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This paper introduces new algorithms for conducting and improving watershed analysis, implemented with the particular goal of improving the ability to measure the shapes of mineral grains to be subsequently be analyzed by mass spectrometry. This application requires a high degree of accuracy and fidelity in terms of both separating all touching grains and preserving their shapes. The algorithms are designed to take advantage of a vector-based programming environment. A new implementation of the Euclidean distance transform utilizes the fact that the distance from any adjacent pair of voxels to the nearest boundary must be within one voxel of each other. In practice, however, this algorithm is outperformed by a smoothed approximate distance transform that is faster to compute and results in less irregular watershed boundaries. A one-pass rainfall-based watershed algorithm is introduced that runs in linear time with the number of segmented voxels, and requires no priority queue. Unlike marker-based watershed algorithms based on the basin-filling approach, the rainfall approach finds watersheds associated with all local maxima in the distance map, even if a marking algorithm is used. A post-watershed smoothing algorithm improves watershed boundaries and eliminates small spurious watersheds. The one-pass watershed and post-watershed smoothing algorithms run in times superior or comparable to basin-fill watershed algorithms implemented in other environments, and offers excellent ability to separate touching objects efficiently while placing watershed boundaries that maximize the preservation of details of particle shape. Further time improvement could come from implementing them in a vector-based environment that allows explicit multi-threading. 

Abstract. Chemical abrasion is a technique that combines thermal annealing and partialdissolution in hydrofluoric acid (HF) to selectively removeradiation-damaged portions of zircon crystals prior to U–Pb isotopicanalysis, and it is applied ubiquitously to zircon prior to U–Pb isotopedilution thermal ionization mass spectrometry (ID-TIMS). The mechanics ofzircon dissolution in HF and the impact of different leaching conditions onthe zircon structure, however, are poorly resolved. We present amicrostructural investigation that integrates microscale X-ray computedtomography (µCT), scanning electron microscopy, and Ramanspectroscopy to evaluate zircon dissolution in HF. We show that µCTis an effective tool for imaging metamictization and complex dissolutionnetworks in three dimensions. Acid frequently reaches crystal interiors viafractures spatially associated with radiation damage zoning and inclusionsto dissolve soluble high-U zones, some inclusions, and material aroundfractures, leaving behind a more crystalline zircon residue. Other acid pathsto crystal cores include the dissolution of surface-reaching inclusions andthe percolation of acid across zones with high defect densities. In highlycrystalline samples dissolution is crystallographically controlled withdissolution proceeding almost exclusively along the c axis. Increasing theleaching temperature from 180 to 210 ∘C results indeeper etching textures, wider acid paths, more complex internal dissolutionnetworks, and greater volume losses. How a grain dissolves strongly dependson its initial radiation damage content and defect distribution as well asthe size and position of inclusions. As such, the effectiveness of anychemical abrasion protocol for ID-TIMS U–Pb geochronology is likelysample-dependent. We also briefly discuss the implications of our findingsfor deep-time (U-Th)/He thermochronology. 

Calibrating human population dispersals across Earth’s surface is fundamental to assessing rates and timing of anthropogenic impacts and distinguishing ecological phenomena influenced by humans from those that were not. Here, we describe the Hartley mammoth locality, which dates to 38,900–36,250 cal BP by AMS 14 C analysis of hydroxyproline from bone collagen. We accept the standard view that elaborate stone technology of the Eurasian Upper Paleolithic was introduced into the Americas by arrival of the Native American clade ∼16,000 cal BP. It follows that if older cultural sites exist in the Americas, they might only be diagnosed using nuanced taphonomic approaches. We employed computed tomography (CT and μCT) and other state-of-the-art methods that had not previously been applied to investigating ancient American sites. This revealed multiple lines of taphonomic evidence suggesting that two mammoths were butchered using expedient lithic and bone technology, along with evidence diagnostic of controlled (domestic) fire. That this may be an ancient cultural site is corroborated by independent genetic evidence of two founding populations for humans in the Americas, which has already raised the possibility of a dispersal into the Americas by people of East Asian ancestry that preceded the Native American clade by millennia. The Hartley mammoth locality thus provides a new deep point of chronologic reference for occupation of the Americas and the attainment by humans of a near-global distribution. 

Features such as particles, pores, or cracks are challenging to measure accurately in CT data when they are small relative to the data resolution, characterized as a point-spread function (PSF). These challenges are particularly acute when paired with segmentation, as the PSF distributes some of the signal from a voxel among neighboring ones; effectively dispersing some of the signal from a given object to a region outside of it. Any feature of interest with one or more dimensions on the order of the PSF will be impacted by this effect, and measurements based on global thresholds necessarily fail. Measurements of the same features should be consistent across different instruments and data resolutions. The PVB (partial volume and blurring) method successfully compensates by quantifying features that are small in all three dimensions based on their attenuation anomaly. By calibrating the CT number of the phase of interest (in this case, gold) it is possible to accurately measure particles down to <6 voxels in data acquired on two instruments, 14 years apart, despite severe artifacts. Altogether, the PVB method is accurate, reproducible, resolution-invariant, and objective; it is also notable for its favorable error structure. The principal challenge is the need for representative effective CT numbers, which reflect not only the features of interest themselves, but also the X-ray spectrum, the size, shape and composition of the enclosing sample, and processing details such as beam-hardening correction. Empirical calibration is the most effective approach. 

Abstract. (U–Th) ∕ He thermochronometry relies on the accurate andprecise quantification of individual grain volume and surface area, whichare used to calculate mass, alpha ejection (FT) correction, equivalentsphere radius (ESR), and ultimately isotope concentrations and age. The vastmajority of studies use 2-D or 3-D microscope dimension measurements and anidealized grain shape to calculate these parameters, and a long-standingquestion is how much uncertainty these assumptions contribute to observedintra-sample age dispersion and accuracy. Here we compare the results forvolume, surface area, grain mass, ESR, and FT correction derived from2-D microscope and 3-D X-ray computed tomography (CT) length and width datafor > 100 apatite grains. We analyzed apatite grains from twosamples that exhibited a variety of crystal habits, some with inclusions. Wealso present 83 new apatite (U–Th) ∕ He ages to assess the influence of 2-D versus 3-D FT correction on sample age precision and effective uranium(eU). The data illustrate that the 2-D approach systematically overestimatesgrain volumes and surface areas by 20 %–25 %, impacting the estimates formass, eU, and ESR – important parameters with implications for interpretingage scatter and inverse modeling. FT factors calculated from 2-D and 3-Dmeasurements differ by ∼2 %. This variation, however, haseffectively no impact on reducing intra-sample age reproducibility, even onsmall aliquot samples (e.g., four grains). We also present a grain-mountingprocedure for X-ray CT scanning that can allow hundreds of grains to be scannedin a single session and new software capabilities for 3-D FT andFT-based ESR calculations that are robust for relatively low-resolutionCT data, which together enable efficient and cost-effective CT-basedcharacterization.