The origin and production of silt are key factors in the formation of loess deposits. Although many processes can potentially lead to silt generation, few are known to produce silt in the volumes and particle-size modes required to form geologically significant loess deposits. Here we investigate the hypothesis that pedogenic weathering in tropical and Mediterranean climates can generate abundant in situ silt, and therefore contribute significantly to loess formation throughout geologic time. We utilize granulometric and geochemical data from soils formed in Puerto Rico (hot-humid) and Southern California (hot-arid) to discern whether the mud fraction (<62.5 μm) is generated from bedrock (autochthonous) or sourced from eolian contributions (allochthonous). Our study demonstrates that the Puerto Rico soil contains abundant (up to 72%) silt- and clay-sized grains compared to the Anza Borrego soil (<6%). However, the silt fraction of the Puerto Rico soil is at least partially derived from eolian inputs, and the silt fraction of the Anza Borrego soil is geochemically indistinguishable from allochthonous dust sources. Furthermore, while intense chemical weathering in a tropical climate can produce abundant fines, the majority are significantly finer (average mode ~15 µm) than the modes of most “typical” loess deposits (modes more than 20 – 30 µm). In contrast, weathering in Mediterranean climates produces volumetrically sparse silt. Hence, pedogenic weathering in hot climates appears to be ineffectual for producing the volume and size distributions of silt-sized material needed to generate significant loess deposits.

High-resolution passive seismic imaging of shallow subsurface structures is often challenged by the scarcity of coherent body-wave energy in ambient noise recorded at surface stations. We show that autocorreleation (AC) of teleseismic P-wave coda extracted from just 1-month of continuous recording at 5 Hz geophones can overcome this limitation. We apply this method to investigate the longitudinal subsurface structure of Unaweep Canyon, a paleovalley in western Colorado (US) with complex evolution. Both fluvial and glacial processes have been proposed to explain the canyon’s genesis and morphology. The teleseismic P-wave coda AC retrieves zero-offset reflections from the shallow (200 – 500 m depth) basement interface at 120 stations along a 5 km long profile. Additionally, we invert interferometrically retrieved surface wave dispersion for the shear-wave structure of the sedimentary fill. Combined interpretation of these results and other geophysical and well data suggests an overdeepened basement geometry due to glacial processes.

Unaweep Canyon (Uncompaghre Plateau, Colorado) represents an enigmatic landscape with a complex evolution. Interpretations for its origin have ranged from ancestral fluvial erosion in the late Cenozoic to glacial erosion in the Paleozoic, or some combination thereof, with significant implications for global climatic and large-scale tectonic reconstructions. To address the conflicting interpretations, we acquired a high-resolution seismic reflection profile to investigate the depth, structure, and sedimentary infill in the canyon. The dataset is further complemented with a high-resolution electrical resistivity survey. Integrated with other geophysical and geological data, the results unambiguously demonstrate an overdeepened Precambrian basement with pronounced transverse U-shape and corroborate the hypothesis of a pre-Quaternary glacial origin. Our data constitute the first detailed and high-resolution image of a buried pre-Quaternary glacial valley in North America, and thus have far-reaching implications for our understanding of global ice houses as well as the tectonic conditions enabling preservation of such systems.

Mass wasting plays an important role on carbon cycling and sequestration by exposing fresh, weatherable bedrock and delivering hillslope sediments to lowlands and fluvial systems. Chemical weathering signatures of landslide-derived fluvial sediments can be used to understand linkages between hillslope and fluvial processes, and thus to characterize spatiotemporal dynamics of sediments. However, chemical signatures of fluvial sediments derived by landslides are yet to be fully understood. Here we compare the bulk chemistry, mineralogy, and grain size of fluvial sediments collected pre- and post-Hurricane Maria (landfall on Sept. 20, 2017) in the Rio Guayanés and Rio Guayabo River watersheds in southeastern Puerto Rico to help fill this knowledge gap. Relative to fluvial muds collected before Hurricane Maria, mud samples collected after the storm exhibit higher weathering index values, but coarser grain size modes. We infer that small landslides triggered by Hurricane Maria transported slope materials from shallow depths, including weathered topsoil and saprolite, as opposed to previous deep-seated landslides which likely sampled regolith and bedrock. The variances in weathering indices observed pre- and post-hurricane do not reflect climate change, but rather subtle differences in transport mechanism which produce significant differences in weathering indices recorded by fluvial sediments. We propose that weathering indices provide a means to understand sediment dynamics in mountainous regions, particularly for sediment transported in the immediate aftermath of landslides triggered by extreme events, such as precipitation and earthquakes, and also provide important datasets required for mapping potential carbon sequestration across a landscape.

The percentage of Geoscience students, faculty, and professionals from historically under-represented minority (URM) groups has been largely unchanged for two decades and remains significantly below general population trends. Diversifying Geosciences, and developing an equitable culture in the discipline requires faculty members (irrespective of race/ethnicity) to engage actively in JEDI (Justice-Diversity-Equity-Inclusion) efforts. Previous studies that focused broadly on faculty experiences in academia indicate that restructuring of existing faculty evaluation frameworks to better value JEDI work may help more URM faculty, researchers and students feel valued, thus enhancing diversity at all academic levels. However, such efforts may not be significantly valued in many Geoscience departments. To better understand faculty perspectives and motivations related to JEDI work, including the effects of faculty evaluation systems on behaviors, we are interviewing a range of faculty members across the US. Preliminary interviews suggest <50% of faculty in the Geoscience departments are actively involved in JEDI work and those who are, are more likely to be women and/or early career professionals. To aid to this data collection process, we will also be conducting a nation-wide survey of Geoscience faculty to better understand the value of JEDI activities in current faculty assessment (evaluations/promotion/tenure/raise etc.) frameworks, and identify potential barriers in engaging more faculty in meaningful DEI work. Using data from the interviews and survey results, we aim to develop example evaluation and reward structures that explicitly value JEDI work that can be adopted/adapted by other Geoscience departments, and to produce webinars focused on helping faculty leaders explicitly value JEDI efforts within hiring and evaluation systems.