Fabrice Minoletti

Separation of sedimentary micron-sized particles for palaeoceanography and calcareous nannoplankton biogeochemistry

A protocol is described for separating sub-20μm-sized particles contained in sedimentary rocks into size fractions. Geochemical data from manually isolated foraminifera are commonly used in the interpretation of marine palaeoenvironments; problems associated with the isolation of calcareous nannofossils hampers their geochemical exploitation. However, geochemistry performed on calcareous nannofossil monotaxic assemblages should provide more meaningful data sets than those generated from the highly heterogeneous bulk carbonate. This protocol is based on cascade filtering steps, using polycarbonate membranes with well-calibrated pores. Strong ultrasonic treatment can further be applied to selectively reduce the size of particles for greater enrichment. Obtained residues frequently comprise near-monotaxic nannofossil assemblages. The application of this technique, which can be achieved within less than 2 days, has provided distinct fractions of coccoliths, calcareous dinoflagellate shells and also diagenetic monocrystals. This protocol is designed for application in reconstructing the history of water-column physicochemistry and diagenesis. It also has the potential to provide insights into the biogeochemistry of calcareous nannoplankton, including vital effects.

Mise au point d'un protocole experimental de separation granulometrique d'assemblages de nannofossiles calcaires; applications paleoecologiques et geochimiques

Calcareous nannofossils play an important role in biostratigraphy and paleoecology, though their study is often complicated by their reduced size. The methodology presented herein allows to separate the nannofossil assemblage into granulometrically and taxonomically homogeneous fractions. This method permits :a) the concentration of rare taxa in order to simplify their morphometric study; b) the geochemical analysis (stable isotopes and trace elements) of these fractions. The latter is helpful in many cases, especially when the analysis of major or minor taxa constituent of the assemblage is greatly complicated by the reduced particles dimensions. In this way, it will be possible to bring light to the ecological parameters of these taxa, as calcification temperature and other ecological requirements (salinity, nutrient concentrations).

Calibration of stable isotope composition of Thoracosphaera heimii (dinoflagellate) calcite for reconstructing paleotemperatures in the intermediate photic zone

In this study we investigate the temperature dependence of oxygen isotope ratios preserved in calcite formed by the dinoflagellate Thoracosphaera heimii, focusing primarily on the development of a geological proxy. Geochemical analysis of the calcite shells produced by this species represents a valuable proxy for reconstructing environmental conditions in the intermediate photic zone. Calibration is based on isotopic analysis from culture experiments performed in very dilute batch conditions, as well as from near-monospecific T. heimii assemblages separated from core top sediments. Results are similar for both approaches and indicate that T. heimii shells have oxygen isotope compositions close to equilibrium values predicted for inorganic calcite precipitation. This calibration of the isotopic composition of dinoflagellate calcite indicates that monospecific assemblages of T. heimii can be used to unravel paleotemperatures in the intermediate photic zone by applying isotopic transfer functions for equilibrium calcite. In culture, however, a δ 18 O offset of A1‰ is observed at temperatures <17°C, which falls below the natural temperature range of this species. Culture analyses also reveal a relationship between temperature and carbon isotope composition of calcite. The mechanisms behind this relationship remain to be explored, but their identification may provide a better understanding of carbon isotope systematics from both biogeochemical and geological perspectives. Comparison of the oxygen isotope composition of T. heimii shells with that of shallower dwelling organisms, such as the coccolithophores, represents a valuable proxy for determining temperature gradients within the photic zone and may enable reconstruction of the evolution of the depth of the thermocline.

Morphometry of selected calcareous nannofossils across the Cretaceous-Paleocene boundary at the Bidart (France) and Elles (Tunisia) sections. Comparison with carbon and oxygen stable isotope ratios

A detailed biometric analysis of five species of calcareous nannofossils was carried out on samples from the uppermost Maastrichtian and lowermost Danian of Bidart (Basque Country, SW France) and Elles (central Tunisia). This study reveals the existence of two morphotypes of A. cymbiformis and an abrupt reduction of the five species immediately below the K-T boundary, likely in relation with a stressful environment, which would have developed just before the deposition of the iridium level, followed by a sudden size increase in the basal Danian reflecting the reworking pattern of the Cretaceous species. In addition, the comparison of biometric and isotopic data seems to indicate that the two morphotypes of A. cymbiformis have different ecologies, the large one being favoured in cool waters.

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

Significance The 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. Growth of the first permanent Antarctic ice sheets at the Eocene−Oligocene Transition (EOT), ∼33.7 million years ago, indicates a major climate shift within long-term Cenozoic cooling. The driving mechanisms that set the stage for this glaciation event are not well constrained, however, owing to large uncertainties in temperature reconstructions during the Eocene, especially at lower latitudes. To address this deficiency, we used recent developments in coccolith biogeochemistry to reconstruct equatorial Atlantic sea surface temperature (SST) and atmospheric pCO2 values from pelagic sequences preceding and spanning the EOT. We found significantly more variability in equatorial SSTs than previously reported, with pronounced cooling from the Early to Middle Eocene and subsequent warming during the Late Eocene. Thus, we show that the Antarctic glaciation at the Eocene−Oligocene boundary was preceded by a period of heat accumulation in the low latitudes, likely focused in a progressively contracting South Atlantic gyre, which contributed to cooling high-latitude austral regions. This prominent redistribution of heat corresponds to the emplacement of a strong meridional temperature gradient that typifies icehouse climate conditions. Our equatorial coccolith-derived geochemical record thus highlights an important period of global climatic and oceanic upheaval, which began 4 million years before the EOT and, superimposed on a long-term pCO2 decline, drove the Earth system toward a glacial tipping point in the Cenozoic.

Extreme strontium concentrations reveal specific biomineralization pathways in certain coccolithophores with implications for the Sr/Ca paleoproductivity proxy

The formation of the coccolith biominerals by a group of marine algae (the Coccolithophores) offers fascinating research avenues both from the biological and geological sides. It is surprising how biomineralisation by a key phytoplanktonic group remains underconstrained, yet is influential on ocean alkalinity and responsible for the built up of our paleoclimatic archive over the last 200 Myrs. Here, we report two close relative coccolith taxa exhibiting substantial bioaccumulation of strontium: Scyphosphaera and Pontosphaera grown in the laboratory or retrieved from Pliocene sediments. This strontium enrichment relative to calcium is one order of magnitude greater than reported in other coccoliths of the orders Isochrysidales and Coccolithales, and extends well beyond established controls on Sr/Ca ratios by temperature and growth rate. We discuss this prominent vital effect in relation with possible specific uptake of strontium relative to calcium from the extracellular environment to the coccolith vesicle in coccolithophores excreting very large scale coccoliths. The report of Sr-rich biominerals challenges our current understanding of the cellular acquisition and intracellular trafficking of alkaline earth cations in phytoplanktonic calcifying eukaryotic algae. The presence of Sr-rich coccolith species in the geological record has to be quantitatively considered in future Sr/Ca-based palaeoceanographic reconstruction.