白亜紀の謎を微化石解析で解明（Tiny fossils crack a Cretaceous cold case）

2026-07-30 ノースウェスタン大学

New research examining hundreds of microscope fossils offers one of the clearest examples yet of how rising CO2 can alter ocean chemistry and harm marine life. Above, Jonathan Chen, the study’s lead author, uses a fine-tipped brush to separate tiny fossils from surrounding sediment.

＜関連情報＞

• https://news.northwestern.edu/stories/2026/07/tiny-fossils-crack-a-cretaceous-cold-case
• https://www.science.org/doi/10.1126/science.aed9359
• https://tiisys.com/blog/2023/01/20/post-115668/
• https://pubs.geoscienceworld.org/gsa/geology/article-abstract/49/6/723/595187/Calcium-isotope-composition-of-Morozovella-over
• https://pubs.geoscienceworld.org/gsa/geology/article-abstract/49/5/515/593341/Stable-Ca-and-Sr-isotopes-support-volcanically
• https://pubs.geoscienceworld.org/gsa/geology/article/48/1/34/574630/Calcium-isotope-evidence-for-environmental

カルシウム同位体分析により、海洋酸性化とアプチアン期～アルビアン期の有孔虫絶滅との関連性が明らかになった Calcium isotopes link ocean acidification to Aptian–Albian foraminiferal extinctions

Jonathan Chen, Andrew D. Jacobson, Brian T. Huber, Kenneth G. MacLeod, […] , and Bradley B. Sageman
Science  Published:30 Jul 2026
DOI:https://doi.org/10.1126/science.aed9359

Abstract

The second-largest extinction event in the evolutionary history of planktic foraminifera occurred at the Aptian–Albian boundary. This extinction may reflect ocean acidification (OA) associated with Oceanic Anoxic Event 1b. As calcium isotope ratios (δ^44/40Ca) can track how biocalcification rates respond to OA, we measured δ^44/40Ca records for planktic and benthic foraminifera, bulk carbonates, and authigenic calcite across the Aptian–Albian boundary in the South Atlantic. Benthic and bulk δ^44/40Ca data display a distinct sequence of negative and positive excursions, similar to δ^44/40Ca variations across other OA events. Planktic δ^44/40Ca values increase markedly, tracking a reduction in calcification rates coincident with decreases in the size, diversity, and shell thickness of planktic foraminifera. These results suggest that OA drove extinctions of planktic foraminifera at the Aptian–Albian boundary.

後期暁新世～前期始新世における モロゾヴェラ のカルシウム同位体組成 Calcium isotope composition of Morozovella over the late Paleocene–early Eocene

Gabriella D. Kitch;Andrew D. Jacobson;Dustin T. Harper;Matthew T. Hurtgen;Bradley B. Sageman;James C. Zachos
Geology  Published:March 04, 2021
DOI:https://doi.org/10.1130/G48619.1

Abstract

Ocean acidification (OA) during the Paleocene-Eocene thermal maximum (PETM) likely caused a biocalcification crisis. The calcium isotope composition (δ^44/40Ca) of primary carbonate producers may be sensitive to OA. To test this hypothesis, we constructed the first high-resolution, high-precision planktic foraminiferal δ^44/40Ca records before and across the PETM. The records employ specimens of Morozovella spp. collected from Ocean Drilling Program Sites 1209 (Shatsky Rise, Pacific Ocean) and 1263 (Walvis Ridge, Atlantic Ocean). At Site 1209, δ^44/40Ca values start at −1.33‰ during the Upper Paleocene and increase to a peak of −1.15‰ immediately before the negative carbon isotope excursion (CIE) that marks the PETM onset. Values remain elevated through the PETM interval and decrease into the earliest Eocene. A shorter-term record for Site 1263 shows a similar trend, although δ^44/40Ca values are on average 0.22‰ lower and decrease shortly after the CIE onset. The trends support neither diagenetic overprinting, authigenic carbonate additions, nor changes in the δ^44/40Ca value of seawater. Rather, they are consistent with a kinetic isotope effect, whereby calcite δ^44/40Ca values inversely correlate with precipitation rate. Geologically rapid Ca isotope shifts appear to reflect the response of Morozovella to globally forced changes in the local carbonate geochemistry of seawater. All data combined suggest that the PETM-OA event occurred near the peak of a gradual reduction in seawater carbonate ion concentrations during a time of elevated atmospheric pCO₂, potentially driven by North Atlantic igneous province emplacement.

安定カルシウムおよびストロンチウム同位体は、海洋無酸素事変1a期における火山活動によって引き起こされた生物石灰化危機を裏付けている Stable Ca and Sr isotopes support volcanically triggered biocalcification crisis during Oceanic Anoxic Event 1a

Jiuyuan Wang;Andrew D. Jacobson;Bradley B. Sageman;Matthew T. Hurtgen
Geology  Published:December 16, 2020
DOI:https://doi.org/10.1130/G47945.1

Abstract

Large igneous province (LIP) eruptions are hypothesized to trigger biocalcification crises. The Aptian nannoconid crisis, which correlates with emplacement of the Ontong Java Plateau and Oceanic Anoxic Event 1a (OAE 1a, ca. 120 Ma), represents one such example. The Ca isotope (δ^44/40Ca) system offers potential to detect biocalcification fluctuations in the rock record because Ca isotope fractionation is sensitive to precipitation rate. However, other primary and secondary processes, such as input-output flux perturbations and early diagenesis, can produce similar signals. Here, we exploit emergent properties of the stable Sr isotope (δ^88/86Sr) system to resolve the origin of δ^44/40Ca variability during OAE 1a. This study reports high-precision thermal ionization mass spectrometry (TIMS) δ^44/40Ca, δ^88/86Sr, and ⁸⁷Sr/⁸⁶Sr records for Hole 866A of Ocean Drilling Program Leg 143 drilled in Resolution Guyot, mid-Pacific Ocean. The samples span ∼27 m.y. from the Barremian (ca. 127 Ma) to the Albian (ca. 100 Ma). The δ^44/40Ca and δ^88/86Sr secular trends differ from the ⁸⁷Sr/⁸⁶Sr record but mimic each other. δ^44/40Ca and [Sr], as well as δ^44/40Ca and δ^88/86Sr, strongly correlate and yield slopes predicted for kinetic control, which demonstrates that variable mass-dependent fractionation rather than end-member mixing dominated the isotopic relationship between carbonates and seawater. Positive δ^44/40Ca and δ^88/86Sr shifts that begin before OAE 1a and peak within the interval are consistent with reduced precipitation rates. All results combined point to a cascade of effects on rate-dependent Ca and Sr isotope fractionation, which derive from the dynamic interplay between LIP eruptions and biocalcification feedbacks.

白亜紀-古第三紀大量絶滅以前および大量絶滅期における環境変動を示すカルシウム同位体証拠 Calcium isotope evidence for environmental variability before and across the Cretaceous-Paleogene mass extinction

Benjamin J. Linzmeier;Andrew D. Jacobson;Bradley B. Sageman;Matthew T. Hurtgen;Meagan E. Ankney;Sierra V. Petersen;Thomas S. Tobin;Gabriella D. Kitch;Jiuyuan Wang
Geology  Published:October 28, 2019
DOI:https://doi.org/10.1130/G46431.1

Abstract

Carbon dioxide release during Deccan Traps volcanism and the Chicxulub impact likely contributed to the Cretaceous-Paleogene (K-Pg) mass extinction; however, the intensity and duration of CO₂ input differed between the two events. Large and rapid addition of CO₂ to seawater causes transient decreases in pH, [CO₃^2–], and carbonate mineral saturation states. Compensating mechanisms, such as dissolution of seafloor sediment, reduced biomineralization, and silicate weathering, mitigate these effects by increasing the same parameters. The calcium isotope ratios (δ^44/40Ca) of seawater and marine carbonates are hypothesized to respond to these perturbations through weathering/carbonate deposition flux imbalances and/or changes in fractionation between carbonate minerals and seawater. We used a high-precision thermal ionization mass spectrometry method to measure δ^44/40Ca values of aragonitic bivalve and gastropod mollusk shells from the K-Pg interval of the López de Bertodano Formation on Seymour Island, Antarctica. Well-preserved shells spanning the late Maastrichtian (ca. 67 Ma) to early Danian (ca. 65.5 Ma) have δ^44/40Ca values ranging from −1.89‰ to −1.57‰ (seawater [sw]). Shifts in δ^44/40Ca inversely correlate with sedimentological indicators of saturation state. A negative excursion begins before and continues across the K-Pg boundary. According to a simple mass-balance model, neither input/output flux imbalances nor change in the globally integrated bulk carbonate fractionation factor can produce variations in seawater δ^44/40Ca sufficient to explain the measured trends. The data are consistent with a dynamic molluscan Ca isotope fractionation factor sensitive to the carbonate geochemistry of seawater. The K-Pg extinction appears to have occurred during a period of carbonate saturation state variability caused by Deccan volcanism.