An overview and brief description of common marine organic-walled dinoflagellate cyst taxa occurring in surface sediments of the Northern Hemisphere

Highlights

• 51 common extant dinoflagellate cyst species and two morphotypes are briefly described and illustrated.

• 2 belong to the Gymnodiniales, 30 to the Gonyaulacales and 21 to the Peridiniales.

• their lowest stratigraphic occurrences are provided.

Abstract

Organic-walled resting cysts of planktonic dinoflagellates occur commonly in modern marine sediment where they represent, with rare exceptions, the only geologically preservable part of the life cycle. Although many species do not produce fossilizable resting cysts, upper Quaternary sediments contain a diverse cyst record that is used frequently for paleoenvironmental reconstruction and stratigraphic analysis. Reconstructions of past sea-surface conditions rely on an understanding of the distributions of dinoflagellate cysts in modern sediments linked to their respective environmental parameters, underpinned by sound taxonomy and standardized nomenclature. Stratigraphic studies additionally require knowledge of morphological details to distinguish extant from similar extinct taxa. Here, 51 dinoflagellate cyst species and two informal cyst morphotypes that are commonly encountered during routine palynological analysis of upper Quaternary marine sediments from the Northern Hemisphere are briefly described taxonomically and illustrated. In addition, their lowest stratigraphic occurrences are provided.

Introduction

Dinoflagellates are protists belonging to the Alveolata (Adl et al., 2019). They are mostly free-living organisms occupying a wide range of environments from fresh water to the open ocean, and from the tropics to high latitudes. Dinoflagellates employ a range of trophic strategies including autotrophy, heterotrophy, and mixotrophy. Of the over 2000 free-living species that have been documented in the marine realm (Gómez, 2012), around 12–15%, as a conservative estimate, produce a geologically preservable organic-walled resting cyst (Head, 1996). A smaller proportion of living dinoflagellate species produce calcareous cysts. Most of these calcareous cysts either do not fully survive the palynological processing used to concentrate organic-walled dinoflagellate cysts or only leave an indistinct membrane without a distinguishable fossil record (Head et al., 2006). They are therefore excluded from the present survey, with the exception of the cysts of Scrippsiella trifida which have a distinctive organic wall that is recognizable as fossil even after the calcareous component has been removed (Head et al., 2006). Nor does the present survey include cysts produced by Alexandrium species. This genus includes various (sometimes cryptic) species that can produce local blooms with abundant cysts accumulating in seed beds (e.g., Miyazono et al., 2012; Lacasse et al., 2013; Natsuike et al., 2013). Most Alexandrium species produce morphologically indistinguishable cysts (e.g., Fukuyo, 1985; John et al., 2014) whose biological affinity can generally only be established through morpho-molecular studies (e.g., John et al., 2014; Fraga et al., 2015, and references therein). A notable exception is the cyst of Alexandrium pseudogonyaulax (Biecheler, 1952) Horiguchi ex Yuki and Fukuyo, 1992 which can be identified morphologically and preserves after palynological treatment (see Mudie et al., 2017, and references therein, for discussion and illustration of the species).

Organic-walled dinoflagellate cysts are frequently used for paleoenvironmental analyses of upper Quaternary marine records in part because assemblages maintain relatively high taxonomic diversities across both inner neritic–oceanic and glacial–interglacial gradients even at relatively high latitudes. Transfer functions using the modern analogue technique are mostly used for such reconstructions, and this requires the detailed mapping of cyst distributions in modern sediments, a task begun by Williams in the 1960s (Williams, 1965, Williams, 1971), and expanded significantly by Wall et al. (1977), and more recently by de Vernal et al., 1992, de Vernal et al., 1993, de Vernal et al., 2001, de Vernal et al., 2020, Marret and Zonneveld (2003) and Zonneveld et al. (2013). The importance of a standardized taxonomy and nomenclature became increasingly apparent during these studies, and was addressed by Rochon et al. (1999) who provided comparative descriptions, illustrations and distributional maps of 43 taxa commonly found in modern sediments of the North Atlantic realm. Since then, numerous taxonomic studies (e.g., Head et al., 2001, Head et al., 2006; Pospelova and Head, 2002; Head, 2003; Matsuoka et al., 2009; Li et al., 2015a; Mertens et al., 2015b; Mertens et al., 2016; Gurdebeke et al., 2018; Limoges et al., 2018; Marret and Mertens, 2018; Potvin et al., 2018; Van Nieuwenhove et al., 2018; Head et al., 2020; Head and Mantilla-Duran, 2020) have advanced our understanding of these cysts. The present work updates Rochon et al. (1999) and incorporates new advances in providing descriptions and lowest stratigraphic occurrences of 51 dinoflagellate cyst species and two informal cyst morphotypes that are encountered on a regular basis in Upper Pleistocene through modern marine sediment samples from the Northern Hemisphere. The taxa illustrated here are considered “common”, meaning that they are present in the surface sediments of at least 40 of the sites from the updated “n = 1968” modern dataset presented by de Vernal et al. (2020). For species occurring rarely (i.e., at fewer than 40 sites of the “n = 1968” dataset), the reader is referred to Limoges et al. (2020) and Mertens et al. (2020). For a footnote regarding the understanding of the concepts of “common” and “rare” as used here, the reader is referred to Van Nieuwenhove et al. (2020). Endemic species found in modern sediments of the Black Sea Corridor are treated in a separate atlas by Mudie et al. (2017), and a brief overview of the global distribution of selected extant dinoflagellate cyst taxa is also presented in Marret et al. (2020). A review of freshwater dinoflagellate cysts is given in Mertens et al. (2012a). All specimens illustrated here are from Holocene sediments unless stated otherwise.

There is a long tradition of placing dinoflagellate cysts obtained from modern sediments within a fossil cyst-defined nomenclature, even after they have been linked to an independently named motile stage by culturing and other experiments (e.g., Reid, 1974). This dual nomenclature is permitted for dinoflagellates under the International Code of Nomenclature for Algae, Fungi and Plants (ICN; Head et al., 2016). The present study uses cyst-defined names where they are available, but provides reference to the equivalent motile-defined (non-fossil) name where it is known. It should be noted that the distinction recognized under the ICN is between fossil and non-fossil taxa, and that dead cysts in sediment can be treated as fossils because they have stratigraphic relations (Head et al., 2016). Since all cyst-defined taxa treated here are based on types recovered from sediment (or rock), they are effectively fossil-defined taxa and their names can co-exist with those of their equivalent (nonfossil-defined) motile stages. The terminology used in the cyst descriptions mostly follows Williams et al. (2000), but we have opted not to use ‘para-’ terminology to distinguish cyst features from their motile analogues since only the cyst morphology is addressed. The suprageneric classification used here follows Fensome et al. (1993) unless otherwise stated. For a full synonymy list of each species, the reader is referred to Fensome et al. (2019) for the cysts and Guiry and Guiry (2019) for the motile stages, except for recently defined species where synonymies are given in the original publications.