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[Rationale] The recent progress in micro-scale isotopic analytical techniques for otoliths has enabled the reconstruction of the experienced water temperature history of fish in every few days resolution using the stable oxygen isotope ratio (δ¹⁸O) of otoliths. We aimed to improve those techniques and extract the daily δ¹⁸O records of otoliths formed during the juvenile period. [Methods] Growth rings were formed daily in fish otoliths. We precisely distinguished the daily rings in otoliths of Japanese jack mackerel Trachurus japonicus, and milled them along daily growth rings using a high-spatial resolution micromilling system (Geomill326). Then, we determined the stable carbon and oxygen (δ¹³C, δ¹⁸O) isotopic compositions using a high-precision micro-scale isotopic analytical system (MICAL3c with IsoPrime 100). [Results] We successfully milled each daily ring with width ranging from 14.0 to 62.9 μm (average 27.0 μm) during the high growth period (30–70 days after hatching), and determined the isotopic compositions of otolith aragonite. [Conclusions] Our improved micro-scale analytical method is the first to determine the daily δ¹⁸O history of fish otoliths. By using our method together with the δ¹⁸O --water temperature equation, the daily history of experienced water temperature can be elucidated. Our high-resolution milling and analytical technique can also be applied to high-resolution isotope analysis for stalactites, clams, and corals., 魚類の1日毎の経験水温を世界で初めて解明 --謎だらけの魚類生態の理解と資源保全へ--. 京都大学プレスリリース. 2022-10-17.

We investigated temperature and growth‐rate dependency of skeletal oxygen and carbon isotopes in primary polyps of Acropora digitifera (Scleractinia: Acroporidae) by culturing them at 20, 23, 27, or 31°C. Calcification was most rapid at 27 and 31°C. We obtained a δ18O‐temperature relationship (−0.18‰ °C−1) consistent with reported ranges for Porites, indicating that juvenile Acropora polyps can be used for temperature reconstruction. A growth‐rate dependency of skeletal isotopes was detected in the experimental polyps cultured at lower water temperatures, when the skeletal growth rate of these polyps was also low. The estimated upper calcification flux limit for a kinetic isotope effect to be observed in the δ18O‐growth rate relationship (∼0.4–0.7 g CaCO3 cm−2 yr−1) was similar to the calcification flux in Porites corresponding to a linear extension rate of 5 mm yr−1, the maximum rate at which the kinetic isotope effect is evident. This result suggests that the calcification flux can be used as a measure of growth rate‐related isotope fractionation, that is, the kinetic isotope effect, in corals of different genera and at different growth stages.

The molecular work was funded by an Advanced Fellowship award to K. Darling (UK Natural Environment Research Council (NERC); NER/J/S/2000/00860 and NE/D009707/1). This work was also supported by European Research Council (ERC) through a research grant to H. Elderfield. A.D.S. acknowledges support from the Indian Space Research Organization through an ISRO-GBP grant sanctioned in 2008. P. Anand is thankful for NERC support (NE/I019891/1). Planktonic foraminifera record oceanic conditions in their shell geochemistry. Many palaeoenvironmental studies have used fossil planktonic foraminifera to constrain past seawater properties by defining species based on their shell morphology. Recent genetic studies, however, have identified ecologically distinct genotypes within traditionally recognized morphospecies, signaling potential repercussions for palaeoclimate reconstructions. Here we demonstrate how the presence of Globigerina bulloides cryptic genotypes in the Arabian Sea may influence geochemical signals of living and fossil assemblages of these morphospecies. We have identified two distinct genotypes of G. bulloides with either cool water (type-II) or warm water (type-I) temperature preferences in the Western Arabian Sea. We accompany these genetic studies with analyses of Mg/Ca and stable oxygen isotope (δ18O) compositions of individual G. bulloides shells. Both Mg/Ca and δ18O values display bimodal distribution patterns. The distribution of Mg/Ca values cannot be simply explained by seawater parameters, and we attribute it to genotype-specific biological controls on the shell geochemistry. The wide range of δ18O values in the fossil assemblage also suggests that similar controls likely influence this proxy in addition to environmental parameters. However, the magnitude of this effect on the δ18O signals is not clear from our data set, and further work is needed to clarify this. We also discuss current evidence of potential genotype-specific geochemical signals in published data on G. bulloides geochemistry and other planktonic foraminiferal species. We conclude that significant caution should be taken when utilizing G. bulloides geochemistry for paleoclimate reconstruction in the regions with upwelling activity or oceanographic fronts. Publisher PDF

International Symposium, "The Origin and Evolution of Natural Diversity". 1–5 October 2007. Sapporo, Japan., For reconstruction of paleoenvironments using the isotopic compositions of shallow-water microfossils, especially benthic microfossils at high latitudes, difficulties arise due to variation in environmental factors, including seasonal temperature, depth, water currents, and habitat. In this study, we applied a microscale technique to determine stable carbon and oxygen isotopic ratios in shallow-water calcareous benthic microfossils (foraminifera and bryozoans), in order to choose suitable species for reconstructing paleoenvironmental changes in a Pleistocene shallow marine setting. We compared isotopic trends in Cibicides refulgens, a shallow water epibenthic foraminifer, with the mean annual range of temperature (MART) based on bryozoan analyses. We found no strong trend in δ13C composition in the foraminifers from stratigraphically sequential sediment samples of the Soebetsu Sandstone Member (~0.8 Ma) of the Pleistocene Setana Formation, but did find δ18O isotopic fluctuation among samples that was larger than the range of isotopic variation among individual foraminifera. MART values were almost identical from two strata showing a large difference in δ18O composition. We interpret our results to mean that the δ18O isotopic composition of benthic foraminifera was affected mainly by global and/or regional environmental changes rather than local temperature variation on the seafloor, in contrast to MART analysis, which reflects only annual range in local temperature. Further studies are needed on possible correlations between MART values and microscale isotopic determinations for calcareous microfossils in high-latitude shallow-water paleoenvironments.

International Symposium, "The Origin and Evolution of Natural Diversity". 1–5 October 2007. Sapporo, Japan., Zooids in bryozoan colonies show an ecophenotypic response in which zooid size varies inversely with temperature at the time of zooid growth. A technique called MART analysis allows estimation of the mean annual range of temperature (MART) in Recent or paleo- marine environments by measuring the mean coefficient of variation (CV) of zooid size in species of cheilostome bryozoans that lived in those environments. We conducted standard MART analyses of Recent specimens from nearshore shelf regions in Akkeshi Bay (Japan) and the western Aleutian Islands (Alaska) as controls on our application of the method, and of fossil specimens from a Pleistocene shallow benthic environment [Nakasato Congomerate Member, Utasai Section, lower Setana Formation (1.2~1.0 Ma), Kuromatsunai, Hokkaido, Japan] to examine paleoseasonality. In some cases, not enough material was available from fossil strata to conduct standard MART analyses. For these cases, we developed a modified technique that we call specimen-limited MART analysis, or SL-MART. We applied this technique to specimens from two stratigraphically sequential sediment samples from the Soebetsu Sandstone Member in the Soebetsu Section of the upper Setana Formation (1.0-0.6 Ma). Regardless of technique or age of the specimens, replicate samples in our analyses provided remarkably similar estimates of seasonality. The utility of the controls was questionable due to uncertainty about present-day temperature ranges in the localities sampled. A decrease in average MART value from the lower to upper Setana Formation (11.8℃ and 8.6℃, respectively) indicates more pronounced seasonality in the lower than the upper Setana, perhaps suggesting a shift to colder conditions in the latter. However, the younger samples could also have lived at a greater depth, which would also reduce the MART. In addition, previous studies of marine mollusks indicate variability in paleoenvironment within both the upper and lower Setana Formation, perhaps related to the warm Tsushima current that flowed into the Sea of Japan during most interglacial periods from 1.71-0.8 Ma. Thus more than a few scattered MART estimates are necessary for reconstruction of paleoseasonality through the depositional interval of the Setana Formation.