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Mg/Ca-temperature and seawater-test chemistry relationships in the shallow-dwelling large benthic foraminifera <i>Operculina ammonoides</i>
Get PDFThe foraminifera Mg/Ca palaeothermometer contributes significantly to our understanding of palaeoceanic temperature variation. However, since seawater Mg/Ca has undergone large secular variation and the relationship between seawater and test Mg/Ca has not been calibrated in detail for any species with a substantial fossil record, it is only possible to assess relative temperature changes in pre-Pleistocene fossil samples. In order to establish the basis of accurate quantitative Mg/Ca-derived deep-time temperature reconstructions, we have calibrated the relationship between test Mg/Ca, seawater chemistry and temperature in laboratory cultures of the shallow-dwelling large benthic species Operculina ammonoides. Operculina has a fossil range extending back to the early Paleogene and is the nearest living relative of the abundant genus Nummulites. We find a temperature sensitivity of 1.7% °C−1 and a linear relationship between the Mg distribution coefficient and seawater Mg/Ca (Mg/Casw) with m = −1.9 × 10-3, within error of the equivalent slope for inorganic calcite. The higher test Mg/Ca of O. ammonoides compared to inorganic calcite may be explained by an elevated pH of the calcifying fluid, implying that these foraminifera do not modify the Mg/Ca ratio of the seawater from which they calcify, differentiating them in this respect from most other perforate foraminifera. Applying these calibrations to previously published fossil data results in palaeo-Mg/Casw reconstruction consistent with independent proxy evidence. Furthermore, our data enable accurate absolute palaeotemperature reconstructions if Mg/Casw is constrained by another technique (e.g. ridge flank vein carbonate; fluid inclusions). Finally, we examine Li, Na, Sr and Ba incorporation into the test of O. ammonoides and discuss the control exerted by temperature, seawater chemistry, saturation state and growth rate on these emerging proxies
The effect of carbonate chemistry on trace element incorporation in high-Mg calcitic foraminifera
No full textThe sodium-to-calcium ratio (Na/Ca) of biogenic CaCO3 has recently been introduced as a proxy for past seawater Ca2+ concentrations ([Ca2+sw]) as demonstrated by a positive correlation between seawater and shell Na/Ca with a minor influence of salinity. In the present study, we investigate the effect of carbonate chemistry on the Na/Ca proxy by conducting a set of experiments in which pH and the concentration of dissolved inorganic carbon (DIC) were independently varied. In addition to Na+, the incorporation of Li+, Mg2+, and Sr2+ into the shells of the large benthic high-Mg calcitic foraminifer Operculina ammonoides was assessed by culturing under constant DIC (∼2170 µmol kg−1) with varying pH (7.5–8.4 NBS scale), and under varying DIC (830–2470 µmol kg−1) with constant pH (∼7.9). Foraminiferal growth rate correlates linearly with calcite saturation state (Ω) of the experimental seawater (SW). The lowest pH and DIC experiments were characterized by low population growth rates, and some of these specimens died and their shells partially dissolved. Na/Cashell and Li/Cashell in O. ammonoides are positively correlated with SW [CO32–] and Ω, whereas Sr/Cashell and Mg/Cashell are much less sensitive to these parameters. The relative sensitivity of Na/Cashell to Ω in O. ammonoides is ∼ 4 % per Ω unit. However, given that past changes in surface water Ω were probably small relative to changes in [Ca2+sw] the correction for this secondary effect over the Cenozoic is likely to be small. Therefore, we conclude that the sensitivity of O. ammonoides Na/Ca to the carbonate system is unlikely to compromise the use of this proxy to reconstruct past [Ca2+sw]. In the case of the low-Mg planktic and benthic foraminifera, a data compilation exercise indicates that no resolvable carbonate chemistry effect exists on Na/Ca. Thus, the Na/Ca proxy in benthic nummulitid and planktic foraminifera can be utilized for past [Ca2+sw] reconstructions. Furthermore, coupling this information with the distribution coefficients of other elemental and isotopic systems (e.g., Li+, Sr2+, Mg2+, K+, B, δ11B) may allow the reconstruction of wider aspects of past ocean chemistry. Finally, comparison of trace and minor element incorporation into low and high-Mg foraminiferal species, coccolithophores, inorganic calcite, and amorphous CaCO3 (ACC), we propose a modified biomineralization model for hyaline foraminifera centered on SW vacuolization. Foraminiferal data can be explained by a biomineralization process in which high-Mg species utilize a precursor phase (ACC) to produce high-Mg calcite whereas low-Mg species actively remove Mg2+ from the site of calcification.</p
Salinity effect on trace element incorporation in cultured shells of the large benthic foraminifer <i>Operculina ammonoides</i>
No full textThe ratio of sodium to calcium in the shells of foraminifera (Na/Cashell) has been experimentally calibrated as a proxy for past ocean Ca concentrations (Hauzer et al., 2018, https://doi.org/10.1016/j.epsl.2018.06.004). In parallel, it has been suggested that Na/Cashell could be used as a proxy for paleo-salinity. In this study, we determined the extent to which foraminiferal Na/Ca (and other elements) change with salinity for the shallow-dwelling large benthic foraminifer Operculina ammonoides, an extant relative of the abundant Eocene Nummulites. The culture experiment was conducted under four salinities between 33 and 43 psu. Shell chemistry was measured by LA-ICPMS with the newly precipitated CaCO3 identified by a 135Ba-spike added to the experimental seawater. Na/Cashell, Mg/Cashell and Li/Cashell in O. ammonoides increased slightly with salinity, while Sr/Cashell showed no resolvable change. The change in Na/Cashell due to salinity was small (∼1.4%/psu) compared to the changes in this ratio caused by varying seawater calcium concentrations (Casw) with a sensitivity of ∼5%/(mmol kg−1) Casw. Moreover, the change in salinity in most regions of the past open oceans is minor compared to the large secular variations in Casw during the Phanerozoic (10–40 mmol kg−1). Thus, if at all, paleo-salinity may be reconstructed based on Na/Cashell only for samples younger than Casw residence time (∼1 Myr). Furthermore, both regional and global changes in ocean salinity over geological time do not pose a significant complication for the use of Na/Cashell as a proxy for past changes in seawater calcium concentrations
Calibration of Na partitioning in the calcitic foraminifer Operculina ammonoides under variable Ca concentration: Toward reconstructing past seawater composition
No full textReconstructions of past changes in the seawater calcium concentration (Casw) are critical for understanding the long-term changes in ocean chemistry, the carbon cycle and for accurate application of elemental proxies (El/CaCaCO3 ) in foraminifera (e.g., Mg/Ca as proxy of temperature). Here we show that Na/Ca ratios in foraminiferal shells could be used for reconstructing Casw and Mg/Ca in the past oceans. Ca has a short residence time in the ocean (∼1 My), whereas Na in seawater has a residence time of ∼100 My. Hence it may be reasonably assumed that Nasw is invariant over the Cenozoic, enabling variations in oceanic Ca to be deduced from foraminiferal Na/Ca (Na/Cashell) if Na incorporation into foraminiferal shells depends on Na/Ca in seawater. Furthermore, the paleo-concentrations of other major and minor elements may then be calculated relative to the Ca in the shells, provided that other environmental or biological factors do not present a further complication. To evaluate this hypothesis, we cultured the benthic foraminifer Operculina ammonoides, an extant relative of the Eocene Nummulites, under varying Casw and temperature. The foraminifera grew well under the experimental conditions and increased their weight by 40–90%. The newly grown calcite (identified using a 135Ba labeling in the experimental seawater) was analyzed by Laser-Ablation ICP-MS for Li, Na, Mg and Sr to Ca ratios. The relationship between Na/Ca and Mg/Ca in the shell and their ratio in the solution are best described as a power function, where the instantaneous distribution coefficient is the derivative of the power fit to the El/Cashell versus El/Casw. In contrast, DSr and DLi are invariant with El/Casw. The influence of temperature on Li, Na and Sr incorporation was smaller than the uncertainty of our measurements. We conclude that Na/Ca in foraminiferal shells can be used to calculate paleo-calcium concentrations in the oceans and also other elements that may change relative to calcium (e.g., Mg, Sr, Li and others)
Assessing foraminifera biomineralisation models through trace element data of cultures under variable seawater chemistry
Get PDFThe process by which foraminifera precipitate calcite from seawater has received much attention, in part because a mechanistic basis for empirical calibrations between shell chemistry and environmental parameters is desirable given their widespread application in palaeoceanography. The incorporation of fluorescent membrane-impermeable molecules into the shell demonstrates that seawater, transported by vacuolisation, is present at the site of calcification. However, recent discussion has focused on whether the calcium required for chamber formation is sourced predominantly by transmembrane Ca transport (TMT), with seawater vacuolisation playing a passive role, or vice versa. This debate has arisen in part because of the need to explain the low Mg/Ca ratio of most foraminifera compared to inorganic calcite. Here, we present trace element data of Operculina ammonoides and Globigerinoides ruber, a high-Mg shallow benthic, and low-Mg planktonic species respectively, cultured under variable seawater carbonate and elemental chemistries. We find that Mg incorporation in high and low-Mg species is characterised by an opposite response to the carbonate system, demonstrating that the negative relationship between Mg/Ca and pH or [ CO32- ] in several low-Mg foraminifera is not an intrinsic feature of foraminiferal (or inorganic) calcite precipitation. Therefore, any biomineralisation model must be able to explain why the mechanism by which seawater Mg/Ca is reduced is impacted by the carbonate system. Moreover, we show that trace element incorporation in G. ruber is consistent with Rayleigh fractionation from unmodified seawater except for Mg-removal, but in very poor agreement with a biomineralisation site [Ca] substantially elevated above that of seawater as required by the TMT hypothesis. In addition, any biomineralisation model must explain the nonlinear relationship between seawater and shell Mg/Ca, and the large number of seawater vacuoles observed in some species. Although there are important inter-species differences in biomineralisation, evident from the observed range of shell Mg/Ca ratios, we argue that these differences are mechanistically related to the degree of Mg exclusion prior to chamber formation. Indeed, whilst our data for both low-Mg and high-Mg species are consistent with biomineralisation via ions sourced through seawater vacuolisation, it is difficult to reconcile many of these observations with a model based on significant transmembrane Ca transport
Insights on the interaction of calcein with calcium carbonate and its implications in biomineralization studies
No full textThe effects of calcein, a fluorescent marker commonly used to assess mineral growth in calcifying organisms, on calcite and aragonite structure have been investigated. Calcein is entrapped within calcite and aragonite and modifies the shape and morphology of both polymorphs. Moreover, in the presence of Mg2+, it inhibits aragonite formation in favor of magnesium calcite
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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