Octracods in Baltoscandia through the Hirnantian crises

Geological records indicative of the Hirnantian glaciation are known from many areas, in both high and low palaeolatitudes. Gradual cooling culminated in the double glacial maximum of the early Hirnantian (Brenchley, 2004) and caused dramatic changes in the composition of brachiopod faunas as well as in other invertebrate fossil groups (Sheehan, 2001 and references therein). Changes in the biodiversity of ostracod faunas through this critical interval of climate change have so far received limited attention, probably because the ostracod records from this interval are scarce, especially when compared to older parts of the Ordovician. The aim of this work is to study the changes in ostracod assemblages through the critical Hirnantian interval and their postglacial recovery in the tropical carbonate basin. Here we present the ostracode data from the Jurmala drillcore, Latvia, and compare their distribution with δ13Ccarb isotope curve. Jurmala section is drilled in central Latvia, in the middle of the Livonian Tongue (Livonian Basin), the eastward extension of the Scandinavian Basin in between the Estonian and Lithuanian carbonate shelves of the Baltoscandian epicontinental sea (Fig. 1). Representing the deeper part of the basin in East Baltic area, the latest Ordovician deposits of the Livonian Tongue show the greatest potential to preserve a complete sedimentary succession from the interval of a glacioeustatic sea level fall. The uppermost Katian beds, the upper part of the Pirgu Regional Stage are represented by micritic limestone (Paroveja Formation) and marl (Kuili Formation). The Porkuni Stage, considered to be the Baltic equivalent of the Hirnantian Stage (Bergström et al., 2009), is characterized by argillaceous limestone (Kuldiga Formation) and overlying sandy oolitic limestone (Saldus Formation). These beds are covered by argillaceous limestone (Stacˇiunai Formation), traditionally correlated with lowermost Llandovery (Juuru Regional Stage; Pasˇkevicˇius, 1997). Thickness of the Porkuni Stage in the Jurmala section is 14 m (Ainsaar et al., 2010).