Geology topics

Bryan Alan Killingsworth

Publications and source records attributed to Bryan Alan Killingsworth.

3 recordsLinked to original sources

USGS2025

Triple-oxygen isotopic evidence of prolonged direct bioleaching of pyrite with O2

Sulfate is often touted as containing atmospheric oxygen whose isotopic signature can constrain redox, environmental conditions, and biological activity. Yet, the amount and isotopic fractionation associated with air-O 2 incorporation during sulfate formation is still debated, making its verification difficult. In this study, we identify a distinct, microbially dominated environment with the potential to preserve maximum signals of air-O 2 in sulfate. We report triple-oxygen isotope data for sulfate produced from pyrite oxidation in microbial and abiotic experiments, and from natural dissolved sulfate from the Rio Tinto, Spain, an acid mine drainage site. The oxygen isotope systematics of sulfate in these environments define a unique kinetic isotope effect associated with initial stage pyrite oxidation by Acidithiobacillus ferrooxidans that preserves >80 % oxygen from air-O 2 in sulfate. Unlike experiments, which evolve toward water-oxygen dominated sulfate on short time scales, Rio Tinto, Spain hosts a microbe rich environment with distinct geochemistry that maintains high O 2 -oxygen in sulfate. Therefore, in addition to containing isotopic records from water and air, sulfates can also contain a biosignature that is promising for understanding conditions on Mars and early Earth.

Earth and Planetary Science Letters↗

USGS2022

Constraints on triple oxygen isotope kinetics

Isotopic disequilibrium is not as well constrained as equilibrium, hindering interpretation of isotopic variations. Kinetic isotope effects, a subset of disequilibrium, are common in nature and have been assumed to be distinct from equilibrium and mass independent isotope effects based on underdeveloped criteria. Using basic physical principles, we provide needed mechanistic constraints on mass-dependent kinetic isotope effects for the triple oxygen isotope system. We find some kinetic isotope effects yield large isotopic variations, exceeding equilibrium, which could be mistaken for mass independent relationships. Meanwhile, other kinetic isotope effects are found to have triple oxygen isotope relationships that could be mistaken for equilibrium isotope effects. Comparison against prior case studies of thermal decomposition of calcite (CaCO 3 ) and brucite (Mg(OH) 2 ) further tests our results. Although oxygen is the focus here, our approach applies to any system with more than two isotopes.

Chemical Geology↗

USGS2022

Towards a holistic sulfate-water-O2 triple oxygen isotope systematics

Triple oxygen isotope (∆ 17 O with δ 18 O) signals of H 2 O and O 2 found in sulfate of oxidative weathering origin offer promising constraints on modern and ancient weathering, hydrology, atmospheric gas concentrations, and bioproductivity. However, interpretations of the sulfate-water-O 2 system rely on assuming fixed oxygen-isotope fractionations between sulfate and water, which, contrastingly, are shown to vary widely in sign and amplitude. Instead, here we anchor sulfate-water-O 2 triple oxygen isotope systematics on the homogeneous composition of atmospheric O 2 with empirical constraints and modeling. Our resulting framework does not require a priori assumptions of the O 2 - versus H 2 O‑oxygen ratio in sulfate and accounts for the signals of mass-dependent and mass-independent fractionation in the ∆ 17 O and δ 18 O of sulfate's O 2 ‑oxygen source. Within this framework, new ∆ 17 O measurements of sulfate constrain ~2.3 Ga Paleoproterozoic gross primary productivity to between 6 and 160 times present-day levels, with important implications for the biological carbon cycle response to high CO 2 concentrations prevalent on the early Earth.

Chemical Geology↗