V21C-01 08:05h
Tephrochronolgical Studies of Late Neogene Sediments in Interior Alaska and the Yukon Territory
Our tephra studies of Late Neogene sediments in interior Alaska and Yukon are motivated by the need to provide a reliable time-stratigraphic framework for on-going palaeoenvironmental projects. Key sites are located in the Fairbanks, Chicken (Alaska) and Klondike (Yukon) goldfields, Old Crow Basin (Yukon), and the numerous bluffs along the Yukon River in Canada and eastern Alaska. Tephra beds are characterized by their field setting, petrography, geochemical composition of glass (majors and traces) and mineral phases (especially FeTi oxides), palaeomagnetic properties, and age (determined mostly by glass-fission-track methods). Two compositional groups are recognized. Type I beds have abundant bubble-wall glass shards and a small crop of crystals with pyroxene $>$ hornblende. Its glass has a rhyolitic to dacitic composition with relatively high FeO$_{t}$, Cs, Hf and low Al$_{2}$O$_{3}$, CaO, and Sr. REE profiles have a well-developed Eu anomaly with La/Yb $<$ 13. Volcanics with this chemical signature are common throughout the Aleutian Alaska Peninsula arc (AAPA), which is, therefore, the presumed source of the type I distal beds. In contrast, type II beds have more abundant crystals (hornblende $>$ $>$ pyroxene) and the rhyolitic glass is mainly in the form of highly inflated pumice with high Al$_{2}$O$_{3}$, CaO, and Sr. REE profiles are steep with low heavy REE content along with a very weakly developed Eu anomaly, if present. The type II beds are unusual and have many of the characteristics of adakites, known to occur at Mount Drum and Mount Churchill in the Wrangell volcanic field (WVF), and at Hayes volcano at the northeastern end of the Alaska Peninsula arc. It is likely, therefore, that the source vents for the type II beds in interior Alaska and Yukon are located in or near the WVF. Twenty-five distinctive tephra beds have been recognized in the Gold Hill Loess at Fairbanks and a comparable number have been discovered in the Klondike goldfields, although few beds are common to both regions. Tephra beds related to large-magnitude explosive eruptions with inferred widespread distributions, given their location, thickness, and presumed source, respectively, include, from the WVF: White River Ash (1-2 ka), Sheep Creek tephra in Alaska (190 ka), Gold Run (700 ka), SP (870 ka), WP (1.0 Ma), Paradise Hill (1.5 Ma), Fort Selkirk (1.5 Ma), Little Timber (2.3 Ma), Lost Chicken (2.8 Ma), and Quartz Creek (3.0 Ma). Corresponding units from the AAPA include: Dawson tephra (24 ka), VT (80 ka), Old Crow (140 ka), Ester (800 ka), Mosquito Gulch (1.5 Ma), PA (2.0 Ma), and Dago Hill (3.2 Ma). Application of the tephrochronological method to the Late Neogene sediments of eastern Beringia has placed several important palaeoenvironmental events into a precise chronologic context. (1) Preglacial vegetation of {\it Pinus} and {\it Picea}, with rare {\it Abies}, {\it Larix}, {\it Alnus}, {\it Betula}, and {\it Corylus} existed in eastern Beringia as late as 2.8 Ma; (2) loess deposition in interior Alaska began $\sim$ 3.0 Ma; (3) permafrost was established in the area by 3.0 Ma; (4) the first continental glacier invaded Yukon between 3.0 to 2.6 Ma; and (5) the characteristic interglacial boreal forest, dominated by {\it Picea}, {\it Abies}, {\it Betula}, and {\it Alnus}, was established by 2.3 Ma.
V21C-02 08:20h
Terrestrial-marine Correlation of the 24 kyr BP Dawson Tephra: Implications for Dispersal and Preservation of Alaskan Tephra Deposits
The Dawson tephra, radiocarbon-dated to ca. 24,000 yr BP, is as much as 50 cm thick in Pleistocene sediments in Yukon Territory of Canada, but is not found in nearby parts of Alaska. Nonetheless, the geochemical characteristics of the tephra suggested derivation from an Aleutian arc volcano (Westgate et al., 2000). Its source was unknown until proximal deposits were recognized at Emmons Lake Volcanic Center (ELVC) on the Alaska Peninsula (Mangan et al., 2003). We have identified Dawson tephra in ODP core 880B, collected in the Northeast Pacific Ocean. The Dawson tephra also occurs in discontinuous small pods in two new localities near Fairbanks in central Alaska. Our data suggest the Dawson tephra was dispersed from the ELVC principally to the southeast over the North Pacific , and then carried to the northeast by high altitude winds. Reconstructions of the volume and distribution of individual tephras or of arc-wide explosive volcanism in Alaska must incorporate data from proximal and distal terrestrial localities, as well as marine cores.
V21C-03 08:35h
Distribution and Origin of the 1883 Augustine Tephra-Fall Deposits, South-Central Alaska
Augustine Volcano, an island in south-central Alaska, is the most frequently active volcano in Cook Inlet basin where almost half the population of Alaska lives. It has had at least five eruptions in historical time (past 200 years; 1812(?), 1883, 1935, 1964, 1976, and 1986), the most notable of which occurred in 1883. The 1883 eruption consisted of a debris avalanche and resultant tsunami, lava dome, block-and-ash pyroclastic flows, pyroclastic surges, and tephra fall. It is the only historical eruption that does not contain pumice; nonetheless, tephra fall from 1883 may be the most widespread of the historical tephras. 1883 tephra-fall deposits are tentatively identified as far south as the Kamishak River on the west side of the inlet and as far north as the Kenai River on the east side of the inlet; deposit thickness off island is generally 2 to 4 cm. Tephra-fall deposits are everywhere fine grained. Particles rarely exceed 1 mm on island (most are finer) and are generally $<=0.25$ mm distally. Tephras on island contain plagioclase, hypersthene, augite, and magnetite in order of decreasing abundance. We speculate that the origin of the tephra fall is chiefly from ash clouds elutriated from the pyroclastic flows. Field identification of 1883 tephra is based on it being the first non-pumiceous tephra within a few centimeters below the 1912 Katmai ash, a widespread prominent white marker bed in Cook Inlet. The only other Cook Inlet eruption that could occupy that position is tephra fall from the 1902 Redoubt eruption of which little is known. Other potential sources are volcanoes on the Alaska Peninsula, but eruptions from those volcanoes would need to be of significant volume to be several centimeters thick in the upper Cook Inlet basin. Studies are underway to look at mineral assemblage, glass chemistry, and oxide compositions to strengthen correlations. Historical eruptions from Augustine contain mixed-magma clasts of andesite and dacite and most glasses contain numerous microlites; thus it may be difficult to obtain satisfactory geochemical correlations for these tephras.
V21C-04 08:50h
Tephrochronology of North Pacific Volcanic Arcs - data from ODP Leg 145
Volcanic ash layers in deep-sea sediment from the North Pacific provide a record of volcanism in North Pacific Island Arcs. 450 ash layers were counted in deep-sea cores from the Ocean Drilling Project Sites 882, 883, and 887, in order to determine frequency of eruption, ash thickness, episodes of regional volcanism, and the impact of multiple eruptions on the intensification of Northern Hemisphere Glaciation. Episodes of explosive volcanism occurred approximately at 0.2-0.5, 1.5-1.7, and 2.5-2.65 Ma in the Kamchatka-Kurile volcanic arc, and 0.15-.4, 1.5-1.7, and 2.55-2.65 Ma in the eastern Aleutian volcanic arc. Aeolian mineral grains and volcanic glass were removed from the deep-sea sediments using a method developed at the University of Michigan. A new method of heavy-liquid separation was developed to separate volcanic glass from terrestrial mineral grains. The glass was isolated in order to calculate the flux of volcanic glass in order to quantify the climatic impact of the eruptions. The amount of volcanic glass in the sediments between visible ash layers was measured and used as a proxy for sulfate aerosols in the atmosphere. Eruption ages were determined by magnetic reversal stratigraphy. Argon-argon geochronology will be used to compare the ages and to further test the usefulness of the argon-argon method for determining the age of volcanic glass.
V21C-05 09:05h
Dendrochemical Dating of Tephra Layers
Dating eruptions in the past 1000 years can be difficult with $^{14}$C, as production rates have varied. Tree-ring dating has been used, with eruptions presumably causing thin rings. We are developing a dendrochemical dating technique that may give more confidence to such dates. When a tephra is deposited, soil chemistry may change and many components of the glassy matrix are mobile and available for leaching into the soil and, hence, to plants. If some components were scarce prior to the eruption, the content in tree rings may increase and serve as a chemical marker of the eruption onset. LA-ICP-MS analysis of tree cores allows earlywood from each ring to be analyzed. We are currently investigating whether acid dissolution will produce similar results and precision with small sample sizes. The 1943-1952 CE eruption of Paricutin, Michoacan, Mexico, deposited a thick tephra in a conifer forest. Analyses of tree cores show that pre-eruption P concentrations of a few ppm increased an order of magnitude within the first two years after the onset of the eruption and then dropped steadily during the rest of the eruption to 10-15 ppm, which was then maintained for decades. Ca and Sr dropped during the eruption but rebounded immediately afterward. These three elements appear immobile in the wood, as steep compositional gradients occur across rings. The Paricutin tephra has 0.2-0.4 wt% P$_{2}$O$_{5}$. Analyses of soils and tephra obtained by the Bray method (a weak HCl leaching intended to mimic leaching in slightly acid soils) yield values of 2-17 ppm available phosphorus, fairly high values for this area. The Sunset Crater eruption near Flagstaff, Arizona, was dated at 1064 CE based on the onset of thin tree rings in three beams from structures at Wupatki village, 20 km north of the volcano. Our analyses of this wood show pronounced increases in P, Ca, Sr, Mg, and Mn in the early 1080s CE. Currently, we are investigating Jeffrey pine reaction to the mid-1600s CE Cinder Cone eruption at Lassen Volcanic National Park, California.
V21C-06 09:20h
A Tephra Database With an Intelligent Correlation System, Mono-Inyo Volcanic Chain, CA
We are assembling a web-accessible, relational database of information on past eruptions of the Mono-Inyo volcanic chain, eastern California. The PostgreSQL database structure follows the North American Data Model and CordLink. The database allows us to extract the features diagnostic of particular pyroclastic layers, as well as lava domes and flows. The features include depth in the section, layer thickness and internal stratigraphy, mineral assemblage, major and trace element composition, tephra componentry and granulometry, and radiocarbon age. Our working hypotheses are that 1) the database will prove useful for unraveling the complex recent volcanic history of the Mono-Inyo chain 2) aided by the use of an intelligent correlation system integrated into the database system. The Mono-Inyo chain consists of domes, craters and flows that stretch for 50 km north-south, subparallel to the Sierran range front fault system. Almost all eruptions within the chain probably occurred less than 50,000 years ago. Because of the variety of magma and eruption types, and the migration of source regions in time and space, it is nontrivial to discern patterns of behaviour. We have explored the use of multiple artificial neural networks combined within the framework of the Dempster-Shafer theory of evidence to construct a hybrid information processing system as an aid in the correlation of Mono-Inyo pyroclastic layers. It is hoped that such a system could provide information useful to discerning eruptive patterns that would otherwise be difficult to sort and categorize. In a test case on tephra layers at known sites, the intelligent correlation system was able to categorize observations correctly 96% of the time. In a test case with layers at one unknown site, and using a pairwise comparison of the unknown site with the known sites, a one-to-one correlation between the unknown site and the known sites was found to sometimes be poor. Such a result could be used to aid a stratigrapher in rethinking or questioning a proposed correlation. This rethinking might not happen without the input from the intelligent system.
http://www.volcano.buffalo.edu/mmvz
V21C-07 09:35h
Major Holocene Marker Ash Layers on Kamchatka Peninsula, NW Pacific, and Their Implication for Dating Paleoseismic Events
The Kamchatka segment of the Pacific "Ring of Fire" is one of the most active seismic and volcanic regions in the world. During the last 10,000 years it has produced more than 30 large explosive eruptions with tephra volumes of 0.5-170 km$^{3}$. Widespread on-land tephra layers formed by these eruptions have been identified, dated and studied to develop their mineralogical and geochemical fingerprints. The direct tracing of marker tephra layers along lengthy traverses has allowed us to define ashfall axes and areas of ash dispersal to produce isopach maps. These data can be used for long-distance correlations with deep-sea cores and Greenland ice cores. Tephra horizons are formed instantaneously and thus can provide precise correlation of certain stratigraphic levels in various depositional sequences. We report here implications of this extensive tephra data set to dating and correlation of paleoseismic events such as tsunami, seismically triggered landslides, and episodes of faulting along a major Kamchatka fault zone. Because the historical record of earthquakes and tsunamis on Kamchatka is short (~200 years), these investigations can make important contributions to evaluating tsunami and seismic hazards. Using key marker tephra as time lines, we can compare tsunami frequency and intensity records along the Kamchatka subduction zone as well as compare records of faulting events in various parts of the fault system. The combined record of paleoseismic events and largest explosive eruptions in Kamchatka offers an unprecedented opportunity to correlate seismic and volcanic constituents of the tectonic process.
V21C-08 09:50h
Past and Future Directions of North Pacific Tephrochronology
The north Pacific Ocean is rimmed by a complex of subduction zones that dip away from the ocean basin toward the fringing island arcs and continents. Inboard of these subduction zones is a belt of persistent volcanic activity--the Pacific "Ring of Fire"--formed by partial melting of subducted oceanic crust and overlying continental crust. Magma bodies of intermediate to silicic composition at many sites along this active volcanic belt have given rise to explosive volcanic eruptions and wide dispersal of tephra throughout the north Pacific Ocean and adjacent land areas. Moreover, along some parts of the north Pacific rim, as well as farther inboard of the subduction zones, are several persistent loci of crustal extension, translation, and hot spot activity that have also been sites of magma generation and large-volume explosive volcanism. Tephra erupted from these combined sources has been carried mostly in the direction of the prevailing winds, generally from west to east, though distributions have been complex, depending on heights of erupting columns and the prevailing weather and climate (including the seasons of eruption, positions of high- and low-pressure areas, and configurations of the jet stream). Tephra deposits formed as a consequence of this volcanic activity provide an important scientific resource for chronostratigraphy and correlation, and contribute significantly to solution of regional and topical studies in earth science. Tephra studies in Japan, the western conterminous United States, and southwestern Canada have advanced to the point that regional spatial-temporal late Noegene tephrochronological reference frameworks exist for these areas. In northwestern Canada, Alaska, Kamchatka, and eastern Siberia, the development of these stratigraphic frameworks lags, owing to lower population density, more difficult access, and shorter field seasons. Tephrochronologic studies of ocean sediment cores have been more sporadic than systematic, and lag behind tephra studies conducted on land. Promising future research directions are (1) systematic studies of ocean cores and correlation of tephra sequences between land and ocean areas, (2) systematic correlation of land and ocean stratigraphic sequences and tephra to the late Quaternary record in Greenland ice cores, (3) improvements in tephrochronometry, including cross-dating of tephrochronological, dendrochronological, and varve records, (4) improved separation and micranalytical techniques of small tephra particles and other aerosol particles, (5) closer and more systematic collaborative interdisciplinary studies between tephrochronologists and scientists studying environmental change and geologic hazards, and (6) intercalibration and integration of tephrochronologic data bases.