Calcareous Dinoflagellate Cysts Research Articles

Mesozoic calcareous nannofossils — state of the art

Calcareous nannofossils originated in the Triassic, radiated in the Jurassic and became a dominant component of the marine biosphere from the earliest Jurassic onward. They can be considered as one of the most important “innovations” of the Mesozoic oceans. Their basic morphology allows the differentiation of three different groups: coccoliths, nannoliths and calcispheres (= calcareous dinocysts). Only coccoliths and nannoliths are discussed in this article in some detail. Coccoliths and nannoliths have contributed greatly in the Interpretation of Mesozoic marine Systems through biostratigraphy and palaeoecology/palaeoceanography. Ever since the late 1960s both coccoliths and nannoliths have proven to be useful and reliable zonal markers for biostratigraphic schemes, allowing detailed zonations for the Jurassic and Cretaceous. Though affected by palaeobiogeographic provincialism, coccoliths and nannoliths have supplied many cosmopolitan biostratigraphic markers. These allow a global correlation of marine sedimentary units both from onshore sections in the classical European and North American areas and pelagic sequences recovered in the course of the DSDP/ODP drilling from the world’s oceans. Thus research on calcareous nannofossils Covers both, regional and global aspects. Research in the last 15 years concentrated on palaeoecological aspects. Apart from dinoflagellates, coccolithophores were the most important primary producers in Mesozoic oceans. As such they heavily relied on autecological factors such as light, nutrients and temperature. Variations in the assemblage composition of these groups may thus be viewed as a key for understanding palaeoecological, palaeoceanographic and palaeoclimatic changes of the past.

Examples for character traits and palaeoecological significance of calcareous dinoflagellates

The palaeoecological significance of calcareous dinoflagellate cysts is illustrated by various examples. Genetically fixed and ecologically triggered character traits are distinguished. A summary is given regarding intraspe-cific variability, cyst size and shape, wall thickness, size and shape of the calcitic wall crystals, paratabulation, and archeopyle morphology based on the knowledge, which has been accumulated during the last two decades. Diversity and characteristic cyst associations from different localities are compared. Information on sea level changes, water temperature, oceanographic distribution, and nutrient conditions can be gained from the investigated character traits of calcareous dinoflagellates.

Geochemistry and time-series analyses of orbitally forced Upper Cretaceous marl–limestone rhythmites (Lehrte West Syncline, northern Germany)

A cyclic marl–limestone succession of Middle–Late Campanian age has been investigated with respect to a Milankovitch-controlled origin of geochemical data. In general, the major element geochemistry of the marl–limestone rhythmites can be explained by a simple two-component mixing model with the end-members calcium carbonate and ‘average shale’-like material. Carbonate content varies from 55 to 90%. Non-carbonate components are clay minerals (illite, smectite) and biogenic silica from sponge spicules, as well as authigenically formed zeolites (strontian heulandite) and quartz. The redox potential suggests oxidizing conditions throughout the section. Trace element and stable isotopic data as well as SEM investigations show that the carbonate mud is mostly composed of low-magnesium calcitic tests of planktic coccolithophorids and calcareous dinoflagellate cysts (calcispheres). Diagenetic overprint results in a decrease of 2% δ18O and an increase in Mn of up to 250 ppm. However, the sediment seems to preserve most of its high Sr content compared to the primary low-magnesium calcite of co-occurring belemnite rostra. The periodicity of geochemical cycles is dominated by 413 ka and weak signals between 51 and 22.5 ka, attributable to orbital forcing. Accumulation rates within these cycles vary between 40 and 50 m/Ma. The resulting cyclic sedimentary sequence is the product of (a) changes in primary production of low-magnesium calcitic biogenic material in surface waters within the long eccentricity and the precession, demonstrated by the CaCO3 content and the Mg/Al, Mn/Al and Sr/Al ratios, and (b) fluctuations in climate and continental weathering, which changed the quality of supplied clay minerals (the illite/smectite ratio), demonstrated by the K/Al ratio. High carbonate productivity correlates with smectite-favouring weathering (semi-arid conditions, conspicuously dry and moist seasonal changes in warmer climates). Ti as the proxy indicator for the detrital terrigenous influx, as well as Rb, Si, Zr and Na, shows only low frequency signals, indicating nearly constant rates of supply throughout the more or less pure pelagic carbonate deposition of the long-lasting third-order Middle–Upper Campanian sedimentary cycle.

Coccolithophore export production and response to seasonal surface water variability in the oligotrophic Cretan Sea (NE Mediterranean)

A single mooring with two sediment traps (trap A at 500 and trap B at 1700 m below sea surface) was deployed at the southern margin of the Cretan Sea at a depth of 1750m. The duration of the deployment was 12 months (from January 2001 to February 2002) with sampling intervals of approximately 15 days. The results obtained from trap A at 500 m depth show that the total coccolithophore export production was generally low. The highest productivity season was recorded from January till late September with the highest flux values observed between late March to late June (maximum flux: 9 x 10 8 coccoliths m -2 day -1 and 9.4 x10 5 coccospheres m -2 day -1 ). During this period the pattern was charac- terized by three main phases that correlate well with the gradual increase in SST from January till September. The lowest fluxes were re- corded between October 2001 and January 2002. The total calcareous dinocysts flux also followed the same pattern. The annual coccolith flux of 9.6 x 10 10 m -2 year -1 at 500 m was one order of magnitude higher compared to that previously recorded in Bannock Ba- sin at 3000 m water depth. The maximum coccosphere flux from trap B at 1700 m depth (3,9x10 5 m -2 day -1 ) occurred in late April, a month later compared to trap A. Apparently the magnitude of the coccosphere flux decreased with depth, but the overall seasonal pattern was preserved in the deeper environments. The annual coccosphere flux at 1700 m was 3,4x10 7 m -2 year -1 and calcareous dinocysts displayed a similar trend with highest values occurring at mid April 2001. Both coccolith and coccosphere sinking assemblages at 500m were characterized by high abundances of Emiliania huxleyi, (average 65% and 46% respectively). Secondary species in coccolith fluxes were Florisphaera profunda (average 9%), Rhabdosphaera clavigera (average 6%), Syracosphaera spp. (average 5%). The calculated total coccolithophore and calcareous dinocyst carbonate flux is relatively low (between 0-26% of the total carbonate flux) attaining higher values during the high productivity period. It is evident that the coccolithophore productivity pattern in the Cretan Sea (highest values at late March - late June) is the result of fertilization in the upper photic zone from the intrusion of the Transition Mediterranean Water (TMW) water masses from the east. Sea surface temperature (SST) and precipitation are environmental parameters directly associated to seasonality and therefore influence the observed coccolithophore productivity patterns.

Effect of temperature on culture growth and cyst production in the calcareous dinoflagellates Calciodinellum albatrosianum, Leonella granifera and Pernambugia tuberosa

Modern and Late Quaternary oceanic assemblages of calcareous dinoflagellate cysts are often dominated by the species Calciodinellum albatrosianum , Leonella granifera and Pernambugia tuberosa . Their distribution in surface sediments of the South and equatorial Atlantic Ocean has been found to be related to a set of environmental parameters such as temperature, nutrient concentration, stratification and salinity. However, the direct influence of single parameters on the species is as yet unknown. In order to determine the effect of temperature on culture growth and cyst production, strains of the species were investigated in a temperature gradient box under controlled laboratory conditions. Whereas culture growth was observed over a relatively broad range of 12.3–30.4°C in C. albatrosianum , 15.5–30.7°C in L. granifera , and 13.3–32.8°C in P. tuberosa , fossilisable cysts were mainly produced in a smaller range of 16.1–21.7°C in C. albatrosianum , 18.0–24.0°C in L. granifera and 22.3–27.6°C in P. tuberosa . By comparing laboratory and field data it is shown that a good correlation exists in P. tuberosa , whereas in C. albatrosianum and L. granifera higher temperatures of fossilisable cyst production were expected from the field data. As possible explanations different depths of vegetative reproduction and cyst production and seasonality in cyst production are discussed.

<i>Tetratropis terrina</i> sp. nov., a new calcareous dinoflagellate cyst from the Upper Campanian <i>polyplocum</i> zone of Lägerdorf (NW Germany)

Abstract. A new calcareous dinoflagellate cyst species, Tetratropis terrina sp. nov., with an apparent stratigraphically narrow range is described from the Upper Campanian Bostrychoceras polyplocum zone of the Lägerdorf chalk sequence (NW Germany). The electron microscopic and light microscopic analyses show that T. terrina has both a pithonelloid wall type with uniformly inclined wall crystallites and a reduced peridiniacean paratabulation pattern. The prominent morphological similarities of T. terrina to the other two Tetratropis species (T. patina and T. corbula) justify the affiliation of the new species to the genus. As a result of the extension of the morphological spectrum by the new species, the genus Tetratropis Willems, 1990 has been emended.

A PROPOSED CLASSIFICATION OF ARCHEOPYLE TYPES IN CALCAREOUS DINOFLAGELLATE CYSTS

Abstract The phylogenetic significance of archeopyles in calcareous dinoflagellates cysts has been evaluated, and a classification model is developed that focuses on the archeopyle categories and types established for organic-walled dinoflagellates by Evitt (1967, 1985). Several of Evitt's archeopyle categories are presently recognized within the calcareous dinoflagellate cysts: apical, intercalary, and combination archeopyles, which are here subdivided into eight archeopyle types and several variations. Archeopyles that cannot be assigned to a distinct type, and those with outlines that do not allow an accurate interpretation, are together placed in a separate category: miscellaneous archeopyles. The stratigraphic distribution of the different archeopyle types reveals a phylogenetic trend characterized by an increase of the number of plates involved in archeopyle formation. The first calcareous dinoflagellate cysts to appear in the late Triassic have a monoplacoid apical archeopyle. The first taxa that s...

Pithonelloid wall-type of the Late Cretaceous calcareous dinoflagellate cyst genus Tetratropis

The late Cretaceous calcareous dinoflagellate genus Tetratropis features both a pithonelloid wall-type (evenly inclined wall-components, proven here by a polarisation optical revision) and a peridinialean paratabulation strongly suggesting a dinoflagellate origin of at least part of the Pithonelloideae. This affinity with dinoflagellates sheds more light on the palaeoecology of the Pithonelloideae (commonly termed “calcispheres”), which are characteristic of the middle to Late Cretaceous. The very short-term stratigraphic occurrence of all Tetratropis species is comparable to the distribution pattern of other calcareous dinoflagellate cyst species with a distinctive paratabulation and is thought to reflect a narrow palaeoecological niche. Tetratropis species can be interpreted either as paratabulated morphotypes of otherwise atabulate Pithonelloideae formed under exceptional palaeoenvironmental conditions or as invaders from a highly specific palaeoecological niche during short-term palaeoceanographic events probably related to the initiation of the Late Cretaceous global cooling.

Different nutrient sources forcing increased productivity during eastern Mediterranean S1 sapropel formation as reflected by calcareous dinoflagellate cysts

Comparison of calcareous dinoflagellate cyst assemblages with Ba, Al, Mn, and Fe records from three sediment cores collected in the eastern Mediterranean Sea indicate that calcareous dinoflagellate cysts are generally resistant to postdepositional dissolution. Cyst association changes during and after sapropel S1 formation can therefore be closely related to variability in surface water productivity. Two groups of cysts are defined: those having highest abundances within the sapropelic and postsapropelic sediments. The temporal cyst distributions suggest increased freshwater input mainly from the Nile and a shallowing of the pycnocline as the most important processes increasing nutrient concentration in the photic zone, thus leading to increased productivity and organic carbon fluxes during sapropel formation. Furthermore, a general warming trend at the beginning of S1 formation and a slight salinity decrease are reconstructed.

Calcareous dinoflagellate cysts in South and equatorial Atlantic surface sediments: diversity, distribution, ecology and potential for palaeoenvironmental reconstruction

Several marine, peridiniphycidean dinoflagellate species produce calcareous cysts during their life cycle, which are relatively resistant to chemical and physical degradation and are therefore often found in large quantities in oceanic bottom sediments. Although the use of these calcareous cysts as proxies for palaeoenvironmental and palaeoclimatic reconstructions has seen many advances over the last decade, until now only relatively patchy and regional information was available on their recent distribution patterns and ecology, especially at the species level. In this paper, comprehensive calcareous cyst diversity and distribution data have been compiled from published and unpublished work for 167 South and equatorial Atlantic Ocean surface sediments, ranging from 20°N to 50°S, and 30°E to 65°W. The main aim has been to focus on the complex, often non-linear, relationships between individual species’ distributions and the physicochemical and trophic conditions of the overlying (sub)surface waters through the use of x–y graphs of cyst abundance vs. (sub)surface water environmental parameters, and detrended correspondence analyses. Ten cyst species and the calcareous vegetative coccoid species Thoracosphaera heimii were observed in the bottom sediments, each species showing its own characteristic distribution pattern in relation to the environmental conditions of the upper water masses above them (e.g. sea surface temperature, productivity, stratification). The sensitive reactions of various species to unique combinations of environmental parameters shows that each species has its own specific ecological traits, thus rejecting the bundled use of ‘calcareous cyst accumulation’ as a general proxy for oligotrophy or stratification in future palaeoenvironmental analyses. The acquired ‘reference’ data set of this study is large and diverse enough to allow its future application in quantitative palaeoenvironmental reconstruction models, and shows that there is still an enormous reconstruction potential concealed in many fossil calcareous dinoflagellate cyst assemblages.

Calcareous dinoflagellate cysts in surface sediments from the Mediterranean Sea: distribution patterns and influence of main environmental gradients

The distribution of calcareous dinoflagellate cysts in surface sediments from the Mediterranean Sea was quantitatively analysed. The samples contain 11 cyst species and the vegetative coccoid Thoracosphaera heimii. Cyst abundance increases towards the deeper parts of the basins and is generally higher in the eastern Mediterranean Sea. Three major distribution characteristics exist: (1) different assemblages in oceanic and neritic regions, (2) little agreement with the associations of areas studied so far like the Atlantic Ocean, and (3) a unique oceanic assemblage in the eastern Mediterranean Sea. A gradual change in cyst assemblages from the western to the eastern Mediterranean Sea was observed and statistically compared with the main environmental gradients in the upper water column. Temperature, nitrate concentration and possibly salinity appear to be the most important factors controlling cyst production. Three groups containing cysts with similar environmental preferences can be distinguished: (1) an eastern Mediterranean group related to relatively high temperature and salinity but low nitrate concentration, (2) a group of more or less consistently abundant cosmopolitan species tolerating or even preferring relatively low temperature and salinity but high nitrate concentration, and (3) a group containing species that are possibly adapted to neritic environments and have probably been transported from coastal areas into the studied regions. In contrast to other calcareous plankton, calcareous dinoflagellate cysts correlate strongly with the main environmental gradients in the Mediterranean Sea, bearing a high potential for palaeoenvironmental reconstructions.

Small acritarchs from the Upper Cretaceous: taxonomy, biological affinities and palaeoecology

A systematic study of small acritarchs (most commonly around 10 μm) from the Late Campanian (Duwi (Phosphate) Formation) and Maastrichtian (Dakhla Shale) of Egypt, based on scanning electron miscroscopy, resulted in the identification of nine genera and 27 species, among them the new genus Recticystis and 14 new species, namely Operculites bigranulatus, Bacillidinium partitum, Mecsekia acuta, M. curvispina, M. triangula, Recticystis hemispinosus, Clavaticystis septatus, C. furcatus, Odontothrix multiornatus, Filisphaeridium paucigranulosum, Cymatiosphaera parvirugosa, C. muralis, C. polyornata and C. triradiata. In addition, the following new combinations are proposed: Cymatiosphaera coronis (Habib and Knapp), Mecsekia minor (Takahashi) and Mecsekia ariakense (Takahashi). Small acritarchs are thought to include cysts of dinoflagellates because of significant morphological similarities (cyst shape, chasmic and tremic archeopyles, spinose and septate ornaments) to certain organic-walled and calcareous dinoflagellate cysts. Small size in acritarchs is regarded as an adaptation to a planktonic and possibly to an interstitial mode of life. Small acritarchs attain the highest diversity and abundance in the well agitated environment of the phosphoritic sandstones. In the pores between the sand grains they were protected from the wave action and currents which moved away most of the larger dinoflagellates. Dinoflagellates are common in the associated shales representing a more quiet marine environment. The interpretation of some small spinulose acritarchs as interstitial dinoflagellates may also be supported by the presence of spinose cysts with a chasmic archeopyle in the modern dinoflagellate Polykrikos which includes interstitial forms.

Production of calcareous dinoflagellate cysts in response to monsoon forcing off Somalia: a sediment trap study

To study the ecology of calcareous dinoflagellates we examined the impact of the SW and NE monsoons on cyst formation using sediment trap material, collected at 1032 m water depth, off Somalia from June 1992 to February 1993. The results do not confirm the relationship between cyst production and lower nutrient concentrations, as highest cyst fluxes were recorded during late SW monsoon under the relatively nutrient-rich and less agitated conditions of mature upwelled water. Lowest cyst fluxes were found under strongly stratified, nutrient-depleted surface waters during the inter-monsoon. Although all of the studied species seem to prefer a stratified water column, an elevated concentration of nutrients appears to be necessary to maintain high cyst production. Comparison of the mean cyst flux to the sediment trap with that into the underlying surface sediments reveals a loss of 81–96%, which can be attributed to calcite dissolution. The relatively small spheres of Thoracosphaera heimii are affected more than the cysts of the other species.

Oxygen availability effects on early diagenetic calcite dissolution in the Arabian Sea as inferred from calcareous dinoflagellate cysts

In oceanic regions with high primary production, such as the Arabian Sea, the primary signals of proxies are often altered by diagenetic processes. The present study aims at assessing the effects of early diagenesis on calcareous dinoflagellate cysts, which represent a relatively new tool for reconstructing the paleoenvironmental conditions within the photic zone. For this purpose, surface sediment samples from within and below the oxygen minimum zone (OMZ) of the northeastern and southwestern Arabian Sea have been analysed quantitatively for their calcareous dinoflagellate cyst content. The calculated cyst accumulation rates (ARs), the relative abundances and cyst fragmentation values were compared to bottom water oxygen (BWO) content and ARs of organic carbon at the sample positions. Different patterns were found in the northeastern and southwestern part of the Arabian Sea. In the SW, no relationship between cyst ARs and BWO is distinguishable, and the distribution of cyst ARs is thought to largely reflect primary cyst production. In the NE, much higher ARs of all species are found in samples from within the OMZ in comparison to samples from below it. This is interpreted to result from better calcite preservation within the OMZ, presumably due to reduced oxic degradation of organic matter. The differential drop of cyst ARs of the individual species at the lower boundary of the OMZ in the NE Arabian Sea, as well as the species-specific change in relative abundance and fragmentation, indicate different sensitivity to calcite dissolution of the different species. These results show that early diagenetic calcite dissolution can change both relative and absolute abundances of calcareous dinoflagellate cysts, which has to be considered if using them for paleoenvironmental reconstructions. Furthermore, it is shown that considerable calcite dissolution can occur above the carbonate saturation horizon in high productive areas. However, calcite preservation can be substantially increased, as soon as oxygen concentrations are too low for oxic degradation of OM. Under low oxic conditions (within and near the OMZ), the main factor controlling organic matter (OM) preservation appears to be BWO concentrations. Under higher oxygen levels (below ∼1500 m depth in the NE Arabian Sea) there seems to be an increasing influence of bioturbation and sedimentation rate on the preservation of OM by controlling its oxygen exposure time. This study presents an example of a highly productive basin in which differences in early diagenetic processes can lead to the preservation of a signal that is either dominated by primary production (off Somalia) or by secondary alteration (off Pakistan), although in both areas, an oxygen depleted zone is present. For estimating the effects of early diagenetic calcite dissolution in a sediment by metabolic CO 2 (and probably by H 2S oxidation), not only the content of organic carbon but also other geochemical proxies for paleoredox-conditions have to be included for paleoenvironmental reconstructions.

Palaeoecology of calcareous dinoflagellate cysts in the mid-Cenomanian Boreal Realm: implications for the reconstruction of palaeoceanography of the NW European shelf sea

A transect from the bathyal to proximal shelf facies of the Boreal Realm was investigated to compare spatial and temporal distribution changes of calcareous dinoflagellate cysts (c-dinocysts) throughout the mid-Cenomanian in order to gain information on the ecology of these organisms. Pithonelloideae dominated the cyst assemblages to more than 95% on the shelf, a prevalence that can be observed throughout most of the Upper Cretaceous. The affinity of this group with the dinoflagellates, which is still controversially discussed, can be confirmed, based on evidence from morphological features and distribution patterns. The consistent prevalence of Pithonella sphaerica and P. ovalis in c-dinocyst assemblages throughout the Upper Cretaceous indicates that they were produced more frequently than cysts of the other species and might, therefore, represent a vegetative dinoflagellate life stage. P. sphaerica and P. ovalis are interpreted as eutrophic species. P. sphaerica is the main species in a marginal-shelf upwelling area, offshore Fennoscandia. Here, sedimentary cyclicity appears to have been reduced to the strongest light/dark changes, while in the outer shelf sediments, light/dark cycles are well-developed and show pronounced temporal assemblage changes. Cyclic fluctuations in the P. sphaerica / P. ovalis ratio reflect shifts of the preferred facies zones and indicate changes in surface mixing patterns. During periods of enhanced surface mixing most parts of the shelf were well-ventilated, and nutrient-enriched surface waters led to high productivity and dominance of the Pithonelloideae. These conditions on the shelf contrasted with those in the open ocean, where more oligotrophic and probably stratified waters prevailed, and an assemblage with very few Pithonelloideae and dominance of Cubodinellum renei and Orthopithonella ? gustafsonii was characteristic. While orbitally-forced light/dark sedimentary cyclicity of the shelf sections was mainly related to surface-water carbonate productivity changes, no cyclic modulation of productivity was observed in the oceanic profile. Therefore, dark layer formation in the open ocean was predominantly controlled by the cyclic establishment of anoxic bottom water conditions. Orbitally-forced interruptions in mixing on the shelf resulted in cyclic periods of stratification and oligotrophy in the surface waters, an expansion of oceanic species to the outer shelf, and a shelfward shift of pithonelloid-facies zones, which were probably related to shelfward directed oceanic ingressions.

Reconstruction of mid-Cenomanian orbitally forced palaeoenvironmental changes based on calcareous dinoflagellate cysts

Mid-Cenomanian, precession-controlled (21 ka) chalk–marl couplets of the Cap Blanc Nez section (Anglo–Paris Basin) have been studied with focus on the effects which Milankovitch cycles have had on the palaeoenvironment. In this paper, we present micropalaeontological and lithological proxies that enable the reconstruction of both the cycle architecture and the transformation of the orbitally forced signal into the sediment. A palaeoecological reconstruction based on changes in calcareous dinoflagellate cysts (c-dinocysts) assemblages was carried out, in which two characteristic ecological assemblages of c-dinocysts were identified. Gradual changes in absolute and relative abundance of the cyst species in these assemblages over several couplets depict a bundling pattern which is interpreted to reflect the modulation of the intensity of the precession cycle by the eccentricity cycle (100 ka). The stacking pattern in the natural gamma ray signal and the carbonate and TOC content has the same period and provides lithological support of the bundling. A shelf basin circulation model is proposed to explain the relation between orbitally forced climate change, its palaeoenvironmental consequences and the resulting sedimentary cyclicity. Variations in surface water circulation are reflected in the sediment by the chalk–marl couplets, the most distinctive couplets ocurring at the base and top of the bundles. While the chalks reflect well-mixed surface water conditions, the marls, particularly those at the bundle boundaries, can be interpreted as the sedimentary expression of stratified water masses. During deposition of these marls, reduced oceanic mixing due to low seasonality during strong precession maxima at the eccentricity maxima caused periods of water column stratification that in turn led to nutrient depletion and decreased productivity in the surface water masses.

On the response of calcareous dinoflagellates to oligotrophy and stratification of the upper water column in the equatorial Atlantic Ocean

Large numbers of calcareous dinoflagellate cysts and the vegetative calcareous coccoid species Thoracosphaera heimii are generally found in sediments underlying oligotrophic and/or stratified (sub)surface water environments. It is difficult to distinguish between the relative importance of these two environmental parameters on calcareous cyst and T. heimii distribution as they usually covary, but this information is essential if we want to apply cysts properly in the reconstruction of palaeoenvironments and past surface water hydrography. In the multi-proxy core GeoB 1523-1 from the Ceará Rise region in the western equatorial Atlantic Ocean (covering the past 155 ka), periods of greatest oligotrophy are not synchronous with periods of greatest stratification (Rühlemann et al., Mar. Geol. 135 (1996) 127–152; Mulitza et al., Geology 25 (1997) 335–338; Mulitza et al., Earth Planet. Sci. Lett. 155 (1998) 237–249), giving us the unique opportunity to differentiate between the effects of both parameters on cyst accumulation. The calcareous cyst record of the core reflects prominent increases in accumulation rate of nearly all observed species only during the nutrient-enriched but more stratified isotopic (sub)stages 5.5, 5.3, 5.1 and 1. In this respect, the distribution trends in the core are more similar to those of the eastern equatorial upwelling region (GeoB 1105-4) than they are to those of the oligotrophic north-eastern Brazilian continental slope (GeoB 2204-2), even though temporal changes in bioproductivity are principally in antiphase between the eastern and western equatorial regions. We conclude that stratification of the upper water column and the presence of a well-developed thermocline are probably the more important factors controlling cyst distribution in the equatorial Atlantic, whereas the state of oligotrophy secondarily influences cyst production within a well-stratified environment.

Calcareous cyst-producing dinoflagellates: ecology and aspects of cyst preservation in a highly productive oceanic region

Abstract Absolute and relative abundances of calcareous dinoflagellate cyst species in surface sediment samples from the Arabian Sea are compared with environmental parameters of the upper 100 m of the water column to gain information on their largely unknown autecology. Ten species or morphotypes were encountered of which four occurred only as accessories. On the basis of the distribution patterns of the six more abundant species or morphotypes, the studied area is subdivided into three provinces, demonstrating a clear relationship to monsoon-controlled upper-ocean conditions. The two dominant species, Thoracosphaera heimii and Orthopithonella granifera , show opposite trends in distribution of both their absolute and relative abundances. In the NE Arabian Sea, low absolute and relative abundances of T. heimii are mainly attributed to enhanced dissolution of the small tests in this region, whereas elevated concentrations of O. granifera seem to be related to higher water temperatures and the influence of the Indus River. Sphaerodinella albatrosiana and Calciodinellum operosum are most abundant in the open ocean, associated with lower nutrient levels, relatively high temperatures and low seasonality. Spiny cysts (mainly represented by Scrippsiella trochoidea ), in contrast, exhibit a more shelf-ward distribution and are most abundant in regions that are influenced by coastal upwelling, characterized by eutrophic and rather unstable conditions with seasonally lower temperatures and a shallow thermocline. A generally negative correlation of calcareous dinoflagellate cysts with primary productivity or high nutrient concentrations, as proposed by other workers, cannot be confirmed. Cyst accumulation rates off Somalia show that strong turbulence and high current speeds are unfavourable for calcareous dinoflagellates, suggesting that these organisms are more successful under rather stratified conditions.

Shifts in the position of the north equatorial current and rapid productivity changes in the western tropical Atlantic during the last glacial

High‐resolution, well‐dated calcareous dinoflagellate cyst and organic carbon records from a 58 kyr sediment core (M35003‐4) located southeast of the island of Grenada show that rapid and pronounced changes in cyst association and accumulation and organic carbon deposition occurred, controlled by (1) a significant southward shift in the position of the North Equatorial Current during the last glacial period and the Younger Dryas cold interval and (2) rapid changes in local productivity in marine isotopic stage 3 that are associated with variations in Orinoco River nutrient discharge and coastal upwelling strength. Prominent cyst accumulation peaks representing extremely oligotrophic and stratified thermocline conditions mimic the Greenland ice core and northern Atlantic Dansgaard/Oeschger stadials and Heinrich events. We provide new evidence for a coupled tropical/high‐latitude Atlantic climate system during the last glacial period and suggest that changes in the zonality of the low‐latitude winds may play an important role in modulating rapid interhemispheric climate variability.

Palaeoenvironmental interpretation of Late Albian calcareous dinoflagellate cysts from the Kirchrode I borehole (Lower Saxony Basin, NW Germany)

Late Albian calcareous dinoflagellate cyst assemblages from samples of the Kirchrode I borehole contains 38 morphospecies, which represent a flora of moderate warm water conditions in an oceanic-controlled epicontinental regime. The vertical changes of cyst associations reflect sea level fluctuations. The regressive phases of the highstand are characterised by the exclusive occurrence of benthic resting cysts, while planktonic counterparts joined during transgressive phases. Floral exchanges via the water masses, which were influenced by the Tethys, were in progress only during phases of relatively strong currents, in particular during transgressions. In contrast, the highstand, during which water circulation decreased, can be characterised by autochthonous morphotypes, reflecting an epicontinental habitat. The corresponding cyclic, intraspecies change of cyst diameter is interpreted to reflect changes in productivity, which were probably controlled by eccentricity cycles.