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The effects of paleoenvironmental changes on nannoplankton biocalcification: mid-Cretaceous size variations of Biscutum constans
No full textThe investigation of the geological record allows to decipher the response of organisms to paleoenvironmental changes occurring at a time scale larger than human observation. One open issue regards the effects of stressing factors on coccolithophore algae calcification process. These planktonic organisms are important primary producers and largely contribute to the organic and inorganic carbon cycle. In the last decades, evidence for a direct response of coccolithophores to stressing factors were provided by experiments on living forms and by studies of extreme Cretaceous events, such as the early Aptian Oceanic Anoxic Event (OAE) 1a and latest Cenomanian OAE 2. These OAEs were marked by altered carbon cycle, trace elements anomalies, rapid and intense warming and peaks in surface water fertility. The studies evidenced fluctuations in the mean size of some selected nannofossil species, among which, Biscutum constans resulted to be the most sensitive. B. constans reached the smallest mean size under the climax of the OAEs coinciding with super greenhouse climate, excess CO2, accelerated nutrient recycling and trace metals peaks. In order to better understand the role of these stressing factor/s on B. constans calcification, we decided to investigate B. constans size variations through a longer time interval (ca. 27 Myrs) spanning the Aptian to the Cenomanian. This time interval includes either periods of stability and episodes of global environmental perturbations such as OAE 1a, OAE 1b, OAE 1d, the Mid-Cenomanian Event and OAE 2. Prior to this study, it was unknown if B. constans was affected by size changes during interludes of “stable” paleoenvironmental conditions. We focused on the Umbria-Marche Basin (central Italy) by investigating the Piobbico core and the Monte Petrano section which are stratigraphically well constrained and a complete characterization of paleotemperature and paleofertility is available. By performing morphometrics analyses we intended to better understand which factor (or combination of factors) was directly altering the biocalcification process in this species. The results revealed indeed changes in the mean size of B. constans. A relatively prolonged interval of smaller specimens was detected after OAE 1a. A recovery in B. constans average size was identified in the Albian although the largest specimens were found in the middle Albian. A relative decrease in size is detected just prior to OAE 1d. The subsequent main shift coincides with OAE 2 marked by dwarf specimens. Statistical analyses were performed to detect any possible dependence from temperature or nutrient variations suggesting no direct connection between these parameters and size, whilst trace elements peaks and phases of most intense volcanism resulted to correlate with minimum B. constans size
Living in a crazy ocean: calcareous nannoplankton response to Oceanic Anoxic Event 2 and comparison with living coccolithophores algae
No full textCoccolithophores algae are phytoplanktonic organisms that play a fundamental role in the ocean food web and in the global carbon cycle. They produced some tiny calcium carbonate plate, coccoliths, that surround the cell and therefore they actively contribute to the marine carbonate cycling. Nowadays the ocean is facing an alteration of seawater carbonate chemistry due to anthropogenic release of carbon dioxide. Laboratory experiments suggest that coccolithophores are very sensitive to seawater pH, dissolved CO2, temperature and nutrient concentration all of which are impacted by human CO2 emissions. Some species might have the ability to adapt but coccolithophores responses to environmental stress in culture experiments are not straightforward. Furthermore studies on large volcanic eruptions, demonstrate that metals contained in volcanic ashes, such as Cd, Cu, Pb and Zn, can have both fertilizing and toxic effects on marine phytoplankton species (diatoms and coccolithophores algae) which will ultimately lead to changes in species composition with implications for community productivity. The geological record is imprinted with numerous examples of natural global perturbations of the global carbon cycle and climate changes some of which are characterized by large-scale release of natural CO2 and ocean acidification. The selected case history we analyzed is the Cenomanian-Turonian Oceanic Anoxic Event (OAE2), during which the formation of the Carribean Plateau Large Igneous Provinces (LIP), induced the degassing of large amounts of CO2 and associated environmental consequences of warming, lower oxygen minimum zone and possibly ocean acidification. Here we present biometric data of 4 coccolith species mostly dominant in the Cenomanian and Turonian. The morphometric analyses showed a species-specific response to the perturbation: W. barnesiae, the most cosmopolitan species, doesn’t show any significant variation in size through all the event. On the contrary the other eutrophic species, respectively B. constans, D. rotatorius and Z. erectus, display a strong reduction in sizes. Dwarfism affected both length and width and mimics the p CO2 fluctuations: a progressive increase in size take place when pCO2 start to decrease while dwarf coccolith are coeval with a strong increase in CO2. Notably the maximum reduction in size occurs with the highest CO2 concentration, very high sea surface temperature and a remarkable biolimiting metal peak. However there is difficulty in unequivocally attributing deformities and size reduction during OAE2 to a single parameter, given the covariation of environmental changes. To better understand which parameter/s influence coccolith formation the most, we performed several culture experiments on four species of living coccolithophores algae taking in account toxic metal content, carbonate chemistry, nutrient content and light intensities. Here we present some preliminary results
Changes in coccolith sizes through Oceanic Anoxic Event 2: a proxy of ocean acidification?
No full textThe latest Cenomanian was a time of global paleoenvironmental changes: the normal pelagic sedimentation was abruptly interrupted by an episode of ocean-wide anoxia, named Oceanic Anoxic Event 2 (OAE2). The associated C isotopic positive excursion, documented in marine carbonate and organic matter as well as in terrestrial records, is caused by a major perturbation of the carbon budget, generally related to enhanced productivity and burial of organic matter. OAE2 was perhaps triggered by the extensive submarine volcanism during the formation of the Caribbean Plateau that acted as a natural source of CO2. The environmental perturbation recorded during OAE2 can be synthesized as follows:
1. The onset of OAE2 correlates with a major volcanic episode, causing global warming, a rise in CO2 and an increase in metals.
2. A weathering spike is followed by a cooling episode and CO2 drop in the interval of C isotopic peak A, under persisting volcanic emissions.
3. At C isotopic peak B, a major volcanic peak is associated with an increase in CO2.
4. The end of OAE2 is marked by the decrease of C isotopic values after peak C with a return to normal metal concentrations, although temperatures remain relative warm.
Here we present morphometric data of four nannofossil species in the OAE2 interval from different areas. The major result is a change to tiny-dwarf coccoliths, although of different amplitude, at the OAE2 onset. The inferred warmer conditions, higher fertility and excess CO2 suggest a potential role on nannoplankton calcification. Coccolith sizes return close to normal values around the C isotopic peak A, where minimum pCO2 and a cooling phase are reconstructed. A major reduction in size is recorded around C isotopic peak B, and coeval to an increase in volcanogenic CO2 based on metal spikes. The end of the C isotopic excursion doesn’t correlate with a return to coccolith normal sizes, suggesting a protraction of anomalous conditions immediately after OAE2 termination. Our results were compared to the morphometric data collected through OAE1a. During OAE1a dwarfism and malformation are restricted to the C isotopic negative shift and most profound paleoenvironmental perturbations. In the OAE2 interval dwarfism is most pronounced in the last part of the C isotopic anomaly, and coccolith malformation is negligible. Based on available data, climatic and fertility changes per se appear to be of marginal relevance to coccolith morphologies. In particular, the nannofossil record of paleo-fertility during OAE2 is not straightforward, since increases or decreases in abundance were documented in different settings.
Similarly to OAE1a, we speculate that during OAE2, excess CO2 played a fundamental role in nannoplankton calcification, and that coccolith dwarfism might be a proxy of ocean acidification. In the analyzed sections, during OAE2, dwarf coccoliths occur at levels with metal peaks, perhaps also-alternatively recording a species-specific intolerance to metal toxicity
Calcareous Nannofossil adaptations and life-strategies during Oceanic Anoxic Event 2: go dwarf, don't die
No full textThe environmental changes associated with anthropogenic CO2 emissions will greatly influence the ecosystem functioning of future oceans: the oceans take up anthropogenic-atmospheric CO2 with the consequence of a strong reduction in carbonate ion concentrations leading to ocean acidification. In this scenario, international research is facing the considerable challenge to predict if and how the oceans will be able to respond/adapt to the rapid environmental change and particularly to the acidification of surface waters.
The Earth has already experienced extreme environmental change and the geological record shows that one of the most pronounced perturbations of the Cretaceous is the so-called Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2). OAE2 induced major changes in the marine environment and severe disturbances of the biosphere. The forcing functions behind such phenomena are still problematic: it has been hypothesis that abnormal volcanism triggered global warming and enhanced primary productivity with consequent oxygen consumption at global scale. OAE2 is thought to be linked to the emplacement of the Caribbean Plateau that acted as a natural source of CO2. The environmental perturbation recorded during OAE2 models with 4 main steps:
1. OAE2 onset correlates with a major volcanic episode, causing global warming and a rise in CO2.
2. Then a weathering spike, followed by a cooling episode and CO2 drop in the interval of δ13C peak A, under persisting volcanic emissions.
3. At δ13C peak B, a major volcanic activity is associated with an increase in pCO2.
4. The end of OAE2 is marked by the decrease of carbon isotopic values after δ13C peak C although temperatures remain relative warm.
Calcareous nannoplankton, as primary producers, influence the marine carbon cycle and, indirectly, the climate system: they contribute to biological processes, such as photosynthesis and to biomineralization; furthermore nannoplankton are sensitive to chemical - physical - trophic parameters providing the potential for monitoring environmental variations.
Here we present morphometric data of 4 nannofossil species during OAE2 interval from sections along an ideal S-N transect through Sicily, France and England. The major result is a size-shift to tiny-dwarf coccoliths of different amplitudes. There is a species - specific response in all the sections: B. constans displays the most pronounced reduction in size, D. rotatorius records a well expressed reduction in size too as opposed to Z. erectus which diminishes in size to a lesser extent. In all the sections, the mean size of W. barnesiae is within the holotype and normal range size. Furthermore, we observe that coccolith size variations follow the main steps of OAE2 perturbation: coccolith sizes return close to normal values around δ13C peak A, where minimum pCO2 and a cooling phase are reconstructed. On the contrary, a major reduction in size is recorded around δ13C peak B, coeval to an increase in CO2. On the other hand the end of the C isotopic excursion doesn’t correlate with a return to coccolith normal sizes, suggesting a protraction of anomalous conditions.
The record of paleofertility during OAE2 is not straightforward: mid-latitude localities seem to have been affected by a decrease in nutrient availability, whereas, in the Atlantic tropical waters, the nutrient content increased. Comparing these data with our morphometric results, we notice a lack of repetitive pattern: specifically, intervals characterized by higher nutrient contents do not show dwarf coccoliths. It seems therefore unlikely that nutrients content have controlled coccoliths dwarfism during OAE2.
We speculate that during OAE2, excess CO2 played a fundamental role in nannoplankton calcification and that coccolith dwarfism might be useful to reconstruct ocean CO2 concentration. In the analyzed sections dwarf coccoliths occur at levels with metal peaks, perhaps recording a species-specific intolerance to metal toxicity
Morphological changes of calcareous nannofossils during Oceanic Anoxic Event 2
No full textThe Cenomanian - Turonian Oceanic Anoxic Event 2 (OAE2) is one of the most pronounced perturbations of the Cretaceous that induced major changes in the marine environment and severe disturbances of the biosphere. It is thought to be linked to extensive volcanism and particularly to the emplacement of a Large Igneous Province (LIP) that acted as a natural source of excess CO2. Particularly, OAE2 seems to be strictly connected with the formation of the Caribbean Plateau, which triggered a chain of events leading to enhance ocean fertility, accelerated organic carbon burial, widespread ocean anoxia, and acidification.
Calcareous nannoplankton is sensitive to chemical - physical - trophic changes and must have reacted to the OAE2 extreme environmental conditions. Indeed, nannofossil data show that they experienced a turnover, a general decrease in species richness and locally a decline in abundance of the fertility indicator species.
In this study we present new results of a detailed morphometric analysis of 4 nannofossil species during OAE2 from two different areas: the Mediterranean area (Novara di Sicilia section, North-eastern Sicily) and the Sussex area (Eastbourne section, United Kingdom). These sections have been chosen based on integrated high-resolution stratigraphy: the C isotopic anomaly is taken as primary tool of dating and correlation of OAE2 events. Furthermore the Novara di Sicilia section represents only the lower portion of OAE2, whereas the Eatsbourne section, covering the entire OAE2 interval, provides the opportunity to characterize changes before, during and after the paleoenvironmental perturbation.
In analogy to the work of (ERBA et alii, 2010) on the Early Aptian OAE1a, biometric analyses were performed at high resolution on B. constans, D. rotatorius, Z. erectus and W. barnesiae in order to evaluate the influence of paleoenvironment on coccolith size and morphology. Size measurements of calcareous nannofossils were carried out on smear slides with a light microscope: in each sample 30 specimens of B. constans, D. rotatorius, Z. erectus and 50 specimens of W. barnesiae have been measured by using the Qcapture Pro suite.
Morphometric analyses highlight that:
1. In both sections dwarf coccoliths are present even before the OAE2 onset.
2. There is a species - specific response in both section: B. constans displays the most pronounced reduction in size, D. rotatorius records a well express reduction in size too as opposed to Z. erectus which diminishes in size to a lesser extend.
3. In both sections, the mean size of W. barnesiae is within the holotype and normal range size
4. In the Novara di Sicilia section dwarf coccoliths record the strongest reduction in size relative to Eastbourne samples.
5. Pronounced dwarfism is observed in discrete intervals before and within the carbon isotope excursion.
6. During OAE2 coccoliths return to normal sizes around the carbon isotopic peak “A”.
Comparison of morphometric analyses through OAE1a (ERBA et alii, 2010) and OAE2 shows analogies and differences. During OAE1a dwarfism and malformation are restricted to the onset of the carbon isotopic anomaly, in the core of the negative shift coeval with most profound paleoenvironmental perturbations. In the OAE2 interval dwarfism is most pronounced in the last part of the C isotopic anomaly, and coccolith deformation is negligible. During the two major Cretaceous OAEs, the ocean experienced extreme CO2 concentrations, acidification and an increase in fertility. Coccolith dwarfism suggests a link between nannoplankton calcification and high pCO2, although enhanced fertility and/or presence of toxic metals in seawater might have been instrumental as well. We speculate that during OAE1a and OAE2 excess CO2 played a fundamental role in ruling nannoplankton calcification efficiency, and that coccolith dwarfism might be a proxy of ocean acidification. Different patterns and degree of dwarfism and malformation during OAE1a and OAE2 suggest unequal volcanic CO2 emissions (rates, pulses, amount). We conclude that analogous causes (LIPs emplacement) have induced partially similar response at different times.
REFERENCE
ERBA E., BOTTINI C., WEISSERT H.J. & KELLER C.E., L., (2010) – Calcareous nannoplankton response to surface- water acidification around Oceanic Anoxic Evenet 1. Science, 329, 428-432
Environmental control on coccolithophore morphology : do modern species yield information that is transferable to the geological past?
Full text linkIt is generally assumed that calcareous nannofossils conserve palaeoenvironmental information from the time of their formation. Changes in coccolith morphology can result from physiological responses to environmental drivers. Temperature, salinity, nutrient concentration, light and carbonate chemistry are among the environmental drivers that impact extant coccolithophores and may alter coccolith size, as well as coccosphere size and morphology. Many palaeoreconstruction studies have assessed the biological responses of living coccolithophore species to environmental drivers with the expectation that it is possible to use this information for calibrating the biomineralisation responses of ancient coccolithophores. However, there is a large uncertainty concerning whether the morphological responses of living coccolithophores to environmental changes are similar to the morphological responses of fossil species, when you consider the fact that millions of years of evolutionary adaptation lie between the extant species and their fossilised ancestors. In order to test this caveat, we examined four extant species (Emiliania huxleyi, Gephyrocapsa oceanica , Coccolithus pelagicus subsp. braarudii and Pleurochrysis carterae), which have been evolutionarily distinct for millions of years. We cultured them under changing environmental conditions in order to evaluate any changes in coccolith morphology. Our underlying hypothesis was that if the species showed a uniform reaction to any of the tested environmental drivers, then this would suggests that the same response may well occur over geological timescales, and that coccolith morphological changes could serve as a palaeo-proxy for that particular driver. Our experiments demonstrated that the four species had no common response to changing light intensity, Mg/Ca, nutrient content or temperature with respect to coccolith size. These results revealed the difficulties in using coccolith size as a proxy for environmental drivers. One exception was an increase in malformations when coccolithophores were grown under excess CO2 , and these data provided evidence that this response variable can be used as a palaeo-proxy for episodes of acute carbonate chemistry perturbations
Emiliania huxleyi sensitivity to ecological changes: a new methodological approach to ascertain if and which environmental parameter influences coccolith sizes and shapes
No full textCoccolithophores are a group of unicellular marine phytoplankton primary producers of biogenic calcite in the open ocean. During their diploid life-cycle stage, coccolithophores produce calcite plates called heterococcoliths. These circular to elliptical coccoliths have very ornate structures and form an exoskeleton, called coccosphere, generally composed of a single layer of plates. Coccolithophore algae produce coccoliths one at a time, with an intracellular growth process. This begins with nucleation of a proto-coccolith ring of simple crystals and continues by upward and outward growth of these crystals into a complex unit to form complete coccoliths. Coccoliths are then extruded to the cell surface and continue to be generated until a complete coccosphere covering is created. The cell recurrently produces incomplete and malformed coccoliths: incomplete coccoliths occur if the growth process is arrested due to premature extrusion of the coccolith from the cell, while malformation is due to “irregular coccolith formation as a result of departure from the normal growth process” (Young and Westbroek 1991), implying the malfunction of the coccolith-shaping machinery per se. Malformations are difficult to evaluate and most studies use a qualitative and subjective approach with the identification of arbitrary categories (e.g. normal, very malformed, malformed and incomplete coccoliths).
The use of morphometrics has the potential to significantly improve the knowledge of shape and size variations in coccolithophore taxa and the ability to describe their evolutionary history and response to ecological changes. Here, we present a new morphometrical method to quantitatively characterize coccolith sizes and shapes and discriminate normal versus malformed specimens and the degree of malformations. This methodology was applied to coccoliths of Emiliania huxleyi grown under different ecological conditions. The obtained results evidenced that E. huxleyi is very sensitive to chemical alterations of seawater and specifically, that introduction of toxic trace metals and increased CO2 concentrations might have the potential to disturb the calcification process causing an increase in the number of aberrant coccoliths, an alteration of the calcite content per coccolith and a general decrease in the cellular calcification rate.
Young, J.R. & Westbroek, P. (1991): Genotypic variation in the coccolithophorid species Emiliania huxleyi. Marine Micropaleontology, 18(1-2), 5-23
Life in extreme Oceans: Calcareous Nannoplankton adaptations and strategies during Oceanic Anoxic Event 2
No full textAt present about one third of the carbon dioxide (CO2) released in the atmosphere from fossil fuel burning is absorbed by the oceans. The invasion of anthropogenic CO2 into the oceans increases seawater acidity and decreases carbonate ion concentration and carbonate saturation. This dramatic change of the carbonate system can have a huge impact on the marine ecosystem and, in particular, can seriously impair marine calcifiers (e.g. corals, foraminifera, coccolithophores). Coccolithophorid algae are sensitive to ocean acidification and most studies show a strong decline in growth and/or reduction in calcification rate and/or increase in coccolith malformation with increasing CO2 concentration.
During the Cretaceous the Earth has already experienced extreme environmental change: the construction of Large Igneous Provinces (LIPs), forming gigantic oceanic plateaus, affected the ecosystem at global scale. LIP volcanism probably triggered global warming and enhanced primary productivity with consequent oxygen consumption and burial of massive amounts of organic matter at global scale: these episodes are known as Oceanic Anoxic Events (OAEs). We investigated calcareous nannoplankton morphological variation through one of the most interesting anoxic event of the Cretaceous, the latest Cenomanian OAE2. This episode of global anoxia is associated with the formation of the Caribbean Plateau (CP) that triggered high release of excess CO2, causing a general global warming. Recent studies of the OAE2 episode have pointed out climatic variability and fluctuations in the atmCO2 concentration. In fact, a cooling episode and CO2 drop in the early phase of OAE2 is connected with a weathering spike, followed by a new increase in CO2 and warming.
We investigated OAE2 sections from Sicily, Southern France, England and Colorado. The major result that we observed is a change to tiny-dwarf coccoliths, although of different amplitude, through OAE2: calcareous nannofossil size variations follow the pCO2 fluctuations and record an increase in size when pCO2 start to decrease while dwarf coccoliths are coeval with a strong increase in CO2. The record of paleofertility during OAE2 is not straightforward: mid-latitude localities seem to have been affected by a decrease in nutrient availability, whereas, in the Atlantic tropical waters, the nutrient content increased. Comparing these data with our morphometric results, we notice a lack of repetitive pattern. It seems therefore unlikely that nutrient content has controlled coccolith dwarfism during OAE2.
Another selected case history is the Aptian OAE1a that is associated with the submarine construction of the Ontong Java Plateau (OJP) and triggered a disruption of the oceanic carbonate system. At the onset of the carbon isotopic anomaly, during the most profound paleoenvironmental perturbation, excess CO2 induced ocean acidification with the consequence of a temporary failure of the rock-forming nannoconids and formation of dwarf and malformed coccoliths.
The inferred warmer conditions and excess CO2, during intervals of LIPs volcanism, suggest a potential role on nannoplankton calcification. Hydrothermal plumes during construction of both OJP and CP introduced biolimiting metals that fertilized the global ocean. However, some toxic metals might have disturbed the functioning of some intolerant coccolithophorid species and perhaps hampered calcification of some species.
Different patterns and degree of dwarfism and malformation during OAE1a and OAE2 suggest unequal volcanic CO2 emissions (rates, pulses, amount) and/or variable combinations of CO2, climate and fertility: analogous causes (LIPs emplacement) have therefore induced only partially similar response at different times
Calcareous nannofossils as tracers of paleoCO2
Full text linkThe Cretaceous has been characterized by intervals of super-greenhouse climate and profound environmental perturbations, including the early Aptian Oceanic Anoxic Event (OAE 1) a and the
latest Cenomanian OAE 2 being episodes of widespread organic matter burial in oxygen-depleted oceans. The OAE 1a and OAE 2 are thought to be related to the emplacement of the Ontong Java
Plateau and the Caribbean Plateau, respectively. The volcanic acrivity of these Large Igneous Provinces (LIPs) introduced in the atmosphere a large amount of CO2 with consequent impact on biota, climate and ocean chemistry. The perturbations of the C cycle are traced in the C isotopic record which shows a negative shift at the beginning of OAE 1a followed by a positive excursion, and a large positive anomaly marking the OAE 2. In this study, we intended to detected if and how these changes in pCO2 affected the biogenic carbonate production of calcareous nannoplankton. Coccolithophore algae are in fact extremely sensitive to changes in physical and chemical conditions of the oceans and laboratory experiments on living forms indicate that coccolith type, abundance and degree of mineralization depend on chemical-physical-trophic conditions of water as well as on pCO2. Our data revealed the presence of dwarf/malformed coccoliths during OAE 1a and OAE 2 interpreted to be the response of some coccolithophore species to increased surface-water acidification, thus providing indication of intervals of excess CO2. Following these observations, that suggest that past calcareous nannoplankton probably responded similarly to extant coccolithophores under fluctuating CO2, we reconstructed nannofossil calcite paleofluxes during the Aptian and used them to calculate paleoCO2. Calcite production resulted to be severly affected during OAE 1a, when the highest CO2 concetrations of the Aptian were reached under the most intense phase of Ontong Java. During the middle-late Aptian, paleofluxes were then characterized by fluctuations, suggestive for variable paleoCO2 depending on the interplay between CO2 emissions and uptake
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