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This study presents a methodological advancement in the field of clumped‐isotope (∆47) thermometry, specifically tailored for application to freshwater ostracods. The novel ostracod clumped isotope approach enables quantitative temperature and hydrological reconstruction in lacustrine records. The relationship between ∆47 and the temperature at which ostracod shell mineralized is determined by measuring ∆47 on different species grown under controlled temperatures, ranging from 4 ± 0.8 to 23 ± 0.5ºC. The excellent agreement between the presented ∆47 ostracod data and the monitored temperatures confirms that ∆47 can be applied to ostracod shells and that a vital effect is absent outside the uncertainty of measurements. Results are consistent with the carbonate clumped‐isotope unified calibration (Anderson et al., 2021, https://doi.org/10.1029/2020gl092069), therefore, an ostracod‐specific calibration is not needed. The ostracod clumped‐isotope thermometer represents a powerful tool for terrestrial paleoclimate studies all around the world, as lakes and ostracods are found in all climatic belts. Plain Language Summary: In the framework of global warming, the reconstruction of past climatic conditions is important to understand the future evolution of climate and its impact. Lake sediments can be used as archives to quantify these effects. This study presents a novel paleo‐thermometer based on the application of clumped‐isotope technique (i.e., measurement of the number of 13C–18O bonds in carbonate minerals that depends on the temperature of carbonate precipitation) on carbonatic microcrustacea, named ostracods that commonly live in lakes. By using ostracods that formed their shells at known temperatures, we demonstrate that they can be easily used to reconstruct water temperature and hydrological conditions (precipitation/evaporation). The ostracod clumped‐isotope thermometer represents a powerful tool for terrestrial paleoclimate studies around the world, as lakes and ostracods are located in all climatic belts. Key Points: ∆47—ostracod signal accurately records the shell calcification temperature∆47—ostracod signal is not affected by the so called "vital effect"The unified calibration of Anderson et al. (2021) can be used to convert the ∆47—ostracod signal into accurate temperatures [ABSTRACT FROM AUTHOR]

Stable isotopes in mollusc shells, together with variable growth rates and other geochemical properties, can register different environmental clues, including seawater temperature, salinity and primary productivity. However, the strict biological control over the construction of biominerals exerted by many calcifying organisms can constrain the use of these organisms for paleoenvironmental reconstructions. Biologically controlled calcification is responsible for the so called vital effects that cause a departure from isotopic equilibrium during shell formation, resulting in lower shell oxygen and carbon compared to the equilibrium value. We investigated shell oxygen and carbon isotopic composition of the bivalve Chamelea gallina in six sites along with a latitudinal gradient on the Adriatic Sea (NE Mediterranean Sea). Seawater δ18O and δ13CDIC varied from North to South, reflecting variations in seawater temperature, salinity, and chlorophyll concentration among sites. Shell δ18O and δ13C differed among sites and exhibited a wide range of values along with the ~400 km latitudinal gradient, away from isotopic equilibrium for both isotopes. These results hampered the utilization of this bivalve as a proxy for environmental reconstructions, in spite of C. gallina showing promise as a warm temperature proxy. Rigorous calibration studies with a precise insight of environment and shell growth are crucial prior to considering this bivalve as a reliable paleoclimatic archive. [ABSTRACT FROM AUTHOR]

Emiliania huxleyi cells were grown in artificial seawater of different Li and Ca concentrations and coccolith Li/Ca ratios determined. Coccolith Li/Ca ratios were positively correlated to seawater Li/Ca ratios only if the seawater Li concentration was changed, not if the seawater Ca concentration was changed. This Li partitioning pattern of E. huxleyi was previously also observed in the benthic foraminifer Amphistegina lessonii and inorganically precipitated calcite. We argue that Li partitioning in both E. huxleyi and A. lessonii is dominated by a coupled transmembrane transport of Li and Ca from seawater to the site of calcification. We present a refined version of a recently proposed transmembrane transport model for Li and Ca. The model assumes that Li and Ca enter the cell via Ca channels, the Li flux being dependent on the Ca flux. While the original model features a linear function to describe the experimental data, our refined version uses a power function, changing the stoichiometry of Li and Ca. The version presented here accurately predicts the observed dependence of DLi on seawater Li/Ca ratios. Our data demonstrate that minor element partitioning in calcifying organisms is partly mediated by biological processes even if the partitioning behavior of the calcifying organism is indistinguishable from that of inorganically precipitated calcium carbonate. Plain Language Summary: Marine shell‐forming organisms such as the minute, but abundant, coccolithophores (single‐celled phytoplankton) and foraminifera (single‐celled zooplankton) are not only ecologically important but also contribute significantly to the global carbonate sink. Minor elements (e.g., Sr and Li) trapped in biogenic carbonate sediments provide a window into past environmental conditions such as temperature, which is relevant for climate change. An understanding of elemental incorporation processes is required in order to correctly translate these minor element signatures into past environmental data. Here we conducted culture experiments with the coccolithophore Emiliania huxleyi to determine its Li incorporation behavior. We compare our results with previously published data on the foraminifer Amphistegina lessonii and data on synthetic calcite. The Li incorporation behavior of biogenic calcites is surprisingly similar to that of synthetic calcite. This is usually taken to mean that Li incorporation into shells should proceed inorganically. By contrast, we conclude that minor element incorporation processes in marine shell‐forming organisms always include biological processes. This is relevant to past climate reconstructions because it excludes any interpretation of minor element signatures in fossil shells based on inorganic processes only. Key Points: The Li partitioning pattern of E. huxleyi resembles that of A. lessonii and inorganic calciteLi partitioning in E. huxleyi is dominated by a coupled transmembrane transport of Li and CaThe vital effect is ubiquitous in calcifying organisms, even if it appears to be absent [ABSTRACT FROM AUTHOR]

Trace element concentrations in fish earstones ('otoliths') are widely used to discriminate spatially discrete populations or individuals of marine fish, based on a commonly held assumption that physiological influences on otolith composition are minor, and thus variations in otolith elemental chemistry primarily reflect changes in ambient water chemistry., We carried out a long-term (1-year) experiment, serially sampling seawater, blood plasma and otoliths of mature and immature European plaice ( Pleuronectes platessa L.) to test relationships between otolith chemistry and environmental and physiological variables., Seasonal variations in otolith elemental composition did not track seawater concentrations, but instead reflected physiological controls on metal transport and biokinetics, which are likely moderated by ambient temperature. The influence of physiological factors on otolith composition was particularly evident in Sr/ Ca ratios, the most widely used elemental marker in applied otolith microchemistry studies. Reproduction also triggered specific variations in otolith and blood plasma metal chemistry, especially Zn/ Ca ratios in female fish, which could potentially serve as retrospective spawning indicators., The influence of physiology on the trace metal composition of otoliths may explain the success of microchemical stock discrimination in relatively homogenous marine environments, but could complicate alternative uses for trace element compositions in biominerals of higher organisms. [ABSTRACT FROM AUTHOR]