In Situ Analysis of Sulfur Isotopic Fractionation in Deep‐Sea Corals Using Secondary‐Ion Mass Spectrometry: Insights Into Vital Effects

Carbonate‐associated sulfate (CAS) δ34S values (δ34SCAS) are generally assumed to reflect S isotopic composition of paleo‐seawater and have been extensively used to reconstruct secular variations in seawater sulfate concentrations during the geological past. However, it has often been documented that δ34SCASrecords are incompatible with seawater sulfur isotopes (20.9 ± 0.1%, 2σ) determined from other archives, such as sulfate evaporites and barite (both of which may also display inconsistencies). A possible explanation for this discrepancy is that δ34SCASvalues can be easily altered by atmospheric sulfate and sulfide re‐oxidation. However, the specific influence of biological factors (vital effects, common in biogenic carbonates) on CAS S isotopic composition remains unresolved, particularly at microscale levels. To elucidate these effects on δ34SCAS, S isotopic profiles were analyzed across two skeletal transects of two modern deep‐sea corals (gorgonia) using a novel secondary‐ion mass spectrometry method. Strong S isotopic fractionation was observed in calcitic skeletons from the most 34S‐depleted center (δ34S = ∼19‰), increasing outward to a relatively constant 22.5‰ in gorgonia sp. coral and 21.6‰ in bamboo coral, suggesting that vital effects are much larger than previous estimated (∼±1‰ fractionation from seawater). Oxygen isotopic and Mg, S, O elemental compositions, and Raman spectral and crystal morphological features indicate that processes such as pH control, Rayleigh fractionation, and organic effects are precluded as causes of such fractionation. Instead, vital effects associated with kinetic processes related to surface entrapment seem plausible as controls on S isotopic fractionations in the coral. This novel method is significant for gaining insights into vital effects, assessing the reliability of biogenic carbonates as high‐resolution environmental archives of S isotopes, and understanding the fundamental mechanisms governing biomineralization. The δ34S values of carbonate‐associated sulfate (CAS) serve as crucial indicators of past environmental conditions, providing insights into historical variations in seawater sulfate and atmospheric oxygen levels. It is commonly assumed that δ34SCASclosely reflects seawater sulfate, except for severely altered samples. However, analysis of S isotopic profiles across two skeletal transects of modern calcitic deep‐sea corals (gorgonia sp.coral and bamboo coral) using a newly developed in situ method based on secondary‐ion mass spectrometry, reveals significant isotopic fractionation. The most 34S‐depleted center (δ34S = ∼19‰) increases outward to a relatively constant value of 22.5‰ for gorgonia sp.coral and 21.6‰ for bamboo coral. These findings suggest that vital effects play a notable role in δ34SCASfractionation, challenging the previous consensus of ∼±1‰. Further investigation, incorporating multiple indices and skeletal δ18O, indicates that vital effects associated with kinetic processes related to surface entrapment contribute to this fractionation. These insights shed light on the processes driving S–O isotopic fractionation in biogenic carbonates, bolstering their utility in reconstructing paleo‐oceanographic conditions. Development of in situ carbonate S isotopic analysis using secondary‐ion mass spectrometryCommonly observed anomalous low δ34S and δ18O values in the axial skeleton of deep‐sea gorgonia coralsContribution of kinetic‐associated vital effects to significant Carbonate‐associated sulfate sulfur isotopic fractionation, supported by multiple geochemical indices Development of in situ carbonate S isotopic analysis using secondary‐ion mass spectrometry Commonly observed anomalous low δ34S and δ18O values in the axial skeleton of deep‐sea gorgonia corals Contribution of kinetic‐associated vital effects to significant Carbonate‐associated sulfate sulfur isotopic fractionation, supported by multiple geochemical indices