Global climate during the Neogene is distinguished by the transition into a colder, more variable world dominated by the onset and intensification of major Northern Hemisphere glaciations. This transition into the icehouse world corresponds to a global increase in erosion rates and sediment delivery to ocean basins. The effects of this increased erosion may be profound, as analyses of orogenic belts worldwide have shown that Earth systems cannot be considered to be the product of a series of distinct, decoupled tectonic and climatic processes. Rather, there is complex interplay between crustal deformation, exhumation, and climate systems. Exhumation plays a key role in controlling the regional distribution of metamorphic rocks, local climate change, and the development of structures throughout an orogen. Tectonic processes influence regional climate conditions by raising mountains that alter orographic precipitation patterns. The Neogene climate changes, in turn, likely affected tectonic processes through changes in erosion rates, which redistributed mass and subsequently altered stress regimes in orogenic wedges. Analytical models examining the coupling between glacial erosion and orogenic processes reveal that glacial erosion can significantly modify the patterns and rates of deformation in an orogenic wedge. A critical question is: At which stage of the deteriorating Neogene climate is an orogen ultimately driven into a subcritical state? And does this state lead to increased exhumation in the glaciated core of a mountain belt, enhanced topographic relief, and migration of the locus of sediment accumulation to the toe of an orogen thus impacting deformation patterns?
Addressing the linkages between global climate change, modification of surficial process dynamics, and subsequent tectonic responses requires integrated studies of orogenic and sedimentary systems in areas where specific end-members of the problem are encountered. The Gulf of Alaska borders the St. Elias orogen of Alaska and Canada, the highest coastal mountain range on Earth and the highest range in North America. This orogen is <30 m.y. in age, and mountain building occurred throughout a period of significant global climate change. This situation allowed Integrated Ocean Drilling Program Expedition 341 to examine the response of an orogenic system to the climatically driven establishment of a highly erosive glacial system. Additionally, the implications of Neogene glacial growth in the circum-North Pacific reach beyond the issue of tectonic response to increased glacial erosion and exhumation. As climate conditions determine the timing and patterns of precipitation, they control glacial dynamics, erosion, and sediment/meltwater and chemical fluxes to the ocean. Establishing the timing of northwestern Cordilleran ice sheet advance–retreat cycles in southern Alaska will address a major challenge in Neogene paleoclimatology: to determine whether glacial advances occurred synchronously around the world and what the driving mechanisms were for global millennial-scale warming–cooling cycles. Evidence for substantial changes in surface productivity in the Gulf of Alaska since the Last Glacial Maximum indicates that millennial-scale climate changes and eustasy in the northeast Pacific Ocean have a first-order effect on primary productivity and marine ecosystems. Thick Pleistocene glacimarine deposits of the Gulf of Alaska continental margin contain a rich history of climate change recorded in both proxy climate data and sediment accumulation rates that can help decipher the architecture of massive Neogene high-latitude Northern Hemisphere continental margin sedimentary sequences. Exceptionally high rates of glacigenic sediment accumulation in this region also allow for the development of a paleomagnetic record of geomagnetic field variability on submillennial scales to assess geomagnetic persistence, a signature of the mantle’s influence on the geodynamo and the paleomagnetic record.
A cross-margin transect was drilled during Expedition 341 to investigate the northeast Pacific continental margin sedimentary record formed during orogenesis within a time of significant global climatic deterioration in the Pliocene–Pleistocene that led to the development of the most aggressive erosion agent on the planet, a temperate glacial system.
Major objectives for drilling in the Gulf of Alaska were as follows:
Document the tectonic response of an active orogenic system to late Miocene to recent climate change;
Establish the timing of advance and retreat phases of the northwestern Cordilleran ice sheet to test its relation to dynamics of other global ice sheets;
Implement 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;
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; and
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.
Drilling during Expedition 341 recovered a 3240 m sedimentary record that extends from the Late Pleistocene/Holocene through the middle Miocene. Drilling at Sites U1417 and U1418 recovered distal and proximal deepwater sedimentary records, respectively. Site U1417 contains a complete and continuous interval from the mudline to 220.4 m core composite depth below seafloor (CCSF-D), the base of which was dated shipboard to 1.7–1.8 Ma, and additional material was recovered to 709 m core depth below seafloor (CSF-A). Site U1417 contains no apparent hiatuses through the late Miocene based on initial shipboard biostratigraphy and magnetostratigraphy. Site U1418 contains a complete and continuous interval from the mudline to 271 m CCSF-D, which was dated shipboard to 0.2–0.3 Ma, and additional material was recovered to 941 m CSF-A. Site U1418 contains no apparent hiatuses through 1.2 Ma based on initial shipboard chronostratigraphy. Drilling at Sites U1419 and U1421 sampled the transitional environment along the continental slope. Site U1419 is located on a small ridge at 780 m water depth between two large shelf-crossing glacial troughs, whereas Site U1421 is located downslope of the Bering Trough. Site U1419 contains a complete and continuous interval from the mudline to 100 m CCSF-D and recovered core to ~177 m CCSF-B. Shipboard chronostratigraphy indicates the cored interval is younger than 0.3 Ma, and oxygen isotopes of foraminifers analyzed immediately postcruise further constrain the recovered interval to span <0.06 m.y. Site U1421 contains a continuous interval to ~30 m CCSF-D, and an interval to 694 m CSF-A was recovered that accumulated in <0.3 m.y. based on shipboard chronostratigraphy. Lastly, Site U1420 is located most proximal to the orogen on the continental shelf within, but near the flank of, the shelf-crossing Bering Trough. Site U1420 cores consist of drilled clasts, lonestones, diamict, and mud that were deposited <0.78 m.y. ago based on shipboard biostratigraphy and magnetostratigraphy. All Expedition 341 sites reveal notable changes in seismic reflection facies and stratigraphy that can be integrated at the nested core–downhole log–seismic reflection profile scales.
A remarkable expedition discovery is the substantial sediment volume accumulating on the shelf, slope, and deep sea fan since the early Pleistocene intensification of Northern Hemisphere glaciation and more significantly since the mid-Pleistocene transition. The Expedition 341 cross-margin transect discovered transitions in sediment accumulation rates from >100 m/m.y. in the distal fan to >800 m/m.y. in the proximal fan to >1000 m/m.y. on the slope and continental shelf that provide a telescoping view of strata formation from the Miocene to the Holocene. All five sites sampled Middle Pleistocene to recent sediment and demonstrate exceptional accumulation rates. The 709 m of sediment recovered at Site U1417 records Miocene to recent deposition in the distal Surveyor Fan, including the onset of glaciation at the Pliocene/Pleistocene boundary when sedimentation rates doubled to ~100 m/m.y. Site U1418 contains an expanded Middle to Late Pleistocene sedimentary record that also includes significant increases in sedimentation from ~400 m/m.y. in the Middle Pleistocene to >1200 m/m.y. in the Late Pleistocene. Slope Site U1421 and shelf Site U1420, both proximal to the Bering Glacier during glaciations, provide cores penetrating thick sequences of poorly sorted glacigenic sediments ranging from mud to boulders, accumulating at a rate of at least 1 km/m.y. Slope Site U1419 is slightly west of the Bering Trough mouth and also has exceptional Late Pleistocene sedimentation rates (~3 km/m.y.).
Stratal lithofacies span from biogenic ooze to clast-rich diamict, both punctuated with ash, indicating a dynamic Neogene depositional environment. Lithofacies were interpreted shipboard as reflecting deposition from suspension fallout, sediment gravity flows, large-scale mass wasting, ice rafting, variation in organic productivity/preservation, and subaerial volcanic eruptions. Pleistocene strata are dominated by glacigenic sediments at all sites. The retrieval of Holocene interglacial sediments and microfossils at slope and fan sites provides a means to identify comparable interstadial periods in the deeper sedimentary record. An exceptional shipboard paleomagnetic chronology and a biosiliceous and calcareous biostratigraphy provide a temporal framework to guide future analyses of particular glacial–interglacial periods. Shipboard analyses indicate that sedimentation at slope and fan sites corresponds to major global Pleistocene climate patterns. A notable discovery at Site U1418 is that proximal deepwater sediment depocenters can contain an expanded record of fjord-like glacimarine facies during periods of maximum glacigenic sediment accumulation. Site U1420 demonstrates the potential for extremely thick Pleistocene depocenters in shelf settings where accommodation space can be maintained; consequently, individual glacial advance–retreat facies cycles can be seismically mapped. Postcruise analyses 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 positive feedback in exhumation in an active orogen.