Alexander Johannes Clark


Last Name

Clark

First Name

Alexander Johannes

ORCID

0000-0002-6949-3187

Organisational unit

09601 - Stoll, Heather / Stoll, Heather

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2024 - 20255
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Publications 1 - 5 of 5
  • Clumped Isotope Temperatures of Coccolithophores From Global Sediment Traps
    Item type: Journal Article
    Clark, Alexander Johannes; Jaggi, Madalina; Bernasconi, Stefano M.; et al. (2025)
    Paleoceanography and Paleoclimatology
    Temperature proxies such as clumped isotope ($\Delta$$_{47}$) thermometry on biogenic carbonates are applied to the past with greatest confidence when the proxy-temperature relationship is shown to be robust within natural temperature conditions of the ocean. Especially well-suited for this purpose are biogenic carbonates sampled from well-constrained production period and oceanographic conditions of sediment traps. Since coccolithophorids have a cosmopolitan distribution and are major biogenic carbonate producers in the surface ocean, their coccoliths usually dominate the inorganic carbon flux in sediment traps and are sufficiently abundant in most traps for clumped isotope analysis. Here, we measured $\Delta$$_{47}$ in the coccolith size fraction of 18 sediment trap samples across a 75° latitudinal gradient and three ocean basins. To identify the upper ocean provenance region of the coccoliths in each trap, a simple model of coccolith transport by ocean currents was constructed. The coccolith $\Delta$$_{47}$ strongly follows upper ocean temperatures, and there is no evidence for a coccolithophore species-specific effect on the $\Delta$$_{47}$-temperature relationship. Applying the coccolith-specific $\Delta$$_{47}$-temperature calibration (Clark, Torres-Romero, et al., 2024) to estimate calcification temperatures shows that inferred calcification depths match the depth of maximum coccolithophore production, and shows a good agreement with the depth of maximum coccolithophore abundance and the expected depth of coccolith production. We also measured dual clumped isotopes ($\Delta$$_{47}$ and $\Delta$$_{48}$) on coccoliths cultured under controlled temperatures, and confirm that the coccolith clumped isotopes are affected by kinetic bias, independently verifying the need for a coccolith-specific calibration for accurate paleotemperature reconstructions by means of $\Delta$$_{47}$.
  • A clumped isotope calibration of coccoliths at well-constrained culture temperatures for marine temperature reconstructions
    Item type: Journal Article
    Clark, Alexander Johannes; Torres Romero, Ismael; Jaggi, Madalina; et al. (2024)
    Climate of the Past
    Numerous recent studies have tested the clumped isotope (Delta 47) thermometer on a variety of biogenic carbonates such as foraminifera and bivalves and showed that most follow a common calibration. However, there may be a difference between biogenic-carbonate-based calibrations and the most recent inorganic carbonate calibrations that are assumed to have formed close to isotopic equilibrium. Biogenic calibrations such as those based on foraminifera from seafloor sediments suffer from uncertainties in the determination of the calcification temperatures. Therefore, well-constrained laboratory cultures without temperature uncertainty can help resolve these discrepancies. Although the sample size requirements for a reliable Delta 47 measurement have decreased over the years, the availability and preservation of many biogenic carbonates are still limited and/or require substantial time to be extracted from sediments in sufficient amounts. Coccoliths, on the other hand, are abundant and often well-preserved in sediments, and they are a potential interesting target for palaeoceanography. We thus determined the Delta 47-temperature relationship for coccoliths due to their relative ease of growth in the laboratory. The carbon and oxygen isotopic compositions of coccolith calcite have limited use in palaeoenvironmental reconstructions due to physiological effects that cause variability in the carbon and oxygen isotopic fractionation during mineralization. However, the relatively limited data available suggest that clumped isotopes may not be significantly influenced by these effects. We cultured three species of coccolithophores under controlled carbonate system conditions with CO2(aq) concentrations between 5 and 45 mu M, pH between 7.9 and 8.6 units, and temperatures between 6 and 27 degrees C.Our well-constrained results agree with a previous culture study that there are no apparent species- or genus-specific vital effects on the Delta 47-temperature relationship in coccolithophores despite significant deviations from equilibrium in the C and O isotopic composition. We find that while varying environmental parameters other than temperature does not have a significant effect on Delta 47, changing the parameters yields coccolith Delta 47-temperature calibrations that agree within 1.2 ppm. Our coccolith-specific Delta 47-temperature calibration with well-constrained temperatures shows a consistent, positive offset of 2-3 degrees C to the inorganic carbonate calibrations, which point to as yet unknown coccolith-specific disequilibrium effects.Thus, we suggest the use of our coccolith-specific calibration for further coccolith palaeoceanographic studies and that calibrations derived from laboratory-grown biogenic carbonates are desirable to reinforce the confidence of clumped-isotope-based temperature reconstructions in palaeoceanography.
  • Hydrogen isotope fractionation is controlled by CO₂ in coccolithophore lipids
    Item type: Journal Article
    Torres Romero, Ismael; Zhang, Hongrui; Wijker, Reto S.; et al. (2024)
    Proceedings of the National Academy of Sciences of the United States of America
    Hydrogen isotope ratios (δ2H) represent an important natural tracer of metabolic processes, but quantitative models of processes controlling H-fractionation in aquatic photosynthetic organisms are lacking. Here, we elucidate the underlying physiological controls of 2H/1H fractionation in algal lipids by systematically manipulating temperature, light, and CO2(aq) in continuous cultures of the haptophyte Gephyrocapsa oceanica. We analyze the hydrogen isotope fractionation in alkenones (αalkenone), a class of acyl lipids specific to this species and other haptophyte algae. We find a strong decrease in the αalkenone with increasing CO2(aq) and confirm αalkenone correlates with temperature and light. Based on the known biosynthesis pathways, we develop a cellular model of the δ2H of algal acyl lipids to evaluate processes contributing to these controls on fractionation. Simulations show that longer residence times of NADPH in the chloroplast favor a greater exchange of NADPH with 2H-richer intracellular water, increasing αalkenone. Higher chloroplast CO2(aq) and temperature shorten NADPH residence time by enhancing the carbon fixation and lipid synthesis rates. The inverse correlation of αalkenone to CO2(aq) in our cultures suggests that carbon concentrating mechanisms (CCM) do not achieve a constant saturation of CO2 at the Rubisco site, but rather that chloroplast CO2 varies with external CO2(aq). The pervasive inverse correlation of αalkenone with CO2(aq) in the modern and preindustrial ocean also suggests that natural populations may not attain a constant saturation of Rubisco with the CCM. Rather than reconstructing growth water, αalkenone may be a powerful tool to elucidate the carbon limitation of photosynthesis.
  • Clumped isotope temperatures of coccolithophores from global sediment traps
    Item type: Working Paper
    Clark, Alexander Johannes; Jaggi, Madalina; Bernasconi, Stefano M.; et al. (2024)
    ESS Open Archive
    Temperature proxies such as clumped isotope (Δ47) thermometry on biogenic carbonates are applied to the past with greatest confidence when the proxy-temperature relationship is shown to be robust within natural temperature conditions of the ocean. Especially well-suited for this purpose are biogenic carbonates sampled from well-constrained production period and oceanographic conditions of sediment traps. Since coccolithophorids have a cosmopolitan distribution and are major biogenic carbonate producers in the surface ocean, their coccoliths usually dominate the inorganic carbon flux in sediment traps and are sufficiently abundant in most traps for clumped isotope analysis. Here, we measured Δ47 in the coccolith size fraction of 18 sediment trap samples across a 75° latitudinal gradient and three ocean basins. To identify the upper ocean provenance region of the coccoliths in each trap, a simple model of coccolith transport by ocean currents was constructed. The coccolith Δ47 strongly follows the upper ocean temperatures, in particular the average temperatures from the maximum production depths of living coccolithophores from their provenance areas. There is no evidence for a coccolithophore species-specific effect on the Δ47-temperature relationship. Applying the recent coccolith-specific Δ47-temperature calibration (Clark et al., 2024a) to estimate calcification temperatures shows that inferred calcification depths match the depth of maximum coccolithophore production in the provenance area. Compared to other calibrations for biogenic carbonates, the coccolith-specific Δ47-temperature calibration yields the best agreement with the depth of maximum coccolithophore abundance and the expected depth of coccolith production.
  • Temperature-dependent carbon isotope fractionation in coccolithophores
    Item type: Journal Article
    Torres Romero, Ismael; Clark, Alexander Johannes; Wijker, Reto S.; et al. (2024)
    Frontiers in Earth Science
    Introduction: The stable carbon isotope ratio of long-chain alkenones produced by marine haptophyte phytoplankton has often been used to estimate past variations in atmospheric CO2 throughout the Cenozoic. However, previous experimental studies and surveys of alkenones from surface sediment and suspended particulate matter document additional environmental and physiological influences on carbon isotopic fractionation in alkenones. Methods: To clarify the non-CO2 effects on the alkenone carbon isotope fractionations, an important alkenone producer, Gephyrocapsa oceanica, was cultured in laboratory. To separate effects of different environment parameters, G. oceanica was grown in continuous cultures under a matrix of environmental conditions in order to explore the influence of temperature independently of CO2(aq). Through careful manipulation of the media carbon system, we can control the variation of the media CO2(aq) independently of temperature solubility. Carbon isotope fractionations from alkenones, coccolith, and particulate organic carbon were measured from this steady state system. Results and Discussion: We find epsilon p in alkenones and particulate organic carbon inversely correlates with temperature, and temperature affects epsilon(p) more strongly than CO2(aq). The magnitude of the temperature effect can be explained by higher growth rates at warmer temperatures with a similar growth rate dependence as observed in previous cultures in which growth rate was regulated by other factors. Where the past temperature influence on growth rate could be constrained using the U-K' (37) alkenone index in the same samples, our finding offers an approach to deconvolve an important physiological factor affecting ancient alkenones epsilon(p), and may therefore improve past pCO(2) estimates.
Publications 1 - 5 of 5