Stable Ca and Sr isotopes support volcanically triggered biocalcification crisis during Oceanic Anoxic Event 1a

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 (delta Ca-44/40) 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 (delta Sr-88/86) system to resolve the origin of delta Ca-44/40 variability during OAE 1a. This study reports high-precision thermal ionization mass spectrometry (TIMS) delta Ca-44/40, delta Sr-88/86, and Sr-87/Sr-86 records for Hole 866A of Ocean Drilling Program Leg 143 drilled in Resolution Guyot, mid-Pacific Ocean. The samples span similar to 27 m.y. from the Barremian (ca. 127 Ma) to the Albian (ca. 100 Ma). The delta Ca-44/40 and d88/86Sr secular trends differ from the Sr-87/Sr-86 record but mimic each other. delta Ca-44/40 and [Sr], as well as delta Ca-44/40 and delta Sr-88/86, 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 delta Ca-44/40 and delta Sr-88/86 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.

Automated summary

This study explores the origin of δCa-44/40 variability during OAE 1a by utilizing the stable strontium isotope system, revealing a strong correlation between δCa-44/40 and δSr-88/86 under kinetic control, rather than end-member mixing. Positive shifts in δCa-44/40 and δSr-88/86 before OAE 1a suggest reduced precipitation rates, indicating a cascade effect on rate-dependent Ca and Sr isotope fractionation induced by the interaction between LIP eruptions and biocalcification feedbacks.