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

Abstract

Carbonate-associated sulfate (CAS) δ³⁴S values (δ³⁴S_CAS) 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 δ³⁴S_CAS records 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 δ³⁴S_CAS values 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 δ³⁴S_CAS, 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.