Yakutat Bay Research Articles

Revisiting the 1899 earthquake series using integrative geophysical analysis in Yakutat Bay, Alaska, USA

AbstractA series of large earthquakes in 1899 affected southeastern Alaska near Yakutat and Disenchantment Bays. The largest of the series, a MW 8.2 event on 10 September 1899, generated an ~12-m-high tsunami and as much as 14.4 m of coseismic uplift in Yakutat Bay, the largest coseismic uplift ever measured. Several complex fault systems in the area are associated with the Yakutat terrane collision with North America and the termination of the Fairweather strike-slip system, but because faults local to Yakutat Bay have been incompletely or poorly mapped, it is unclear which fault system(s) ruptured during the 10 September 1899 event. Using marine geophysical data collected in August 2012, we provide an improved tectonic framework for the Yakutat area, which advances our understanding of earthquake hazards. We combined 153 line km of 2012 high-resolution multichannel seismic (MCS) reflection data with compressed high-intensity radar pulse (Chirp) profiles, basin-scale MCS data, 2018 seafloor bathymetry, published geodetic models and thermochronology data, and previous measurements of coseismic uplift to better constrain fault geometry and subsurface structure in the Yakutat Bay area. We did not observe any active or concealed faults crossing Yakutat Bay in our high-resolution data, requiring faults to be located entirely onshore or nearshore. We interpreted onshore faults east of Yakutat Bay to be associated with the transpressional termination of the Fairweather fault system, forming a series of splay faults that exhibit a horsetail geometry. Thrust and reverse faults on the west side of the bay are related to Yakutat terrane underthrusting and collision with North America. Our results include an updated fault map, structural model of Yakutat Bay, and quantitative assessment of uncertainties for legacy geologic coseismic uplift measurements. Additionally, our results indicate the 10 September 1899 rupture was possibly related to stress loading from the earlier Yakutat terrane underthrusting event of 4 September 1899, with the majority of 10 September coseismic slip occurring on the Esker Creek system on the northwest side of Yakutat Bay. Limited (~2 m) coseismic or postseismic slip associated with the 1899 events occurred on faults located east of Yakutat Bay.

Stress Promotion of the 1958 Mw∼7.8 Fairweather Fault Earthquake and Others in Southeast Alaska by Glacial Isostatic Adjustment and Inter‐earthquake Stress Transfer

AbstractWe assess how recent glacial isostatic adjustment (GIA) and inter‐earthquake stress transfer modulated the state of stress on major faults in Southeast Alaska, and how these effects may have influenced recent large and moderate earthquakes. The Glacier Bay Icefield has lost >3,000 km3 of ice since ca. 1770, with ice thinning up to 1.5 km, and the rest of southeast Alaska has been deglaciating since ca. 1900. The resulting GIA response in the solid earth produces present‐day surface uplift rates of up to ∼4 cm/yr, among the fastest on Earth. This rapid deformation occurs directly alongside and atop the Fairweather Fault, which accommodates 4–5 cm/yr of right‐lateral motion through slip in frequent large earthquakes, recently including a Mw∼7.8 earthquake in 1958. We find that the 1958 earthquake nucleated very close to the location of maximum GIA‐induced Coulomb stress increase on the Fairweather Fault between 1770 and 1958 (estimated at ∼0.2–0.6 MPa), on the only section of fault where GIA‐induced stress changes had involved both right‐lateral shearing and unclamping. The September 10, 1899 Mw∼8.1 Yakutat Bay earthquake also promoted failure on at least the northwest part of the eventual 1958 rupture. In turn, the 1899 and 1958 earthquakes and GIA promoted failure along the St. Elias compressional margin, the site of a Mw∼7.4 event in 1979. While rapid tectonic loading is very likely the main driver of earthquakes in southeast Alaska, we estimate that 23 of 30 instrumentally constrained Mw ≥ 5.0 earthquakes in this region were also promoted by post‐1770 GIA.

A unified three-dimensional model of the lithospheric structure at the subduction corner in southeast Alaska: Summary results from STEEP

AbstractWe merge structural results from the ST. Elias Erosion/tectonics Project (STEEP), other studies, and seismicity data to build a comprehensive, three-dimensional model of the lithosphere of the subduction corner in southern Alaska. The model is defined by three surfaces: (1) a top of the subducting lithosphere surface, (2) Moho surfaces, and (3) a base of subducting lithosphere surface. We model the eastern edge of the subducting lithosphere using the southern tip of the Yakutat microplate as an anchor. Kinematic reconstructions using that anchor suggest the modern Fairweather fault is likely inherited from motion of the margin in the 6–10 Ma period. We constructed a 4D kinematic model of crustal deformation in the vicinity of Mount St. Elias. We call this model the middlebuster model because the geometry is similar to a two-sided plow with that name. The west side of the plow is the eastern limit of the Aleutian megathrust constructed from the union of constraints from STEEP seismic results and slip models of the 1979 St. Elias earthquake. The east side is inferred from geologic mapping and slip models of the 1899 Yakutat Bay earthquake sequence. The top of the plow is near the Seward Glacier, where previous studies showed near world-record exhumation rates. GPS velocity vectors show a large rotation across the syntaxis at Mount St. Elias. West of the syntaxis, faults inferred from inversion of the GPS data are above the megathrust inferred from seismic imaging. That and other evidence suggest the presence of a wedge of ductile crust that partially decouples the subducting mantle lithosphere from the upper crust in the area near the suture with the Yakutat microplate.

Biospheric and petrogenic organic carbon flux along southeast Alaska

Holocene fjords store ca. 11–12% of the total organic carbon (OC) buried in marine sediments with fjords along southeast (SE) Alaska possibly storing half of this OC (Smith et al., 2015). However, the respective burial of biospheric (OCbio) and petrogenic OC (OCpetro) remains poorly constrained, particularly across glaciated versus non-glaciated systems. Here, we use surface sediment samples to quantify the sources and burial of sedimentary OC along SE Alaska fjord-coastal systems, and conduct a latitudinal comparison across a suite of fjords and river-coastal systems with distinctive OC sources. Our results for SE Alaska show that surface sediments in northern fjords (north of Icy Strait) with headwater glaciers are dominated by OCpetro, in contrast to marine and terrestrially-derived fresh OC in non-glaciated southern fjords. Along the continental shelf of the Gulf of Alaska, terrestrial OC is exported from rivers. Using end-member mixing models, we determine that glaciated fjords have significantly higher burial rates of OCpetro (∼1.1×103 gOC m−2yr−1) than non-glaciated fjords and other coastal systems, making SE Alaska potentially the largest sink of OCpetro in North America. In contrast, non-glaciated fjords in SE Alaska are effective in burying marine OC (OCbio-mari) (13–82 g OC m−2yr−1). Globally, OC in fjord sediments are comprised of a mixture of OCpetro and fresh OCbio, in contrast to the pre-aged OC from floodplain river-coastal systems. We find that there may be a general latitudinal trend in the role of fjords in processing OC, where high-latitude temperate glacial fjords (e.g., Yakutat Bay, SE Alaska) rebury OCpetro and non-glacial mid-latitude fjords (e.g., Doubtful Sound, Fiordland) sequester CO2 from phytoplankton and/or temperate forests. Overall, we propose that fjords are effective in sequestering OCbio and re-burying OCpetro. Based on our study, we hypothesize that climate change will have a semi-predictable impact on fjords' OC cycling in the near future.

Ice, Seals, and Guns: Late 19th-Century Alaska Native Commercial Sealing in Southeast Alaska

Starting in about 1870, indigenous residents of southeast Alaska intensified their traditional hunting of harbor seals ( Phoca vitulina richardii ) in order to produce surpluses of skins and oil for trade with the Alaska Commercial Company. The most important hunting ground was the head of Yakutat Bay, where thousands of seals were taken annually in June and July at the ice floe rookery near Hubbard Glacier and processed on shore at Keik’uliyaa sealing camp. Firearms obtained in trade were essential tools for mass harvesting. A multisource study of Keik’uliyaa and the historical ecology of 19th-century sealing at Yakutat was undertaken during 2011 and 2013 by the Smithsonian Institution in collaboration with the Yakutat Tlingit Tribe, employing oral history, archaeology, archival sources, and photographic documentation from the 1899 Harriman Alaska Expedition. Economic, social, and ecological dimensions of this historical mode of production are examined and compared with both earlier and later eras.

Traditional knowledge and historical and opportunistic sightings of beluga whales, Delphinapterus leucas, in Yakutat Bay, Alaska, 1938-2013

Traditional knowledge and historical and opportunistic sightings of beluga whales, Delphinapterus leucas, in Yakutat Bay, Alaska, 1938-2013

Acoustic monitoring and prey association for beluga whale, Delphinapterus leucas, and harbor porpoise, Phocoena phocoena, off two river mouths in Yakutat Bay, Alaska

Acoustic monitoring and prey association for beluga whale, Delphinapterus leucas, and harbor porpoise, Phocoena phocoena, off two river mouths in Yakutat Bay, Alaska

The genetic ecology and population origins of the beluga whale, Delphinapterus leucas, of Yakutat Bay, Alaska

The genetic ecology and population origins of the beluga whale, Delphinapterus leucas, of Yakutat Bay, Alaska

Imaging evidence for Hubbard Glacier advances and retreats since the last glacial maximum in Yakutat and Disenchantment Bays, Alaska

AbstractHigh‐resolution 2‐D multichannel seismic data, collected during the 2012 UTIG‐USGS National Earthquake Hazards Reduction Program survey of Disenchantment and Yakutat Bays in southeast Alaska, provide insight into their glacial history. These data show evidence of two unconformities, appearing in the form of channels, and are interpreted to be advance pathways for Hubbard Glacier. The youngest observable channel, thought to have culminated near the main phase of the Little Ice Age (LIA), is imaged in Disenchantment Bay and ends at a terminal moraine near Blizhni Point. An older channel, thought to be from an advance that culminated in the early phase of the LIA, extends from Disenchantment Bay into the northeastern edge of Yakutat Bay, turning southward at Knight Island and terminating on the southeastern edge of Yakutat Bay. Our interpretation is that Hubbard Glacier has repeatedly advanced around the east side of Yakutat Bay in Knight Island Channel, possibly due to the presence of Malaspina Glacier cutting off access to central Yakutat Bay during times of mutual advance. We observe two distinct erosional surfaces and retreat sequences of Hubbard Glacier in Yakutat Bay, supporting the hypothesis that minor glacial advances in fjords do not erode all prior sediment accumulations. Interpretation of chaotic seismic facies between these two unconformities suggests that Hubbard Glacier exhibits rapid retreats and that Disenchantment Bay is subject to numerous episodes of outburst flooding and morainal bank collapse. These findings also suggest that tidewater glaciers preferentially reoccupy the same channels in bay and marine settings during advances.

Low-temperature thermochronology of the Yakutat plate corner, St. Elias Range (Alaska): bridging short-term and long-term deformation

This study investigates the long-term deformation and rock exhumation of the subducting Yakutat microplate corner at the St. Elias syntaxis region with a special focus on the area east and west of Yakutat Bay, where major coseismic surface uplift occurred during a MW 8.2 earthquake in 1899. We use low-temperature thermochronology to quantify the long-term activity of thrust and reverse faults. We present new apatite and zircon (U–Th–Sm)/He ages from 35 bedrock samples collected along profiles crossing major faults that compose a one-sided flower structure at the northern end of the Fairweather Fault. The cooling ages reveal that rapid rock exhumation is accommodated by the eastern Esker Creek Fault and the Yakutat Fault with rates of 2–3 mm/yr and slower rates of <1.5 mm/yr occur along the Boundary Fault. Exceptionally high exhumation rates of 3–5 mm/yr are recorded by <1 Ma apatite ages at the western part of the Esker Creek Fault and the Chaix Hills Fault. This area is closest to the intersection line of the northwest striking reverse faults associated with the northern Fairweather Fault and the shallow dipping northeast striking thrusts of the Pamplona-Malaspina fault zone. The long-term exhumation patterns derived from this new thermochronology data are compared with the observed coseismic uplift pattern caused by the 1899 earthquake, revealing good agreement and supporting the speculation that the 1899 rupture propagated along the western part of the Esker Creek Fault and the Chaix Hills Fault. Thus, this study illustrates that low-temperature thermochronology can support seismic hazard analyses particularly in area where preservation of paleoseismic field evidence is challenged.

Seismic stratigraphic evidence for glacial expanse during glacial maxima in the Yakutat Bay Region, Gulf of Alaska

[1] Modern shelf bathymetry bordering the Gulf of Alaska exhibits shelf-crossing sea valleys that suggest focused pathways for ice flow during glacial conditions. Using an integrated seismic data set between the present Yakutat and Alsek Sea Valleys, we investigate the glacial stratigraphic record in order to improve our understanding of the regional glacial system during maximum glacial conditions. Our investigations reveal four glacial unconformities, of which, the latter two are overlain by sediment packages hundreds of meters thick. We suggest that these unconformities are indicative of ice advance phases during the Little Ice Age (LIA), the Last Glacial Maxima (LGM), and two pre-LGM advances with glacial retreat sequences preserved from the youngest two. The advances were dominated by ice expanse from the Malaspina Glacial system and Alsek River districts rather than the Hubbard Glacial system and only show distinctive morainal bank development near the shelf edge and near the mouth of, or within, modern bays, fjords, or river valleys. This observation strongly supports rapid and continuous retreat from glacial maxima conditions during climatic warming. All, except the inferred LIA sequence, transgress the shelf and exhibit concentrated erosion in overdeepened troughs, analogous to cross-shelf troughs similar to those observed on other high-latitude glaciated shelves. The two Alaskan troughs discussed here may be end-member examples of high-sediment flux systems due to the temperate glacial setting combined with an actively exhuming orogen.

The Yakutat terrane: Dramatic change in crustal thickness across the Transition fault, Alaska

We present new constraints on the crustal structure of the Yakutat terrane and evidence of the role of the Transition fault in southern Alaska. The Yakutat terrane south of Yakutat Bay includes crystalline crust that is 24–27 km thick overlain by sedimentary units that are 4.5–7.5 km thick. The Yakutat terrane crustal thickness and velocity structure are consistent with an oceanic plateau origin. The southern edge of the Yakutat terrane is bounded by the Transition fault, which is imaged as a near-vertical fault zone ∼1 km wide. The Transition fault is coincident with a dramatic change in Moho depth from 32 km for Yakutat oceanic plateau crust to 11.5 km for Pacific Ocean crust occurring over a horizontal distance of 0–5 km. There is no evidence for underthrusting of the Pacific Ocean crust beneath the Yakutat terrane at the Transition fault. We argue that the Yakutat terrane formed on the Kula or Farallon plate and was later juxtaposed next to younger Pacific Ocean crust by the Transition fault.

The Tlingit Indians in Russian America, 1741–1867 (review)

The Tlingit Indians in Russian America, 1741-1867. By Andrei V. Grinev. Translated by Richard L. Bland and Katerina G. Solovjova. (Lincoln and London: University of Nebraska Press, 2005. Pp. 386. Illustrations. Maps. Tables. Cloth, $24.95.)Reviewed by Lucien J. FraryFor more than two centuries the lifestyle and culture of the Tlingit, who occupy the rugged mainland and fjord-ridden archipelago of the southeastern Alaskan panhandle, have attracted attention from folklorists, ethnologists, and historians. Dense forests, turbulent rivers, and abundant marine and plant resources helped create a flourishing society, the origin of which can be traced to the first millennium of the Common Era.Driven by the quest for furs, Russian adventurers began exploring the wild coasts of Alaska in the waning decades of the eighteenth century. The Russian-American Company (RAC) led the charge by regulating settlements and exchanging merchandise. In 1788, Russians reached Yakutat Bay, thus inaugurating a long-lasting relationship with the Tlingit. The establishment of a fort at Sitka (Novo-Arkhangel'sk) in 1799 led to the first violent encounters; the pugnacious Tlingit were initially successful in defeating the Russians. The Tlingit maintained their independence in subsequent decades, even as their interest in European products increased. In contrast to the experience of other Native Americans, the Tlingit were able to prosper, thanks, in part, to the administrative and organizational activities of the RAC.The pioneering efforts of Frederica De Laguna (who has published a stream of works in recent years, including the three-volume Under Mount Saint Elias) marked a turning point in the study of Tlingit culture. Bland and Solovjova's translation of Andrei V. Grinev's monograph (originally published in Novosibirsk in 1991) is a welcome addition to the literature on Native Americans. What makes the study admirable is Grinev's weaving of ethnographic and historical sources into a critical synthesis touching upon nearly every aspect of Tlingit life during the Russian- Ameri can period.Grinev's introduction assesses De Laguna's work and other major studies dealing with the Tlingit, underscoring the significance of Russian archives for Native American history. The core of the book is divided into three sections: the Tlingit before European contact, Tlingit life during the Russian- American period, and the influence of European contact on Tlingit culture.Before the arrival of Europeans the Tlingit never formed a single ethnosocial organism (although various clans maintained ties due to common language, customs, and territory). The name - tlinkit - meaning human, helped distinguish them from other indigenous peoples. The fundamental unit of Tlingit society consists of matrilineal clans of two exogamous moieties. Tribes were often formed through the unification of several clans that inhabited neighboring territories and intermarried. Marriage was almost always with a member of the father's clan, and polygamy was a rather common occurrence, especially among the aristocracy. Grinev shows how the fluid, heterogeneous Tlingit society was divided into elites, commoners, slaves, shamans, and other units based on wealth, talent, and status.Well before European contact, the Tlingit maintained a highly developed economy based on fishing and handicrafts and engaged in intertribal trade. An interesting aspect of Tlingit economy was the potlatch. Often lasting several days, the potlatch included dancing, joking, costume display, and (most importantly) the lavish bestowal of gifts among the guests by the host. Grinev's discussion of the potlatch sheds light on many topics, from gender roles and wealth distribution, to social mores and spiritual life (70-73, 219-21).As Russian explorers soon discovered, the Tlingit were warlike and lived by the law of blood feud. …

OCCURRENCE OF THE MILLIPED GENUS SCYTONOTUSC. L. KOCH, 1847 (POLYDESMIDA: POLYDESMIDAE)IN ALBERTA, CANADA

The milliped genus Scytonotus C. L. Koch, 1847, occupies four areas of North America, one east of the Central Plains and three west of the Continental Divide (one coastal and two in the interior) (Shelley 1993, Hoffman 1999, Shelley et al., 2005). The western coastal area extends along the Pacific from Yakutat Bay, Alaska, to Marin and San Joaquin counties, California, including all intervening offshore islands except the Queen Charlottes, British Columbia (BC). One interior area encompasses the Wasatch and Teton mountains, and associated ranges, in western Wyoming, eastern Idaho, and northern Utah, and the other centers on the Columbia River Valley and the Rocky and Selkirk Mountains from Revelstoke and Yoho National Parks, BC, to southeastern Washington, northern Idaho, and western Montana (Kevan 1983; Shelley 1990, 1993, 2002). Three species inhabit the last area, but only S. columbianus Chamberlin, 1920, occurs in BC, where it extends northward to the vicinity of Takkakaw Falls in Yoho. While recently perusing samples in the Virginia Museum of Natural History (VMNH), Martinsville, I discovered one with three unidentifiable females of Scytonotus from “Sta. 2,” 4,000', Jasper National Park, Alberta, the first record of the genus from this Canadian province. The specimens were collected on 2-4 October 1964 by the late D. R. Whitehead, an experienced arthropod field collector, so there is no reason to suspect a labeling error; Jasper is also where he routinely collected beetles in the course of doctoral program at the University of Alberta. The precise location of “Sta. 2” is unknown, but Dr. Whitehead sampled extensively in the vicinity of Mt. Edith Cavell, some 144 km (90 mi) NNW of the site in Yoho. “Sta. 2” may not have been here, but the southern boundary of Jasper Park is ca. 88 km (55 mi) NNW of the Yoho locality, and the northward generic range extension is probably between these distances because the part of the Park north of Jasper village and highway 16 is largely roadless and inaccessible. Males are necessary for a specific determination, but considering the magnitude of this extension and the fact that the locality lies east of the Divide, a formidable distribution barrier that runs along the border between BC & Alberta, the species seems more likely to be undescribed than S. columbianus. Canadian biologists should be aware of the possibility of an undescribed milliped species in Jasper, and if true, it will be the second that is known only from the Park, the other being Austrotyla borealis Shear, 1971 (Chordeumatida: Conotylidae), whose holotype and only known specimen was collected by Dr. Whitehead in 1967 at “Sta. 5,” precise location also unknown (Shear 1971; Shelley 1990, 2002; Hoffman 1999). Volume 118, Number 3, May and June 2007 319

Climatology of Barrier Jets along the Alaskan Coast. Part I: Spatial and Temporal Distributions

Abstract This paper investigates the temporal and spatial climatology of coastal barrier jets in the Gulf of Alaska. The jets are divided into two categories based upon the origin of the air involved: “classic” barrier jets fed primarily by onshore flow and “hybrid” jets fed primarily by gap flow from the continental interior. The analyses are compiled from five years (1998–2003) of synthetic aperture radar images from the Radarsat-1 satellite totaling 3000 images. Thermodynamic and kinematic data from the NCEP reanalysis is used in the interpretation of the results. The majority of coastal barrier jets occur during the cool season, with the coastline near Mount Fairweather and the Valdez–Cordova mountains experiencing the greatest number of barrier jets. Hybrid jets are even more strongly restricted to the cool season and are commonly found to the west of Cross Sound, Yakutat Bay, and Icy Bay. Some interannual variability in the total number of jets is observed. Coastal barrier jet formation is associated with onshore wind directions and maximum terrain heights exceeding 2 km within 100 km of the coast, features that support low-level flow blocking. Hybrid jet formation requires the additional condition of an abnormally large offshore-directed pressure gradient force. Half of the barrier and hybrid jets exhibit surface wind speeds in excess of 20 m s−1 (strong gale), although their widths are typically less than 100 km. The maximum speed of both types of jet tends to be 2–3 times that of the ambient synoptic flow. A small percentage of the jets detach from the coastline, with the typical detachment distance being 10 km.

Seasonal migration and environmental conditions of Pacific halibut Hippoglossus stenolepis, elucidated from pop-up archival transmitting (PAT) tags

Pop-up archival transmitting (PAT) tags were used to study the fall migration of halibut in the Gulf of Alaska (GOA). We tagged 6 Pacific halibut Hippoglossus stenolepis on summer feeding grounds in the eastern GOA and another 6 in the western GOA from June 13 to August 6, 2002. The tags were programed to be released from the fish on January 15, 2003, at the height of the winter spawning season: 10 tags successfully detached, transmitted archived environmental data (depth and temperature), and generated accurate latitude-longitude coordinates shortly after pop-up; 2 tags deployed off SE Alaska were lost. The tags revealed that 6 fish had moved a considerable distance (>200 km) between tagging and pop-up, and all of these had moved northward to some extent. The longest of the observed migrations was from the southern Alaska Peninsula to Yakutat Bay, a linear displacement of 1153 km; 4 fish showed little evidence of geographic displacement, exhibiting migra- tions that ranged only from 30 to 69 km. Although 2 fish had moved inshore by the end of the tagging period, all other fish had moved offshore regardless of their overall migration distance. The precise timing of offshore movements varied, beginning as early as August and as late as January. These observations generally corroborate conventional tagging, indicating migration of halibut toward winter spawning grounds in the northern GOA, and movement of fish to deep water in fall. However, no single stereotypic migration behavior was apparent, and a variety of vertical movement patterns and temperature profiles were observed. Halibut spent most time in waters of 5 to 7°C, but experi- enced temperatures ranging from 2.6 to 11.6°C. Depth observations ranged from 0 to 736 m, with summertime activity concentrated in depths from 0 to 400 m, and halibut that exhibited offshore movement were typically observed at 300 to 700 m by mid-winter. Vertical movement (short-period changes in depth) varied among fish and over time, with some fish displaying little vertical activity, others displaying short periods of activity, and still others displaying considerable activity throughout their time at liberty.

Strain Signatures of Fjord Sediment Sliding: Micro-Scale Examples from Yakutat Bay and Glacier Bay, Alaska, U.S.A.

Abstract Samples of gravity cores recovered from Yakutat Bay and Glacier Bay in southeast Alaska were examined in thin section. The samples, representing rhythmically laminated plume deposits, current-derived silts and sands, and iceberg-rafted diamicts, exhibit microstructures indicative of vertical deformation such as loading and water escape, as well as boudinage and other features, which are indicative of simple shear deformation. Narrow zones in many of the fine-grained core intervals show subtle patterns of preferred clay-mineral orientations (plasmic fabrics). We interpret these intervals as being low-strain simple shear zones. They all show a consistent and regular internal configuration that is formed by sets of conjugate shear planes. The setting and conditions in the Alaskan fjords allow the assumption that the shear zones are exclusively associated with penecontemporaneous movements of the unconsolidated sediment on subaqueous slopes. Analyzing characteristics and trends relating to symmetry and strain intensity, we argue that these are diagnostic identifiers for sliding mass movements. We present a conceptual model in which deformation resulting from gravity-driven sliding is described. We suggest that strain signatures can be used to recognize types of shear zones and be helpful in distinguishing between, for example, gravity-driven deformation and subglacial deformation in sedimentary records.

Polycyclic Aromatic Hydrocarbon Sources Related to Biomarker Levels in Fish from Prince William Sound and the Gulf of Alaska

Seafloor sediments in Prince William Sound (PWS) and the eastern Gulf of Alaska (GOA) have a substantial regional hydrocarbon background from natural sources including oil seeps and eroding sedimentary rocks along the eastern GOA coast. Polycyclic aromatic hydrocarbons (PAH) from that background appear to be bioavailable to fish. Fish collected from PWS and the GOA in a 1999--2000 biomarker study (bile fluorescent aromatic contaminants and liver ethoxyresorufin O-deethylase) show evidence of exposure to low levels of PAH at all categories of sites sampled. Seafloor sediments at fish sampling sites in the GOA east of PWS and at three PWS site categories (nonspill path, spill path oiled, and spill path not oiled) contain hydrocarbons from four principal sources: regional background, combustion products, residues from the 1989 Exxon Valdez oil spill (EVOS), and Monterey (CA) petroleum residues. GOA sediments between PWS and Yakutat Bay, approximately 350 km to the east, are dominated by regional petrogenic background hydrocarbons (total PAH (TPAH) range approximately 60-3400 ng/g) that are the probable cause of low biomarker levels measured in halibut from this area. PWS sediments contain varying proportions of regional background, combustion products, Monterey residues, and EVOS residues at some spill path sites. Rockfish caught in PWS embayments in 1999 have liver EROD activities that correlate positively with the pyrogenic PAH indicator ratio (FI+Py)/C24Ph. Although traces (<5-100 ng/g TPAH) of EVOS residues were detected in seafloor sediments at some nearshore spill path sites, biomarker levels in fish from those sites are not elevated relative to other sites in PWS.

Holocene history of Hubbard Glacier in Yakutat Bay and Russell Fiord, southern Alaska

Stratigraphic and geomorphic data defined by radiocarbon ages, tree-ring dates, and historical observations provide evidence of three major Holocene expansions of Hubbard Glacier. Early in each advance the Hubbard Glacier margin blocked Russell Fiord to create Russell lake, raising base level and causing stream beds and fan deltas throughout the Russell drainage basin to aggrade. Each Hubbard Glacier expansion continued with an ice lobe advancing through Disenchantment and Yakutat Bays in the west, and an eastern lobe advancing into Russell Fiord. The earlier two Holocene expansions were, respectively, under way at 7690 and 5600 calibrated yr B.P., and each advance culminated more than 1 k.y. later. The late Holocene advance was under way by 3100 yr ago and reached ∼13 km farther south in Russell Fiord than the preceding two expansions. Late Holocene deglaciation of Yakutat and Disenchantment Bays was complete before A.D. 1791; Nunatak Glacier flowing from neves east of Russell Fiord became the primary ice source to the Russell Fiord lobe at or before this date. Ice retreat from the southern end of Russell Fiord began in the late eighteenth century and the penultimate Russell lake drained ca. A.D. 1860. The relatively slow advances and more rapid retreats of Hubbard Glacier are consistent with the model of the iceberg-calving glacier cycle. Hubbard Glacier is currently advancing and will likely reestablish Russell lake in the near future, affecting local fisheries. However, glacier lobes are unlikely to reach the area of the town of Yakutat, built on late Holocene glacial deposits, in the next 1 k.y.

Holocene coastal glaciation of Alaska

Holocene fluctuations of the three cirque glaciers on the Seward Peninsula and five groups of tidewater- and land-terminating glaciers along the northernmost Gulf of Alaska, provide a proxy record of late Holocene climatic change. Furthermore, the movements of the coastal glaciers were relevant to late Holocene native American migration. The earliest expansion was recorded about 6850 yr BP by Hubbard Glacier at the head of Yakutat Bay in the Gulf of Alaska; however, its down-fjord advance to the bay mouth was delayed until ∼2700 BP. Similarly, expansions of the Icy Bay, Bering, and McCarty glaciers occurred near their present termini by ∼3600–3000 BP, compatible with marked cooling and precipitation increases suggested by the Alaskan pollen record. Decrease in glacier activity ∼2000 BP was succeeded by advances of Gulf coastal glaciers between 1500 and 1300 BP, correlative with early Medieval expansions across the Northern Hemisphere. A Medieval Optimum, encompassing at least a few centuries prior to AD 1200 is recognized by general retreat of land-terminating glaciers, but not of all tidewater glaciers. Little Ice Age advances of land-based glaciers, many dated with the precision of tree-ring cross-dating, were centered on the middle 13th or early 15th centuries, the middle 17th and the last half of the 19th century A.D. Strong synchrony of these events across coastal Alaska is evident.