Research paperDynamics of phytoplankton in relation to the upper Homerian (Lower Silurian) lundgreni event – An example from the Eastern Baltic Basin (Western Lithuania)
Graphical abstract
Introduction
The Silurian period was a time of large-scale geobiological changes in the functioning of the Earth's ecosystems. There was a series of sudden extinction and radiation events which were causally related to changes in the oceanic states and sea level (Aldridge et al., 1993, Cramer et al., 2015, Jeppsson, 1987, Jeppsson, 1993, Jeppsson, 1998, Loydell, 2007, Melchin et al., 2012, Munnecke et al., 2003, Spiridonov et al., 2015). One of the most significant perturbations of this time interval was the so-called mid-Homerian “big crisis” or the lundgreni event of graptolites (Jaeger, 1991, Koren’, 1987) which was approximately contemporaneous with the Mulde conodont extinction event (Calner et al., 2012, Jeppsson et al., 1995, Jeppsson and Calner, 2002), and which preceded the subsequent upper Homerian Mulde positive carbon isotopic excursions (CIEs) (Cramer et al., 2012, Samtleben et al., 1996, Wenzel and Joachimski, 1996).
It was determined in earlier studies that this perturbation left a strong mark on the graptolite clade, forcing extinction magnitudes at the species level to reach the 95% mark (Lenz and Kozlowska-Dawidziuk, 2002, Porębska et al., 2004). This event was followed by the highest reduction in graptolite species richness since the Floian epoch (Cooper et al., 2014). This event also influenced microevolutionary regimes, causing a phyletic size reduction in some surviving graptolite lineages (the so-called “Lilliput effect” (Urbanek, 1993)) and enabling subsequent macroevolutionary iterative speciation events (Urbanek et al., 2012). Other organic groups were affected as well – the conodont diversity of the Silurian Baltic basin dropped to a few remaining species due to permanent extinctions and long-term regional extirpations (Jarochowska and Munnecke, 2015, Jeppsson, 1998, Radzevičius et al., 2016, Radzevičius et al., 2014c). Comparable patterns have been determined in other paleocontinents and regions (Jeppsson et al., 1995, Slavík, 2014). This demonstrates the global extent of perturbation. The chitinozoans – a form of microplankton of disputed phylogenetic affinities – during the discussed mid-Homerian event apparently experienced the largest single drop in diversity during the whole Silurian period (Nestor, 1997, Paluveer et al., 2014). Similar patterns have been observed in the radiolarian fossil record – similar to other pelagic groups they were heavily affected, with just two species (out of the 28) recorded in the Arctic Canadian record surviving the perturbation (Lenz et al., 2006).
Coarse grained synoptic studies of Gotland material show that acritarchs were also to some extent affected by the lundgreni mass extinction event (Kaljo et al., 1996). There are indications based on the qualitative studies of the Polish, Swedish and Canadian sections that during the lundgreni event the acritarch paleocommunities experienced a sudden reorganisation in their species composition and dominance patterns (Calner et al., 2006, Lenz and Kozlowska-Dawidziuk, 2002, Porębska et al., 2004). However, in general the quantitative patterns of the impact of the mid-Homerian event on the acritarchs and green algae – one of the most important components of the Silurian phytoplankton – are definitely underexplored.
In this article we present the first to date detailed quantitative description of palynomorph (acritarchs and green algae) diversity and compositional change in relation to the lundgreni geobiological event in the Viduklė-61 core. The studied section has so far yielded one of the richest arrays of information on the mid- to late Homerian global change, which includes the stratigraphic distribution of graptolites, conodonts, lithology, stable carbon isotopic trends and also the cyclic patterns of sedimentary succession (Martma et al., 2005, Radzevičius et al., 2014a, Radzevičius et al., 2014b). This enables us to precisely correlate the revealed changepoints in the acritarch paleoecological dynamics to the regional and ultimately to the global time scales.
Section snippets
Geological setting
The studied section is located in Lithuania. This territory corresponds to the western part of Baltica palaeocontinent and the eastern part of the Baltic Silurian sedimentary Basin (Fig. 1). During the Late Wenlock, the Silurian Baltic Basin was located in the southern hemisphere near the equator (Cocks and Torsvik, 2002).
The facies of the basin vary from shallow marine carbonatic in the eastern part of Lithuania to a deep water clayey facies in the western part (Fig. 1). The Viduklė-61
Palynomorph taxonomy
The majority of the studied samples contained well-preserved organic remains of ancient palynomorphs. According to Le Hérissé et al. (2009), it is useful to divide organic-walled phytoplankton into three major categories in order to facilitate a comparison of the compositional fluctuations in the stratigraphic section. These categories, based on the overall morphology, are marine chlorophytes (including prasinophytes), marine acritarchs and nonmarine types, including coenobial forms. This
Discussion
The present quantitative study reveals changes in acritarch and prasinophyte diversity that are generally similar to that revealed in other parts of the Baltic basin (Bartoszyce well, Poland) (Porębska et al., 2004). The upper Jaagarahu and lower Gėluva regional stages were characterized by high to moderately diverse assemblages; the middle of the Gėluva regional Stage was characterized by low abundance assemblages generally dominated by prasinophytes. The upper-most Gėluva regional Stage is
Conclusions
The constrained clustering analysis study of the middle and upper Homerian micro-phytoplankton revealed five distinct assemblages (A1 to A5). The first (A1 – upper Jaagarahu to lower Gėluva) and the last (A5 – upper Gėluva) assemblages are characterized by moderate to high genus diversity levels. The three assemblages (A2, A3, and A4), which correspond to the middle Gėluva regional Stage, are characterized by very low diversity levels.
The statistical comparison of the diversity estimates (SACE)
Acknowledgments
We would like to thank Alain Le Hérissé for his advice and help in the palynomorph taxonomy, and also two anonymous reviewers and the editor Frans Jorissen for their constructive comments and language corrections that significantly improved the manuscript. We also greatly appreciate Giedrė Vaikutienė for her support in the preparation of the samples. We are grateful to Gailė Žalūdienė and Laurynas Šiliauskas for their assistance with the SEM imaging. This study is a part of the project “Event
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