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| Titel |
Volcanically far-flung lake sediments in New Zealand and their diatom contents. |
| VerfasserIn |
Margaret Harper, Shirley Pledger, Euan Smith, Alexa Van Eaton, Colin Wilson |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250102365
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| Publikation (Nr.) |
 EGU/EGU2015-1678.pdf |
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| Zusammenfassung |
| Explosive volcanic eruptions from the Taupo Volcanic Zone in New Zealand such as
the Okaia (28.6 ka) and Oruanui (25.4 ka) through paleolake Huka and the Taupo
eruption (1.8 ka) through paleolake Taupo dispersed measurable quantities of diatom
valves (remains of siliceous microscopic algae) along with the tephra (Van Eaton
et al. 2013). Diatoms preserved in tephra can inform us about the past histories
of freshwater floras and lakes. For instance the possibly extinct endemic diatom
Cyclostephanos novaezealandiae is 20 times commoner in the Late Pleistocene Oruanui
and Okaia tephras than in recent lake sediment. We also found Aulacoseira valves
(mainly A. ambigua) were generally shorter in the older tephras, possibly due to more
turbid conditions in the glacial period due to stronger winds or less availability of
nutrients.
Some information on eruptive processes can also be derived from diatoms. Nearly all
diatom assemblages in the Oruanui samples were remarkably similar, indicating they were
well mixed by turbulence in the eruptive column. The exceptions were a proximal sample (11
km from vent) and the clasts of slightly older lake sediment enclosed in the ignimbrite. One
clast was dominated by different Aulacoseira taxa (A. granulata and A. granulata var.
angustissima). A vent in a shallower area of paleolake Huka could well have been the source
of these diatoms. The proximal sample was deposited early in the eruption and on a
ridge close to the rim of the paleolake, and is also likely to have come from a local
vent.
Caution should be used in interpreting diatoms in phreatomagmatic tephra in lake basins.
Earlier Harper & Collen (2002) interpreted diatoms associated with the Okaia and Oruanui
tephras in the Poukawa basin (125 km from vent) as indicating the existence of lakes formed
when drainage was blocked by the tephra. However the strong resemblance of the
assemblages and morphometry of Aulacoseira valves to those measured in our new samples
shows they arrived with the tephra.
Diatoms dispersed by volcanic eruption through lake sediment and subsequently
preserved in tephra have the potential to supply information about past lake environments.
Harper, M. A. & Collen, J. D. 2002. Global and Planetary Change 33: 243-256.
Van Eaton A. R., Harper M. A, Wilson C. J. N 2013. Geology 41: 1187-1190 |
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