Geochemical characters of Quaternary tephra beds and their stratighraphic position in the sedimentary core drilled at the site U1343 in the central Bering Sea
Abstract
The project IODP exp.323 in the Bering Sea focused on analyzing long-term ocean and climate trends during 5 Ma, and drilled seven sites (sites 1339-1345). Up to now, the studies of long-term tephrochronology in this area are very rare, though a part of histories of several volcanoes and late Pleistocene to Holocene volcanisms and studies for geochemistry of magma were reported in detail. Our objectives are to reveal how many widespread tephras are found in the Bering Sea and which of volcanoes or volcanic zones provided them. First of all, we analyzed forty-eight tephra samples in the sedimentary core collected at site U1343, near the Bering self sloop. Sediments in site U1343 (57°33.4'N, 175°49.0'E, water depth 1956 mbsf, core length 779.18 m) include three paleomagnetic events (the BM boundary;0.788 Ma, Jaramillo; 0.998 Ma, Cobb Mountain;1.173 Ma). The bottom datum event is reported as 2.0-2.2 Ma (diatom) at 716.4 m. All tephra samples were washed by flesh water and decanting, dried up naturally, sieved by the mesh of dia.250, 125 and 63 micrometer. We observed every tephra under the binocular/polarizing microscopes, and analyzed major-element composition of volcanic glass shards by EPMA (10nA, 15kV, probe dia.10 micrometer). All samples include many kinds of volcanic glass shards (color: colorless to dark brown, form: bubble-wall type, pumice type, fiber type). Diameter of grain size is normally less 125 micrometer, and volcanic glass size in some layers is concentrated in the less 63 micrometer. Thickness of tephra samples is approximately 0.5 cm to 4 cm. In the basis of geochemicalc analysis of volcanic glass shards in 48 samples, though every sample includes volcanic glass shards, we can distinguish the two groups roughly; glass-rich samples (31 samples) and contaminated samples (17 samples). Contaminated samples include course sands (lithic fragments, rounded minerals, fossil fragments), besides volcanic glass shards. Number of contaminated samples remarkably increases after BM boundary. Furthermore, during middle to late Pleistocene, the oxygen isotopic stratigraphy of benthic foranminiferal fossils in this core is clear and their oscillation is wider than early Pleistocene and Pliocene. So, volcanic glass shards in 'contaminated samples' which are expected to be provided from plural sources might be transported by seasonal sea ices, and ice berg which collapsed from the ice sheet around the Beringia. On the other hand, general character and geochemistry of glass-rich samples is as follows: The size of every tephra material is under 250 micrometer and there are rarely heavy minerals. Color of tephras has many varieties (white, brown, gray, and black) by geochemistry of volcanic glass shards. Content of SiO2 in volcanic glass shards varied approximately 58-72 wt%, Na2O is ~5.2 wt%, and K2O is ~3.2 wt% (basaltic andesite, andesite, dacite, and rhyolite). There is no sample in alkali rock series, 5 samples in tholeiitic rock series and the other 26 samples in calc-alkalic rock series. Six tephra layers of 0.378Ma, 0.518-0.529Ma, 0.822-0.824Ma, 1.008Ma, 1.108Ma, and 1.547Ma are several centimeters thickness, so they are expected to be recognized as time marker beds in Bering Sea widely.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V43B2828A
- Keywords:
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- 1000 GEOCHEMISTRY;
- 8404 VOLCANOLOGY / Volcanoclastic deposits