Disenchantment Bay Research Articles

Spatially Estimating Disturbance of Harbor Seals (Phoca vitulina).

Tidewater glacial fjords in Alaska provide habitat for some of the largest aggregations of harbor seals (Phoca vitulina), with calved ice serving as platforms for birthing and nursing pups, molting, and resting. These fjords have also been popular destinations for tour ships for more than a century, with dramatic increases in vessel traffic since the 1980s. Seals on ice are known to flush into the water when approached by tour ships, but estimating the exposure to disturbance across populations is difficult. Using aerial transect sampling while simultaneously tracking vessel movements, we estimated the spatial overlap between seals on ice and cruise ships in Disenchantment Bay, Alaska, USA. By integrating previously estimated rates of disturbance as a function of distance with an ‘intensity surface’ modeled spatially from seal locations in the surveys, we calculated probabilities of seals flushing during three separate ship visits. By combining our estimate of seals flushed with a modeled estimate of the total fjord population, we predict that up to 14% of the seals (up to 11% of pups) hauled out would have flushed into the water, depending on the route taken by ships relative to seal aggregations. Such high potential for broad-scale disturbance by single vessels (when up to 4 ships visit per day) was unexpected and underscores the need to 1) better understand long-term effects of disturbance; 2) regularly monitor populations exposed to high vessel traffic; and 3) develop conservation measures to reduce seal-ship overlap.

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.

Natural and human effects on harbor seal abundance and spatial distribution in an Alaskan glacial fjord

AbstractTidewater glacial fjords support the largest populations of harbor seals (Phoca vitulina richardii) in Alaska and are a prime destination for tour ships. Chronic disturbance from ships, however subtle, could impact long‐term population stability. We examined variation in abundance and distribution of harbor seals on floating ice in Disenchantment Bay, Alaska, a tour ship destination for over a century with near daily visitation by ships in the spring/summer over the last decade. Counts of seals by aerial transect showed a sharp decline in May, prior to pupping and the first ships arriving; counts rebounded by the end of June remaining high until August. Seal distribution and abundance peaked in 5–7 tenths ice cover; total area of ice cover showed no effect. Despite regular flushing of seals by ships, we found no broad‐scale patterns in seal abundance and distribution that could be explained by ship presence. We cannot rule out mechanisms of long‐term disturbance, difficult to detect and that might explain notable differences with other, similar sites. Population declines at disturbed glacial sites and the still rising popularity of vessel‐based tourism indicate a need for individual‐based studies on how seals respond to the dynamics of glacial ice environments and human‐caused stresses.

Morainal bank progradation and sediment accumulation in Disenchantment Bay, Alaska: Response to advancing Hubbard Glacier

Morainal banks are primary features at the margins of advancing and stable to quasi‐stable temperate tidewater glaciers, yet their roles in glacier dynamics and terminus stability are poorly defined by submarine observations. Analysis of new and archival multibeam data and Landsat images of the advancing Hubbard Glacier, southeast Alaska, reveal that between 1978 and 2010 the ice face and morainal bank advanced together at an average rate of ∼34 m/yr, varying spatially and temporally between 14 and 80 m/yr. Morphological features including gullies and a boulder lag suggest cyclical deposition and gravitational erosion on the proximal slope of the morainal bank (15–18°), and possible ice pushing in an area without recent sedimentation. In contrast, the morainal bank of the nearby, quasi‐stable (surging) Turner Glacier advanced steadily since 1978 by proximal sedimentation on the steep fjord wall below its hanging valley. Sedimentation in the deep (>220 m) basin of Disenchantment Bay increased from 0.88 m/yr spanning 1978 to 1999, to 1.22 m/yr thereafter. This change appears to be a combined response to glacier advance and sediment dispersal farther down‐fjord, and to an increase in sediment yield from other glacial and non‐glacial sources. Analysis of Hubbard Glacier illustrates the direct correlation between movement of the terminus and morainal bank in advancing the grounding line of a marine‐terminating glacier, and that morainal banks provide a fundamental stabilizing role for advance into a deep‐water fjord, compensating for changes in water depth at the grounding line.

Glacial outburst flood sediments within Disenchantment Bay, Alaska: Implications of recognizing marine jökulhlaup deposits in the stratigraphic record

Glacial outburst floods have been observed or inferred in most glaciated regions of the world. Outburst flooding events along past ice-sheet margins have been discussed as possible causes for changes in ocean circulation while subglacial lake drainage under ice sheets may influence ice-sheet stability. The recognition of these geologically instantaneous events in the marine sediment or rock record is critical in order to characterize the magnitude and frequency of past outburst events as well as to predict the likelihood of future occurrences. In this paper, we characterize outburst lithofacies and infer depositional processes from two outburst floods within Disenchantment Bay, Alaska, and compare these deposits to other recognized outburst deposits within marine depositional settings. Here we show that the outburst deposits are lithologically similar and are composed of stratified mud, sand, and diamicton. We interpret these facies to have been formed by sediment gravity flows and hypopycnal plume sedimentation. These results show that the volume of sediment, lithofacies, and inferred depositional processes cannot be easily distinguished from normal sediment accumulation within the proximal temperate glacimarine depositional environment. The paucity of fluvial subglacial discharge in polar glacimarine environments suggests the potential for recognition of such deposits in the Antarctic.

Reaction of Harbor Seals to Cruise Ships

The largest aggregations of harbor seals (Phoca vitulina) in Alaska, USA, haul out on floating ice in tidewater glacial fjords. Seals use these fjords in peak numbers during the critical periods of pupping, breeding, and molting when visits by tour ships also peak. Documented and suspected declines of harbor seals in fjords with rising vessel traffic underscore the need to better understand possible impacts, particularly in areas where ship visits have risen substantially in the past 2 decades. We examined the interruption of haul-out bouts of harbor seals due to approaching cruise ships in Disenchantment Bay, Alaska. We conducted observations from cruise ships and focused on disturbance of seals as evidenced by seals flushing into the water from the floating ice on which they rested. We investigated rate of flushing in relation to vessel distance, approach angle, group size, and seal type (mother, pup, or other). Using a survival-regression analysis, we found that the risk of disturbing harbor seals increased when ships approached within 500 m; seals approached as close as 100 m were 25 times more likely to enter the water than seals 500 m from a ship. Seals were 4 times more prone to enter the water when ships were approaching directly rather than passing abeam. Seals responded similarly regardless of group size or seal type. Energetic models indicated a potential to disrupt energy balance and cause thermal stress in disturbed pups if they spent >50% of their time in ice-chilled water. Studies at non-glacial sites suggest that pups spend 40–70% of their time in the water. Voluntary guidelines for approaching seals in Alaska recommend that cruise ships approach ≥91 m (100 yards), a distance at which we show 90% of seals would flush into the water. Our findings indicate a need to develop regulations to maintain a 500-m separation between cruise ships and seals in all Alaskan glacial fjords.

Reaction of Harbor Seals to Cruise Ships

Abstract: The largest aggregations of harbor seals (Phoca vitulina) in Alaska, USA, haul out on floating ice in tidewater glacial fjords. Seals use these fjords in peak numbers during the critical periods of pupping, breeding, and molting when visits by tour ships also peak. Documented and suspected declines of harbor seals in fjords with rising vessel traffic underscore the need to better understand possible impacts, particularly in areas where ship visits have risen substantially in the past 2 decades. We examined the interruption of haul‐out bouts of harbor seals due to approaching cruise ships in Disenchantment Bay, Alaska. We conducted observations from cruise ships and focused on disturbance of seals as evidenced by seals flushing into the water from the floating ice on which they rested. We investigated rate of flushing in relation to vessel distance, approach angle, group size, and seal type (mother, pup, or other). Using a survival‐regression analysis, we found that the risk of disturbing harbor seals increased when ships approached within 500 m; seals approached as close as 100 m were 25 times more likely to enter the water than seals 500 m from a ship. Seals were 4 times more prone to enter the water when ships were approaching directly rather than passing abeam. Seals responded similarly regardless of group size or seal type. Energetic models indicated a potential to disrupt energy balance and cause thermal stress in disturbed pups if they spent >50% of their time in ice‐chilled water. Studies at non‐glacial sites suggest that pups spend 40–70% of their time in the water. Voluntary guidelines for approaching seals in Alaska recommend that cruise ships approach ≥91 m (100 yards), a distance at which we show 90% of seals would flush into the water. Our findings indicate a need to develop regulations to maintain a 500‐m separation between cruise ships and seals in all Alaskan glacial fjords.

Intra-annual Variability in Benthic Foraminiferal Abundance in Sediments of Disenchantment Bay, an Alaskan Glacial Fjord

Sediment deposited from turbid meltwater from Hubbard Glacier dominates the benthic environment in Disenchantment Bay, a glacial fjord in southern Alaska. Sedimentation rates during the meltwater season average 22 cm yr−1 at a station 12 km from the glacier. Samples were collected for foraminiferal analyses from multicores and a piston core. Samples from multicores show annual trends in abundance of Elphidium spp. and Textularia earlandi. Lithofacies consist of couplets of laminated mud deposited by meltwater discharge in summer and diamicton beds deposited as ice-rafted debris during winter. Within mud layers, counts of Elphidium spp. and T. earlandi increase upcore until the winter diamicton layer, where they are absent to rare. Evidence for this variation in abundance can be seen at depth in the piston core. High C∶N ratios (30–90) indicate that carbon flux is from refractory, terrestrial sources rather than labile organic matter. We hypothesize that the pattern of seasonal variation is related to vertical migration of foraminifera toward a preferred sediment depth, then death following reproduction in late summer. Episodic events reduce total abundance, impacting the intra-annual pattern. The high resolution record from an Alaskan glacial fjord can be used to better understand in situ foraminiferal ecology.

Space—time zero‐inflated count models of Harbor seals

AbstractEnvironmental data are spatial, temporal, and often come with many zeros. In this paper, we included space‐time random effects in zero‐inflated Poisson (ZIP) and ‘hurdle’ models to investigate haulout patterns of harbor seals on glacial ice. The data consisted of counts, for 18 dates on a lattice grid of samples, of harbor seals hauled out on glacial ice in Disenchantment Bay, near Yakutat, Alaska. A hurdle model is similar to a ZIP model except it does not mix zeros from the binary and count processes. Both models can be used for zero‐inflated data, and we compared space‐time ZIP and hurdle models in a Bayesian hierarchical model. Space‐time ZIP and hurdle models were constructed by using spatial conditional autoregressive (CAR) models and temporal first‐order autoregressive (AR(1)) models as random effects in ZIP and hurdle regression models. We created maps of smoothed predictions for harbor seal counts based on ice density, other covariates, and spatio‐temporal random effects. For both models predictions around the edges appeared to be positively biased. The linex loss function is an asymmetric loss function that penalizes overprediction more than underprediction, and we used it to correct for prediction bias to get the best map for space‐time ZIP and hurdle models. Published in 2007 by John Wiley & Sons, Ltd.

Fine-grained sediment flocculation below the Hubbard Glacier meltwater plume, Disenchantment Bay, Alaska

A study in Disenchantment Bay, Alaska, demonstrates that fine sediment beneath a meltwater plume is flocculated and that floc sizes and fraction of mass bound within flocs exhibit a pronounced increase with depth rather than down fjord. This spatial pattern of variability likely is due to the longer depositional timescale of flocs compared to their horizontal advection timescale within the meltwater plume. The flux of mass within flocs also increases with depth. These observations have implications for sedimentation models as sedimentation rates estimated from surface waters underestimate those at depth, and could result in the inaccurate prediction of the position of suspension depocenters. The results also may explain the behavior of fine sediment in more complex environments where floc properties are difficult to observe.

The slow advance of a calving glacier: Hubbard Glacier, Alaska, U.S.A.

AbstractHubbard Glacier is the largest tidewater glacier in North America. In contrast to most glaciers in Alaska and northwestern Canada, Hubbard Glacier thickened and advanced during the 20th century. This a typical behavior is an important example of how insensitive to climate a glacier can become during parts of the calving glacier cycle. As this glacier continues to advance, it will close the seaward entrance to 50 km long Russell Fjord and create a glacier-dammed, brackish-water lake. This paper describes measured changes in ice thickness, ice speed, terminus advance and fjord bathymetry of Hubbard Glacier, as determined from airborne laser altimetry, aerial photogrammetry, satellite imagery and bathymetric measurements.The data show that the lower regions of the glacier have thickened by as much as 83 m in the last 41 years, while the entire glacier increased in volume by 14.1 km 3. Ice speeds are generally decreasing near the calving face from a high of 16.5 md −1in1948 to11.5 md −1in 2001. The calving terminus advanced at an average rate of about 16 m a−1 between 1895 and 1948 and accelerated to 32 m a−1 since 1948. However, since 1986, the advance of the part of the terminus in Disenchantment Bay has slowed to 28 m a−1. Bathymetric data from the lee slope of the submarine terminal moraine show that between 1978 and 1999 the moraine advanced at an average rate of 32 m a−1, which is the same as that of the calving face.

Terrestrial gypsum from Alaska and Greenland in glacially influenced marine sediments

Abstract Gypsum grains were identified in Miocene–Pleistocene sediment cores from two deep-water ODP sites, Site 918 off the SE Greenland margin and Site 887 in the Gulf of Alaska, and in Holocene sediment cores from shallow-water localities in Disenchantment Bay and Muir Inlet in southern Alaska. Although initial morphologic and textural observations suggested a complex system in which the gypsum may have had more than one origin, quantitative sulfur isotope analyses of the gypsum provide evidence of its detrital nature. δ34S values in gypsum from southern Alaska range between +0.0 and +7.1‰. Gypsum has δ34S values between −27.1 and −27.5‰ in the Gulf of Alaska and values between −28.5 and +0.2‰ off the SE Greenland margin. All of these isotopic signatures are too highly depleted in δ34S to have precipitated from seawater, present or past. In addition there is no significant change in δ34S values for gypsum crystals with differing physical characteristics (abraded vs. unabraded) from the same stratigraphic horizon, suggesting all the gypsum is detrital regardless of the degree of abrasion. The isotopic and physical evidence, in combination with the onshore geology the environmental setting, and site characteristics of the gypsum-bearing marine localities, lead us to propose that the ultimate source of the gypsum is precipitation from freeze-induced terrestrial sediment or soil brines. Furthermore the combined evidence suggests that the subsequent occurrence of gypsum in glacimarine sediments results from ice-rafting (by icebergs or sea ice) of the frozen regolith and/or, in the proximal glacimarine setting of southern Alaska, very rapid burial via turbidity currents.

Temperate Glacimarine Varves: An Example from Disenchantment Bay, Southern Alaska

ABSTRACT The sediment record of Disenchantment Bay, Southern Alaska, a large marine calving embayment, contains distinctive annual deposits. Each year a glacimarine couplet forms with a clast-rich stratified to massive diamicton deposited in winter by intense iceberg rafting and a summer, meltwater deposit of thinly laminated mud and turbidite sand beds. Although iceberg rafting occurs throughout the year, coarse debris is deposited in high concentrations and forms diamicton only during the winter because of minimal fine sediment from meltwater discharges and a longer residence time of icebergs in the Bay due to winter fjord circulation and meteorological factors. When meltwater discharge commences in summer, laminated mud with dropstones is deposited. Spring and fall conditions are recorded a transitional phases between winter and summer by clast-poor diamictons. Average sediment accumulation rates calculated from varve thicknesses range from 48 cm/yr, 3.4 km from Hubbard Glacier at the head of the Bay, to 14 cm/yr, 15 km away. Sediment accumulation rates estimated from 210Pb dating of core sediments are in the same range. Deposits from the 1986 Russell Fiord outburst flood are identified from anomalously high 210Pb activities in sediments indicating an alternative sediment source to Disenchantment Bay. Glacimarine varves are a dating tool for the barren proximal sediments commonly deposited near calving glacier termini. They yield paleoclimatic information because they are a product of both glacial ice in the sea and laminated sediments deposited from significant melt-water discharge found in a temperate to subpolar climate.

The marine record of the Russell Fiord outburst flood, Alaska, U.S.A.

The advance of Hubbard Glacier, near Yakutat, Alaska, U.S.A., in spring 1986 blocked the entrance to Russell Fiord with an ice-and-sediment dam, behind which a lake formed. The water level in Russell Lake rose to 25.5 m a.s.l. The dam catastrophically failed in October 1986, releasing 5.4 km3 of water into Disenchantment Bay. High-resolution seismic-reflection profiles show a 7.5 km long channel system cut into and buried by glacimarine sediment, represented by continuous, parallel reflections. The chaotic seismic facies filling the channel is interpreted to be debris flow deposits. A gravity core from channel-overbank deposits contained sandy diamicton with mud clasts. Above the channel a 1–2 m thick sediment drape extends across the bay. Laminated mud, fining-upward sand beds and diamicton were recovered from this unit. The sediment-drape deposits were produced by suspension settling from turbid plumes and non-channelized turbidity currents generated by the outburst flood.

The marine record of the Russell Fiord outburst flood, Alaska, U.S.A.

The advance of Hubbard Glacier, near Yakutat, Alaska, U.S.A., in spring 1986 blocked the entrance to Russell Fiord with an ice-and-sediment dam, behind which a lake formed. The water level in Russell Lake rose to 25.5 m a.s.l. The dam catastrophically failed in October 1986, releasing 5.4 km3of water into Disenchantment Bay. High-resolution seismic-reflection profiles show a 7.5 km long channel system cut into and buried by glacimarine sediment, represented by continuous, parallel reflections. The chaotic seismic facies filling the channel is interpreted to be debris flow deposits. A gravity core from channel-overbank deposits contained sandy diamicton with mud clasts. Above the channel a 1–2 m thick sediment drape extends across the bay. Laminated mud, fining-upward sand beds and diamicton were recovered from this unit. The sediment-drape deposits were produced by suspension settling from turbid plumes and non-channelized turbidity currents generated by the outburst flood.

The terminus of Hubbard Glacier, Alaska

Hubbard Glacier advanced across the entrance of Russell Fiord in May 1986, transforming the fiord into a lake, which began filling with fresh water. The dam failed in October 1986. The calving terminus of Hubbard Glacier has been mapped with increasing frequency for nearly a century. A mapping interval of a few years establishes that the terminus has made a slow, but accelerating, advance. Recently the terminus has been mapped several times per month using vertical photography, ground surveys, and time-lapse cameras. At this frequency of observation, the terminus position is found to fluctuate seasonally. These seasonal fluctuations are compared with those of Columbia Glacier, where a longer detailed record is available. Although Columbia Glacier is now undergoing a drastic retreat, it continues to have seasonal length changes similar to those it experienced before the retreat began. The lengths of both Columbia and Hubbard Glaciers are extended in the spring, and retracted in the fall. The relatively long period of record for Columbia Glacier shows consistent seasonal variation in length and, when compared with the short period of record for Hubbard Glacier, suggests that there are consistent seasonal differences in length at Hubbard also. The lower Hubbard Glacier diverges, feeding ice into Disenchantment Bay and Russell Fiord, and advances and retreats synchronously over most of its width. The tidal current, which passes through the 250–500 m wide entrance to Russell Fiord, does not affect the seasonal advance and retreat in that area significantly.

The terminus of Hubbard Glacier, Alaska

Hubbard Glacier advanced across the entrance of Russell Fiord in May 1986, transforming the fiord into a lake, which began filling with fresh water. The dam failed in October 1986. The calving terminus of Hubbard Glacier has been mapped with increasing frequency for nearly a century. A mapping interval of a few years establishes that the terminus has made a slow, but accelerating, advance. Recently the terminus has been mapped several times per month using vertical photography, ground surveys, and time-lapse cameras. At this frequency of observation, the terminus position is found to fluctuate seasonally. These seasonal fluctuations are compared with those of Columbia Glacier, where a longer detailed record is available. Although Columbia Glacier is now undergoing a drastic retreat, it continues to have seasonal length changes similar to those it experienced before the retreat began. The lengths of both Columbia and Hubbard Glaciers are extended in the spring, and retracted in the fall. The relatively long period of record for Columbia Glacier shows consistent seasonal variation in length and, when compared with the short period of record for Hubbard Glacier, suggests that there are consistent seasonal differences in length at Hubbard also. The lower Hubbard Glacier diverges, feeding ice into Disenchantment Bay and Russell Fiord, and advances and retreats synchronously over most of its width. The tidal current, which passes through the 250–500 m wide entrance to Russell Fiord, does not affect the seasonal advance and retreat in that area significantly.

The Upheaval of the Sea Coast by Earthquakes

THE question so long discussed by geologists concerning the upheaval of the land by earthquakes has been impresively revived by recent events. In the San Francisco Argonaut of November 3, 1906, Prof. H. D. Curtis, of the D. O. Mills Expedition of the Lick Observatory at Santiago, Chile, reports that the harbour at Valparaiso is now 10 feet shallower than before the earthquake of August 16, 1906, and he concludes that the movement was mainly vertical. In the Bulletin of the Geological Society of America for May, 1906, Messrs. Tarr and Martin give a memoir on the changes of level at Yakutat Bay, Alaska, produced by the great earthquake of September 3–20, 1899, two of the most terrible shocks of which occurred on September 10 and 15. The investigators prove conclusively that an uplift occurred extending along the whole Yakutat Coast for more than a hundred miles, the maximum movement in Disenchantment Bay being 47 feet 4 inches. Uplifts of 7 feet to 20 feet were common, while slight subsidences also occurred in a few places.