Microphytoplankton from middle palaeolatitudes of the Southern Hemisphere – a record from climate change strata of Baltica’s O/S boundary

Abstract

In the present study, data on the diversity of acritarch and prasinophyte microphytoplankton were collected from Ordovician–Silurian transitional strata at three different geological localities in Poland: southern part of the Holy Cross Mountains (Małopolska Block, southern Poland), East European Platform (Baltica), and Koszalin–Chojnice Zone (NW Poland). The material represents the mucronata (trilobite) to vesiculosus (graptolite) biozones. The Ordovician microphytoplankton assemblages are characterized by low frequency and low diversity: up to 110 specimens and 12 genera per slide, in contrast to Silurian ones that are more frequent and more diverse – >3000 specimens and 16 genera per slide (diversity at the species level is also higher). Throughout the Ordovician part of the succession, typical Ordovician genera occur, such as Acanthodiacrodium, Ordovicidium, Orthosphaeridium and large Baltisphaeridium, together with isolated occurrences of  typical Silurian genera, especially in the upper part of the Hirnantian (e.g., Diexallophasis). The Silurian assemblages are typified by high frequency of prasinophytes followed by typical Silurian acritarchs: Tylotopalla, Ammonidium, Domasia and Oppilatala. The palynological material from Poland is compared with material known from other localities: the Rapla and Valga boreholes (Estonia) and Anticosti Island (Canada), as well as others placed in different palaeocontinents and different bathymetric zones. Palynological assemblages obtained from Polish material confirm that bathymetry played the key role in taxonomic diversification of the microphytoplankton assemblage. This supports the existing models of distribution: dominance of prasinophytes (leiospheres) and cryptospores is characteristic for shallow-water environments. In deeper water, dominance of acanthomorphs is observed. In the deepest-water zones – mixed assemblages occur. For the Holy Cross Mountains, microphytoplankton frequency is compared with a TOC curve in the same interval.