetic record (Gubbins et al., 2007; Stoner, 2009; Amit et al., 2010), and to facilitate a test of the hypothesis that heterogeneities of the lowermost mantle influence the structure of the geodynamo and, therefore, the behavior of the geomagnetic field (Cox and Doell, 1964).

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