Reactions that occur during the hydration and aqueous alteration of ultramafic rocks (collectively, serpentinization reactions) are important where fluids circulate in seafloor oceanic crust and uplifted mantle rocks on Earth. Evidence of serpentinization has also been found in carbonaceous chondrite meteorites (Velbel et al., 2012), and serpentine minerals have been detected spectrally on a variety of rocky bodies in the solar system (Mars,
The Oman Drilling Project established an "Active Alteration" multi-borehole observatory in peridotites undergoing low-temperature serpentinization in the Samail Ophiolite. The highly serpentinized rocks are in contact with strongly reducing fluids. Distinct hydrological regimes, governed by differences in rock porosity and fracture density, give rise to steep redox (Eh +200 to −750 mV) and pH (pH range 8.5-11.2) gradients within the 300-400 m deep boreholes. The serpentinites and fluids host an active subsurface ecosystem. Microbial cell abundances in serpentinite vary at least six orders of magnitude, from ≤3.5 × 10 1 to 2.9 × 10 7 cells/g. Low levels of biological sulfate reduction (2-1,000 fmol/ cm 3 /day) can be detected in rock cores, particularly in rocks in contact with reduced groundwaters with pH < 10.5. Thermodesulfovibrio is the predominant sulfate reducer identified via metagenomic sequencing of adjacent groundwater communities. We infer that transport and reaction of microbially generated sulfide with the serpentine and brucite assemblages gives rise to optical darkening and sulfide overprinting, including the formation of tochilinite-vallerite group minerals, potentially serving as an indicator that this system is inhabited by microbial life. Olivine mesh-cores replaced with ferroan brucite and minor awaruite, abundant veins containing hydroandradite garnet and polyhedral serpentine, and late-stage carbonate veins are suggested as targets for future spatially resolved life-detection investigations. The high-quality whole-round core samples that have been preserved can be further probed to define how life distributes itself and functions within a system where chemical disequilibria are sustained by lowtemperature water/rock interaction, and how biosignatures of in situ microbial activity are generated.Plain Language Summary Ultramafic rocks undergoing water/rock interaction, and storing fluids that are far from chemical equilibrium, may be one of the most common habitats in our solar system. Through the Oman Drilling Project we collected >1 km of intact serpentinite in contact with groundwaters. These cores capture parts of the rock-hosted biosphere and show how cells are distributed within serpentinites that vary in their mineralogical, physical and chemical properties. The cores are also biologically active, enabling us to detect specific metabolisms, such as when microorganisms combine hydrogen as reductant and sulfate as an oxidant to fuel their metabolism. Although the distribution of microbial cells in the rock cores is very heterogeneous, there are many intervals where the abundance of cells constitutes robust biomass. In the deeper cores, slow, albeit detectable, microbial sulfate reduction proceeds. We suggest that this pervasive biological activity releases byproducts such as sulfide that can react with the serpentinite and change the optical and chemical properties of the rocks. The feedbacks between the rock alteration and microbial activity produce markers that enable us to focus our sea...
Abstract. Apatite (U-Th)/He (AHe) dating generally assumes that grains can be accurately and precisely modeled as geometrically perfect hexagonal prisms or ellipsoids in order to compute the apatite volume (V), alpha-ejection corrections (FT), equivalent spherical radius (RFT), effective uranium concentration (eU), and corrected (U-Th)/He date. It is well-known that this assumption is not true. In this work, we present a set of corrections and uncertainties for V, FT, and RFT aimed at 1) “undoing” the systematic deviation from the idealized geometry and 2) quantifying the contribution of geometric uncertainty to the total uncertainty budget on eU and AHe dates. These corrections and uncertainties can be easily integrated into existing laboratory workflows at no added cost, can be routinely applied to all dated apatite, and can even be retroactively applied to published data. To quantify the degree to which real apatite deviate from geometric models, we selected 267 grains that span the full spectrum of commonly analyzed morphologies, measured their dimensions using standard 2D-microscopy methods, and then acquired 3D scans of the same grains using high-resolution computed-tomography. We then compared the V, FT, and RFT calculated from 2D-microscopy measurements with those calculated from the ‘real’ 3D measurements. We find that apatite V, FT, and RFT values are all consistently overestimated by the 2D microscopy method, requiring correction factors of 0.74–0.83 (or 17–26 %), 0.91–0.99 (or 1–9 %), and 0.85–0.93 (or 7–15 %), respectively. The 1s uncertainties on V, FT, and RFT are 20–23 %, 1–6 %, and 6–10 %, respectively. The primary control on the magnitude of the corrections and uncertainties is grain geometry, with grain size exerting additional control on FT uncertainty. Application of these corrections and uncertainties to a real dataset yields 1s analytical and geometric uncertainties of 15–16 % on eU and 3–7 % on the corrected date. These geometric corrections and uncertainties are substantial and should not be ignored when reporting, plotting, and interpreting (U-Th)/He datasets. The Geometric Correction Method presented here provides a simple and practical tool for deriving more accurate FT and eU values, and for incorporating this oft neglected geometric uncertainty into AHe dates.
Extreme precipitation can have profound consequences for communities, resulting in natural hazards such as rainfall-triggered landslides that cause casualties and extensive property damage. A key challenge to understanding and predicting rainfall-triggered landslides comes from observational uncertainties in the depth and intensity of precipitation preceding the event. Practitioners and researchers must select from a wide range of precipitation products, often with little guidance. Here we evaluate the degree of precipitation uncertainty across multiple precipitation products for a large set of landslide-triggering storm events and investigate the impact of these uncertainties on predicted landslide probability using published intensity-duration thresholds. The average intensity, peak intensity, duration, and NOAA-Atlas return periods are compared ahead of 177 reported landslides across the continental United States and Canada. Precipitation data are taken from four products that cover disparate measurement methods: near real-time and post-processed satellite (IMERG), radar (MRMS), and gauge-based (NLDAS-2). Landslide-triggering precipitation was found to vary widely across precipitation products with the depth of individual storm events diverging by as much as 296 mm with an average range of 51 mm. Peak intensity measurements, which are typically influential in triggering landslides, were also highly variable with an average range of 7.8 mm/h and as much as 57 mm/h. The two products more reliant upon ground-based observations (MRMS and NLDAS-2) performed better at identifying landslides according to published intensity-duration storm thresholds, but all products exhibited hit ratios of greater than 0.56. A greater proportion of landslides were predicted when including only manually verified landslide locations. We recommend practitioners consider low-latency products like MRMS for investigating landslides, given their near-real time data availability and good performance in detecting landslides. Practitioners would be well-served considering more than one product as a way to confirm intense storm signals and minimize the influence of noise and false alarms.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.