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The d13C
anomalies at different levels of Permian-Triassic carbonates in the Primorye
region, North Caucasus, Transcaucasia and the Alps, which are usually characterized
by high Mg contents, seem to be related to high biological productivity
of the Tethyan marine basins caused by conditions of transgressions and
warm climate during the following epochs: (1) early Dorashamian (Paratirolites
kittli Zone), (2) middle Olenekian (Tirolites-Amphistephanites Zone), (3)
early Anisian, (4) late Ladinian-?earliest Carnian, (5) late Carnian, (6)
early Norian, and (7) early Rhaetian. The highest bioproductivity during
Triassic took apparently place during the middle Olenekian (Fig.
1).
The existence of thermal maxima in the Tethys during early Dorashamian, middle Olenekian, early Anisian, and early Norian times seems to be in agreement with some radiolarian di versification events. Abundance and high taxonomic diversity of the Albaillellaria from lower Dorashamian cherts of the Pantovyj and Skalistyj Creek basins and Amba Mount area (Sikhote-Alin) and from conteporaneous flyschoid strata of the Orel Mount area (South Primorye) seem to be caused by optimal temperature conditions. Accumulation of significant masses of radiolarian cherts in Sikhote-Alin during the Olenekian is probably related to similar conditions. It is possible that appearance of the polysegmental Nassellaria was related to the Anisian warmth and transgression, although the spherical Spumellaria, as was recently recognized in North Caucasus (Kapustina and Svinyach'ya Ravine, Mamryuk, Rufabgo) and South Primorye (Abrek Bay), was a dominant group among the microfossils in both Induan-Olenekian strata and during the Olenekian-Anisian transition. The sharp changes in taxonomic diversity of both Nassellariaand the Spumellaria in cherts of Sikhote-Alin (Skalistayj Creek, Dalnegorsk and Breevka village areas) and the development of planktonic limestone in the Dzhaurskaya Suite and its equivalents in Sikhote-Alin appear to have been related to the Carnian and Norian warm periods.
Judging from the isotopic data, the calculated palaeotemperature for the early Dorashamian shallow-water carbonate facies of Transcaucasia (Zakharov et al., 1997) and North Caucasus reaches 23.8°C. In the beginning of Urushtenian (late Dorashamian) time temperature of near bottom waters of the shallow sea in North Caucasus (23.8-24.2°C) was similar to those for the early Dorashamian. The same temperature conditions of tropic and subtropic seas are suspected to be at least in middle Olenekian time proceeded from some isotopic and Ca-Mg ratio data for the Tethys.
Conditions for the middle Olenekian seem to be compared also with the early Aptian climatic optimum (with palaeotemperature for shallow water terrigenous facies in North Caucasus about 13.7-23.9°C). Morante & Hallam (1996) have reconstructed tropical conditions for the Eastern Alps in early Rhaetian from oxygen-isotopic investigation of the Kössen Limestone ( d18O = -1.76‰ to -2.89‰; T = 19-24°C). Similar result for the Upper Triassic of the Alps (values of d18O range from -0.05‰ to -2.83‰) from ammonoid aragonitic material was obtained early by Fabricius et al. (1970).
The range of d18O values in aragonitic ammonoid shells from the Lower and Middle Triassic in Arctic Siberia suggests the average temperature values for early Olenekian, late Olenekian and late Anisian of 8.8?°, 16.2?° and 15.4?°C correspondingly (Zakharov et al., in prep.), which is consistent with palaeotemperatures obtained from Olenekian bivalves probably living in some fully saline basins of Arctic Siberia (Kurushin & Zakharov, 1995).
We indicate the temperature values with
question-marks because they were obtained using using R.V. Teiss' "water
correction" (Zakharov et al., 1975). For reconstruction of late Palaeozoic
and early Mesozoic environments the data from reef distribution seem to
be very important, because the reefs consider to be very sensible indicator
for marine environment changes. As an example of a prospering reef is that
of the end-Permian strata of the Urushtenskaya Suite in North Caucasus.
It is known that at the start of the Triassic, reefs disappeared from the
face of the earth and a reef formation was not renew in any region of world
in both the middle Olenekian climatic optimum (transgression) and the similar
condition of the beginning of Middle Triassic. After the Permian-Triassic
boundary ecological crisis they have arisen in the tropical zone only in
the Late Triassic (although scleractinian corals made their first appearance
in Middle Triassic). Lack of reefs in the low latitudes during the beginning
of the Triassic more logically to connect with O2
deficit both in the atmosphere and the hydrosphere of that time as consequence
of the anoxic event across the Permian Triassic boundary (Baud et al.,
1989; Berner, 1989; Gruszczynski et al., 1989; Hallam, 1994; Holser et
al., 1989). The absence of visible sighns of organic SiO2
- accumulation just in the Permian-Triassic transition time and the low
rates in reconstruction of the radiolarian taxonomic diversity through
the Induan and Olenekian to the early Anisian seem to be caused by the
same reason.
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