V23A-0616 1340h
Tephra Studies by the Alaska Volcano Observatory: Present and Future Research
Tephra from Aleutian arc volcanoes constitutes an important volcanic hazard for Alaska, western Canada, and some parts of the conterminous U.S. where even small amounts of airborne ash may have dire consequences for jet aircraft traversing North Pacific and western U.S. air routes. Motivated by the need to address volcanic ash hazards on a regional scale, we have initiated a program of tephra studies within the auspices of the Alaska Volcano Observatory (AVO) of the U.S. Geological Survey. A concentrated focus on tephra problems and a new laboratory facility within AVO will help facilitate studies of Quaternary age tephra at Alaskan volcanoes by providing a regional center for laboratory analyses of volcanic ash and standardized web-based reporting and archiving of tephra data. In its first year of operation, the laboratory has been engaged in research at Veniaminof, Mt. Spurr, and Augustine volcanoes, has sponsored research on Holocene tephra deposits of upper Cook Inlet, and has initiated analytical studies of tephra deposits on Adak and Kanaga Islands in the western Aleutians. The objective of these studies is to develop multiparameter techniques for characterization and correlation of tephra deposits, establish radiocarbon-controlled tephrostratigraphic frameworks, and to evaluate the magnitude and frequency of tephra-producing eruptions. In the upper Cook Inlet region of Alaska, we and our colleagues have begun developing a comprehensive record of ash fall by systematically selecting and coring shallow lakes and evaluating the tephra preserved in the lacustrine sediment. Sediment cores from these lakes contain numerous tephra deposits of Holocene age in datable context that can be correlated with proximal tephra deposits on the flanks of their source volcanoes. By combining tephra data from lacustrine deposits and natural exposures we hope to develop a robust geologic catalog of tephra deposits that will enable long-distance correlation of tephras, provide greater detail on the chronology of eruptions, and establish a longer-term context for tephra hazards. Future work will be focused on improving correlation of tephras, identification of source volcanoes, developing reference datasets, and developing a web-served database of tephra data.
V23A-0617 1340h
Tephras and the History of Volcanism, Recent Tectonism, and Tsunamis in Lower Cook Inlet, Alaska
Cook Inlet is located on the western side of the Kenai Peninsula in south-central Alaska. Past eruptions from Augustine volcano and other volcanoes have had significant impacts on the lower Cook Inlet communities. Tephra layers from prehistoric and historic eruptions are preserved within peat sequences along Kachemak Bay, near Homer, Alaska. Evidence of active tectonism is found where thick marine muds overlie peat deposits along Kachemak Bay. The marine mud deposits record co-seismic subsidence that occurred during a prehistoric great earthquake on the northern pacific subduction zone. Two layers of sand and beach gravel also occur within the peat deposits. The sedimentology is identical to classic tsunami deposits identified in other tectonically active areas, and we interpret these deposits as evidence of prehistoric tsunamis. The upper sand is directly overlain by a 1.2 cm thick grayish white tephra. The sand and beach gravel associated with the tephra most likely records a tsunami triggered at the nearby Augustine volcano, while the other sand deposit may be evidence of a tectonically triggered tsunami. The recognition of tephras, tsunami deposits, and evidence of prehistoric co-seismic subsidence has significant implications for geohazards in the lower Cook Inlet area.
V23A-0618 1340h
Life on the Edge: Holocene Tephra Stratigraphy of Tanginak Anchorage, Sitkalidak Island, Kodiak Archipelago, Alaska
Geologic hazards associated with volcanism in the North Pacific have profound if usually temporary effects on the environment and human populations. Ash falls associated with these events are often preserved across large areas providing time specific markers. In the past century, volcanic activity and its effects in the North Pacific have been recorded, but much of the Holocene volcanic record in the Alaskan region is still being investigated. The Kodiak Archipelago, while not volcanic itself, is located near both Aleutian and Alaskan peninsula volcanoes. However, little has been published about the Holocene tephrochronology of the Kodiak region. This study focuses on the area around Tanginak Spring Site (KOD481). Located on Sitkalidak Island it is the earliest known human occupation in the Kodiak archipelago. We are documenting Holocene environmental changes on Sitkalidak Island and relating these changes to the archaeological record. As part of this work, we will establish a local tephrochronology using stratigraphy and geochemistry which will allow us to better correlate sedimentary changes across large areas as well as study human interaction with ashfall events. Herein we report a preliminary tephrochronology in peat excavations on Sitkalidak Island dating back to the earliest Holocene. Dates are radiocarbon years BP on peat directly below tephra. Marker tephra present in our reference sections are Katmai 1912, light gray (historic?), medium gray (3370), medium gray (3720), beige 1 (4340), apricot (5390), beige 3 (6790), black (9280), and white (11,520). Geochemical and petrographic analysis will help to determine with which volcanic events these tephra are associated. Establishing a local tephrochronology is important not only for local correlation but also to ascertain the tephra stratigraphy of the Kodiak Archipelago and beyond. The frequency of tephra in Tanginak Anchorage sections suggests that tephra will be a very useful stratigraphic tool in this region.
V23A-0619 1340h
Using Kettle Lake Records to Date and Interpret Holocene Ash Deposition in Upper Cook Inlet, Anchorage, AK
Fourteen sediment cores recovered from three kettle lakes (Goose, Little Campbell and Lorraine) near Anchorage, AK were used to document and date Holocene volcanic ash deposition in the upper Cook Inlet area. Small lakes ($<$0.5 km$^2$) with small ($<$1.5 km$^2$), low relief ($<$50 m), and well-vegetated drainage areas were selected in order to minimize ash remobilization by mass wasting and fluvial processes. The resulting stratigraphic records are interpreted as primary terpha-fall stratigraphies. Relative to the surrounding lacustrine sediments, the ash layers exhibit low organic-matter content (as determined by loss-on-ignition, LOI), high magnetic susceptibility (MS), increased density (X-radiographs), and bubble-wall glass shards. Some ash layers are up to 1 cm thick (macrotephra) consisting of pure glass, some occur as light bands, while others (microtephra) can only be located using non-visual techniques (MS, LOI and X-radiography). The thinnest microtephras observed occur either as discrete (1 mm) layers or diffuse laminations composed of tephra mixed with ambient lake sediment. Forty-five AMS C-14 dates on terrestrial macro fossils were used to constrain sedimentation-rate models for the cores, and to assign absolute ages to ash units. Comparison of inferred tephra ages corroborates our intra and inter basin stratigraphic correlations (+/- 200 yrs) based on physical and MS stratigraphy. Ten out of 12 macrotephras can be confidently correlated among all three lakes, whereas, two of the prominent tephras occur in one basin but not in the others. This suggests subtle differences in ash plume extents or differences in tephra preservation between lakes. A total of 24 Holocene ash units (12 macro and 12 micro) have been recognized and dated in the Anchorage area, suggesting an ash-fall frequency of about 2.4/1000 yrs. By comparison, historical records suggest more frequent ash-fall events (120/1000 yrs). Our data indicate that, either the ash layers are not consistently preserved in the kettle basins, or more likely, these records lack the resolution to differentiate closely spaced ash-fall events. Core top stratigraphies support the latter interpretation: The 10-12 historically observed ash-fall events are represented by two diffuse zones in the upper 15 cm of the cores. As such, ash records from small kettle lakes should be regarded as conservative statements of ash deposition. Further, ash plumes can have narrow geographic distributions and ash-fall thicknesses can change markedly over short distances. Therefore distal ash-fall stratigraphies underestimate eruption frequencies.
V23A-0620 1340h
Intrabasin Variability of Volcanic Ash Stratigraphy in a Small Kettle Lake; Lorraine Lake, Anchorage, Alaska
Lorraine Lake is a small (0.53 km$^{2}$) shallow (ca. 8 m) kettle located on the Elmendorf Moraine (Pleistocene age) 11 km northwest of Anchorage, Alaska. Situated in an area of low relief (49 m), the basin has a small drainage basin (1.3 km$^{2}$), no inflow and remains ice covered for approximately six months of the year. This study was initiated to resolve the volcanic ash-fall record preserved in the Holocene lake sediments from this basin, and to evaluate intrabasinal variability of ash stratigraphies. It was hypothesized that tephra deposition varies spatially across the lake and that some locations exhibit a more complete record of ash fall than others. This variation may possibly be due to tephra being redistributed by wind on the frozen or open-water surface, carried by currents once it sinks, or mixed by bioturbation following deposition. Six sediment cores between 3.2 and 5.8 m long were recovered from the north, south, east, and west parts of the lake, which is divided into two (north and south) sub-basins. A total of 21 AMS $^{14}$C ages were obtained on terrestrial macrofossils and basal ages from three cores are greater than 14,500 cal yr. BP, confirming that the cores contain the entire postglacial sedimentary record. Eleven tephra deposits, ranging from invisible to several centimeters in thickness, were correlated among the cores based on their relative depths, spacing, color, texture, thickness, high magnetic susceptibility (MS), low loss-on-ignition, X-ray gray scale value, and abundance of magnetic minerals. Although other diffuse tephra units occur, these 11 clearly defined units are used to compare tephra deposition within the lake. Several physical characteristics were compared to evaluate possible intrabasin variability including stratigraphic thickness, and X-ray density stratigraphy. A numerical classification scheme was developed ranking visual and stratigraphic prominence based on thickness, purity of ash and nature (sharpness and continuity) of stratigraphic contacts. Despite variability in sedimentation rates (ranging from 0.69 to 0.29 mm/yr) the physical characteristics (thickness, MS, and purity) of the tephra units display minimal variation. No consistent pattern of variability was recognized when comparing cores recovered from different depths and areas within the basin. The stratigraphic prominence of the tephra layers and their similarity between core sites implies that probable depositional complexities (e.g., aeolian reworking, wave action, and lake ice) and post-depositional processes (e.g., bioturbation, bathymetric focusing) have a minimal impact on the deposition and preservation of tephra units in small kettle lakes similar to Lorraine Lake.
V23A-0621 1340h
Correlation Confidence: A Systematic Approach to Presenting and Evaluating Tephra Data
Tephra deposits reflect the source, volume, and explosivity of a volcanic eruption, the geochemical composition and physical character of its parent magma, and the age of eruption. In addition and owing to the widespread distribution of tephra deposits, they are often used to establish age, stratigraphic position, and correlation over long distances for associated deposits. To make reliable tephrostratigraphic correlations, it is important to demonstrate equivalence among tephra outcrop and core locations when tephra layers cannot be continuously traced. It is also important to demonstrate dissimilarity between candidate sample pairs of known different ages. Identification and correlation of unknown tephra layers is commonly done using glass-shard chemistry. This approach may be inconclusive or unreliable, especially where geochemical differences between tephra deposits are small (as in the case of tephra erupted from the same vent or eruptive source area), or where tephra is chemically heterogeneous. Attempts at quantifying the similarity between tephra samples is commonly limited to use of the similarity coefficient (SC) and cluster analyses generated from geochemical data which do not include the role of other important tephra and stratigraphic characteristics including 1) field characteristics and stratigraphic sequence 2) petrography, 3) physical glass shard characteristics, 4) geochemical trends (degree of scatter, polymodality, linearity, replicate analyses) and 5) associated qualitative and quantitative chronologic information. Correlation of tephra deposits is often based on a high SC value and other important and distinguishing characteristics of the tephra that could increase the confidence level of the correlation are often overlooked. We propose here a means of assigning an index of correlation probability by presenting data in the form of a matrix and assigning numerical values to candidate pairs by independently weighting tephra characteristics as parameters for correlation. This technique shows that multi-parameter characterization and comparison among tephra samples is an improved means of reaching a more confident correlation. Each parameter, however, does not hold the same weight and should be independently weighted to reflect its significance to the correlation. This method encourages users to use a maximum number of parameters for correlation that increases the level of confidence in the resulting correlation.
V23A-0622 1340h
Kaguyak to Katmai: Post-Glacial Tephras in Katmai National Park, Alaska
At least 15 explosive eruptions from the Katmai volcanoes on the Alaska Peninsula are preserved as tephra layers in syn- and post-glacial (Last Glacial Maximum) loess and soil sections in Katmai National Park, AK. About 400 tephra samples from 150 measured sections have been collected between Kaguyak volcano and Mt Martin and from Shelikof Strait to Bristol Bay ($\sim$8500 km$^{2}$). Five tephra layers are distinctive and widespread enough to be used as marker horizons in the Valley of Ten Thousand Smokes area, and 140 radiocarbon dates on enclosing "soils" have established a time framework for entire soil-tephra sections to 10 ka; the white rhyolitic ash from the 1912 plinian eruption of Novarupta caps almost all sections. Stratigraphy, distribution and tephra characteristics (grainsize, thickness, color and mineralogy) have been combined with microprobe analyses of glass and Fe-Ti oxide minerals to correlate ash layers with their source vents including Mounts Martin, Mageik and Katmai, as well as Snowy Mountain and Kaguyak volcano. Microprobe analyses (25-70 analyses per glass or oxide sample) show oxide compositions to be more definitive than glass in distinguishing one tephra from another; oxides from the Kaguyak caldera-forming event are so compositionally coherent and distinctively low in TiO$_{2}$ relative to the rest of the Katmai group that they have been used as internal standards throughout this study. Other than the Novarupta and Trident eruptions of the last century, the youngest locally derived tephra yet recognized is associated with emplacement of the Snowy Mtn summit dome ($<$250 C$^{14}$ years). East Mageik erupted at least twice (2 and 4 ka), Mt Martin's blocky lava coulees were emplaced $\sim$6 ka, and Mt Katmai has three times produced very large explosive events (Novarupta 1912; plinian to sub-plinian "Lethe Assemblage" 12-16 ka; and a plinian rhyodacite 23 ka). Kaguyak's caldera-forming event (5.8 ka) generated a distinctive bright orange fine ash that is distributed widely throughout the Katmai district. Radiocarbon dating of loess, soil and peat enclosing this tephra are in good agreement despite their varying organic contents.
V23A-0623 1340h
The Tephra Stratigraphy of two Lakes in South-Central British Columbia, Canada and its Implications for the Middle-Late Holocene Volcanic Activity at Glacier Peak, Washington, USA
Several mid-late Holocene Glacier Peak tephras along with Mazama and Mount St. Helens Wn and P tephras were found in cores from Cooley and Rockslide Lakes in southeastern British Columbia, Canada approximately 300 km northeast of Glacier Peak. The sediments in Cooley Lake host the early Holocene Glacier Peak A tephra (2010 cal years BP), four separate, closely timed airfalls of Glacier Peak Dusty Creek tephra (5780-5830 cal years BP) and a Glacier Peak set D tephra (6060 cal years BP). This is the first report of Glacier Peak A and D tephras from British Columbia. The A tephra has been correlated on the basis of glass composition and age to a late Holocene Glacier Peak tephra in the sediments of Big Twin Lake, 75 km northeast of Glacier Peak. The glasses in the four Glacier Peak Dusty Creek tephra layers from Cooley Lake are compositionally indistinguishable from those in Mount Barr Cirque and Frozen Lakes in southwestern British Columbia. The layers represent four eruptions taking place over approximately 50 years (5780-5830 cal years BP). Although set D tephra has not been correlated to a known proximal/distal deposit, its glass bears the Glacier Peak glass compositional signature and its interpolated age (6060 cal. years BP) corresponds to the initiation of the set D eruptive period. The presence of Dusty Creek tephra in lake sediments across southern British Columbia and northern Washington State suggests a broad plume trajectory to the north and northeast. In contrast the apparent presence of the Glacier Peak A in only Cooley Lake in British Columbia and in lakes in Washington on the same linear trajectory suggests a narrow plume with a northeasterly direction.
V23A-0624 1340h
Tephra study in the Lake Biwa
Lake Biwa is one of important reservoirs for the volcanic ash in Japan. During 1982-1983, the 1422 m core was drilled in southern basin of the lake (35>X13'06"N, 136>X00'49"E). The most upper part of core was a continuous 250 m thick lacustrine clay and dated as within 430 ka (Meyers et al.,1993; Takemura, 1990). Total of 18 tephra layers were distinguished from the upper 640 m of the core. However, the fore 14 tephra layers were from the most upper 250 m. Pure glass shards were handpicked out from each tephra layer under the microscope and analyzed by the electron microprobe in the IES for chemical compositions. The K2O vs. SiO2 plot of the glass shards was used to verify the source of each tephra layer. Tephra Layer-2 at depth 13.5m with lower SiO2 and higher K2O was supposed from Ulreung-Oki in southern Japan Sea. Furuta (1986) claimed that glass shards of the caldera eruptions in central Kyushu revealed the high alkali contents (K2O + Na2O = 8.8wt%). Therefore, the tephra Layer-4 (K2O =4.6wt%) at depth of 66.85 m can be corresponded to the Aso-4 eruption well, while the tephra Layer-10 (K2O =4.8wt%) and tephra Layer-11 (K2O =4.98wt%) at around 169.2-170.9 m depth can be identified as the product of the Aso-1 eruption. Comparison of chemical compositions of glass shards in tephra layers in Lake Biwa with those in marine cores around Japan (Machida, 1999), they clearly showed that tephra Layer-1 at depth of 12.9 m, tephra Layer-5 at depth of 74.5 m, and tephra Layer-9 at depth of 158.6 m can be correlated with the caldera eruptions of K-Ah, Ata and Ata-Th in southern Kyushu, respectively. However, some tephra layers with lower K2O content ($<$4 wt%) are hard to distinguish their sources. Advanced isotopic analyses are needed to distinguish their sources in the future.