Transportation history of subantarctic volcanic ashes derived from the ...

Transportation history of subantarctic volcanic ashes derived from the Scotia Arc during the past I million years

sson in press), been attributed to the South Sandwich Islands. To the east, three distal cores (10 1176-36, -41, and -68) contain mostly dispersed ashes invisible to the unaided eye, although some ashes are concentrated to form fine laminae. To quantify the ashes and to obtain detailed ash distributions in the cores, the technique developed by Huang, Watkins, and Shaw (1975) was adopted. This method involves a separation of ashes from other sediments and an analysis of ash particle sizes.

T. C. Huc

Figure 2 is an example of the ash distribution determined by this method. Figure 2a shows the downcore variations of the ash accumulation rates in three ranges (greater than 88 micrometers, 88 to 36 micrometers, and 36 to 11 micrometers) in core 10 1176-41. These rates were calculated on the basis of weights of ashes and the ages determined by the magnetic polarity and the extinction of a radiolarian, Sty!artractus universus in the core. There are at least 22 ash maxima, probably produced by separate eruptions on the South Sandwich Islands, during the last 750,000 years.

Graduate School of Oceanography University of Rhode Island Kingston, Rhode Island 02881

Recent extensive study of deep-sea volcanic ashes has provided much information useful in interpreting many geological and paleo-oceanographic phenomena. As a part of the University of Rhode Island study of the history of bottom current scour in the southern oceans, the volcanic ashes derived from the Scotia Arc have been examined. The purpose is to see if the volcanic ashes in the subantarctic deep-sea floor were redistributed by high bottom-water activities after their deposition. In addition, the ashes may provide information on explosive volcanism, time-stratigraphy, paleowind conditions, and oceanic circulations.

Two significant characteristics of the ash distribution can be envisaged. 1. Multiple dynamic processes such as differential settling, bottom current, and bioturbation have resulted in diachronous ash distribution in different ash-size ranges from a single eruption. Twelve out of 22 ash layers are diachronous (as shown by dash lines, figure 2a); in most cases the coarse ash (greater than 88 micrometers) concentrates in the lower portion of an ash maximum. The vertical and horizontal differential settling of the ashes in the water column may have played a major role in forming the size gradation. Bioturbation may further blur the ash records and has resulted in broad ash-accumulation-rate curves. The broad ash maxima, such as layers 9 and 10 (figure 2a), may have been caused mainly by bioturbation. High bottom-current velocity can lead to further ash redistribution. An examination of glass shards by means of a

Seven ARA Islas Orcadas and Eltanin sedimentary cores east of the South Sandwich Islands were selected for this study (figure 1). The islands, located on the eastern part of the Scotia Arc, are volcanic and are believed to be 4 million years old (Baker 1968). The cores proximal to the islands (cores E 8-14, -19, and -27 and 10 775-27) contain distinct ash layers as much as 5 centimeters thick. Between the ash layers, abundant volcanogenic debris, probably of icerafted origin, are scattered in other sediments. The source of the ash layers has, on the basis of a petrographic study (Ninkovich, Heezen, Conolly, and Burckle 1964) and a chemical analysis of ashes (Federman, Watkins, and Sigurd-

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Figure 1. Location of the Islas Orcadas and Eltanin sedimentary cores in the east of the Scotia Arc. Arrows show the directions of the present Antarctic Circumpolar Current and the antarctic bottom water (AABw).

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scanning electron microscope (sEM) shows that the pits formed by bottom-current transports occur in some ash layers. 2. There is a marked contrast in the ash accumulation rates during the two periods, 0 to 410,000 years and 410,000 to 690,000 years. The high contrast is not due to change in the pelagic sedimentation rates. As a check, the downcore apparent ash-accumulation rates were computed on the basis of the linear sedimentation rate obtained from the Brunhes/Matayama boundary of the core (figure 2b). There is a marked change in the apparent rates at about 410,000 years (figure 2b). An estimate shows that the sedimentation rates of nonvolcanics (mostly siliceous planktons) remained about constant during the two periods but that the rates of volcanic ashes accumulated during the past 410,000 years is about one-eighth of those deposited during the preceding period. The ratios of the rates between the two periods, 0 to 410,000 years and 410,000 to 690,000 years, are about the same for the three size ranges: 0.103, 0.121, and 0.116 for greater than 88 micrometers, 88 to 36 micrometers, and 36 to 11 micrometers size ranges, respectively (figure 2a). The volcanic eruption characteristic alone is unlikely to have caused such a contrast. A plausible explanation is that the Antarctic Circumpolar Current (Acc) had a much higher velocity before 410,000 years ago. A similar conclusion has been drawn from a study of sediment particle size distribution in core E 49-18, which is from a site under the present ACC in the Southern Indian Ocean (Ledbetter personal corn106

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Figure 2s. The volcanic ash accumulation rate variations In core 10 1176-41. The ash is In three six. fractions (greater than 88 micrometers, 88 to 36 mIcrometers, and 36 toll micrometers). The magnetic polarity, the presence and absence of Stylatractus universus (Morley and Shackleton 1978), and the ash layers are added. Note that some ash layers are diachronous In different six.. (dash lines) and others are synchronous (broken lines). Figure 2b. The apparent ash-accumulation rate variations In the same core. The rates are calculated on the basis of a linear sedimentation rate determined by the magnetic polarity method (see text).

munication). it is conceivable that the high-altitude geostrophic westerlies may have been a higher velocity prior to that time. This work was supported by National Science Foundation grant DPP 78-08511. Senior research personnel on the project included T. C. Huang and James Kennett at the University of Rhode Island. The magnetic polarity data were provided by M. Ledbetter of the University of Georg. References Baker, P. E. 1968. Comparative volcanology of the Atlantic Island Arcs. Bulletin Volcanologique, 32(1), 189-206. Federman, A., Watkins, N. D., and Sigurdsson, H. in press. Scotia Arc volcanism recorded in abyssal piston cores downwind from

the islands. Third Symposium on Antarctic Geology and Geophysics.

Huang, T. C., Watkins, N. D., and Shaw, D. M. 1975. Atmospherically transported volcanic glass in deep-sea sediments: Development of a separation and counting technique. Deep-Sea Research, 22(3),185-1%. Ledbetter, M. T. May 10, 1979. Personal communication. Morley J., and Shackleton, N. J . 1978. Extinction of the radiolarian Stylatractus universus as a biostratigraphic datum to the Atlantic Ocean. Geology, 6(5), 309-311. Ninkovich, D., Heezen, B. C., Conolly,J. R., and Burckle, LH. 1964. South Sandwich tephra in deep-sea sediments. Deep-Sea Research, 11(8), 605-619. ANTARCTIC JOURNAL