New Geochemical Insights from Electron-Spin- Resonance Studies of Mn²⁺ and SO₃- in Calcites: Quantitative Analyses of Chicxulub Crater Ejecta from Belize and Southern México with Comparison to Limestones from Distal Cretaceous-Tertiary-Boundary Sites

Abstract

The solid-state-physics technique of electron spin resonance (ESR) has been employed in an exploratory study of marine limestones and impact-related deposits from Cretaceous-Tertiary (KT) boundary sites including Spain (Sopelana and Caravaca), New Jersey (Bass River), the U.S. Atlantic continental margin (Blake Nose, ODP Leg 171B/1049/A), and several locations in Belize and southern Mexico within ~600 km of the Chicxulub crater. The ESR spectra of SO^- ₃ (a radiation-induced point defect involving a sulfite ion substitutional for CO^2- ₃ which has trapped a positive charge) and Mn²⁺ in calcite were singled out for analysis because they are unambiguously interpretable and relatively easy to record. ESR signal strengths of calcite-related SO^- ₃ and Mn²⁺ have been studied as functions of stratigraphic position in whole-rock samples across the KT boundary at Sopelana, Caravaca, and Blake Nose. At all three of these sites, anomalies in SO^- ₃ and/or Mn²⁺ intensities are noted at the KT boundary relative to the corresponding background levels in the rocks above and below. At Caravaca, the SO^- ₃ background itself is found to be lower by a factor of 2.7 in the first 30,000 years of the Tertiary relative to its steady-state value in the last 15,000 years of the Cretaceous, indicating either an abrupt and quasi-permanent change in ocean chemistry (or temperature) or extinction of the marine biota primarily responsible for fixing sulfite in the late Cretaceous limestones. An exponential decrease in the Mn²⁺ concentration per unit mass calcite, [Mn²⁺], as the KT boundary at Caravaca is approached from below (1/e characteristic length =1.4 cm) is interpreted as a result of post-impact leaching of the seafloor.

Absolute ESR quantitative analyses of proximal impact deposits from Belize and southern Mexico group naturally into three distinct fields in a two- dimensional [SO^- ₃]-versus-[Mn²⁺] scatter plot. These fields contain (I) limestone ejecta clasts, (II) accretionary lapilli, and (III) a variety of SO^- ₃-depleted/Mn²⁺- enriched impact deposits. Data for the investigated non-impact-related Cretaceous and Tertiary marine limestones (Spain and Blake Nose) fall outside of these three fields. With reference to these non-impact deposits, fields I, II, and III can be respectively characterized as Mn²⁺-depleted, SO^- ₃-enhanced, and SO^- ₃-depleted. It is proposed that (1) field I represents calcites from the Yucatán Platform, and that the Mn²⁺-depleted signature can be used as an indicator of primary Chicxulub ejecta in deep marine environments and (2) field II represents calcites that include a component formed in the vapor plume, either from condensation in the presence of CO₂/SO₃-rich vapors, or reactions between CaO and CO₂/SO₃ rich vapors, and that this SO^- ₃-enhanced signature can be used as an indicator of impact vapor plume deposits. Given these two propositions, the ESR data for the Blake Nose deposits are ascribed to the presence of basal coarse calcitic Chicxulub ejecta clasts, while the finer components that are increasingly represented toward the top are interpreted to contain high-SO^- ₃ calcite from the vapor plume. The apparently-undisturbed Bass River deposit may contain even higher concentrations of vapor-plume calcite. None of the three components included in field III appear to be represented at distal, deep marine KT-boundary sites; this field may include several types of impact-related deposits of diverse origins and diagenetic histories.

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