Constraints on the vital effect in coccolithophore and dinoflagellate calcite by oxygen isotopic modification of seawater

Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 141 (2014): 612-627, doi:10.1016/j.gca.2014.05.002.In this study, we show that there are independent controls of 18O/16O and 13C/12C fractionation in coccolithophore and dinoflagellate calcite due to the contrasting kinetics of each isotope system. We demonstrate that the direction and magnitude of the oxygen isotope fractionation with respect to equilibrium is related to the balance between calcification rate and the replenishment of the internal pool of dissolved inorganic carbon (DIC). As such, in fast growing cells, such as those of Emiliania huxleyi and Gephyrocapsa oceanica (forming the so-called “heavy group”), calcification of the internal carbon pool occurs faster than complete isotopic re-adjustment of the internal DIC pool with H2O molecules. Hence, coccoliths reflect the heavy oxygen isotope signature of the CO2 overprinting the whole DIC pool. Conversely, in large and slow growing cells, such as Coccolithus pelagicus ssp. braarudii, complete re-equilibration is achieved due to limited influx of CO2 leading to coccoliths that are precipitated in conditions close to isotopic equilibrium (“equilibrium group”). Species exhibiting the most negative oxygen isotope composition, such as Calcidiscus leptoporus (“light group”), precipitate coccolith under increased pH in the coccolith vesicle, as previously documented by the “carbonate ion effect”. We suggest that, for the carbon isotope system, any observed deviation from isotopic equilibrium is only “apparent”, as the carbon isotopic composition in coccolith calcite is controlled by a Rayleigh fractionation originating from preferential incorporation of 12C into organic matter. Therefore, species with low PIC/POC ratios as E. huxleyi and G. oceanica are shifted towards positive carbon isotope values as a result of predominant carbon fixation into the organic matter. By contrast, cells with higher PIC/POC as C. braarudii and C. leptoporus maintain, to some extent, the original negative isotopic composition of the CO2. The calcareous dinoflagellate Thoracosphaera heimii exhibits different behaviour for both isotopic systems, in particular with respect to its very negative carbon isotope composition, owing to coeval intra and extracellular biomineralisation in this group. In this study, we also investigate the sensitivity of 18O/16O fractionation to varying ambient oxygen isotope composition of the medium for inorganic, coccolithophore, and dinoflagellate calcite precipitated under controlled laboratory conditions. The varying responses of different taxa to increased oxygen isotope composition of the growth medium may point to a potential bias in sea surface temperature reconstructions that are based on the oxygen isotopic compositions of sedimentary calcite, especially during times of changing seawater oxygen isotopic composition. Overall, this study represent an important step towards establishing a mechanistic understanding of the “vital effect” in coccolith and dinoflagellate calcite, and provides valuable information for interpreting the geochemistry of the calcareous nannofossils in the sedimentary record, at both monospecific and interspecies levels.MH is grateful to the Natural Environment Research Council (NERC) for funding through Postdoctoral Fellowship (NE/H015523/1). TJH is supported by the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution, with funding provided by the Doherty Foundation. REMR was supported through European Research Council (ERC) grant SP2-GA-2008-200915

Seawater carbonate chemistry and mass and fine-scale morphological changes of Coccolith Gephyrocapsa oceanica

Coccolithophores have been extensively studied to understand the environmental control on calcification in a key biological group influencing the alkalinity of seawater. Previous studies have established that bulk calcification scales with cell division rates under a wide range of pH conditions. Yet, the fine scale ultrastructural changes of the coccoliths and therefore the pH‐sensitive underlying mechanisms altering biomineralization of the coccoliths remain largely under‐constrained. Using circularly polarized light and high resolution microscopy, we have generated mass estimates of cultured Gephyrocapsa oceanica coccoliths grown in medium with pH values ranging from 7.4 to 9.0. These mass estimates representing a bulk calcification response were related to the morphological changes within the coccoliths. From optimal (pH 8.6) down to pH 7.4 conditions, we have observed that impaired cell growth and lower calcite quota are accompanied by a 35% decrease in mean coccolith mass. The data further show that seawater acidification does not homogenously affect calcification of the coccoliths, as a clear reduction in the breadth of the tube (a structure surrounding the central area of the coccoliths) was detected, whereas all other ultrastructural components were far less impacted. We discuss this specific sensitivity to acidification as the consequence of the altered interaction of the acidic polysaccharides used for biomineralization and ambient concentration of protons released by calcification that substantially modify the growth patterns, the morphology and ultimately the mass of the coccoliths

Mass and Fine-Scale Morphological Changes Induced by Changing Seawater pH in the Coccolith Gephyrocapsa oceanica

International audienceCoccolithophores have been extensively studied to understand the environmental control on calcification in a key biological group influencing the alkalinity of seawater. Previous studies have established that bulk calcification scales with cell division rates under a wide range of pH conditions. Yet, the fine scale ultrastructural changes of the coccoliths and therefore the pH-sensitive underlying mechanisms altering biomineralization of the coccoliths remain largely under-constrained. Using circularly polarized light and high-resolution microscopy, we have generated mass estimates of cultured Gephyrocapsa oceanica coccoliths grown in media with pH values ranging from 7.4 to 9.0. These mass estimates representing a bulk calcification response were related to the morphological changes within the coccoliths. From optimal (pH 8.6) down to pH 7.4 conditions, we have observed that impaired cell growth and lower calcite quota are accompanied by a 35% decrease in mean coccolith mass. The data further show that seawater acidification does not homogenously affect calcification of the coccoliths, as a clear decrease of the breadth of the tube (a structure surrounding the central area of the coccoliths) was detected, whereas all other ultrastructural components were far less impacted. We discuss this specific sensitivity to acidification as the possible consequence of the altered interaction of the acidic polysaccharides used for biomineralization and ambient concentration of protons released by calcification that substantially modify the growth patterns, the morphology, and ultimately the mass of the coccoliths

Equatorial heat accumulation as a long-term trigger of permanent Antarctic ice sheets during the Cenozoic

International audienceThe long-term cooling trend of the Cenozoic is punctuated by shorter-term climatic events, such as the inception of permanent ice sheets on Antarctica at the Eocene−Oligocene Transition (∼33.7 Ma). Taking advantage of the excellent state of preservation of coccolith calcite in equatorial Atlantic deep-sea cores, we unveil progressive tropical warming in the Atlantic Ocean initiated 4 million years prior to Antarctic glaciation. Warming preceding glaciation may appear counterintuitive, but we argue that this long-term climatic precursor to the EOT reinforced cooling of austral high latitudes via the redistribution of heat at the surface of the oceans. We discuss this new prominent paleoceanographic and climatic feature in the context of overarching pCO2 decline and the establishment of an Antarctic circumpolar current

Reconstruction des pCO2 atmosphériques quaternaires par la géochimie des coccolithes

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Black shale deposition during Toarcian super-greenhouse driven by sea level

One of the most elusive aspects of the Toarcian oceanic anoxic event (T-OAE) is the paradox between carbon isotopes that indicate intense global primary productivity and organic carbon burial at a global scale, and the delayed expression of anoxia in Europe. During the earliest Toarcian, no black shales were deposited in the European epicontinental seaways, and most organic carbon enrichment of the sediments postdated the end of the overarching positive trend in the carbon isotopes that characterises the T-OAE. In the present study, we have attempted to establish a sequence stratigraphic framework for Early Toarcian deposits recovered from a core drilled in the Paris Basin using a combination of mineralogical (quartz and clay relative abundance) and geochemical (Si, Zr, Ti and Al) measurements. Combined with the evolution in redox sensitive elements (Fe, V and Mo), the data suggest that expression of anoxia was hampered in European epicontinental seas during most of the T-OAE (defined by the positive carbon isotope trend) due to insufficient water depth that prevented stratification of the water column. Only the first stratigraphic occurrence of black shales in Europe corresponds to the "global" event. This interval is characterised by >10% Total Organic Carbon (TOC) content that contains relatively low concentration of molybdenum compared to subsequent black shale horizons. Additionally, this first black shale occurrence is coeval with the record of the major negative Carbon Isotope Excursion (CIE), likely corresponding to a period of transient greenhouse intensification likely due to massive injection of carbon into the atmosphere–ocean system. As a response to enhanced weathering and riverine run-off, increased fresh water supply to the basin may have promoted the development of full anoxic conditions through haline stratification of the water column. In contrast, post T-OAE black shales during the serpentinum and bifrons Zones were restricted to epicontinental seas (higher Mo to TOC ratios) during a period of relative high sea level, and carbon isotopes returning to pre-T-OAE values. Comparing palaeoredox proxies with the inferred sequence stratigraphy for Sancerre suggests that episodes of short-term organic carbon enrichment were primarily driven by third-order sea level changes. These black shales exhibit remarkably well-expressed higher-frequency cyclicities in the oxygen availability in the water column whose nature has still to be determined through cyclostratigraphic analysis