Introduction Cosmic airbursts and impacts produce a wide range of surface effects, with high-altitude airbursts, such as the 1908 Tunguska event, primarily generating blast damage without forming craters [1]. In contrast, low-altitude “touch-down” airbursts may induce surface melting, spherule formation, shocked quartz, and shallow cratering [2]. Due to preservation challenges, few airburst signatures are documented in the geologic record, limiting our understanding of these events. Here, we report findings from a site approximately 5.8 km east of Perkins, Louisiana (30.3980° N, 93.3535° W) (Figure 1), where an anomalous 300-m-long seasonal lake/depression (Figure 2) is associated with extensive deposits of likely impact-related materials (Figure 3). We tested the hypothesis that an airburst/impact at the Perkins site produced these materials. To understand these deposits, we conducted a multi-disciplinary study employing a comprehensive suite of microscopy and geochemical techniques to characterize the material, investigate its origin, and determine whether the Perkins site represents an airburst/impact cratering event. Figure 1: Figure 2: Figure 3:
Research history In ~1938, Walter Leo Fitzenreiter, Jr., father of the lead author (R.F.), first speculated that a seasonal lake on his property was an “impact crater” based solely on its shape and crater-like rim raised ~1 m above the surrounding terrain. Later, in 2006, lead author R.F. began to test his father’s suggestion of a crater and discovered several locations around the lake displaying large semi-consolidated masses of spherules associated with multi-cm-sized fragments of black vesicular glass, which, he reasoned, might result from a cosmic airburst/impact event. Beginning in 2006, lead author R.F. found very large quantities of meltglass and spherules in two deposits near the raised rim of the lake, ~300 m long and 120 m wide. From 2006 through 2024, lead author R.F. also hand-extracted 32 cores (~4 to 13 cm in diameter) and excavated two trenches (~3 m wide). Locations and results are described in the sections below. In 2007, after identifying numerous spherules and fragments of meltglass in the Vee and Pond deposits, R.F. first contacted one of the co-authors (K.E.), who analyzed the spherules and meltglass. His analyses suggested that they resulted from a cosmic impact event, and he recommended additional research. In 2011, R.F. next contacted members of the Comet Research Group, who analyzed these materials to investigate their characteristics and origin.
Information on airbursts Cosmic airbursts can occur at various altitudes, producing distinctly different surface effects. High-altitude airbursts, like the 1908 Tunguska event (~5-10 km altitude) [1], primarily cause widespread devastation through blast waves but typically leave no craters [2]. Even so, the airburst evidence at Tunguska has been reported to include spherules, shocked quartz, glass-filled fractured quartz, melted feldspar, carbon spherules, and glasslike carbon [1]. Such events, called “touch-down airbursts,” occur when a bolide explodes sufficiently close to Earth’s surface that high-velocity fragments, the shockwave, and the thermal pulse form meltglass, create spherules, generate shocked quartz, and potentially create shallow ephemeral craters [2]. Due to the scarcity of documented cases of airbursts and the frequent lack of evidence preserved in the geologic record, the effects of airbursts are still poorly understood. However, understanding them is vitally important because, as Boslough and Crawford [3, 4] wrote, “Low-altitude airbursts are by far the most frequent impact events that have an effect on the ground.” To provide additional insights into airburst processes in the geologic record, we document our discoveries at this new proposed airburst location near Perkins, SW Louisiana (Figure 1).
Methods We employed standard protocols for eight complementary techniques to characterize spherules, meltglass, and shocked quartz (for more details, see Supporting Information, https://zenodo.org/records/15496666 [5]). Optical microscopy (OPT). Transmission microscopy is used for 3D imaging. It also uses crossed polarizers to identify isotropic areas of melted silica in quartz grains. EPI-illumination microscopy (EPI) is used for 3D imaging of spherules and meltglass. It also reveals if a fracture in a quartz grain is filled with material but not the nature of the material. Scanning electron microscopy (SEM): Used to image and analyze spherules and meltglass. It also reveals if quartz fractures are filled with silica without determining if it was melted. Energy dispersive spectroscopy (EDS): A standardless technique used to determine the elemental and oxide compositions of spherules, meltglass, and the quartz fracture-filling material (e.g., melted silica, hydrated silica, carbon, other minerals, or polishing compounds). Laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS): A laser beam and mass spectrometer were used to analyze the elemental and isotopic composition of spherules and meltglass. Cathodoluminescence (CL): Used to differentiate between amorphous and crystalline areas of quartz grains; non-luminescent (black) areas indicate melted silica [6–8]. Neutron activation analysis (INAA): This nuclear technique measures the elemental concentrations in a material by irradiating it with neutrons and measuring the resulting gamma rays. Universal stage (u-stage): This technique was used to index candidate shocked quartz grains, previously identified using a petrographic microscope. Indexing some quartz grains with a universal stage was problematic because they appeared thermally distorted. However, indexing was possible in those areas where the lamellae remained planar and parallel. The u-stage is considered the definitive technique for identifying shocked quartz. Argon-argon dating (40Ar/39Ar): This radiometric dating technique can determine the formation age of rocks that underwent rapid cooling to below their closure temperature since impact-related heating likely reset previous ages. Samples were neutron-irradiated for 6 hours, which is appropriate for ages from 1 Ma to 100 Ma.
Study objectives Material composition: analyze the material with a focus on the morphology and composition of Fe- and Si-rich spherules, meltglass, carbon spherules, melted minerals, and fractured quartz grains, with the latter as a widely accepted indicator of shock metamorphism [2, 9–12]. Dating: produce an age-depth model for Trench #1 to determine the age of the spherule-and-meltglass abundance peak and acquire calibrated radiocarbon ages on the carbon-rich meltglass surrounded by very large amounts of spherules. Lastly, determine whether or not the meltglass is co-eval with the spherules and meltglass in Trench #1. Formation conditions: estimate the temperatures and pressures required to produce these materials. Potential formation mechanisms: compare anthropogenesis, authigenesis, volcanism, tectonism, lightning discharges, and cosmic airbursts/impact. Hydrocode modeling: produce a cosmic impact model consistent with the observed evidence.
Results This section documents our analyses of the wide range of materials observed at the Perkins site. To investigate the characteristics of these materials, we used eight analytical techniques, as described in the Methods section. Below, we offer a short introduction before presenting the results for each proxy. Also, to assist in understanding the complex, multidisciplinary data presented in this study, we provide the following Table of Contents of the Results and Discussion sections: Table of Contents Results 1. Sedimentology and Stratigraphy
2. Age of Materials 2.1 Radiocarbon Dating 2.2 Argon-Argon Dating
3. High-Temperature Melted Spherules 3.1 Spherule Abundances 3.2 Spherule Distribution 3.3 Spherule Morphologies
4. Carbon Spherules and Glasslike Carbon
5. Meltglass
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