Coccolith Size Research Articles

Biometric analysis of the calcareous nannofossil group reticulofenestrids from the Oligocene to the Miocene

The calcareous nannofossil genus Reticulofenestra and its related taxa are called “reticulofenestrids,” and they were one of the major nannofossil groups during the Cenozoic. The reticulofenestrid specimens are composed of two shields, proximal and distal, which are circular or subcircular in shape and consist of tiny calcite plates. Cyclic changes in the size and shape of reticulofenestrid coccoliths have been used to characterize their evolution. The major taxonomic turnover in this group was related to environmental changes during the Cenozoic. From the Oligocene to the Miocene, there were several different types of reticulofenestrids. The primary taxon of reticulofenestrids disappeared during the Oligocene-Miocene boundary, and elliptical species emerged during the early Miocene. However, it is uncertain exactly when and how the change in reticulofenestrids occurred. To understand the evolutionary trends of reticulofenestrids, we conducted biometric analyses from the Oligocene to the Miocene. Detailed observation and measurement of reticulofenestrid coccoliths under a scanning electron microscope revealed a significant size reduction in the group near the Oligocene–Miocene, which corresponds to a period of rapid global cooling. Furthermore, regarding the reticulofenestrid taxa in this study, central structures characterize the Oligocene genus Dictyococcites, and the long axis length of the coccolith outline and central areas can be effective characteristics for distinguishing between theMiocene Cyclicargolithus and Reticulofenestra. These results suggest that morphological differences of reticulofenestrids are expressed not only in the shape and size of the coccolith but also in the size and shape of its central area.

Coccolith-calcite Sr/Ca as a proxy for transient export production related to Saharan dust deposition in the tropical North Atlantic

Atmospheric dust deposition can modulate the earth’s climate and atmospheric CO2 through fertilising the ocean (nutrient source) and by accelerating the biological carbon pump through fuelling the ballasting process. To distinguish the biogeochemical effects of Saharan dust with respect to fertilization and ballasting, and to gain a broader perspective on the coccolith calcite Sr/Ca in relation to the drivers of coccolith export production, we determined the coccolith-Sr/Ca from a one-year (2012–2013) time-series sediment trap record in the western tropical North Atlantic (M4—49°N/12°W). High Sr/Ca were linked to enhanced export production in the upper part of the photic zone, most notably under windier, dry, and dustier conditions during spring. Attenuated Sr/Ca in the autumn probably reflect a combination of lower Sr-incorporation by dominant but small-size placolith-bearing species and the presence of “aged” coccoliths rapidly scavenged during a highly productive and usually fast export event, likely added by (wet) dust ballasting. Higher Sr/Ca observed in the large coccolith size fractions support the existing notion that larger-sized coccolithophores incorporate more Sr during calcification under the same environmental conditions. The presence of the abnormally Sr-rich species Scyphosphaera apsteinii is also shown in the separated large fraction of our Sr/Ca seasonal data.

Alisphaera crostae sp. nov. (Prymnesiophyceae), a new extant coccolithophore from the Southwest Indian Ocean

The analysis of water samples collected during the MD218 CROTALE expedition (February–March, 2019), and the 4th Indian Southern Ocean Expedition (SOE) (January–February, 2010) (re-visited samples) resulted in the occurrence of a new coccolithophore species belonging to the genus Alisphaera, and is named Alisphaera crostae sp. nov. This new species appears monomorphic and shows distinctive morphological diff erences in terms of its coccolith size, fl ange structure, and nodules, compared to other known Alisphaera species. In this study, morphological diff erences between A. crostae and other Alisphaera species are illustrated and compared. We suggest that the genus Alisphaera may have a higher diversity and may possess a wider distribution than previously evidenced, which requires more careful observation.

Reduction in size of the calcifying phytoplankton Calcidiscus leptoporus to environmental changes between the Holocene and modern Subantarctic Southern Ocean

The Subantarctic Zone of the Southern Ocean plays a disproportionally large role on the Earth system. Model projections predict rapid environmental change in the coming decades, including ocean acidification, warming, and changes in nutrient supply which pose a serious risk for marine ecosystems. Yet despite the importance of the Subantarctic Zone, annual and inter-annual time series are extremely rare, leading to important uncertainties about the current state of its ecosystems and hindering predictions of future response to climate change. Moreover, as the longest observational time series available are only a few decades long, it remains unknown whether marine pelagic ecosystems have already responded to ongoing environmental change during the industrial era. Here, we take advantage of multiple sampling efforts – monitoring of surface layer water properties together with sediment trap, seafloor sediment and sediment core sampling – to reconstruct the modern and pre-industrial state of the keystone calcifying phytoplanktonCalcidiscus leptoporus, central to the global marine carbonate cycle. Morphometric measurements reveal that modernC. leptoporuscoccoliths are 15% lighter and 25% smaller than those preserved in the underlying Holocene-aged sediments. The cumulative effect of multiple environmental factors appears responsible for the coccolith size variations since the Last Deglaciation, with warming and ocean acidification most likely playing a predominant role during the industrial era. Notably, extrapolation of our results suggests a future reduction in cell and coccolith size which will have a negative impact on the efficiency of the biological pump in the Southern Ocean through a reduction of carbonate ballasting. Lastly, our results tentatively suggest thatC. leptoporuscoccolith size could be used as a palaeo-proxy for growth rate. Future culture experiments will be needed to test this hypothesis.

High‐Resolution Coccolithophore Morphological Changes in Response to Orbital Forcings During the Early Oligocene

AbstractThe global climate of the early Oligocene was characterized by initiated Antarctic glaciation and meridional overturning circulation, which then led to profound eutrophication in the upper ocean. Generating a high‐resolution coccolith record helps to understand the responses of marine phytoplankton to the newly established environment. Using highly resolved (∼6 kyr time‐resolution) marine sediment samples from Deep Sea Drilling Project Site 522 in the South Atlantic Ocean, we conducted a comprehensive morphological study on coccoliths from the genera Reticulofenestra, Dictyococcites, and Coccolithus, which dominated the study interval between ∼33.1 and 32.8 Ma. Our results showed that the size variations of the three measured genera were significantly correlated (p < 0.01) with each other, indicating homogeneous responses to the environmental changes. Moreover, spectrum analysis on integrated morphologic data of all measured coccoliths showed distinct obliquity (∼40‐kyr) and precession (∼23‐kyr and ∼18‐kyr) cycles. We suggest that these variations were mainly driven by temperate, short‐term ecological fluctuations, which periodically altered the nutrient conditions in the common living habitats of the studied coccolithophores. We proposed two tentative explanations focusing on the obliquity signal. First, the cyclic variation could result from obliquity‐modulated changes in ice volume and variations in ocean circulation intensity, which influenced nutrient export from deep waters to the upper ocean. Alternatively, the changes in coccolith size may indicate the strength of seasonality that influenced upper ocean mixing on the west coast of South Africa.

Origin of the long-term increase in coccolith size and its implication for carbon cycle and climate over the past 2 Myr

A long-term increase in coccolith size over the Pleistocene has long been discovered in global oceans. Such a long-term and evolutionary change is expected to exert substantial impact on the carbon cycle as coccolithophores are one of the major components of marine carbonate counter pump (CCP). Here, we present coccolith morphological data as well as coccolith calcite export at Ocean Drilling Program Site 1143 in the South China Sea over the past 2 Myr. The results confirm the idea that Earth eccentricity controlled coccolith size diversity and calcite export production in tropical oceans in the Pleistocene. In addition, we consider a possible origin of the long-term increase in coccolith size as a result of increased oceanic alkalinity availability. As coccolithophores evolved simultaneously in global oceans, the increased coccolith calcification may have promoted global marine particulate inorganic carbon production, especially for the interglacial stages. Therefore, the enhanced coccolith-based CCP may account for a stepwise increase in interglacial CO2 levels over the middle Pleistocene. High CO2 levels at the onset of the interglacial stages may reduce the glaciation rate, thereby prolonging an entire interglacial-glacial cycle after the middle Pleistocene.

Palaeoecologic and Palaeoclimatic Inferences from Calcareous Nannofossils in Western Lobe Offshore, Niger Delta

In support of the ongoing temporal palaeoenvironment and palaeoclimatic reconstructions of the Neogene sediments, this study attempts to detail the paleo-proxies recovered from DEL-1 Well, western offshore Niger Delta. The standard smear slide method enabled the recovery of well-preserved calcareous nannofossils that depict early to mid–Miocene (NN4–NN5) sediments. The up-hole relationships between the nannofossil accumulation rate (NAR), the relative abundance of Discoaster and coccolith size of Reticulofenestra show step by step collapse of sea surface stability from early to middle Miocene. The lower horizons (8000–9460 ft) exhibit a low NAR, relatively high Discoaster abundance and relatively large Reticulofenestra size to suggest a deep thermocline and nutricline that characterise oligotrophic conditions in less warm-water induced climate. Conversely, upper horizons (5225–6550 ft) exhibit a high NAR, relatively low Discoaster abundance and relatively small Reticulofenestra size to suggest a shallow thermocline and nutricline that characterise eutrophic conditions in warm-water induced climate. The relative abundance of Helicosphaera carteri within the mid-NN5 suggests mesotrophic conditions within a stressed environment, with the possible occurrence of carbonate crash events. The combined parameters indicate gradual eutrophication and collapse of sea surface stability favouring nutrients and influx of terrestrial sediments in the ocean water as it progressed from early to middle Miocene. The abundance of the palaeo-proxies assemblages suggests hyposaline waters in a neritic environment that prevailed during the warm climatic condition.

Coccolith size rules – What controls the size of coccoliths during coccolithogenesis?

Heterococcoliths are calcite platelets produced inside diploid coccolithophore cells and extruded to form a covering on the cell surface called a coccosphere. The size of coccoliths is an important parameter sometimes used to identify species, and it is observed to be influenced in extant species by abiotic parameters (e.g., CO2, light). However, the variable distribution of coccolith sizes occurring within a single coccosphere questions the mechanisms controlling coccolith size. A relationship between cell/coccosphere size and mean coccolith size was previously identified, called the “coccolithophore size rules”. In this study, we query the mechanisms controlling the size of a coccolith during coccolithogenesis. A culture experiment on Gephyrocapsa huxleyi strain RCC1216 shows that coccolithogenesis occurs during the cell growth G1 interphase and newly produced coccoliths get bigger as the cell grows. These observations provide parameters for the development of two numerical models used to simulate the coccolith size distribution within a coccolithophore population. Neither model can accurately reproduce an empirical monoclonal coccolith size distribution, indicating that additional factors influence coccolith size. According to our results, coccolith size is only clearly related to cell size at the time of its formation. We confirm that application of the coccolithophore size rules model should be limited to inferring average cell dimensions from (fossil) coccolith biometry, and that comparisons are valid only in multipopulational studies. The coccolith size rule model – the constraining effect of coccolith production during G1 interphase cell growth on coccolith size – proposed here is applicable only for some placolith-forming species.

Strain-specific morphological response of the dominant calcifying phytoplankton species Emiliania huxleyi to salinity change.

The future physiology of marine phytoplankton will be impacted by a range of changes in global ocean conditions, including salinity regimes that vary spatially and on a range of short- to geological timescales. Coccolithophores have global ecological and biogeochemical significance as the most important calcifying marine phytoplankton group. Previous research has shown that the morphology of their exoskeletal calcified plates (coccoliths) responds to changing salinity in the most abundant coccolithophore species, Emiliania huxleyi. However, the extent to which these responses may be strain-specific is not well established. Here we investigated the growth response of six strains of E. huxleyi under low (ca. 25) and high (ca. 45) salinity batch culture conditions and found substantial variability in the magnitude and direction of response to salinity change across strains. Growth rates declined under low and high salinity conditions in four of the six strains but increased under both low and high salinity in strain RCC1232 and were higher under low salinity and lower under high salinity in strain PLYB11. When detailed changes in coccolith and coccosphere size were quantified in two of these strains that were isolated from contrasting salinity regimes (coastal Norwegian low salinity of ca. 30 and Mediterranean high salinity of ca. 37), the Norwegian strain showed an average 26% larger mean coccolith size at high salinities compared to low salinities. In contrast, coccolith size in the Mediterranean strain showed a smaller size trend (11% increase) but severely impeded coccolith formation in the low salinity treatment. Coccosphere size similarly increased with salinity in the Norwegian strain but this trend was not observed in the Mediterranean strain. Coccolith size changes with salinity compiled for other strains also show variability, strongly suggesting that the effect of salinity change on coccolithophore morphology is likely to be strain specific. We propose that physiological adaptation to local conditions, in particular strategies for plasticity under stress, has an important role in determining ecotype responses to salinity.

Adaptations of Coccolithophore Size to Selective Pressures During the Oligocene to Early Miocene High CO2 World

AbstractCulture experiments with coccolithophore algae—the dominant group of marine calcifying phytoplankton—imply a strong sensitivity in growth rate, degree of cellular calcification, and cell size to changes in the carbon chemistry of their growth environment. These results underpin recent studies that have explored how these physiological parameters have varied on geological time scales, in response to changing surface ocean habitats and the concentrations of carbon in the ocean‐atmosphere system. Here, we add to this work with a study of reticulofenestrid coccolith size—the dominant coccolithophore family of the Cenozoic—over the Oligocene to Early Miocene time interval. We examine sediments from contrasting latitudes and regional environmental settings, comparing sites using coherent, updated age models to distinguish globally synchronous trends in cell size from regional trends. Our results confirm several changes in coccolith size—which is strongly correlated to cell size—that are globally reproducible within the ~1 Myr age uncertainty, including a reduction in mean size by >2 μm from 30.2 to 27 and 24.5 to 23 Ma, and then increase in mean size after 20 Ma. The main difference among regions is the presence/absence of coccoliths larger than 8 μm. We evaluate which scenarios of change in carbon dioxide, temperature, and nutrient availability could have exerted selective pressure on cell size for different size classes to produce the observed size trends at each studied site. These million‐year scale adaptations of ancient coccolithophores contribute to the understanding of phytoplankton physiology in the transition to the modern “icehouse” world.

High resolution spatial analyses of trace elements in coccoliths reveal new insights into element incorporation in coccolithophore calcite

Coccolithophores are phytoplanktonic algae which produce an exoskeleton made of single platelets of calcite named coccoliths. They are widespread in all oceans and directly impact the short- and long-term C cycle. The study of coccolith size, morphology and elemental composition reveals important information regarding the ability of the cell to calcify and on the factors that influence this process. In this regard, very little is known about coccolith composition and its changes under altered environmental conditions. Here, we present high resolution (50 × 50 nm) elemental spatial distribution in pristine coccoliths of Coccolithus pelagicus and Gephyrocapsa oceanica reconstructed via X-ray fluorescence analyses at synchrotron. The studied specimens are from control culture and metal-enriched (V, Ni, Zn and Pb) experiments. The analysed specimens produced under stress conditions, display an irregular shape and are thinner, especially in the external rim, with ca. 1/3 lower Ca concentrations compared to specimens from the control. The same specimens also have higher Sr/Ca ratio with highest values in the coccolith external rim, suggesting that difficulty in calcification is additionally reflected in increased Sr/Ca ratios. Selenium is found in the coccolith as possible substitute of carbonate in the calcite. V and Pb apparently did not interact with the coccoliths while Zn and Ni were deposited on the coccolith surface.

Biscutum constans coccolith size patterns across the mid Cretaceous in the western Tethys: Paleoecological implications

The morphometric record of the calcareous nannofossil species Biscutum constans from the western Tethys indicates that this species underwent marked size changes across the latest Barremian-Cenomanian interval (~ 28 Myrs). Significant size reduction occurred during the core of the early Aptian Oceanic Anoxic Event (OAE) 1a (~ 120 Ma) and soon after OAE 1a followed by a 9 Myr-long interval characterized by small specimens. Larger coccoliths marked the middle to late Albian and, particularly, the lower part of the nannofossil subzone NC10a was characterized by abundant large specimens. A minor size decrease coincided with the late Albian OAE 1d onset (~ 100 Ma), but very small size was reached again during the latest Cenomanian OAE 2 (~ 94.4 Ma). The sequence of relatively long (millions of years) intervals marked by different average sizes is interpreted to reflect adaptation to changing paleoecological conditions. Temperature and fertility were not directly influencing the size of B. constans. On shorter time scale, abrupt changes in ocean chemistry related to Large Igneous Provinces submarine volcanism played a central role in secretion of small B. constans coccoliths to face ocean acidification during OAE 1a and OAE 2. Contrarily, during the late early Aptian-early Albian time interval, smaller B. constans size was related to cooler temperature which possibly also promoted CO2 sequestration in cooler surface waters forcing B. constans to produce small coccoliths to survive lowered pH. In addition, pulses of subaerial volcanism of the Southern Kerguelen Plateau may have further affected the ocean chemistry. The termination of intense volcanism and warmer temperature in the middle Albian-early Cenomanian coincided with restoration of bigger B. constans specimens.

A 15-million-year-long record of phenotypic evolution in the heavily calcified coccolithophore <i>Helicosphaera</i> and its biogeochemical implications

Abstract. The biogeochemical impact of coccolithophores is defined not only by their overall abundance in the oceans but also by wide ranges in physiological traits such as cell size, degree of calcification and carbon production rates between different species. Species' sensitivity to environmental forcing has been suggested to relate to their cellular PIC : POC (particulate inorganic carbon : particulate organic carbon) ratio and other physiological constraints. Understanding both the short-term and longer-term adaptive strategies of different coccolithophore lineages, and how these in turn shape the biogeochemical role of the group, is therefore crucial for modeling the ongoing changes in the global carbon cycle. Here we present data on the phenotypic evolution of a large and heavily calcified genus Helicosphaera (order Zygodiscales) over the past 15 million years (Myr), at two deep-sea drill sites in the tropical Indian Ocean and temperate South Atlantic. The modern species Helicosphaera carteri, which displays ecophysiological adaptations in modern strains, was used to benchmark the use of its coccolith morphology as a physiological proxy in the fossil record. Our results show that, on the single-genotype level, coccolith morphology has no correlation with growth rates, cell size or PIC and POC production rates in H. carteri. However, significant correlations of coccolith morphometric parameters with cell size and physiological rates do emerge once multiple genotypes or closely related lineages are pooled together. Using this insight, we interpret the phenotypic evolution in Helicosphaera as a global, resource-limitation-driven selection for smaller cells, which appears to be a common adaptive trait among different coccolithophore lineages, from the warm and high-CO2 world of the middle Miocene to the cooler and low-CO2 conditions of the Pleistocene. However, despite a significant decrease in mean coccolith size and cell size, Helicosphaera kept a relatively stable PIC : POC ratio (as inferred from the coccolith aspect ratio) and thus highly conservative biogeochemical output on the cellular level. We argue that this supports its status as an obligate calcifier, like other large and heavily calcified genera such as Calcidiscus and Coccolithus, and that other adaptive strategies, beyond size adaptation, must support the persistent, albeit less abundant, occurrence of these taxa. This is in stark contrast with the ancestral lineage of Emiliania and Gephyrocapsa, which not only decreased in mean size but also displayed much higher phenotypic plasticity in their degree of calcification while becoming globally more dominant in plankton communities.

Climate and paleoceanography controlled size variations of calcareous nannofossils during the Valanginian Weissert Event (Early Cretaceous)

Abstract Strata of Valanginian age (Early Cretaceous, 139.8–132.9 Ma) record a 1.5‰ positive carbon isotope excursion (CIE), the Weissert Event. It coincides with volcanic activity of the Parana-Etendeka large igneous province. Unlike the Mesozoic Oceanic Anoxic Events (OAEs), no organic-rich deposits documenting widespread perturbation of the marine carbon-cycle are associated with the Weissert Event. To study the impact of paleoceanographic changes in the Valanginian on calcareous nannofossils, this study analyzes coccolith size data of three species (Biscutum constans, Watznaueria barnesiae, Zeugrhabdotus erectus) from northern Germany and the western North Atlantic. Further it presents a new Valanginian carbon isotope record of organic material (δ13Corg) from northern Germany. The dataset records the Weissert Event CIE in the late Valanginian. At both locations biometric analyses reveal size decreases of B. constans coccoliths during the Weissert Event. A size reduction of ~1 μm has been observed for northern Germany (37°N) and of 0.4 μm for the North Atlantic (23°N). The different magnitudes of size decrease possibly reflect the different paleogeographic positions of the sites and the paleoceanographic response to the late Valanginian climate variations. Proxy data support a link between a humid climate with increased weathering, and the observed coccolith size variations. This impacted nannoplankton communities either through the availability of light or by eutrophication and, possibly, nutrient limitation. In addition to increased terrigenous input, emissions of the Parana-Etendeka volcanism possibly acted fertilizing. Furthermore, widely dispersed volcanic emissions possibly caused increased concentrations of toxic trace metals in the surface ocean which affected marine primary producers.

Coccolithus pelagicus subsp. braarudii morphological plasticity as a response to variations in the upwelling regime of the west coast of Iberia

Taxonomy of fossil coccolithophores is based primarily on the heterococcolith life cycle stagemorphology and, at species level, on fine variations in size and shape. Coccolith size is relevant on routine identification of certain calcareous nannofossil species, but morphometry can also be a tool to study their morphological plasticity. Here we used a new morphometric statistical method to reassess Coccolithus pelagicus s. l. data from the Holocene of the west coast of Iberia. Our results suggest that both C. pelagicus subsp. braarudii and C. pelagicus subsp. pelagicus present morphological plasticity in response to (paleo)environmental changes, mainly variations in the upwelling regime in the west coast of Portugal and (paleo)oceanographic conditions in the North Atlantic linked to glacial periods of the Holocene.

Reconstruction of Miocene to Pleistocene sea‐surface conditions in the eastern Indian Ocean on the basis of calcareous nannofossil assemblages from ODP Hole 757B

AbstractWe studied the calcareous nannofossil assemblages from Ocean Drilling Program Hole 757B in the eastern Indian Ocean to reconstruct the sea‐surface conditions, especially changes in thermocline/nutricline depth, from the middle Miocene to the Pleistocene. Nutrient levels at the sea surface were reconstructed from the nannofossil accumulation rate, abundance of Discoaster, and coccolith size variations in Reticulofenestra. The sea‐surface conditions were likely to have been oligotrophic with a deep thermocline/nutricline during 11.8–7.5 and 6.7–6.2 Ma. The change from oligotrophic to eutrophic conditions at 7.5–6.7 Ma was caused by an influx of nutrient‐rich terrestrial material or upwelling of nutrient‐rich water induced by intensified southeast trade winds. This conclusion is supported by a marked increase in the magnetic susceptibility of the sediment that is coincident with the change from oligotrophic to eutrophic assemblages. The diminished abundance of Discoaster after ~ 3.0 Ma indicates shallowing of the thermocline/nutricline after that time, which confirms the reduced Indonesian Throughflow resulting from tectonic restriction and intensified cooling of the Benguela upwelling system. Decreases in the modal coccolith size of Reticulofenestra, indicating eutrophic conditions, are observed in three horizons. Two of these events (at 7.5 and 5.0 Ma) correspond to biogenic blooms in the Indian Ocean at 7.6 Ma and 6.0–5.0 Ma, respectively.

Low‐Latitude Calcareous Nannofossil Response in the Indo‐Pacific Warm Pool Across the Eocene‐Oligocene Transition of Java, Indonesia

AbstractWe reconstruct the calcareous nannofossil response to the Eocene‐Oligocene Transition (EOT)—the most significant climate transition of the Cenozoic—in the Indo‐Pacific Warm Pool. Data from south central Java consist of species relative abundance counts of well‐preserved nannofossil assemblages. From the late middle Eocene to early Oligocene species biodiversity declined, with the most rapid species loss occurring across the latest Eocene rosette‐shaped discoaster extinction event (DEE; ~34.44–34.77 Ma). A decline in abundance of oligotrophic indicator taxa across the DEE indicates increased nutrient supply to the tropical surface ocean in the early stages of the EOT. The mean lith size of reticulofenestrids also increases across the DEE driven by a marked reduction in the abundance of small Reticulofenestra morphotypes (<3.5 μm). There is no preferential loss of larger Reticulofenestra cell sizes (coccoliths > 8 μm) across the EOT, indicating that coccolith size was apparently not limited by atmospheric CO2 concentrations at this time. Overall, the main phase of tropical phytoplankton ecosystem change preceded the interval of rapid Antarctic ice sheet growth and is closely associated with the biotic perturbations that define the start of the EOT. This suggests that enhancement of Southern Ocean controls on tropical ocean biogeochemistry and nutrient pathways may have played a role in triggering the transition to an icehouse climate state.

The colonization of the oceans by calcifying pelagic algae

Abstract. The rise of calcareous nannoplankton in Mesozoic oceans has deeply impacted ocean chemistry and contributed to shaping modern oceans. Nevertheless, the calcareous nannoplankton colonization of past marine environments remains poorly understood. Based on an extensive compilation of published and unpublished data, we show that their accumulation rates in sediments increased from the Early Jurassic (∼200 Ma) to the Early Cretaceous (∼120 Ma), although these algae diversified up to the end of the Mesozoic (66 Ma). After the middle Eocene (∼45 Ma), a decoupling occurred between accumulation rates, diversity and coccolith size. The time series analyzed points toward a three-phase evolutionary dynamic. An invasion phase of the open-ocean realms was followed by a specialization phase occurring along with taxonomic diversification, ended by an establishment phase where a few small-sized species dominated. The current hegemony of calcareous nannoplankton in the world ocean results from a long-term and complex evolutionary history shaped by ecological interactions and abiotic forcing.

Size patterns of the coccolith Watznaueria barnesiae in the lower Cretaceous: Biotic versus abiotic forcing

Abstract Body size is a major trait allowing the assessment of the respective control of evolution vs. (palaeo)environment on unicellular micro-organisms. Size patterns can be retraced for fossilisable micro-organisms, especially for coccolithophores, over geological time spans. Here, we present coccolith size patterns of Watznaueria barnesiae, which was a dominant and ubiquitous taxon in the Mesozoic. Measurements were conducted at two sites spanning the Upper Berriasian to Lower Hauterivian, namely the Vergol-La Charce section in the epicontinental Vocontian Basin, and the ODP Leg 185 Hole 1149B located in the Nadezhda Basin, in the Pacific Ocean. The obtained results were treated using multivariate analysis in order to detect significant trends of the measured parameters. Size values of coccoliths from both sites are remarkably similar and display comparable patterns, namely a statistically significant increase in the Lower Valanginian until maximum sizes recorded in the aftermath of the Weissert event (Upper Valanginian). Although W. barnesiae coccolith-size fluctuations, in the short term, might be related to environmental changes, the observed long-term size patterns are interpreted as the result of directional selection, in which a single phenotype is favored through time. Comparison with data available in the literature shows a return to a smaller size after the Turonian.

X-ray nanotomography of coccolithophores reveals that coccolith mass and segment number correlate with grid size

Coccolithophores of the Noëlaerhabdaceae family are covered by imbricated coccoliths, each composed of multiple calcite crystals radially distributed around the periphery of a grid. The factors that determine coccolith size remain obscure. Here, we used synchrotron-based three-dimensional Coherent X-ray Diffraction Imaging to study coccoliths of 7 species of Gephyrocapsa, Emiliania and Reticulofenestra with a resolution close to 30 nm. Segmentation of 45 coccoliths revealed remarkable size, mass and segment number variations, even within single coccospheres. In particular, we observed that coccolith mass correlates with grid perimeter which scales linearly with crystal number. Our results indirectly support the idea that coccolith mass is determined in the coccolith vesicle by the size of the organic base plate scale (OBPS) around which R-unit nucleation occurs every 110–120 nm. The curvation of coccoliths allows inference of a positive correlation between cell nucleus, OBPS and coccolith sizes.