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Abstract Otoliths are frequently used to infer environmental conditions or fish life history events based on trace-element concentrations. However, otoliths can be comprised of any one or combination of the three most common polymorphs of calcium carbonate—aragonite, calcite, and vaterite—which can affect the ecological interpretation of otolith trace-element results. Previous studies have reported heterogeneous calcium carbonate compositions between left and right otoliths but did not provide quantitative assessments of polymorph abundances. In this study, neutron diffraction and Raman spectroscopy were used to identify and quantify mineralogical compositions of Chinook salmon Oncorhynchus tshawytscha otolith pairs. We found mineralogical compositions frequently differed between otoliths in a pair and accurate calcium carbonate polymorph identification was rarely possible by visual inspection alone. The prevalence of multiple polymorphs in otoliths is not well-understood, and future research should focus on identifying otolith compositions and investigate how variations in mineralogy affect trace-element incorporation and potentially bias environmental interpretations.

Constraining the rise in atmospheric oxygen through the early Earth is important to understand the evolution of complex life. Here, the authors find that a major rise in atmospheric oxygen level occurred after the Great Oxidation Event, followed by pO2 within 1% of present atmospheric level through most of the Proterozoic Eon (2.4 to 0.65 Ga).

We tested rover science operations strategies to determine best practices for interrogating geologic sections where the bulk composition is igneous but depositional/emplacement processes range from sedimentary to volcanic. This scenario may mirror locations on Mars interrogated by mobile vehicles (e.g., Perseverance rover in Jezero crater). Two field teams studied a 60 m vertical outcrop on Iceland’s Tjörnes peninsula as an analog for a Martian site containing interleaved layers of sedimentary and volcanic units. A Rover team commanded a human rover to execute observations based on common Mars rover sequences; the resulting data were used to characterize the geologic history of the location. Results were compared to that of a Tiger team using traditional terrestrial field methods to interrogate the same site. The goal was to understand which instruments, at what resolution, are required to assess the provenance of volcanic or sedimentary layers of similar chemical composition. Results suggest that, in a succession dominated by rocks having basaltic composition, current rover-driven decision-making protocols are sufficient to support a first-order interpretation of a sequence of sedimentary and volcanic layers. Two crucial data sets in maximizing science return in this scenario are (1) handlens-scale images revealing grain morphology and relationships and (2) data sets that allow comparison between surface and bulk geochemistry. Certain sedimentary features can be difficult to confidently identify if not viewed at a specific angle and resolution, and confident interpretations appear to require lateral scanning of beds at meter scales. This work illuminates the need for strategic planning, particularly of resource-intensive observations.