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doi:10.2204/iodp.proc.341.101.2014

Expedition 341 preliminary scientific assessment

Expedition 341 was an outstanding success in terms of sediment recovered and objectives addressed across the Gulf of Alaska margin. Regional seismic data indicate transitions in seismic facies and sequences within the Surveyor Fan and continental margin that lead to interpretations of climate-driven increases in sediment transport and deposition with potential feedback on the orogenic belt. Testing these interpretations required recovery of sediment across this glaciated margin. Meeting the scientific objectives by drilling a high–depositional rate continental margin transect demanded essentially complete recovery of important climate transitions recorded in the uppermost strata and adequate recovery of challenging deep-penetration targets exceeding 700 m CSF-A in both deep (>4000 m) and shallow (<300 m) waters. In both aspects, the resultant drilling was a notable success. Material was recovered along a cross-margin transect from the deepwater Surveyor Fan at both distal (Site U1417) and proximal (Site U1418) locations, the mid-slope region (Sites U1419 and U1421), and the continental shelf (Site U1420). Recovery was very good (70% at Site U1417) to excellent (86% at Site U1418) on the Surveyor Fan sites and very good on the upper slope Site U1419 (77%). At these three sites, real-time correlation among holes developed long intervals of verified 100% recovery that provide the opportunity for ultrahigh-resolution studies. Recovery was limited both at shelf Site U1420 (14%) and slope Site U1421 (23%) because of the expected challenges of deep subseafloor drilling on a glaciated margin setting. Nevertheless, recovered intervals proved critical to the overall expedition objectives. At Sites U1417–U1419, a complete stratigraphic splice was developed to depths exceeding 100 m CCSF-B, spanning a telescoping temporal record with increasing resolution from ~1.8 Ma at Site U1417 to ~0.2 Ma at Site U1418 and ~0.04 Ma at Site U1419. The corresponding sedimentation rates (8–13 cm/k.y. at Site U1417 and 100–150 cm/k.y.) within the spliced intervals at the two fan sites provide the potential to resolve Middle–Late Pleistocene changes in sedimentation and climate on glacial–interglacial and higher resolution timescales. On the shelf and slope, Sites U1420 and U1421 recovered sediments that are all <0.781 Ma based on magnetostratigraphy, with the slope site biostratigraphically constrained to be <0.3 Ma; these ages correspond to sedimentation rates of >100 cm/k.y. The sedimentary record from Site U1417 allows us to examine the paleoceanography, regional tectonics, and deep-sea sedimentary environment from the late Miocene to the present through key Neogene climatic transitional intervals, including the MPW, the early Pleistocene INHG, and the MPT. We observed 24 tephra layers at Site U1417 that record Neogene and Quaternary volcanic activity in the Gulf of Alaska, and we expect that these tephra will provide important independent chronological constraints for the Pleistocene. The Site U1418 record provides the opportunity to resolve how paleoceanography and sedimentation varied within and following the MPT at exceptionally high (submillennial) temporal resolution. Sites U1419–U1421 contain glacially influenced strata that have accumulated since the MPT on the shelf and slope and provide a chronostratigraphy and sedimentary record to test Expedition 341’s fundamental climate-tectonic hypotheses. The continuous spliced record at Site U1419 provides a key proximal record of contrasting intervals of glacial and biogenic sedimentation in the Late Pleistocene at intermediate water depths.

The overarching goal of Expedition 341 was to document continental margin sedimentation during a period of strong tectonic and climatic forcing. In this respect, Expedition 341 has given us a unique opportunity to understand the interaction of tectonics, climate, and sedimentation on a high-latitude continental margin. Our success demonstrates that scientific ocean drilling has the potential to recover similar records from other optimally located high-latitude settings. All primary science objectives were addressed. Below we explore these objectives and highlight major results within each.

Documenting the tectonic response of an active orogenic system to late Miocene to recent climate change

A fundamental hypothesis tested by Expedition 341 is that the onset of an erosive temperate glacial environment can radically alter mass fluxes within an orogenic wedge, which in turn leads to a tectonic setting that is out of equilibrium, causing a positive feedback response of exhumation, erosion, and sedimentation. The sedimentary record collected during Expedition 341 spans several important climate transitions (MPW, INHG, and MPT) and contains a high-quality biostratigraphic and paleomagnetic record, leading toward a refined chronostratigraphy that allows us to test this hypothesis. A remarkable discovery from the expedition is the apparently very young depositional age and extraordinarily high Pleistocene sediment accumulation rates of the shelf and slope strata recovered at Sites U1420 and U1421. A significant mass flux from the orogen since the MPT is demonstrated by (1) the existence of >1 km of Middle to Late Pleistocene poorly sorted (from mud to boulders) glacigenic sediment on the shelf and (2) ~700 m on the upper slope that is partly coeval with accumulation of >1 km thick finer grained glacimarine and hemipelagic strata in the proximal fan at Site U1418. Burial of an outer shelf thrust fault by >130 cm/k.y. glacial sedimentation likely resulted in cessation of thrusting on this structure while shortening continued both onshore and seaward from the locus of accumulation within the Bering Trough (Pavlis et al., 2012; Worthington et al., 2010). These exceptionally high Pleistocene near-shore fluxes are supported by the notable discovery from the Surveyor Fan at Site U1417 that the boundary between the lowermost seismic unit and the overlying strata (Sequence I/II) dates to only the INHG (~2.56 Ma; Glacial Unit B of Lagoe et al., 1993), rather than the older ~5–6 Ma onset of Glacial Unit A of Lagoe et al. (1993). This finding implies much greater sediment delivery from onshore sources in the Pleistocene than previously calculated. Additionally, sediment accumulation rates apparently increased in both the distal and proximal Surveyor Fan across the MPT and further increased into the Late Pleistocene, when extremely high sedimentation rates (100–300 cm/k.y.) are also observed on the shelf and slope. Another finding is that some key stratigraphic changes are observed within the Middle Pleistocene, including the first advance of the Bering Glacier to the shelf edge, the associated change to a possible trough-mouth fan depositional system on the slope, and a transition in facies and accumulation rate within the proximal fan, which may or may not be coeval. These increases in sediment accumulation rates possibly reflect enhanced exhumation due to positive feedback between glacial erosion and tectonic shortening within the St. Elias orogen. Pleistocene sediment on the margin and fan is potentially derived from both the Bering Glacier (Sites U1418–U1421) and the Malaspina/Seward/Hubbard Glaciers (Site U1417) based on seismic reflection mapping (Reece et al., 2011; Worthington et al., 2010); the recovered sediments reflect bedrock sources that both glacial systems are currently eroding. The recovery of a long and relatively continuous sequence of sediment that spans from the Miocene to Holocene allows us to evaluate the relative influence of climate change on exhumation throughout the St. Elias orogen. Postcruise analysis of sediment provenance will constrain this locus of erosion, linking it to onshore patterns of exhumation to ultimately test whether rapid erosion has the potential to lead to a positive feedback in exhumation in an active orogen.

Establishing the timing of advance and retreat phases of the NCIS to test its relation to dynamics of other global ice sheets

Expedition 341 drilling recovered an unprecedented North Pacific Pleistocene sedimentary record of glacigenic sediments, accumulating at rates that likely resolve orbital- and suborbital-scale influences. An important first-order observation of the expedition is that glacial conditions dominated the region during the Pleistocene. Interglacial events were brief and extreme. Within the glacial intervals, variations in lithofacies indicate dynamic advances and retreats of regional outlet glaciers and likely also the northern Cordilleran ice sheet feeding into them. The glacial signal in the deep sea is recorded as thin beds of diamict and laminated mud, whereas on the shelf and slope it is dominated by clast-bearing to laminated mud and diamict. The retrieval at Sites U1417–U1419 and U1421 of Holocene interglacial sediments and microfossils provides a means to identify comparable interstadial intervals in the deeper sedimentary record. The shipboard chronology allows us to place the stratigraphy and proxies of glacial–interglacial intervals into an initial temporal context. The good preservation of calcareous microfossils at all sites is potentially sufficient to provide radiocarbon and oxygen isotope chronostratigraphic control. An exceptional shipboard paleomagnetic reversal chronology provides a temporal framework to guide future analyses of particular glacial–interglacial intervals. In particular, the influx of fine-grained glacial material combined with high accumulation rates leads to exceptional paleomagnetic records that are likely to result in highly resolved RPI chronologies.

The depth-transect drilling approach allowed us to address this particular objective at four scales. First, at Site U1417 the Gauss/Matuyama paleomagnetic polarity transition (2.581 Ma) occurred slightly deeper than the first evidence of ice-rafted sediment deposition at the site, providing a key chronostratigraphic marker that will allow Alaskan tidewater glaciation to be put into a global context. This increase of glacial sediment input in the northeast Pacific first occurred near the Pliocene/Pleistocene boundary and accelerated in the Middle Pleistocene. These results demonstrate long-term synchronicity of local impacts with global climate and demand a global cause. Second, Site U1418 provides an ultrahigh-resolution (potentially millennial-scale) record of climate and glacial erosion over the past ~1 m.y. With high-resolution chronologies being developed postcruise, this site will demonstrate possible relationships between Cordilleran and global glaciation in response to orbital and greenhouse gas forcing. Site U1419 provides an ice-proximal view of glacier advances, with potentially century-scale resolution over the past ~60 k.y. This site provides key material for detailed study of glacier dynamics over the past glacial cycle, with the potential for a highly resolved chronology. Finally, Sites U1420 and U1421 demonstrate the existence of extremely thick Pleistocene depocenters of glacigenic sediment in shelf/slope settings where tectonic forcing drives accommodation. Seismic images linking these sites show a stratigraphic transition from an interval of a glaciated inner shelf, aggradational outer shelf, and associated slope sequences to a time of glacial advances to the shelf edge and prograding grounding line wedges that transition into a possible trough-mouth fan on the slope. Examination of these sequences and correlation to δ¹⁸O chronostratigraphy may allow the determination of the timing of glacial advances to the outer shelf edge that coincided with the angular unconformity and the transition in sequences on the slope that reflects shelf-edge progradation (Fig. F9). This unconformity and associated change in slope sequences, which is clearly younger than the MPT and not associated directly with it, may or may not tie with the changes in sedimentation and sediment routing on the Surveyor Fan, which are observed at Site U1418 within the Middle Pleistocene section.

Consequently, the sedimentary record has the temporal fidelity to regionally map individual advance–retreat cycles of grounded ice and the ice-proximal and ice-distal facies. These drilled sequences may provide global analogs for shelf/slope records of glacial cycles.

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

The integration of existing high-quality seismic reflection data with the sedimentary and downhole record collected during Expedition 341 allows for postcruise interpretation of seismic facies based on the ability to correlate with lithostratigraphic units and downhole logs. By assigning lithologic facies, representing various sedimentary processes, to seismic facies mappable throughout the Surveyor Fan and by tying these facies relationships into high-resolution chronology and proxy records of paleoenvironment and sediment provenance, we have a truly unique opportunity to constrain the timing and mechanisms of sediment transfer and accumulation from a high-latitude continental shelf to the deep sea. The establishment of an independent chronology from the MPT to the present along a depth transect of sites allows for unprecedented ability to link the transfer from shelf-to-slope/inner fan to distal fan throughout the Late Pleistocene. By mapping Expedition 341 age-calibrated seismic reflectors between Sites U1417 and U1418 and throughout the Surveyor Fan, insight into how sediment is routed from shelf sources to deep-sea sinks is possible. Notable examples of how this objective was addressed include the following:

• We have established the relative timing of a transition from a distributed, likely nonglacial Miocene–Pliocene deep-sea fan depocenter to the glacial Pleistocene Surveyor Channel, creating a channel-lobe system with the lobe extending into, and being truncated and subducted along, the Aleutian Trench.
• Results from Site U1418 provide evidence for the potential relationship between the Surveyor Fan MTD within the proximal fan, dated at ~1.0–1.2 Ma, and the subsequent formation of wide-channel systems that may be conduits of glacigenic sediment into the Aleutian Trench.
• We have documented the rapid formation of a middle to Late Pleistocene glacially dominated shelf depocenter beneath the modern Bering Trough.
• The combined results from Sites U1418, U1420, and U1421 indicate that there was a Middle–Late Pleistocene continental margin stratigraphic transition, possibly driven by the glacigenic filling of tectonically created accommodation space, that ultimately led to the development of shelf-crossing troughs and the direct delivery of sediments to the Surveyor Fan.
• Lastly, we note that the variable contribution from, and likely composition of, organic matter found from proximal to distal sites indicates evolving source regions of sediment supplied to the Gulf of Alaska and/or evolving transport pathways for such sediment (e.g., ice rafting, mass transport, or marine primary productivity).

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

The sedimentary, micropaleontological, and geochemical observations made during Expedition 341 indicate a dynamic Neogene setting spanning from the late Miocene to the present. Variable accumulation of biosiliceous and calcareous material in both space and time attests to a surface ocean that is sensitive to forcing by regional controls on productivity. Preliminary pore water, headspace gas, and sediment geochemistry data reveal that the accumulation rates of organic matter and the intensity of its remineralization within the sediments have changed with time. Although total carbon (organic and carbonate) contents are similar between sites, higher sediment accumulation rates at proximal sites indicate greater burial of biogenic matter at these locations. There is downcore evidence at all sites for fluctuating concentrations of marine and terrestrial organic matter, confirming that productivity and carbon cycling have changed over a range of timescales. The diverse micro- and sometimes macrofossil content at most sites records a biogenic transect from the deep sea to the shelf from the modern oligotrophic (iron-limited and nitrate-replete) central gyre to the eutrophic (iron-replete and nitrate-limited) shelf. This transect opens opportunities for postcruise study of biogenic sedimentation related to overlying production. Site U1419 on the continental slope offers an excellent opportunity to constrain the history of intermediate water ventilation over the past 60 k.y. Regarding freshwater input to the northeast Pacific, pore waters at ice-proximal Sites U1419–U1421 show significantly reduced salinity and chlorinity values from a few tens of meters subseafloor to several hundred meters depth. This finding indicates that glacial meltwater was stored, or is actively flowing, through these deposits and thus documents a yet-unstudied component to glacial meltwater dynamics in glacially influenced continental shelf/slope deposits beyond ocean-surface meltwater plumes.

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

The Gulf of Alaska is a location where paleomagnetic records are likely to be sensitive to important geomagnetic features such as high-latitude flux lobes and the Pacific dipole window. With a glaciated, tectonically active margin that ensures an almost continuous supply of fine-grained lithogenic material accumulating at exceptionally high rates, quality paleomagnetic records that can provide much-needed stratigraphic information are likely to exist. The sites drilled during Expedition 341 provide, for the first time, the quality of material required to explore such geomagnetic concepts. Shipboard measurements of NRM from half-core sections constrained by, and enriched from, the determination of physical, chemical, biological, and lithostratigraphic properties highlight the paleomagnetic potential of this expedition. At each site, we recovered paleomagnetic records of normalized remanence and paleomagnetic secular variation, and at Sites U1417 and U1418, we recovered several polarity transitions, some in exceptional temporal detail and in replicate core sections. There is high-fidelity magnetic material to the recovered material, with relatively little diagenetic overprint and little magnetic dissolution. Sites U1417 and U1418 within the Surveyor Fan represent the highest resolution deep-sea paleomagnetic record yet recovered from the North Pacific Ocean. Site U1417 recorded an expanded record across the Matuyama/Brunhes boundary, the upper and lower Jaramillo Subchron, and several reversals within the Gauss Chron. Site U1418 sediments include an extremely high resolution record of paleointensity variations within the Brunhes Chron. Site U1418 also includes one of the highest-resolution upper and lower Jaramillo events globally. Site U1419 largely contains a well-resolved Late Pleistocene paleomagnetic record that can potentially be tied to radiocarbon and oxygen isotope chronologies to examine paleomagnetic secular variation and paleointensity during this time interval. At Sites U1420 and U1421, NRM indicating normal polarity of the Brunhes Chron is a key temporal component of our age model for these critical sites in terms of testing climate-tectonic interactions. The potential for radiometric and δ¹⁸O age control likely suggests that we will tie these new high-resolution views of magnetic field variability to independent chronologies, providing additional constraints on the process of magnetic reversals and secular variations, practical chronostratigraphic constraints based on relative paleointensity, and opportunities for environmental reconstruction.

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