Biotic response to explosive volcanism: Ostracod recovery after Ordovician ash-falls Vincent Perrier ⁎, Tõnu Meidla, Oive Tinn, Leho Ainsaar University of Tartu, Institute of Ecology and Earth Sciences, Department of Geology, Ravila 14A, 50411 Tartu, Estonia abstract article info Article history: Received 1 June 2012 Received in revised form 19 September 2012 Accepted 26 September 2012 Available online 4 October 2012 Keywords: Ordovician Baltoscandia Explosive volcanism Ostracods Recovery Extinction The impact of two Ordovician ash-falls of different intensities was studied in order to determine the recovery pat- terns of benthic ostracods. The studied sections are of Sandbian age in NW Estonia: at Põõsaspea, the Kinnekulle K-bentonite (~40 cm, derived from a major volcanic event) and at Ristna, the Grimstorp bentonite (~6 cm). In Põõsaspea, important faunal changes are observed: the three pre-crisis dominant species (i.e. Tetrada memorabilis, Steusloffina? sp. nov. and Olbianella braderupensis) were not recovered above the bentonite and at least two of them became extinct. The post-crisis ecosystem shows strong perturbations of diversity and abundance for a long period of time. In Ristna the impact of the ash-fall is less dramatic: all of the dominant pre-crisis species sur- vived, although some changes in abundance and diversity are observed. These results show that significant ash-falls lead to marked rearrangement of assemblages and extinction of some taxa while less prominent volcanic episodes only result in temporary changes in the assemblage structure. In both cases, recovery follows several steps: – Post-event ecosystem with ‘survival faunas’ (very low abundance; low diversity); one/two species (opportu- nistic taxa?; e.g. Sigmoopsis rostrata, Circulinella nuda) dominate the assemblage. – Early stages of recolonization with ‘recovery faunas’ (increasing abundance and diversity; recolonization by pre-event and immigrant species). – After a short ‘disturbance period’, return to ‘climax assemblages’ with specialized taxa (high abundance; high diversity). Although assemblages recovered rapidly after the crises, the recovery periods still seem much longer than ob- served in Recent foraminiferal assemblages. Causal links between the volcanic events and subsequent diversifi- cations could not be demonstrated. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The impact of volcanism on the environment and biodiversity has been extensively studied (e.g. Vogt, 1972; Officer et al., 1987; Rampino and Stothers, 1988; Huff et al., 1992; Stothers, 1993; Renne et al., 1995; Waśkowska, 2011). Volcanism associated with meteorite impacts is thought to be responsible for several major extinctions in Earth history (White and Saunders, 2005; Keller et al., 2009; Saunders and Reichow, 2009). Such major reorganizations of the biosphere are mainly ascribed to huge and rare long-lasting flood basalt events capable of influencing the atmospheric/hydrospheric composition and carbon cycle. The consequences of large-scale events of explosive volca- nism, however, are less clearly understood. The biotic effects of ash falls and other short term environmental crises on benthic invertebrates is well documented both in Recent and past environments (e.g. Harper et al., 1995; Heikoop et al., 1996; Hess et al., 2001; Galeotti et al., 2002; Keller, 2003; Kuhnt et al., 2005; Lokier et al., 2009; Wetzel, 2009; Waśkowska, 2011) but most of these studies concentrated on foraminifera. Scant attention has been paid to the re- sponse of ostracods to such crises (Swain, 1996; Botting, 1999, 2002; Hints et al., 2003; Botting and Muir, 2008). Swain (1996), Botting (1999, 2002) and Botting and Muir (2008) suggested that there is a link between volcanic episodes and the diversification of ostracods in Or- dovician sections in North America and in Wales. The volcanic episodes appear to have provided nutrient-rich habitats and fractioning of the ecosystems that resulted in the diversification of the ostracods. Across the Kinnekulle bentonite (Sandbian, Upper Ordovician), Hints et al. (2003) demonstrated that the ostracod assemblage underwent a major reorganization including the replacement of dominant taxa, a drop in abundance and diversity and extinction of several species at the event level. Swain (1996) recorded extinctions of species, followed by remark- able diversification episodes. How animal communities reacted to rapid and intense past environ- mental changes is crucial to understanding the origin and the organiza- tion of present-day ecosystems. The investigation of post-crisis faunal recovery requires a precise study based on a diverse, abundant and Palaeogeography, Palaeoclimatology, Palaeoecology 365 (2012) 166–183 ⁎ Corresponding author. Tel.: +372 7376695; fax: +372 7375836. E-mail addresses: vincent.perrier@ut.ee (V. Perrier), tonu.meidla@ut.ee (T. Meidla), oive.tinn@ut.ee (O. Tinn), leho.ainsaar@ut.ee (L. Ainsaar). 0031-0182/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2012.09.024 Contents lists available at SciVerse ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo