CONTENTS & ABSTRACTS

In English. Summaries in Estonian

Proceedings of the Estonian Academy of Sciences.

Geology

Volume 53 No. 4 December 2004

Diversity of late Ordovician rugose corals in Baltoscandia: role of environmental changes and comparison with other areas; 233–245

Dimitri Kaljo

Abstract. Regional biodiversity curves are controlled environmentally and reveal details that can be correlated with stable isotope data, sea level curves, etc. This paper uses such ties for better understanding of the environmental background of certain palaeobiological events. The first appearance of rugose corals in the shelf seas of Baltica in the Middle Caradoc seems to be linked to the warming of sea water and climate due to the drift of the continent closer to the equator. The same factor also favoured a further biodiversity rise in which both biology and ecology should be considered. The general diversity rise was slowed down by repeated coolings in the late Caradoc and early Ashgill and caused a diversity low within the early Ashgill. Along with a favourable temperature regime, changes in oceanic circulation, nutrient flow, and availability of suitable habitats were of great importance. The Hirnantian glaciation, accompanied by a pronounced sea level fall, brought about favourable conditions for coral evolution on many subequatorial cratonic shelves, which helped corals to survive the severe conditions of the end-Ordovician glacial time. The Baltoscandian rugose coral assemblage is the most diverse by comparison with those of Australasia and Laurentia thanks to the abundance of old lambelasmatids living in relatively cooler and deeper habitats.

Key words: rugose corals, biodiversity, carbon isotopes, Ordovician, Baltoscandia.

Apatite varieties in the shell of the Cambrian lingulate brachiopod Obolus apollinis Eichwald; 246–256

Jüri Nemliher, Tiia Kurvits, Toivo Kallaste, and Ivar Puura

Abstract. An XRD study of heat-treated shell samples of the lingulate brachiopod Obolus apollinis Eichwald from the upper Cambrian (Furongian) of NW Russia (basal Tosna Formation, Lava River section) revealed the presence of two phases of apatite persisting the temperatures from 200 to 700 °C. At higher temperatures, 900 and 1000 °C, only one phase was detected. The lattice parameters of the two phases suggest possible preservation of carbonate-hydroxylapatite within the compact laminae of initially organo-phosphatic lingulate shells. The formation of another phase, carbonate-fluorapatite, is attributed to the precipitation of apatite to the free space available after disintegration of organic tissues of the shell and to partial recrystallization of initial shell apatite.

Key words: brachiopods, XRD, apatite, taphonomy, Cambrian, NW Russia.

The earliest known Trypanites borings in the shells of articulate brachiopods from the Arenig (Ordovician) of Baltica; 257–266

Olev Vinn

Abstract. Several specimens of the Arenig (Volkhov Stage) brachiopod Antigonambonites from Northwest Russia and North Estonia bear the earliest known Trypanites borings in articulate brachiopod shells. Two basic types of borings are recorded: rare large rounded borings that penetrate the shell of the brachiopod, and abundant borings that follow the shell, almost never intersecting it. Rare shell repair marks are associated with some Trypanites borings. Presumably some shell-boring worms inhabited also the living Antigonambonites brachiopods, suggesting that diverse shell boring strategies had evidently evolved by the Arenig.

Key words: Trypanites, shell borings, brachiopods, palaeoecology, Arenig, Northwest Russia, North Estonia.

Application of OSL and ¹⁰Be techniques to the establishment of deglaciation chronology in Estonia; 267–287

Anto Raukas

Abstract. The deglaciation history of Estonia has been under research for about a century. Despite the great number of publications devoted to this subject and marked improvements in study methods, many problems of topical interest have not been solved yet, especially due to the lack of good direct dating methods. In this paper the suitability of OSL and ¹⁰Be dating techniques for establishing accurate deglaciation chronology for Estonia is assessed. Turbidity and water depth, velocity of outwash streams and transport length, possible fast sedimentation at night hours or below the ice, incorporation of older, unbleached particles, and other factors affected the extent of the bleaching of the TL signal in different ways, causing great variability of dates. Surface inclination, height of the surface over ground, snow and vegetation cover, and evolution of water bodies influenced the calculation of reliable exposure ages of objects dated using the ¹⁰Be method. It means that age determinations of both glaciofluvial deposits with the OSL method and erratic boulders with the ¹⁰Be method are highly problematic, especially for glaciofluvial intertill sediments where the exact genesis of deposits is unknown and for boulders, which have been in the forest, under the waters of proglacial lakes and/or the Baltic Sea, or under snow cover for a long time.

Key words: ice-marginal formations, glaciofluvial deposits, OSL dating, deglaciation chronology, erratic boulders, cosmogenic ¹⁰Be.

CHRONICLE

The Eighth Marine Geological Conference “The Baltic – 8th” (23–28 September 2004); 288–290

Igor Tuuling

Copyright Transfer Agreement; 291

Contents of volume 53; 292–293