Western Aleutian Islands Research Articles

Changes in sea urchins and kelp following a reduction in sea otter density as a result of the Exxon Valdez oil spill

Interactions between sea otters Enhydra lutris, sea urchins Strongylocentrotus droebachiensis, and kelp were investigated following the reduction in sea otter density in Prince William Sound, Alaska, after the Exxon Valdez oil spill in 1989. At northern Knight Island, a heavily oiled portion of the sound, sea otter abundance was reduced by a minimum of 50 % by the oil spill, and from 1995 through 1998 remained at an estimated 66 % lower than in 1973. Where sea otter densities were reduced, there were proportionally more large sea urchins. However, except in some widely scattered aggregations, both density and biomass of sea urchins were similar in an area of reduced sea otter density compared with an area where sea otters remained about 10 times more abundant. Furthermore, there was no change in kelp abundance in the area of reduced sea otter density. This is in contrast to greatly increased biomass of sea urchins and greatly reduced kelp density observed following an approximate 90 % decline in sea otter abundance in the western Aleutian Islands. The variation in community response to a reduction in sea otters may be related to the magnitude of the reduction and the non-linear response by sea urchins to changes in predator abundance. The number of surviving sea otters may have been high enough to suppress sea urchin populations in Prince William Sound, but not in the Aleutians. Alternatively, differences in response may have been due to differences in the frequency or magnitude of sea urchin recruitment. Densities of small sea urchins were much higher in the Aleutian system even prior to the reduction in sea otters, suggesting a higher rate of recruitment.

Global block kriging to estimate biomass from acoustic surveys for zooplankton in the western Aleutian Islands

Global block kriging to estimate biomass from acoustic surveys for zooplankton in the western Aleutian Islands

MECHANISMS OF POPULATION DIFFERENTIATION IN MARBLED MURRELETS: HISTORICAL VERSUS CONTEMPORARY PROCESSES

Mechanisms of population differentiation in highly vagile species such as seabirds are poorly understood. Previous studies of marbled murrelets (Brachyramphus marmoratus; Charadriiformes: Alcidae) found significant population genetic structure, but could not determine whether this structure is due to historical vicariance (e.g., due to Pleistocene glaciers), isolation by distance, drift or selection in peripheral populations, or nesting habitat selection. To discriminate among these possibilities, we analyzed sequence variation in nine nuclear introns from 120 marbled murrelets sampled from British Columbia to the western Aleutian Islands. Mismatch distributions indicated that murrelets underwent at least one population expansion during the Pleistocene and probably are not in genetic equilibrium. Maximum-likelihood analysis of allele frequencies suggested that murrelets from “mainland” sites (from the Alaskan Peninsula east) are genetically different from those in the Aleutians and that these two lineages diverged prior to the last glaciation. Analyses of molecular variance, as well as estimates of gene flow derived using coalescent theory, indicate that population genetic structure is best explained by peripheral isolation of murrelets in the Aleutian Islands, rather than by selection associated with different nesting habitats. No isolation-by-distance effects could be detected. Our results are consistent with a rapid expansion of murrelets from a single refugium during the early–mid Pleistocene, subsequent isolation and divergence in two or more refugia during the final Pleistocene glacial advance, and secondary contact following retreat of the ice sheets. Population genetic structure now appears to be maintained by distance effects combined with small populations and a highly fragmented habitat in the Aleutian Islands.Corresponding Editor: J. Neigel

Productivity, diets, and environmental contaminants in nesting bald eagles from the aleutian archipelago

AbstractWe studied productivity, diets, and environmental contaminants in nesting bald eagles from the western Aleutian Islands, Alaska, USA, during the summers of 1993 and 1994. Productivity on Adak, Tanaga, and Amchitka Islands ranged from 0.88 to 1.24 young produced per occupied site and was comparable to that of healthy populations in the lower 48 United States. However, productivity on Kiska Island was depressed, averaging 0.67 young per occupied site. The lower reproductive success on Kiska was associated with elevated levels of dichlorodiphenyldichloroethylene and other organochlorine pesticides. Many of the organochlorine pesticides were elevated in bald eagle eggs from the four islands, but concentrations of these contaminants and Hg were significantly higher in eggs from Kiska Island than in eggs from the other islands. In contrast, polychlorinated biphenyl concentrations were higher in eggs from Adak, Amchitka, and Kiska (where military facilities have been installed) than in those from Tanaga (which has had little military activity). The most likely source of these contaminants in bald eagles was from their diets, which were variable spatially and temporally. Fish made up most (56%) of the eagles' diet on Adak and Tanaga Islands, followed by birds (25%) and mammals (19%). In contrast, birds comprised the majority (60%) of bald eagle diets on Amchitka and Kiska Islands, followed by mammals (30%) and fish (10%). The high proportion of seabirds in the diet of eagles from Kiska Island could be the major source of organochlorine and Hg contamination. Elevated concentrations of organochlorines in bald eagle eggs from the Aleutian Archipelago was surprising, because of the distance to agricultural areas. The results indicate that these contaminants can be transported long distances and affect wildlife populations in remote and pristine areas. We also discuss potential sources and transport mechanisms of these contaminants to the Aleutian Islands.

AGE, GROWTH, AND REPRODUCTIVE PATTERNS OF DALL'S PORPOISE (PHOCOENOIDES DALLI) IN THE CENTRAL NORTH PACIFIC OCEAN

AbstractLife history parameters were estimated for Dall's porpoise, Phocoenoides dalli, from biological specimens collected in the western Aleutian Islands, during 1981–1987. Of 2,033 males and 3,566 females examined, reproductive data were available for 1,941 males and 1,906 females; ages were determined for 813 males and 1,297 females. Female sexual maturity was based on the presence of one or more corpus on either ovary; 845 were sexually immature and 1,061 were sexually mature. Two estimates of female average age at sexual maturity (ASM) were 3.8 and 4.4 yr; average length at sexual maturity (LSM) was 172 cm. Males were considered sexually mature when evidence of spermatogenesis was detected; 1,136 were sexually immature and 805 were sexually mature. Two estimates of male ASM were 4.5 and 5.0 yr; LSM was 179.7 cm. Physical maturity was assessed for 246 males and 446 females by examining the degree of fusion in thoracic vertebral epiphyses. For both sexes, the average age at physical maturity was 7.2 yr. Average length at physical maturity was 202.6 cm for males and 192.7 cm for females. Average lengths of physically mature males (x̄= 198.1 cm, SE = 0.8566) and females (x̄= 189.7 cm, SE = 0.4002) were significantly different(P < 0.0001). Early postnatal growth was rapid in both sexes. A secondary growth spurt in both mass and length was characteristic for both sexes; the increase in length preceded the mass increase by 1–2 yr. Average length at birth (LOB) was approximately 100 cm; birth mass averaged 11.3 kg (SE = 0.0772). By the time the umbilicus had healed (<2 mo), the average length and mass had increased to 114.1 cm and 23.8 kg, respectively. Gestation period based on projections using LOB was 12 mo, but this was considered an overestimate. Calving was modal, centered in early July; an annual reproductive interval was indicated. Among the sexually mature females, 120 were pregnant, 55 were pregnant and lactating, 321 were pregnant with colostrum, and 33 were “resting.” By 3 July (95% CI =x̄ 1 d), 50% of births had occurred, during each of the seven years sampled. The ovulation rate was estimated at 0.914 ovulations per average reproductive year. Enlarged follicles and recent ovulations were observed in postpartum females in late July.

Distribution of acoustically determined biomass and major zooplankton taxa in the upper mixed layer relative to water masses in the western Aleutian Islands

Hydrographic, acoustic and zooplankton data were collected during June 1992 and June 1995 along transects to the north and south of Kiska and Buldir Islands in the western Aleutian Islands to look for mesoscale relationships between zooplankton community composition, biomass, and associated frontal systems and pycnoclines. Hydrographic data revealed a front dividing the Bering Sea water mass from the mixed regime in the passes and Alaska Stream water south of the islands. Chlorophyll concentration and ADB (acoustically determined biomass) were consistently higher on the Bering Sea side of the front in June 1992. Elevated ADB in the Bering Sea was due to aggregates of Neocalanus cristatus copepodid V in the pycnocline separating the upper mixed layer from the underlying cold intermediate layer. N. cristatus abundance was significantly higher (p < 0.05) in Bering Sea water in June 1992 than in Pacific water in June 1992 and Bering Sea water in June 1995. N. fleminger abundance was significantly higher in Bering Sea water than in Pacific water in both 1992 and 1995. N. plumchrus and Eucalanus bungil stages III and IV were more abundant in Pacific water than in Bering Sea water. The results clearly demonstrate the utility of interfacing acoustics, net samples and hydrographic data to elucidate mesoscale associations of zooplankton to fronts, water masses and pycnoclines.

Neocalanus scattering layers near the western Aleutian Islands

Dense copepod scattering layers were detected in the upper 50 m of the water column in the Bering Sea north of the western Aleutian Islands during June 1992. Acoustic data were collected at frequencies of 200 and 420 kHz, and compared with simultaneous estimates of wet weight biomass from net tows. Regression of net and acoustic data produced a scaling factor of-69.6 dB gr1 to convert volume scattering to biomass. Application of the fluid sphere scattering model produced an average equivalent spherical radius for the targets of 0.54 mm. The major sound-scattering organism, Neocalanus cristatus copepodid stage V, dominated the biomass in the pycnocline between the upper mixed layer and the underlying cold intermediate layer. Populations were dominated by Neocalanus plumchrus and Neocalanus flemingeri, with greatest abundance above the pycnocline.

Comparative foraging ecology of planktivorous auklets in relation to ocean physics and prey availability

We tested the hypothesis that the spatial distributions of foraging least, crested and parakeet auklets (Aethia pusilla, A. cristatella and A. psittacula, respectively) in the shallow passes of the Aleutian Islands would be determined by physical mechanisms that control near-surface prey concentrations. We recorded currents using an Acoustic Doppler Current Profiler, volume scattering using 200 and 420 kHz scientific echosounders, and the numbers of foraging birds. Zooplankton were sampled using a multiple opening/closing net and environmental sampling system (MOCNESS). Prey choice of birds was ascertained by collecting foraging birds and examining their stomach contents. Most sampling occurred between 8 July and 6 August 1993, when we conducted 50 passages along a transect that crossed a sill between Unalga and Kavalga Islands, western Aleutian Islands, thereby sampling the foraging activity of auklets at a variety of times of day and tidal phases. We found that the abundance of foraging individuals of each of the 3 auklet species was a function of tidal speed. Auklet species were selective about the species of prey taken. Regardless of tidal direction, crested auklets foraged on euphausiids upwelled on the upstream side of the pass, whereas least auklets consumed copepods concentrated in near-surface convergences on the downstream side. Parakeet auklets foraged over the top of the pass and took fish and invertebrates. Tidal speed and direction influenced the distance between the peak numbers of some, but not all, species of auklets. Auklet prey preferences dictated where they foraged in the pass and the physical mechanisms exploited for successful foraging. Thus, in this instance, resource partitioning by these closely related planktivores was enhanced by a spatial segregation forced by the physical processes that enhanced the availability of prey. Our findings emphasize the important role of physical processes in the structuring of marine communities.

Large‐scale deformation related to the collision of the Aleutian Arc with Kamchatka

The far western Aleutian Island Arc is actively colliding with Kamchatka. Westward motion of the Aleutian Arc is brought about by the tangential relative motion of the Pacific plate transferred to major, right‐lateral shear zones north and south of the arc. Early geologic mapping of Cape Kamchatka (a promontory of Kamchatka along strike with the Aleutian Arc) revealed many similarities to the geology of the Aleutian Islands. Later studies support the notion that Cape Kamchatka is the farthest west Aleutian “island” and that it has been accreted to Kamchatka by the process of arc‐continent collision. Deformation associated with the collision onshore Kamchatka includes gravimetrically determined crustal thickening and formation of a narrow thrust belt of intensely deformed rocks directly west of Cape Kamchatka. The trend of the thrust faults is concave toward the collision zone, indicating a radial distribution of maximum horizontal compressive stress. Offshore, major crustal faults trend either oblique to the Kamchatka margin or parallel to major Aleutian shear zones. These offshore faults are complex, accommodating both strike‐slip and thrust displacements as documented by focal mechanisms and seismic reflection data. Earthquake activity is much higher in the offshore region within a zone bounded to the north by the northernmost Aleutian shear zone and to the west by an apparent aseismic front. Analysis of focal mechanisms in the region indicate that the present‐day arc‐continent “contact zone” is located directly east of Cape Kamchatka. In modeling the dynamics of the collision zone using thin viscous sheet theory, the rheological parameters are only partially constrained to values of n (the effective power law exponent) ≥ 3 and Ar (the Argand number) ≤ 30. These values are consistent with a forearc thermal profile of Kamchatka, previously determined from heat flow modeling. The thin viscous sheet modeling also indicates that onshore thrust faulting is a consequence, not only of compressive stresses resulting from the west directed collision, but also of sediment‐induced coupling of the subducting Pacific plate.

Food Habits of the Whiskered Auklet at Buldir Island, Alaska

We studied the food habits of Whiskered Auklets (Aethia pygmaea) at Buldir Island, western Aleutian Islands, Alaska, during the summer of 1976. At Buldir, these birds fed primarily in and near convergent tidal fronts in the passes between three islets just offshore; feeding occurred throughout the day. Whiskered Auklets ate at least 10 different species of zooplanktonic prey, of which the copepod Neocalanus plumchrus was the most important. Next in importance were chaetognaths (probably Sagitta elegans), the amphipod Parathemisto pacifica, megalopae of the crab Erimacrus isenbeckii, and an unidentified amphipod. Pteropods (probably Limacina helicina), larval cephalopods, and a larval fish occurred in trace amounts. During most of the summer, Whiskered Auklets were nearly monophagous on N. plumchrus; we do not consider a difference in prey during late incubation/early chick rearing to represent a biologically-significant trend. We collected Least (A. pusilla), Crested (A. cristatella), and Parakeet (Cyclorrhynchus psittacula) auklets during late chick rearing, to compare their food habits with those of Whiskered Auklets. At this time, both Least and Whiskered auklets were monophagous on smaller N. plumchrus, whereas Crested and Parakeet auklets mainly ate larger N. cristatus and P. pacifica; Parakeet Auklets ate the widest diversity of zooplankton. Selection of prey by the four auklet species appeared to occur in relation to size, with the smaller auklets eating smaller zooplankters.

Seismicity and tectonics of the far Western Aleutian Islands

Study of focal mechanisms of earthquakes in the Near and Komandorsky Islands indicate that there are several distinct zones of tectonic activity. South of the Near Islands, normal faulting occurs in the trench east of 172°E and low-angle thrusting dominates the Aleutian ridge. Mechanisms indicate underthrusting as far west as Mednyy Island with strike-slip faulting restricted to the south and west of Beringa Island. A zone of northeast striking left-lateral faulting near 1645.°E is proposed to separate the Aleutian Ridge from Kamchatka Peninsula. This motion, as well as faulting north of the Komandorsky Islands, may be related to the existance of a buffer plate comprising the Aleutian Ridge in the Komandorsky Islands. Active subduction terminates near 173°E and the faulting north of the Komandorsky Islands may, in part, be due to the bouyancy of a remnant slab. Depth phase modelling indicates bulletin-reported depths are overestimated due to a misidentification of depth phases.

Foraging Strategies of Glaucous-Winged Gulls in a Rocky Intertidal Community

Foraging strategies of Glaucous-winged Gulls (Larus glaucescens) were studied in rocky intertidal habitats of the western Aleutian Islands, Alaska. Daily foraging activity was most intense at maximum low tide, and was concentrated in the lowest intertidal zones available to the birds. Barnacles (Balanus glandula) and mussels (Mytilus edulis) comprised most of the gull's diet during neap low tides, but these species were almost entirely abandoned during spring low tides in favor of sea urchins (Strongylocentrotus polyacanthus), chitons (Katharina tunicata), and limpets (Collisella pelta and Notoacmaea scutum). Sea urchins, chitons, and limpets, which had positive prey selection indices, were most abundant in the lower intertidal zones; barnacles and mussels, which had negative prey selection indices, were most abundant in the upper zones. Gulls also generally selected the larger individuals from each prey species, although sea urchins larger than the commissural bill width were avoided and limpets were selected in proportion to availability. Variation in prey availability also occurred among study areas with varying densities of sea otters (Enhydra lutris). With increasing depression of invertebrate prey by sea otter predation, gulls fed on a more diverse prey resource, and they switched to neritic fishes under intense sea otter predation. Preference experiments were conducted in the field, in which the common species and sizes of prey were made equally available to foraging gulls, thus eliminating search and capture times. In comparison with natural food choice, where sea urchins were most preferred, chitons became most preferred. We suggest that chitons are infrequent in natural diets because they adhere more strongly to the substratum than do sea urchins. Benefits of selective foraging were determined by comparing the net rate of energy gain of simulated random foragers with energy gained by selection of intertidal zones, prey species, and prey sizes. Observed selection patterns provided increased energy as gulls became more selective, and averaged 155% more than that obtained by the simulated random foragers. Results of the study support the two main predictions of optimal foraging theory in that (1) foraging patches (intertidal zones) and diets were selected such that net rates of energy gain were maximized, and (2) gulls became more selective foragers when energetically more profitable prey were more available.

Parasites of the Dall's porpoise (Phocoenoides dalli True).

The prevalences of three helminths, Campula oblonga, Halocercus dalli and Crassicauda sp., recovered from Dall's porpoises which were net-entrapped incidentally in the vicinity of the Western Aleutian Islands in the northwest Pacific are reported. Specimens of Campula oblonga were found within the bile ducts of 46% of 127 livers examined. The prevalence of hepatic trematodiasis increased with the age of the host. Pulmonary nodules associated with Halocercus dalli were noted in 71% of the Dall's porpoises. Adult H. dalli were recovered from the main stem bronchi of heavily infected lungs. Younger animals exhibited a relatively higher prevalence. Specimens of Crassicauda sp. were found within the main lactiferous canal of 69% of 29 mammary glands examined. The prevalence was highest in mature porpoises. Possible detrimental effects and the modes of transmission of the three species of parasites are also considered.

Flow of the Alaskan Stream and its variations

Data from cruises in winter 1980 and summer 1981, which traversed the region from the head of the Gulf of Alaska to the western Aleutian Islands, are used to examine the flow of the Alaskan Stream and its changes. The physical property distributions reveal seasonal changes, interactions with shelf and offshore waters, and the effects of large-scale variations in circulation. Peak geostrophic speeds, referred to 1500 db, were typically 100 cm s −1; consideration of the baroclinic structure below 1500 db would increase the speeds slightly, but the usual volume transports (about 12 × 10 6 m 3 s −1, referred to 1500 db) would increase by about 3 × 10 6 m 3 s −1 near 150°W and by about 8 × 10 6 m 3 s −1 near 180°. In winter 1980 the Alaskan Stream had essentially constant transport (referred to 1500 db) from the head of the gulf to the western Aleutians, with some southward recirculation of upper waters near 175°W and a southward flow through Amchitka Pass. In summer 1981 the circulation pattern was markedly different; only about half the normal flow entered the upper gulf, and the rest was deflected northward across the southern stream boundary. The two flows converged near 165°W to produce typical transports along the Aleutians. It appears that the rare flow structure of summer 1981 was related to an anomalous distribution of wind-stress curl, which may have altered isopycnal slopes and the geostrophic flow.

Alcataenia fraterculae sp. n. from the horned puffin, Fratercula corniculata (Naumann), Alcataenia cerorhincae sp. n. from the rhinoceros auklet, Cerorhinca monocerata (Pallas), and Alcataenia larina pacifica ssp. n. (Cestoda: Dilepididae) in the North Pacific basin.

Three Cestodes representing two species of the genus Alcataenia Spasskaia, 1971 and a subspecies of Alcataenia larina (Krabbe, 1869) are described. Alcataenia fraterculae sp. n. (Cestoda: Dilepididae) was found in horned puffins, Fratercula corniculata (Naumann), and other species of seabirds from localities in the western Aleutian Islands, Gulf of Alaska, Bering Sea, and Chukchi Sea. Alcataenia cerorhincae sp. n. is described from the rhinoceros auklet, Cerorhinca monocerata (Pallas) in the eastern North Pacific Ocean and western Aleutian Islands. Alcataenia larina pacifica ssp. n. is recognized from species of Laridae and other seabirds in the North Pacific Ocean, Sea of Okhotsk, Bering Sea and the region of the Arctic Ocean near Bering Strait. It is distinguished from A. larina larina (Krabbe, 1869) by a greater number of testes, a longer cirrus sac, and variation in the position of the genital ducts which may be either dorsal to or between the osmoregulatory canals. A fraterculae and A. cerorhincae are most similar to A. larina and particularly to the North Pacific form A. l. pacifica. Generally specimens of A. fraterculae can be distinguished from the other taxa by larger rostellar hooks, a longer cirrus sac, and a combination of other characters. A. fraterculae, A. cerorhincae, and A. l. pacifica however represent a complex of cryptic species in which there is extensive overlap in some morphological characters. Results of a discriminant analysis among these nominal taxa were significant and, in combination with data about other morphological characters and host and geographic distribution, clearly indicated that these represent three species in the North Pacific basin.

Diet of the Snowy Owl in the Absence of Small Mammals

Snowy Owls (Nyctea scandiaca) prey mostly on small mammals (Pitelka et al. 1955, Watson 1957, Taylor 1974), though they are known to eat birds (Murie 1929, Gross 1944, Wiggins 1953, Campbell and MacColl 1978). On Agattu Island in the Near Island group of the western Aleutian Islands, the owls occur in the absence of small mammals and subsist entirely on a diet of birds. From June to August 1975 one of us (LGF) collected 141 pellets which were analyzed to determine the Snowy Owl diet in this unusual ecological situation. Snowy Owls have been seen sporadically throughout the Aleutians and are found consistently on Agattu, Buldir, and Attu (Turner 1886, Murie 1959). While they are known to breed on the Commander Islands (Murie 1959) and on Attu where Norway rats (Rattus norvegicus) are present, they did not appear to breed on Agattu in 1974-77 (J. L. Trapp, pers. comm.). We saw no evidence of nesting, breeding displays, or territorial behavior in 14 h of observation in June 1975. No immatures were seen on Agattu, although they have been seen on Buldir.

Sea Otter Predation and Community Organization in the Western Aleutian Islands, Alaska

Predation by the sea otter (Enhydra lutris) limits epibenthic invertebrates, especially sea urchins (Strongylocentrotus polyacanthus), in turn allowing a luxuriant development of the macroalgal canopy. Where sea otters are abundant, sea urchins are small and scarce in shallow water, and the association of fleshy macroalgae apparently is regulated by competition. Sea urchins are larger and more abundant in deeper water, where they are less accessible to sea otters. Macroalgae are most abundant, and competition in the plant association is severest, near the sublittoral fringe where sea otters can remove sea urchins most efficiently. In deep water, competition among maccroalgae is reduced because the light intensity is lower and grazing by sea urchins increases. On islands where sea otters are absent, sea urchins are abundant, large, and are probably limited by intraspecific competition; and they have eliminated fleshly macroalgae. Available data suggest that the association of Laminaria spp. and Agarum cribrosum contributes most to primary production in nearshore areas of the western Aleutian Islands. Where sea otters are absent and sea urchins have eliminated this plant association, some higher trophic forms also are absent or less abundant than where sea otters are common and the plant association is well developed. Earlier studies of sea otter food suggested that low—density populations of sea otters consume primarily sea urchins and mollusks in the western Aleutian Islands. Later studies of high—density populations showed a wider variety of foods consumed, with fish an important component of the diet. These studies support our observations on the differences in availability of these foods between islands with and without sea otters.

Sunspot activity and oceanic conditions in the northern north Pacific Ocean

During periods of sunspot maxima (approximately every 11 years) the mean winter position of the center of the Aleutian Low pressure system shifts from the Gulf of Alaska to the western Aleutian Islands, and mean, cyclonic, wind-stress transport in the Gulf of Alaska is reduced by roughly 20%. Coastal sea level data in the gulf do not reflect an 11-year cycle but spectral energy densities indicate an approximate 6-year periodicity also present in transpacific annual mean sea surface temperatures that, in the last one or two decades, parallels large year classes of Pacific herring in southeastern Alaska, large escapements of sockeye salmon fry in the Bristol Bay area, and maxima in the January catch of Dungeness crab in Alaska.

K-Ar ages from the Near Islands, western Aleutian Islands, Alaska: Indication of a mid-Oligocene thermal event

Deformed and metamorphosed basaltic volcanic rocks (one pillow lava, one amphibolite) from the basement on Attu Island have yielded whole-rock K-Ar ages of about 30 m.y. The greenschist (or higher grade) assemblage of these rocks and their presumed correlation with late Eocene or older early-series rocks elsewhere in the Aleutian arc suggest that these dates indicate a time of metamorphism. Similar ages (≅30 m.y.) for two mafic intrusions into the basement may also signify a time of metamorphism. These four ages place an older limit on, and may coincide with, the mid-Tertiary tectonism that deformed rocks of the early series along the length of the Aleutian arc. One post-early-series intrusion on Agattu is 14 m.y. old, in agreement with previous results from Shemya on a similar rock and in accord with the observation of a relatively abrupt start of late Tertiary magmatism throughout the arc about 15 m.y. ago. The youngest recognized igneous activity in the Near Islands is represented by hornblende-bearing rocks (one andesite lava, one dacite dike) on Attu that have yielded ages of 6 to 8 m.y.

Earth-science studies of a nuclear test area in the western Aleutian islands, Alaska : An interim summary of results GEOL. SOC. AMER. BULL. V82, MAR. 1971, P699–705

Earth-science studies of a nuclear test area in the western Aleutian islands, Alaska : An interim summary of results GEOL. SOC. AMER. BULL. V82, MAR. 1971, P699–705